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Spin-up of rubble-pile asteroids: disruption, satellite formation, and equilibrium shapes

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus

Kevin J. Walsh, Derek C. Richardson, Patrick Michel

We present results from numerical experiments testing the behavior of cohesionless gravitational aggregates experiencing a gradual increase of angular momentum. The test bodies used in these numerical simulations are gravitational aggregates of different construction, distinguished by the size distribution of the particles constituting them, parameterized in terms of the angle of friction (ϕ). Shape change and mass loss are found to depend strongly on ϕ, with results ranging from oblate spheroids forming binary systems to near-fluid behavior characterized by mass shedding bursts and no binary formation. Bodies with the highest angle of friction, ϕ∼40^°, evolve to shapes with average axis ratios of c/a ∼0.70 and b/a ∼0.90 (a ≥b ≥c), and are efficient at forming satellites. Bodies with lower angle of friction, ϕ∼20^°, evolve to shapes with average axis ratios of c/a ∼0.61 and b/a ∼0.83, and are less efficient at forming satellites. The most fluid-like bodies tested, with ϕnear zero, become very elongated, with average axis ratios c/a ∼0.40 and b/a ∼0.56, and do not form satellites in any simulation. In all but 2 fluid-like cases out of 360, no more than 5% of the total mass was ejected in a single event. Bodies with substantial cores were also tested under slow spin-up, and cases with cores larger than ∼30% of the total mass were successful at forming binaries. The binary systems created in all simulations are analysed and compared against observed binary near-Earth asteroids and small Main Belt asteroids. The shape and rotation period of the primary, orbital and rotational period of the secondary, and the orbital semi-major axis and eccentricity are found to closely match the observed population.

Highlights

► We model the slow spin-up of rubble pile asteroids. ► Asteroids with high angles of friction (>40 deg) can maintain spherical/oblate shapes at critical Rotation. ► Asteroids that can maintain spherical/oblate shapes can build satellites in orbit via mass lost from their equators

The application of new methane line absorption data to Gemini-N/NIFS and KPNO/FTS observations of Uranus’ near-infrared spectrum

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus

P.G.J. Irwin, C. de Bergh, R. Courtin, B. Bézard, N.A. Teanby, G.R. Davis, L.N. Fletcher, G.S. Orton, S.B. Calcutt, D. Tice, J. Hurley

New line data describing the absorption of CH4 and CH3D from 1.26 – 1.71 μm (Campargue et al. 2012, building upon previous papers by Campargue et al. 2010; Wang et al. 2010, 2011) have been applied to the analysis of Gemini-N/NIFS observations of Uranus made in 2010 and compared with earlier disc-averaged observations made by KPNO/FTS in 1982. The new line data are found to improve greatly the fit to the observed spectra and present a huge advance over previous methane absorption tables by allowing us to determine the CH3D/CH4 ratio and also start to break the degeneracy between methane abundance and cloud top height. The best fits are obtained if the cloud particles in the main cloud deck at the 2-3 bar level become less scattering with wavelength across the 1.4 – 1.6 μm region and we have modelled this variation here by varying the extinction cross-section and single-scattering albedo of the particles. Applying the new line data to the NIFS spectra of Uranus, we determine a new estimate of the CH3D/CH4 ratio of, which is consistent with the estimate of de Bergh et al. (1986) of, made by fitting a disc-averaged KPNO/FTS spectrum measured in 1982, but much better constrained. The NIFS observations made in 2010 have been disc-averaged and compared with the 1982 KPNO/FTS spectrum and found to be in excellent agreement. Using k-tables fitted to the new line data, the central meridian observations of Uranus H-band spectrum (1.49 – 1.64 μm) made by Gemini-N/NIFS in 2010 have been reanalyzed. The use of the new methane absorption coefficients and the modified scattering properties of the cloud particles in the main cloud deck appears to break the degeneracy between cloud height and methane abundance immediately above it in this spectral region and we find that both vary with latitude across Uranus’ disc. Overall, we find that the main cloud deck becomes higher, but thinner from equator to poles, with a local maximum in cloud top height in the circumpolar zones at 45° N and 45°S. At the same time, using the ‘D’ temperature pressure profile of Lindal et al. (1987) and a deep methane abundance of 1.6% (Baines et al., 1995) we find that the relative humidity of methane is high near the equator (∼60%) and decreases sharply towards the poles, except near the circumpolar zone at 45° N, which has brightened steadily since 2007, and where there is a local maximum in methane relative humidity. In tests conducted with the warmer ‘F1’ profile of Sromovsky et al. (2011) we find a similar variation of methane abundance above the main cloud, although for this warmer temperature profile this abundance is dependent mostly on the fitted deep methane mole fraction.

Highlights

► Revised methane line data give very good fit to Uranus H-band spectra. ► Reflectivity of particles in main cloud deck is found to decrease with wavelength from 1.4 – 1.6 μm. ► Uranus’ CH3D/CH4 ratio is determined to be
${2.9}_{- 0.5}^{+ 0.9} \times 10^{- 4}$

. ► New line data allow differentiation between methane humidity and cloud top height. ► Methane humidity is found to be high in the tropics, but to decrease rapidly polewards of 45° N and S.

Sustainability of a Subsurface Ocean within Triton’s Interior

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus

Jodi Gaeman, Saswata Hier-Majumder, James H. Roberts

We present a study of coupled thermal and structural evolution of Neptune’s moon, Triton, driven by tidal dissipation and radiogenic heating. Triton’s orbital history likely involves capture from a binary system by Neptune, followed by a period of circularization. This work investigates Triton’s evolution past its circularization. We examine the rate of ice shell growth as a function of different orbital eccentricities, in the presence of radiogenic heating. Tidal dissipation in the ice shell, proportional to orbital eccentricity squared, concentrates heating near the base, reducing the basal heat flux. As the growth of the ice shell is proportional to the basal heat flux, increased tidal heating creates a blanketing effect, reducing the rate of ice shell growth. Radiogenic heating from Triton’s core is the other, more dominant, source of heat to the shell. Despite being several orders of magnitude higher than the tidal dissipation, radiogenic heating alone fails to sustain an ocean within Triton over 4.5 Ga. For orbital eccentricities of
$5 \times 10^{- 7}$

and
$3 \times 10^{- 5}$

it takes approximately 2 Ga and 3 Ga, respectively, to completely freeze the ocean. For higher values of orbital eccentricities, an ocean can be sustained in Triton’s interior over 4.5 Ga. If Triton’s history past circularization involves a slow decrease in orbital eccentricity to the current value, a thin, possibly
${NH}_{3}$

-rich ocean exists beneath Triton’s icy shell.

Highlights

► Coupled thermal-structural evolution. ► Tidal blanket. ► Triton’s ocean

The fate of sub-micron circumplanetary dust grains II: Multipolar Fields

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus

Daniel Jontof-Hutter, Douglas P. Hamilton

We study the radial and vertical stability of dust grains launched with all charge-to-mass ratios at arbitrary distances from rotating planets with complex magnetic fields. We show that the aligned dipole magnetic field model analyzed by Jontof-Hutter and Hamilton (2012) is an excellent approximation in most cases, but that fundamentally new physics arises with the inclusion of non-axisymmetric magnetic field terms. In particular, large numbers of distant negatively-charged dust grains, stable in a magnetic dipole, can be driven to escape by a more complex field. We trace the origin of the instability to overlapping Lorentz resonances which are extremely powerful when the gravitational and electromagnetic forces on a dust grain are comparable. These resonances enable a dust grain to tap the spin energy of the planet to power its escape. We also explore the relatively minor influence of different launch speeds and the far more important effects of variable grain charge. Only the latter are capable of significantly affecting the micron-sized grains that dominate visible and infrared images of faint dust rings. Finally, we present full stability maps for Earth, Jupiter, Saturn, Uranus, and Neptune with magnetic fields modeled out to octupole order. Not surprisingly, dust in the tortured magnetic fields of Uranus and Neptune show the greatest instability.

Highlights

► Planetary magnetic fields affect the orbits of Kepler-launched dust grains. ► We explore the dynamics of grains at Jupiter, Saturn, Uranus, Neptune and Earth. Non-zero launch impulses have little effect on orbital stability. ► Lorentz resonances can destabilize motions causing negative grains to escape. ► We explore the sensitivity of dust grain orbits to time-variable charging.

Problems determining relative and absolute ages using the small crater population

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus

Zhiyong Xiao, Robert G. Strom

Abstract The small crater populations (diameter smaller than 1 km) are widely used to date planetary surfaces. The reliability of small crater counts is tested by counting small craters at several young and old lunar surfaces, including Mare Nubium and craters Alphonsus, Tycho and Giordano Bruno. Based on high-resolution images from both the Lunar Reconnaissance Orbiter Camera and Kaguya Terrain Camera, small craters in two different diameter ranges are counted for each counting area. Large discrepancies exist in both the cumulative (absolute model ages) and relative plots for the two different size ranges of the same counting areas. The results indicate that dating planetary surfaces using small crater populations is highly unreliable because the contamination of secondaries may invalidate the results of small crater counts. A comparison of the size-frequency distributions of the small crater populations and impact ejected boulders around fresh lunar craters shows the same upturn as typical Martian secondaries, which supports the argument that secondaries dominate the small crater populations on the Moon and Mars. Also, the size-frequency distributions of small rayed lunar and Martian craters of probable primary origin are similar to that of the Population 2 craters on the inner solar system bodies post-dating Late Heavy Bombardment. Dating planetary surfaces using the small crater populations requires the separation of primaries from secondaries which is extremely difficult. The results also show that other factors, such as different target properties and the subjective identification of impact craters by different crater counters, may also affect crater counting results. We suggest that dating planetary surfaces using small crater populations should be with highly cautious.

Highlights

► We count small craters on both young and old surfaces using LROC and Kaguya data. ► Craters in 2 diameter ranges are counted for each counting area and then compared. ► Discrepancies occur in the results and the contamination of secondaries is one reason. ► Steep upturns in size distributions are caused by secondaries but not primaries. ► Lunar and Martian small primaries have flat slopes like the Population 2 craters.

Upper Limits on the Size of Satellites of Asteroid (4) Vesta from 2007 Hubble Space Telescope Observations

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus

Lucy A. McFadden, Fabienne A. Bastien, Max Mutchler, Carolyn A. Crow, Heather Weir, Jian-Yang Li, Douglas P. Hamilton

We imaged the region around asteroid (4) Vesta in nine long exposures using the Wide Field Planetary Camera 2 on the Hubble Space Telescope on May 14 and 16, 2007 to conduct a deep search for satellites in support of NASA’s Dawn mission that orbited (4) Vesta in 2011-2012. Several previous search efforts have been undertaken, but no satellites were detected. Our search covered distances from 14 to 260 Vesta radii and searched to a limiting magnitude of 22.5 ± 0.4 in HST’s wide-band red filter (F702W). Our upper limit for possible satellites corresponds to a satellite just 22 ± 4m in radius, assuming the same optical properties as Vesta. Our upper limit is ∼10 times smaller than the best limit of previous searches. In situ satellite searches by NASA’s Dawn spacecraft will probe regions closer to Vesta than our effort reported here.

Highlights

► Vestan satellite upper limit is 22 ±4 m, ∼10 times smaller than previous searches. ► Volunteer searchers found almost all objects brighter than 22.5 ±0.4 magnitude. ► With albedo at 700 nm of 0.47 (Vesta’s), this magnitude corresponds to a radius of 22 ±4 meters. ► The region searched does not extend to the altitude of the Dawn mission’s spacecraft. ► There remains scientific reason to search for satellites with the Dawn spacecraft.

A Parametric Study of Io’s Thermophysical Surface Properties and Subsequent Numerical Atmospheric Simulations Based on the Best Fit Parameters

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus

Andrew C. Walker, Chris H. Moore, David B. Goldstein, Philip L. Varghese, Laurence M. Trafton

Io’s sublimation atmosphere is inextricably linked to the SO2 surface frost temperature distribution which is poorly constrained by observations. We constrain Io’s surface thermal distribution by a parametric study of its thermophysical properties in an attempt to better model the morphology of Io’s sublimation atmosphere. Io’s surface thermal distribution is represented by three thermal units: sulfur dioxide (SO2) frosts/ices, non-frosts (probably sulfur allotropes and/or pyroclastic dusts), and hot spots. The hot spots included in our thermal model are static high temperature surfaces with areas and temperatures based on Keck infrared observations. Elsewhere, over frosts and non-frosts, our thermal model solves the one-dimensional heat conduction equation in depth into Io’s surface and includes the effects of eclipse by Jupiter, radiation from Jupiter, and latent heat of sublimation and condensation. The best fit parameters for the SO2 frost and non-frost units are found by using a least-squares method and fitting to observations of the Hubble Space Telescope’s Space Telescope Imaging Spectrograph (HST STIS) mid- to near-UV reflectance spectra and Galileo PPR brightness temperature. The thermophysical parameters are the frost Bond albedo, α F , and thermal inertia, Γ F , as well as the non-frost surface Bond albedo, α NF , and thermal inertia, Γ NF . The best fit parameters are found to be α F ≈ 0.55 ± 0.02 and Γ F ≈ 200 ± 50 J m^-2 K^-1 s^-1/2 for the SO2 frost surface and α NF ≈ 0.49 ± 0.02 and Γ NF ≈ 20 ± 10 J m^-2 K^-1 s^-1/2 for the non-frost surface. These surface thermophysical parameters are then used as boundary conditions in global atmospheric simulations of Io’s sublimation-driven atmosphere using the direct simulation Monte Carlo (DSMC) method. These simulations are unsteady, three-dimensional, parallelized across 360 processors, and include the following physical effects: inhomogeneous surface frosts, plasma heating, and a temperature-dependent residence time on the non-frost surface. The DSMC simulations show that the sub-Jovian hemisphere is significantly affected by the daily solar eclipse. The simulated SO2 surface frost temperature is found to drop only ∼5 K during eclipse due to the high thermal inertia of SO2 surface frosts but the SO2 gas column density falls by a factor of 20 compared to the pre-eclipse column due to the exponential dependence of the SO2 vapor pressure on the SO2 surface frost temperature. Supersonic winds exist prior to eclipse but become subsonic during eclipse because the collapse of the atmosphere significantly decreases the day-to-night pressure gradient that drives the winds. Prior to eclipse, the supersonic winds condense on and near the cold nightside and form a highly non-equilibrium oblique shock near the dawn terminator. In eclipse, no shock exists since the gas is subsonic and the shock only reestablishes itself an hour or more after egress from eclipse. Furthermore, the excess gas that condenses on the non-frost surface during eclipse leads to an enhancement of the atmosphere near dawn. The dawn atmospheric enhancement drives winds that oppose those that are driven away from the peak pressure region above the warmest area of the SO2 frost surface. These opposing winds meet and are collisional enough to form stagnation point flow. The simulations are compared to Lyman-α observations in an attempt to explain the asymmetry between the dayside atmospheres of the anti-Jovian and sub-Jovian hemispheres. Lyman-α observations indicate that the anti-Jovian hemisphere has higher column densities than the sub-Jovian hemisphere and also has a larger latitudinal extent. A composite “average dayside atmosphere” is formed from a collisionless simulation of Io’s atmosphere throughout an entire orbit. This composite “average dayside” atmosphere without the effect of global winds indicates that the sub-Jovian hemisphere has lower average column densities than the anti-Jovian hemisphere (with the strongest effect at the sub-Jovian point) due primarily to the diurnally averaged effect of eclipse. This is in qualitative agreement with the sub-Jovian / anti-Jovian asymmetry in the Lyman-α observations which were alternatively explained by the bias of volcanic centers on the anti-Jovian hemisphere. Lastly, the column densities in the simulated average dayside atmosphere agree with those inferred from Lyman-α observations despite the thermophysical parameters being constrained by mid- to near UV observations which show much higher instantaneous SO2 gas column densities. This may resolve the apparent discrepancy between the lower “average dayside” column densities observed in the Lyman-α and the higher instantaneous column densities observed in the mid- to near UV.

Highlights

► Io’s surface thermophysical properties are constrained by a parametric study. ► Best fit albedos are α F ≈ 0.55 and α NF ≈ 0.49. ► Best fit thermal inertias are Γ F ≈ 200 J m^-2 K^-1 s^-1/2 and Γ NF ≈ 20 J m^-2 K^-1 s^-1/2. ► Atmospheric structure of the sub-Jovian side is strongly affected by Jovian eclipse. ► Sub-Jovian / anti-Jovian hemisphere asymmetry is likely partially caused by eclipse.

Sublimation-driven erosion on Hyperion. Topographic analysis and landform simulation model tests

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus

Alan D. Howard, Jeffrey M. Moore, Paul M. Schenk, Oliver L. White, John Spencer

The unique appearance of Hyperion can be explained in part by the loss to space of ballistic ejecta during impact events, as was proposed by Thomas et al (2007a). We conclude that such loss is a partial explanation, accounting for the lack of appreciable intercrater plains on a saturation-cratered surface. In order to create the smooth surfaces and the reticulate, honeycomb pattern of narrow divides between old craters, appreciable subsequent modification of crater morphology must occur through mass-wasting processes accompanied by sublimation, probably facilitated by the loss of co2 as a component of the relief-supporting matrix of the bedrock. During early stages of crater degradation, steep, crenulate bedrock slopes occupy the upper crater walls with abrupt transitions downslope onto smooth slopes near the angle of repose mantled by mass wasting debris, as can be seen within young craters. Long-continued mass wasting eventually results in slopes totally mantled with particulate debris. This mass wasting effectively destroys small craters, at least in part accounting for the paucity of sub-kilometer craters on Hyperion. Surface temperatures measured by Cassini CIRS range from 58°K to 127°K and imply a surface thermal inertia of 11 ± 2 J m^-2 K^-1 s^-1/2 and bolometric albedo ranging from 0.05 to 0.33. Resulting H2O sublimation rates are only tens of cm per billion years for most of the surface, so the evolution of the observed landforms is likely to require sublimation of more volatile species such as CO2.

Highlights

► The lack of inter-crater plains is in part due to loss of fall back ejecta. ► Weathering and mass wasting creates honeycomb pattern of old craters. ► Fresher craters exhibit upper slopes that are still undergoing erosional retreat. ► This style of erosion is consistent with sublimation loss of topography-supporting cement.

Compositional Analysis of Hyperion with the Cassini Visible and Infrared Mapping Spectrometer

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus

J. Brad Dalton III, Dale P. Cruikshank, Roger N. Clark

Compositional mapping of the surface of Hyperion using Cassini Visible and Infrared Mapping Spectrometer (VIMS) observations reveals a heterogeneous surface dominated by water ice accompanied by additional materials. Carbon dioxide, as evidenced by a prominent absorption band centered at 4.26 μm, is distributed over most of the surface, including icy regions. This does not represent exposures of pure CO2 ice, but concentrations of CO2 molecules adsorbed on other materials or complexed in H2O, perhaps as a clathrate (Cruikshank et al. 2010). Localized deposits of low-albedo material in subcircular depressions exhibit spectral absorptions indicative of C-H in aromatic (3.29 μm) and aliphatic (3.35-3.50 μm) hydrocarbons. An absorption band at 2.42 μm that is also seen on other Saturnian satellites, tentatively identified as H2 (Clark et al., 2011, 2012) adsorbed on dark material grains, is also prominent. Our best spectral models included H2O and CO2 ice, with small amounts of nanophase Fe and Fe2O3. Weaker and more spatially scattered absorption features are also found at 4.48, 4.60, and 4.89 μm, although no clear molecular identifications have yet been made. While strongest in the low-albedo deposits, the CO2, hydrocarbon and putative H2 bands vary in strength throughout the icy regions, as do the 4.48-, 4.60- and 4.89-μm bands, suggesting that this background ice is laced with a complex mixture of non-ice compounds.

Highlights

► We examine Cassini visible and near-infrared imaging spectroscopy of Hyperion. ► Surface mostly H2O ice together with CO2 and both aliphatic and aromatic hydrocarbons. ► CO2, H2, carbon-and nanophase Fe-bearing material concentrated in dark lag deposits. ► Carbon dioxide and other contaminants are mixed into the water ice everywhere. ► Hyperion bears primordial material and was probably formed elsewhere prior to capture.

A noon-to-midnight electric field and nightside dynamics in Saturn’s inner magnetosphere, using microsignature observations

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus

M. Andriopoulou, E. Roussos, N. Krupp, C. Paranicas, M. Thomsen, S. Krimigis, M.K. Dougherty, K.-H. Glassmeier

We have created a new, updated catalogue of energetic electron microsignature events caused by the moons Tethys and Dione. We used electron data of the MIMI-LEMMS detector that is onboard the Cassini spacecraft, in the energy range 20-300 keV and for the period from July 2004 to January 2011. The present study looks at how the location of a moon’s wake deviates from the nearly circular orbital path of the body. The radial deviation of the wake from the moon’s orbit is a very sensitive tracer of plasma motion in the magnetosphere including its small radial components. The positions of the dropouts the spacecraft detects when it flies through the wakes, or microsignatures, cannot be explained in our study by asymmetric magnetic fields in the inner magnetosphere. Instead, we hypothesize a uniform electric field of around 0.11 to 0.18 mV/m within 4.4-7.0 Rs approximately, oriented roughly from noon to midnight, to explain the persistent radial offsets of the microsignatures from their expected positions. This corresponds to a radial speed that is at most a few percent of rigid corotation and therefore very difficult to measure by direct means. We additionally report a tendency for microsignatures with non-monotonic energy dispersion to have drifted across the post-midnight sector more than those with zero or monotonic energy dispersion.

Highlights

► We report systematic asymmetries of the microsignature radial displacements with respect to local time in Saturn, inward in the nightside and outward in the dayside. ► Magnetic field asymmetries are much smaller than the observed microsignature displacements. ► We require a uniform electric field, pointing towards noon to midnight and with strength of not more than 1.0 mV/m to account for these displacements. ► A flow with a dusk-to-dawn orientation and radial velocity of few km/sec must exist in the inner magnetosphere of Saturn.

Wave signature in the Venus dayside cloud layer at 58-64 km observed by ground-based infrared spectroscopy

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus

M. Hosouchi, T. Kouyama, N. Iwagami, S. Ohtsuki, M. Takagi

We performed infrared spectroscopic measurements of the Venus’ dayside to find out atmospheric wave structures at 58-64 km in the cloud layer by quantifying carbon dioxide absorption. This is the first investigation of atmospheric waves at this altitude region. This effective absorption altitude lies in between that of 70 km by the ultraviolet measurements and of 50 km by the infrared thermal emission measurements. The apparent rotation periods of wave-like signature found are 3.5, 4.9 and 8.4 days, respectively, in May 2007, November 2007 and August 2010 assuming dominance of wavenumber one structure. Those apparent periods may be interpreted as superposition of the mean zonal flow and waves such as the Kelvin and the Rossby.

Highlights

► We performed infrared spectroscopic measurements of the Venus dayside. ► Atmospheric waves are discovered in the cloud layer by quantifying CO2 absorption. ► The apparent rotation periods are found with wavenumber one structure. ► They may be superposition of the mean flow and the Kelvin and/or Rossby waves.

S-burst storms of jovian decametric emission as a test of hypotheses

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus

Oleksiy V. Arkhypov, Helmut O. Rucker

The two-dimensional correlation analysis is applied to the study of hidden patterns in the broadband S-burst storms of jovian decametric emission. It was found that the phenomenon of S-bursts is much more complex than generally assumed. Thus, there are clear ”fingerprints” of the low-altitude acceleration of the radio-sources in the standing dispersive Alfvén wave. This implies that the S-burst emission, like AKR near the Earth, is generated inside depleted cavities with tenuous and hot plasma. In addition, the correlation patterns of radio storms revealed the considerable dispersion (
$- 17.3 \pm 1.4$

MHz/s at 25 MHz) of S-burst emission. These finds are new ingredients for the dominating theory of S-bursts.

Highlights

► Jovian decametric S-bursts shows hidden zebra pattern. ► Dispersive Alfvén waves accelerate S-burst radio sources. ► S-burst emission is generated inside depleted cavities. ► Considerable dispersion of S-burst emission is revealed.

Water Ice Abundance and CO2 Band Strength on the Saturnian Satellite Phoebe from Cassini/VIMS Observations

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus

Gary B. Hansen, Emily C. Hollenbeck, Katrin Stephan, Sean K. Apple, Eun-Ju Z. Shin-White

We have studied the near-infrared spectrum of the Saturn satellite Phoebe, a distant satellite observed before Cassini’s Saturn orbit insertion, using data from the Visual and Infrared Mapping Spectrometer (VIMS) on the Cassini orbiter. We have done a critical calibration of the data set that involves careful correction of dark artifacts. We model areally mixed water ice and non-ice (assumed segregated because of the low ∼3% albedo of the non-ice material) for several high and medium resolution observations of Phoebe made near closest approach. Using a Hapke roughness factor of 15^o, we find ice abundances from ∼0.1% to over 4%. The ice grain radii vary from 1 to 10 μm. These are displayed on a projected map of Phoebe with about 50% coverage (about 33% coverage at better than 5km spatial resolution). Detailed looks at the water ice spectral fits shows that the weak 1.05 and 1.25-μm bands are missing in most of the spectra, implying that the ice endmember is not pure ice, but has a dark material mixed with it that lowers the albedo and suppresses these bands. We made a model of ice contaminated with Phoebe-like dark material showing that a few percent of dark material lowers the albedo to ∼50% and suppresses the bands. The dirty ice model produces better fits to the spectra and implies that the amount of dirty ice is about 1.5 times the amount of pure ice. We have also calculated the CO2 band depth for these same observations and projected the results. The CO2 band depth varies inversely with water ice abundance.

Highlights

► Using Cassini-VIMS spectra, we map the water ice spatial abundance on Phoebe. ► The measured abundances are 0.1 to 4% with grain radii between 1 and 10 μm. ► We used dirty ice models to better fit Phoebe spectra. ► These models increase the measured spatial abundances by 1.6–12 times. ► We measured and mapped the strength of the CO2 band on Phoebe.

Reconstructing the Distribution and Depositional History of the Sedimentary Deposits of Arabia Terra, Mars

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus

Kelsey Zabrusky, Jeffrey C. Andrews-Hanna, Sandra M. Wiseman

The sedimentary deposits of Meridiani Planum formed during the martian climate transition at the Noachian-Hesperian boundary between warm-wet and cold-dry conditions, and give valuable insight into how and when this transition occurred. We show that these deposits share characteristics with sedimentary outcrops across Arabia Terra. Despite near-ubiquitous dust cover across much of Arabia Terra, spectral signatures of polyhydrated sulfate minerals resembling those in Meridiani were identified in Schiaparelli and another unnamed crater. An erosional morphology analysis using both image and topographic data was then used to identify morphologies characteristic of Meridiani-type deposits, and catalogue their occurrences throughout Arabia Terra. The occurrences of deposits with compositions and morphologies resembling the Meridiani deposits throughout Arabia Terra suggest that Meridiani-type sedimentary rocks were once more widespread. Elevations of the eroded remnants were used to reconstruct the pre-erosional paleo-surface of the deposits. Within this study area, these deposits once covered ∼2.5-3.6×10⁶ km² and represent an eroded volume of 0.9-1.7×10⁶ km³ of sediment. Crater retention ages using craters of a range of preservation states and stratigraphic levels reveal that the deposits were laid down and subsequently eroded during a ∼270 Myr period between ∼3.83 and 3.56 Ga. The deposits formed following the transition from fluvial dissection to evaporite deposition at the end of the Noachian. The high erosion rates (∼3×10^-6 m/yr) suggest that Mars may have maintained a thick atmosphere relative to today even as it dried out in the early Hesperian.

Highlights

► Sedimentary deposits across Arabia Terra were compared to those near Meridiani Planum. ► The original extent and volume of these deposits was estimated. ► The timing and rate of deposition was inferred from crater counting. ► The deposits were found to have been laid down and eroded between ∼3.83 – 3.56 Ga.

Low temperature production and exhalation of methane from serpentinized rocks on Earth: a potential analog for methane production on Mars

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus

Giuseppe Etiope, Bethany L. Ehlmann, Martin Schoell

We evaluate, based on terrestrial analogs, the potential flux, origin and isotopic signature of methane (CH4) from serpentinized or serpentinizing rocks on Mars. The Tekirova ophiolites, in Turkey, have been shown to release, either via focused vents or through diffuse microseepage, substantial amounts of CH4 which could be produced via catalyzed abiotic methanation (Sabatier reaction) at low temperatures (<50°C). Serpentinized ultramafic rocks on Mars are likely to have necessary chemical constituents for methane production and fractures for release of gas to the atmosphere, similar to those on Earth. A simple, first-order estimation gas-advection model suggests that methane fluxes on the order of several mg m^-2 d^-1, similar to microseepage observed in terrestrial ophiolites, could occur in Martian rocks. High temperature, hydrothermal conditions may not be necessary for abiotic CH4 synthesis on Mars: low temperature (<50°C) methanation is possible in the presence of catalysts like ruthenium, rhodium or, more commonly, chromium minerals, which occur in terrestrial ophiolites as in Martian mantle meteorites. The terrestrial analog environment of abiotic microseepage may thus explain production of methane on Mars in the ancient past or at present. The wide range of Martian ¹²C/¹³C and D/H ratios and the potential secondary alteration of CH4 by abiotic oxidation, as observed on Earth, could result in large isotope variations of methane on Mars. CH4 isotopic composition alone may not allow definitive determination of biotic versus abiotic gas origin. Using our terrestrial versus Martian analysis as guide to future Mars exploration we propose that direct methane and ethane gas detection and isotopic measurements on the ground over serpentinized/serpentinizing rocks should be considered in developing future strategies for unravelling the source and origin of methane on Mars.

Highlights

► Terrestrial ophiolitic CH4 can be produced by low T (<50°C) catalyzed methanation. ► Low T catalyzed methanation can potentially occur in olivine-rich rocks on Mars. ► Martian CH4 fluxes from serpentinized rocks could be similar to those on Earth. ► Isotopic values of ophiolitic CH4 are a basic reference for Mars. ► Determination of ethane and its isotopic composition is critical to unravel gas origin.

Trapped Ar isotopes in meteorite ALH 84001 indicate Mars did not have a thick ancient atmosphere

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus

William S. Cassata, David L. Shuster, Paul R. Renne, Benjamin P. Weiss

Water is not currently stable in liquid form on the Martian surface due to the present mean atmospheric pressure of ∼7 mbar and mean global temperature of ∼220 K. However, geomorphic features and hydrated mineral assemblages suggest that Mars’ climate was once warmer and liquid water flowed on the surface. These observations may indicate a substantially more massive atmosphere in the past, but there have been few observational constraints on paleoatmospheric pressures. Here we show how the ⁴⁰Ar/³⁶Ar ratios of trapped gases within Martian meteorite ALH 84001 constrain paleoatmospheric pressure on Mars during the Noachian Era [∼4.56-3.8 billion years (Ga)]. Our model indicates that atmospheric pressures did not exceed ∼1.5 bar during the first 400 million years (Ma) of the Noachian era, and were <400 mbar by 4.16 Ga. Such pressures of CO2 are only sufficient to stabilize liquid water on Mars’ surface at low latitudes during seasonally warm periods. Other greenhouse gases like SO2 and water vapor may have played an important role in intermittently stabilizing liquid water at higher latitudes following major volcanic eruptions or impact events.

Forbidden Oxygen Lines in Comets C/2006 W3 Christensen and C/2007 Q3 Siding Spring at Large Heliocentric Distan Implications for the Sublimation of Volatile Ices

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus

Adam J. McKay, Nancy J. Chanover, Jeffrey P. Morgenthaler, Anita L. Cochran, Walter M. Harris, Neil Dello Russo

We present observations and analysis of the forbidden oxygen lines in comets C/2006 W3 (Christensen) and C/2007 Q3 (Siding Spring) at heliocentric distances of 3.13 and 2.96 AU, respectively. We obtained the observations using the ARCES echelle spectrometer, which is mounted on the Astrophysical Research Consortium 3.5-m telescope at Apache Point Observatory. We detected the 5577, 6300, and 6364 Å atomic oxygen lines in Christensen and we calculated the ratio of the green (5577 Å) line flux to that of the red doublet (6300 and 6364 Å). We obtained a ratio of 0.24 ± 0.08, a number higher than previous measurements for other comets at smaller heliocentric distance. This shows that CO and/or CO2 made detectable contributions to the O I population in the coma of Christensen. We only detected the 6300 Å line in Siding Spring, resulting in an upper limit for the oxygen line ratio of 0.20. Based on the derived flux for the 6300 Å line, we constrain the CO2 and H2O production rate in Siding Spring. A comparison of our results at ∼3 AU to previous findings for comets at this approximate heliocentric distance suggests that a heliocentric distance of 3 AU is a transition region between H2O and CO2 dominated activity in comets.

Highlights

► We present observations of the atomic oxygen lines in two comets. ► Both comets were 3 AU from the Sun at the time of observation. ► Can use oxygen line observations to contrain CO2 and H2O production. ► We find evidence for enhanced CO2 activity in these comets.

Corrigendum to “Numerical simulations of granular dynamics II: Particle dynamics in a shaken granular material” [Icarus 219 (2012) 321–335]

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus

Naomi Murdoch, Patrick Michel, Derek C. Richardson, Kerstin Nordstrom, Christian R. Berardi, Simon F. Green, Wolfgang Losert

Instability zones for satellites of asteroids: The example of the (87) Sylvia system

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 220, Issue 1

Julien Frouard, Audrey Compère

The stability of the (87) Sylvia system and of the neighborhood of its two satellites is investigated. We use numerical integrations considering the non-sphericity of Sylvia, as well as the mutual perturbation of the satellites and the solar perturbation. Two numerical models have been used, which describe respectively the short and long-term evolution of the system. We show that the actual system is in a deeply stable zone, but surrounded by both fast and secular chaotic regions due to mean-motion and evection resonances. We then investigate how tidal and BYORP effects modify the location of the system over time with respect to the instability zones. The conclusion is that the system will cross the evection resonance before 1Gyr. We generalize this study to other known triple systems, investigate possible evolutions of the systems under tidal and BYORP effects, and discuss their distance from instability regions. In particular, it is possible to show how systems in a joint opposing evolution can be destroyed depending on the masses of the satellites and their dissipative parameters.

Highlights

► We study the dynamics and stability of the satellites of the Asteroid (87) Sylvia. ► The two satellites are in a stable zone, but bounded by chaotic resonances. ► Tides/BYORP effects are driving the system through the evection resonance. ► The BYORP effect allows a wide diversity of evolutions for triple systems.

An impact-induced, stable, runaway climate on Mars

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 220, Issue 1

Teresa L. Segura, Christopher P. McKay, Owen B. Toon

Large asteroid and comet impacts on Mars, such as the one that formed the Argyre basin, delivered considerable amounts of kinetic energy to the planet and raised the surface temperature hundreds of degrees. The impact that formed the Argyre basin occurred 3.8–3.9byr ago (Werner, S.C. [2008]. Icarus 195, 45–60; Fassett, C.I., Head, J.W. [2011]. Icarus 211, 1204–1214), during the time of formation of fluvial features on the early martian surface, and was capable of causing global-scale precipitation and warming of the surface. Dual solutions to the climate of early Mars, one cold like present Mars and the other in a hot runaway state, exist for the pressure range of 0.006–1bar of CO2, and for water inventories 6.5bars or greater. A large impact event may have pushed Mars to a long-lasting hot runaway state. The runaway state would persist until escape processes reduced water vapor and forced the planet to return to a cold climate.

Highlights

► We show numerically two temperature solutions can exist for one radiative flux. ► One solution is at a cool temperature and one is at a warm temperature. ► Mars may equilibrate at the warm solution after cooling from perturbation. ► This warm solution represents a long-term greenhouse state. ► Impacts may push Mars to this state; rainfall is a result of cooling and collapse.

Correlation of deep moonquakes and mare basalts: Implications for lunar mantle structure and evolution

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 220, Issue 1

Chuan Qin, Alicia C. Muirhead, Shijie Zhong

The genesis of mare basalts and deep moonquakes are important events that have major implications for understanding the thermal evolution and interior dynamics of the Moon. The eruption of mare basalts predominantly from 3.9Ga to 3Ga ago represents one of the most important events in lunar geological history. Deep moonquakes recorded by the Apollo Seismic Network show the dynamic nature of the present-day lunar mantle. In this study, we have correlated the presence of the mare basalts, using FeO concentration as a proxy, with the epicenters of 52 well-located deep moonquake (DMQ) clusters. We determine FeO concentrations of 13wt.% or higher to be representative of the mare basalt deposits. Our analysis shows that over 63% of the DMQs occur within 1° from the mare basalt deposits, while over 80% of the DMQs are within 5° from the mare basalt deposits. Our analysis also shows that for the same amount of randomly distributed DMQs within a spherical cap on the nearside that encompasses all the nearside DMQs, the probability of over 80% of the DMQs occurring within 5° from the mare basalt deposits is about 0.01, thus rejecting a random distribution of the DMQs with respect to the mare basalts. The correlation between mare basalts and the DMQs from our analysis suggests that the mare basalts may be derived from melting processes at relatively large depths, consistent with previous petrology and geodynamic studies. We propose that the water and volatiles in the mare basalt source material (i.e., a mixture of ilmenite cumulates and olivine orthopyroxene, together called MIC) play an important role in causing the DMQs and that the DMQs delineate the present-day locations of MIC in the deep mantle. Since the mare basalts are predominately distributed on the nearside, our results further suggest that the DMQs may indeed be largely nearside features, which is a prediction that can be tested in future lunar seismic exploration.

Highlights

► We demonstrate the correlations of deep moonquakes with mare basalts distribution. ► We reject the random distribution of deep moonquakes relative to mare basalts. ► We propose a new hypothesis for lunar mantle structure and evolution.

Dust levitation as a major resurfacing process on the surface of a saturnian icy satellite, Atlas

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 220, Issue 1

Naoyuki Hirata, Hideaki Miyamoto

A small inner satellite of Saturn, Atlas, has an enigmatic saucer-like shape explained by an accumulation of particles from A-ring of Saturn. However, its unusual smooth surface remains unexplained. Gardening through continuous particle impact events cannot be a unique explanation for the smoothness, because Prometheus does not exhibit a similar surface, though it too would have experienced a similar bombardment. Here, a detailed investigation using close-up images of Atlas reveals the surface to be (1) covered by fine particles (i.e., probably as small as several tens of micrometers); (2) mostly void of impact craters (i.e., only one has been thus far identified); and (3) continuously smooth, even between the equatorial ridge and the undulating polar region. These findings imply that some sort of crater-erasing process has been active on the surface of Atlas. From electro-static analyses, we propose that the upper-most layer of the fine particles can become electro-statically unstable and migrate as a result of dust levitation, which resulted in erasing craters on the surface of Atlas. If true, Atlas would represent the first recognized body where resurfacing is dominated by dust levitation.

Highlights

► Atlas is covered by fine particles, which explains its unusually-smooth appearance. ► Fine particles are accumulated from A-ring, forming the enigmatic shape of Atlas. ► The fine particles on Atlas electro-statically levitate, migrate, and eventually deposit. ► This process is working globally and erasing surface features, including craters.

Mutual events in the Cold Classical transneptunian binary system Sila and Nunam

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 220, Issue 1

W.M. Grundy, S.D. Benecchi, D.L. Rabinowitz, S.B. Porter, L.H. Wasserman, B.A. Skiff, K.S. Noll, A.J. Verbiscer, M.W. Buie, S.W. Tourtellotte, D.C. Stephens, H.F. Levison

Hubble Space Telescope observations between 2001 and 2010 resolved the binary components of the Cold Classical transneptunian object (79360) Sila–Nunam (provisionally designated 1997 CS29). From these observations we have determined the circular, retrograde mutual orbit of Nunam relative to Sila with a period of 12.50995±0.00036days and a semimajor axis of 2777±19km. A multi-year season of mutual events, in which the two near-equal brightness bodies alternate in passing in front of one another as seen from Earth, is in progress right now, and on 2011 February 1UT, one such event was observed from two different telescopes. The mutual event season offers a rich opportunity to learn much more about this barely-resolvable binary system, potentially including component sizes, colors, shapes, and albedo patterns. The low eccentricity of the orbit and a photometric lightcurve that appears to coincide with the orbital period are consistent with a system that is tidally locked and synchronized, like the Pluto–Charon system. The orbital period and semimajor axis imply a system mass of (10.84±0.22)×10¹⁸ kg, which can be combined with a size estimate based on Spitzer and Herschel thermal infrared observations to infer an average bulk density of
${0.72}_{- 0.23}^{+ 0.37} g {cm}^{- 3}$

, comparable to the very low bulk densities estimated for small transneptunian binaries of other dynamical classes.

Highlights

► We report the orbit of transneptunian binary system Sila–Nunam. ► The system is currently undergoing mutual events. ► These events offer an opportunity to constrain system parameters.

Lunar iron abundance determination using the 2-μm absorption band parameters

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 220, Issue 1

Megha Bhatt, Urs Mall, Roberto Bugiolacchi, Susan McKenna-Lawlor, Marek Banaszkiewicz, Andreas Nathues, Kjetil Ullaland

In this work we report the first employment of the 2-μm absorption band parameters to estimate FeO weight percentage (wt.%) abundances from high-resolution Near-Infrared (NIR) lunar data, as measured by the SIR-2 instrument on board Chandrayaan-1. Our method is based on the algorithm originally developed by Le Mouélic et al. (Le Mouélic, S., Langevin, Y., Erard, S., Pinet, P., Chevrel, S., Daydou, Y. [2000]. J. Geophys. Res. 105, 9445–9456) for assessing FeO wt.% in lunar surface materials analyzing Clementine UVVIS and NIR data. A small fresh-looking crater was selected as a test bench to understand the detrimental effect on spectral parameters caused by the prolonged exposure of surface materials to space weather. Using both 1-μm and 2-μm absorption band parameters we found a correlation of about 90% between iron abundances estimated by our method and actual laboratory-measured values (from Apollo and Luna data-sets). Also, Moon Mineralogy Mapper (M³) data collected at the same Coordinated Universal Time (UTC) were used to verify the SIR-2 data-set. Iron abundance estimations based on these instruments were compared with the Clementine iron abundance map produced by the algorithm developed by Lucey et al. (Lucey, P.G., Blewett, D.T., Hawke, B.R. [1998]. J. Geophys. Res. 103, 3679–3699). We selected crater Tycho as a case study for comparing our FeO wt.% estimates against published ones based on Clementine data and found them in good agreement. This study confirms that the 2-μm absorption band can be interrogated effectively to estimate the FeO wt.% content of exposed lunar surface materials through their NIR reflectance characteristics. Applications of this method would potentially be of great interest to those missions to the Moon and other planetary bodies carrying spectrometers ranging above the 0.9μm point.

Highlights

► The SIR-2 data is used for the first time to estimate lunar iron abundances. ► Our method is compared with two other algorithms based on 1-μm absorption band. ► Comparison of SIR-2, M³, and Clementine iron maps show similar trends.

The effects of electric fields on wind driven particulate detachment

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 220, Issue 1

C. von Holstein-Rathlou, J.P. Merrison, C.F. Brædstrup, P. Nørnberg

The effects of external electric fields upon the wind driven detachment of granular material has been investigated experimentally using an environmental wind tunnel facility. It has been shown that for sand sized grains on an electrically conductive surface there is a significant reduction in the threshold shear stress required for detachment at electric fields above 200kV/m. However, if the surface is insulating then dielectric effects cause an increase in surface adhesion and an increase in the detachment threshold. This would be relevant in arid environments where the surface conductivity is low.

Highlights

► Surface conductivity is vital to effects of electric fields on aeolian detachment. ► Sand at a conductive surface is easier to detach in an electric field. ► Sand at an insulating surface is more difficult to detach in an electric field. ► A simple stress balance model reasonably describes this behaviour.

Phase reddening on near-Earth asteroids: Implications for mineralogical analysis, space weathering and taxonomic classification

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 220, Issue 1

Juan A. Sanchez, Vishnu Reddy, Andreas Nathues, Edward A. Cloutis, Paul Mann, Harald Hiesinger

Phase reddening is an effect that produces an increase of the spectral slope and variations in the strength of the absorption bands as the phase angle increases. In order to understand its effect on spectroscopic observations of asteroids, we have analyzed the visible and near-infrared spectra (0.45–2.5μm) of 12 near-Earth asteroids observed at different phase angles. All these asteroids are classified as either S-complex or Q-type asteroids. In addition, we have acquired laboratory spectra of three different types of ordinary chondrites at phase angles ranging from 13° to 120°. We have found that both, asteroid and meteorite spectra show an increase in band depths with increasing phase angle. In the case of the asteroids the Band I depth increases in the range of ∼2°< g <70° and the Band II depth increases in the range of ∼2°< g <55°. Using this information we have derived equations that can be used to correct the effect of phase reddening in the band depths. Of the three meteorite samples, the (olivine-rich) LL6 ordinary chondrite is the most affected by phase reddening. The studied ordinary chondrites have their maximum spectral contrast of Band I depths at a phase angle of ∼60°, followed by a decrease between 60° and 120° phase angle. The Band II depths of these samples have their maximum spectral contrast at phase angles of 30–60° which then gradually decreases to 120° phase angle. The spectral slope of the ordinary chondrites spectra shows a significant increase with increasing phase angle for g >30°. Variations in band centers and band area ratio (BAR) values were also found, however they seems to have no significant impact on the mineralogical analysis. Our study showed that the increase in spectral slope caused by phase reddening is comparable to certain degree of space weathering. In particular, an increase in phase angle in the range of 30–120° will produce a reddening of the reflectance spectra equivalent to exposure times of ∼0.1×10⁶–1.3×10⁶ years at about 1AU from the Sun. This increase in spectral slope due to phase reddening is also comparable to the effects caused by the addition of different fractions of SMFe. Furthermore, we found that under some circumstances phase reddening could lead to an ambiguous taxonomic classification of asteroids.

Highlights

► We studied the phase reddening on near-Earth asteroids and lab samples spectra. ► Phase reddening has no significant impact on the mineralogical analysis. ► Phase reddening is comparable to certain degree of space weathering. ► Phase reddening can lead to an ambiguous taxonomic classification.

Quantitative analysis of Mars surface radar reflectivity at 20MHz

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 220, Issue 1

Cyril Grima, Wlodek Kofman, Alain Herique, Roberto Orosei, Roberto Seu

The radar surface echoes taken from SHARAD observations are extracted to obtain a reflectivity map covering almost half of the martian surface, and then compared to available roughness maps. Then, we used a 2-step method, based on a stochastic description of reflectivity, in order to (i) separate the coherent/incoherent components of the signal by means of a probability density function fitting of amplitude distributions, and (ii) express these components with respect to roughness/permittivity values by adapting common backscattering models to the nadir case. Scattering is found to be the most important process dominating reflectivity over the martian terrains. Reflectivity is best correlated with roughness maps derived from the slope parameter. The stochastic behavior of reflectivity is confirmed by the excellent agreement of our models and demonstrates the low probability of retrieving permittivity from scattering surfaces. The non-stationary nature of most martian terrains is consolidated by a very good fit to K-distributions, justifying the use of the fractal theory for describing surface roughness on Mars. A few slightly-rough regions exhibit both a coherent and incoherent component. When surface correlation length is non-significant within the resolution cell, we show that the RMS roughness at a decameters baseline can be derived with sub-decimeter sensitivity without prior signal calibration, improving the capabilities of SHARAD in determining surface roughness with a view to landing site selection. Sets of derived dielectric constants are obtained and analyzed with regard to the reference signal used for calibration. Given the very different implications of each set to the composition of martian terrains, we emphasize the importance of good signal calibration as a major issue for SHARAD and the next interplanetary radar missions.

Highlights

► A reflectivity map of Mars is obtained from the radar sounder SHARAD. ► We apply statistical models to retrieve surface roughness and dielectric constant. ► Scattering is the most important process dominating reflectivity over Mars. ► The non-stationary nature of most martian terrains is confirmed. ► RMS roughness can be obtained for slightly rough surfaces without any calibration.

Radiative heating of carbonaceous near-Earth objects as a cause of thermal metamorphism for CK chondrites

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 220, Issue 1

Noël Chaumard, Bertrand Devouard, Marco Delbo, Ariel Provost, Brigitte Zanda

Metamorphic CK carbonaceous chondrites display matrix textures that are best explained by a transient thermal event with temperatures in the 550–950K range and durations in the order of days to years, longer than what is commonly admitted for shock events but shorter than what is required for nuclide decay. We propose that radiative heating of small carbonaceous meteoroids with perihelia close to the Sun could account for the petrological features observed in CK chondrites. Numerical thermal modeling, using favorable known NEOs orbital parameters (perihelion distances between 0.07 and 0.15AU) and physical properties of CV and CK chondrites (albedo in the range 0.01–0.1, 25% porosity, thermal diffusivity of 0.5–1.5Wm⁻¹ K⁻¹), shows that radiative heating can heat carbonaceous meteoroids in the meter size range to core temperatures up to 1050K, consistent with the metamorphic temperatures estimated for CK chondrites. Sizes of known CV and CK chondrites indicate that all these objects were small meteoroids (radii from a few cm to 2.5m) prior to their atmospheric entry. Simulations of dynamic orbits for NEO objects suggest that there are numerous such bodies with suitable orbits and properties, even if they are only a small percentage of all NEOs. Radiative heating would be a secondary process (superimposed on parent-body processes) affecting meteoroids formed by the disruption of an initially homogeneous CV3-type parent body. Different petrologic types can be accounted for depending on the sizes and heliocentric distances of the objects in such a swarm.

Highlights

► We studied thermal evolution of C-type NEOs with perihelia close to the Sun. ► Radiative heating can heat carbonaceous meteoroids at temperatures up to 1050K. ► NEOs can be heated at metamorphic temperatures for thousands of years. ► Our results are consistent with CK metamorphic temperatures and durations. ► Radiative heating can be a secondary process affecting small fragments of asteroids.

The effect of shock loading on the survival of plant seeds

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 220, Issue 1

J.A. Leighs, P.J. Hazell, G.J. Appleby-Thomas

Meteorite and asteroid impacts into planet Earth seem rare but over the lifetime of our planet have been relatively frequent. Such collisions (involving very large impactors) have been blamed for mass extinctions during Earth’s history. It has also been postulated that impactors could carry life with them throughout the universe and seed our planet. This is the basis of the theory of panspermia (‘life everywhere’) and suggests that life could be spread throughout the universe by ‘piggy-backing’ on inter-planetary bodies, e.g. asteroids, which then collide with other planets, thus seeding them with life. The shock behaviour of organic matter has an important role to play in helping to inform the feasibility of such theories. An example of a model carrier for life in seeding mechanisms is the plant seed. Here we present the development of an experimental technique in which plant seed samples are shock-loaded and their viability subsequently assessed post-shock. This technique was tested on Lepidium sativum (cress) seed samples. Experimentally, shocked seeds showed positive viability in all tests performed until shocked with a maximum peak shock pressure of ca. 0.8GPa. These results suggest it is unlikely that the plant seeds tested would be able to survive the extreme conditions on an asteroid during impact, but may be able to survive shock waves that would be generated from such collisions when existing on a planetary body.

Highlights

► Seeds have been subjected to shock loading using the plate-impact technique. ► Seeds were sealed in a capsule and 100% of shocked samples were recovered. ► Positive seed viability was seen up to peak pressures of 0.8GPa. ► Seed kill radii on Earth calculated using pressure wave decay techniques. ► Calculations show potential survival in asteroid collision, where ν ⩽45kms⁻¹.

Episodic bright and dark spots on Uranus

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 220, Issue 1

L.A. Sromovsky, H.B. Hammel, I. de Pater, P.M. Fry, K.A. Rages, M.R. Showalter, W.J. Merline, P. Tamblyn, C. Neyman, J.-L. Margot, J. Fang, F. Colas, J.-L. Dauvergne, J.M. Gómez-Forrellad, R. Hueso, A. Sánchez-Lavega, Thomas Stallard

The northern mid-latitudes of Uranus produce greater episodes of bright cloud formation than any other region on the planet. Near 30°N, very bright cloud features were observed in 1999, 2004, and 2005, with lifetimes of the order of months. In October 2011, Gemini and HST observations revealed another unusually bright cloud feature near 23°N, which was subsequently identified in July 2011 observations and found to be increasing in brightness. Observations obtained at Keck in November 2011 revealed a second bright spot only 2°N of the first, but with a substantially different drift rate (−9.2°E/day vs −1.4°E/day), which we later determined would lead to a close approach on 25 December 2011. A Hubble Target of Opportunity proposal was activated to image the results of the interaction. We found that the original bright spot had faded dramatically before the HST observations had begun and the second bright spot was found to be a companion of a new dark spot on Uranus, only the second ever observed. Both spots exhibited variable drift rates during the nearly 5months of tracking, and both varied in brightness, with BS1 reaching its observed peak on 26 October 2011, and BS2 on 11 November 2011. Altitude measurements based on near-IR imaging in H and Hcont filters showed that the deeper BS2 clouds were located near the methane condensation level (≈1.2bars), while BS1 was generally ∼500mb above that level (at lower pressures). Large morphological changes in the bright cloud features suggest that they are companion clouds of possibly orographic nature associated with vortex circulations, perhaps similar to companion clouds associated with the Great Dark Spot on Neptune, but in this case at a much smaller size scale, spanning only a few degrees of longitude at their greatest extents.

Highlights

► A spot of high and increasing brightness was discovered on Uranus on 26 October 2011. ► This 24°N spot was found about 500 mb above the 1.2bar methane condensation level. ► A second and 500mb deeper brightspot was discovered just 2°N of the first. ► Their different drift rates measured from July to Dec led to many close approaches. ► Hubble imaging revealed a rare dark spot near their 25 December 2011 close approach.

UV spectrum of Enceladus

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 220, Issue 1

Mark Zastrow, John T. Clarke, Amanda R. Hendrix, Keith S. Noll

We present a far ultraviolet (FUV) spectrum of Saturn’s moon Enceladus from the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST). We have put upper limits on emission from C, N, and O lines in Enceladus’ atmosphere and column densities for the C lines assuming solar resonance scattering. We find these upper limits to be relatively low—on the order of tens to thousands of Rayleighs and with C column densities on the order of 10⁸–10¹⁵ cm⁻², depending on the assumed source size. We also present a segment of a reflectance spectrum in the FUV from ∼1900–2130Å. This region was sensitive to the different ice mixtures in the model spectra reported by Hendrix et al. (Hendrix, A.R., Hansen, C.J., Holsclaw, G.M. [2010]. Icarus, 206, 608). We find the spectrum brightens quickly longward of ∼1900Å, constraining the absorption band observed by Hendrix et al. from ∼170 to 190nm. We find our data is consistent with the suggestion of Hendrix et al. of the presence of ammonia ice (or ammonia hydrate) to darken that region, and also possibly tholins to darken the mid-UV, as reported by Verbiscer et al. (Verbiscer, A.J., French, R.G., McGhee, C.A. [2005]. Icarus, 173, 66).

Highlights

► Observations of Enceladus from the Cosmic Origins Spectrograph. ► Airglow emissions are found to be relatively low—tens to thousands of Rayleighs. ► Column densities for C assuming solar resonance scattering are 10⁸–10¹⁵ cm⁻² (depending on assumed source size). ► Reflectance spectrum is consistent with presence of small amount ammonia ice.

Seasonal activity and morphological changes in martian gullies

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 220, Issue 1

Colin M. Dundas, Serina Diniega, Candice J. Hansen, Shane Byrne, Alfred S. McEwen

Recent studies of martian dune and non-dune gullies have suggested a seasonal control on present-day gully activity. The timing of current gully activity, especially activity involving the formation or modification of channels (which commonly have been taken as evidence of fluvial processes), has important implications regarding likely gully formation processes and necessary environmental conditions. In this study, we describe the results of frequent meter-scale monitoring of several active gully sites by the High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter (MRO). The aim is to better assess the scope and nature of current morphological changes and to provide improved constraints on timing of gully activity on both dune and non-dune slopes. Our observations indicate that (1) gully formation on Mars is ongoing today and (2) the most significant morphological changes are strongly associated with seasonal frost and defrosting activity. Observed changes include formation of all major components of typical gully landforms, although we have not observed alcove formation in coherent bedrock. These results reduce the need to invoke recent climate change or present-day groundwater seepage to explain the many martian gullies with pristine appearance.

Highlights

► Martian gully landforms are actively forming in the present climate. ► Gully activity includes formation of new channels and alcoves. ► Activity is seasonally controlled. ► Major activity tends to occur near the time that seasonal CO2 frost is removed.

On liquid phases in cometary nuclei

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 219, Issue 2

Richard Miles, George A. Faillace

In this paper we review the relevant literature and investigate conditions likely to lead to melting of H2O ice, methanol (CH3OH) ice, ethane (C2H6) ice and other volatile ices in cometary nuclei. On the basis of a heat balance model which takes account of volatiles loss, we predict the formation of occasional aqueous and hydrocarbon liquid phases in subsurface regions at heliocentric distances, r h of 1–3AU, and 5–12AU, respectively. Low triple-point temperatures and low vapour pressures of C2H6, C3H8, and some higher-order alkanes and alkenes, favour liquid phase formation in cometary bodies at high r h. Microporosity and the formation of a stabilization crust occluding the escape of volatiles facilitate liquid-phase formation. Characteristics of the near-surface which favour subsurface melting include; low effective surface emissivity (at low r h), high amorphous carbon content, average pore sizes of ∼10μm or less, presence of solutes (e.g. CH3OH), mixtures of C2–C6 hydrocarbons (for melting at high r h), diurnal thermal cycling, and slow rotation rate. Applying the principles of soil mechanics, capillary forces are shown to initiate pre-melting phenomena and subsequent melting, which is expected to impart considerable strength of ∼10⁴ Pa in partially saturated layers, reducing porosity and permeability, enhancing thermal conductivity and heat transfer. Diurnal thermal cycling is expected to have a marked effect on the composition and distribution of H2O ice in the near-surface leading to frost heave-type phenomena even where little if any true melting occurs. Where melting does take place, capillary suction in the wetted zone has the potential to enhance heat transfer via capillary wetting in a low-gravity environment, and to modify surface topography creating relatively smooth flat-bottomed features, which have a tendency to be located within small depressions. An important aspect of the “wetted layer” model is the prediction that diurnal melt–freeze cycles alter the mixing ratio vs. depth of solutes present, or of other miscible components, largely through a process of fractional crystallization, but also potentially involving frost heave. Wetted layers are potentially durable and can involve significant mass transport of volatile materials in the near-surface, increasing in extent over many rotations of the nucleus prior to and just after perihelion passage, and causing stratification and trapping of the lowest-melting mixtures at depths of several metres. A possible mechanism for cometary outbursts is proposed involving a heat pulse reaching the liquid phase in the deepest wetted zone, leading to supersaturation and triggering the sudden release under pressure of dissolved gases, in particular CO2, CO, CH4 or N2, contained beneath a consolidated near-surface layer. This study indicates that liquid water can persist for long periods of time in the near-surface of some intermediate-sized bodies (10²–10³ km radius) within protoplanetary discs.

Highlights

► Wetted layer model proposed for cometary nuclei. ► Melting conditions for ices of H2O, methanol and hydrocarbons identified. ► Physical change dependent on microporosity and diurnal thermal cycling. ► Novel mechanism for cometary outbursts proposed. ► Liquid water possible in intermediate-sized bodies within protoplanetary discs.

Io: Volcanic thermal sources and global heat flow

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 219, Issue 2

Glenn J. Veeder, Ashley Gerard Davies, Dennis L. Matson, Torrence V. Johnson, David A. Williams, Jani Radebaugh

We have examined thermal emission from 240 active or recently-active volcanic features on Io and quantified the magnitude and distribution of their volcanic heat flow during the Galileo epoch. We use spacecraft data and a geological map of Io to derive an estimate of the maximum possible contribution from small dark areas not detected as thermally active but which nevertheless appear to be sites of recent volcanic activity. We utilize a trend analysis to extrapolate from the smallest detectable volcanic heat sources to these smallest mapped dark areas. Including the additional heat from estimates for “outburst” eruptions and for a multitude of very small (“myriad”) hot spots, we account for ∼62×10¹² W (∼59±7% of Io’s total thermal emission). Loki Patera contributes, on average, 9.6×10¹² W (∼9.1±1%). All dark paterae contribute 45.3×10¹² W (∼43±5%). Although dark flow fields cover a much larger area than dark paterae, they contribute only 5.6×10¹² W (∼5.3±0.6%). Bright paterae contribute ∼2.6×10¹² W (∼2.5±0.3%). Outburst eruption phases and very small hot spots contribute no more than ∼4% of Io’s total thermal emission: this is probably a maximum value. About 50% of Io’s volcanic heat flow emanates from only 1.2% of Io’s surface. Of Io’s heat flow, 41±7.0% remains unaccounted for in terms of identified sources. Globally, volcanic heat flow is not uniformly distributed. Power output per unit surface area is slightly biased towards mid-latitudes, although there is a stronger bias toward the northern hemisphere when Loki Patera is included. There is a slight favoring of the northern hemisphere for outbursts where locations were well constrained. Globally, we find peaks in thermal emission at ∼315°W and ∼105°W (using 30° bins). There is a minimum in thermal emission at around 200°W (almost at the anti-jovian longitude) which is a significant regional difference. These peaks and troughs suggest a shift to the east from predicted global heat flow patterns resulting from tidal heating in an asthenosphere. Global volcanic heat flow is dominated by thermal emission from paterae, especially from Loki Patera (312°W, 12°N). Thermal emission from dark flows maximises between 165°W and 225°W. Finally, it is possible that a multitude of very small hot spots, smaller than the present angular resolution detection limits, and/or cooler, secondary volcanic processes involving sulphurous compounds, may be responsible for at least part of the heat flow that is not associated with known sources. Such activity should be sought out during the next mission to Io.

Highlights

► Thermal emission from 240 volcanic features on Io equals 55% of Io’s total heat flow. ► Energy emanating from paterae alone represents 46% of Io’s total heat flow of ∼10¹⁴ W. ► Paterae heat flow has a bimodal longitudinal distribution (peaks at 315W and 105W). ► Slight bias towards volcanic heat flow at mid-latitudes; also, a minimum at ∼200W. ► Peak thermal emission offset eastwards from asthenospheric tidal heating models.

Statistical distribution of atmospheric dust devils

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 219, Issue 2

M.V. Kurgansky

Theoretical predictions with regard to dust devil (apparent) angular size–frequency distribution are made and critically compared with Mars Exploration Rover (MER) Spirit optical observations. For an idealized horizontal viewing geometry one should expect that the number of dust devils having the apparent angular diameter greater than a given angle α is inversely proportional to α squared. The actual dependency for Spirit dust devils is in between the inverse-squared and simple inverse laws, and close to the latter one for small and moderate angles α. It is emphasized that such a comparison can be considered as a benchmark for completeness and adequateness of dust-devil optical observations and correctness of competing analytical formulations for dust devil size–frequency distribution.

Highlights

► Predicts “minus 2” cumulative distribution for dust devil’s apparent size, in idealized conditions. ► Critically compares the predicted and the actual distribution for MER Spirit dust devils on Mars. ► Proposes this method for assessing the completeness of future dust-devil optical observations.

Sodium atoms in the lunar exotail: Observed velocity and spatial distributions

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 219, Issue 2

Michael R. Line, E.J. Mierkiewicz, R.J. Oliversen, J.K. Wilson, L.M. Haffner, F.L. Roesler

The lunar sodium tail extends long distances due to radiation pressure on sodium atoms in the lunar exosphere. Our earlier observations measured the average radial velocity of sodium atoms moving down the lunar tail beyond Earth (i.e., near the anti-lunar point) to be ∼12.5km/s. Here we use the Wisconsin H-alpha Mapper to obtain the first kinematically resolved maps of the intensity and velocity distribution of this emission over a 15°×15° region on the sky near the anti-lunar point. We present both spatially and spectrally resolved observations obtained over four nights bracketing new Moon in October 2007. The spatial distribution of the sodium atoms is elongated along the ecliptic with the location of the peak intensity drifting 3° east along the ecliptic per night. Preliminary modeling results suggest the spatial and velocity distributions in the sodium exotail are sensitive to the near surface lunar sodium velocity distribution. Future observations of this sort along with detailed modeling offer new opportunities to describe the time history of lunar surface sputtering over several days.

Highlights

► Lunar exospheric sodium atoms escape the Moon forming a comet-like tail via solar radiation pressure. ► We measure the intensity and velocity distribution of the lunar sodium tail. ► Data/model comparisons indicate sodium is sensitive to the near surface velocity distribution, abundance and photoionization lifetime. ► Measurements of the lunar sodium tail can help constrain lunar exospheric source mechanisms.

Outgassing of icy bodies in the Solar System – II: Heat transport in dry, porous surface dust layers

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 219, Issue 2

Bastian Gundlach, Jürgen Blum

In this work, we present a new model for the heat conductivity of porous dust layers in vacuum, based on an existing solution of the heat transfer equation of single spheres in contact. This model is capable of distinguishing between two different types of dust layers: dust layers composed of single particles (simple model) and dust layers consisting of individual aggregates (complex model). Additionally, we describe laboratory experiments, which were used to measure the heat conductivity of porous dust layers, in order to test the model. We found that the model predictions are in an excellent agreement with the experimental results, if we include radiative heat transport in the model. This implies that radiation plays an important role for the heat transport in porous materials. Furthermore, the influence of this new model on the Hertz factor are demonstrated and the implications of this new model on the modeling of cometary activity are discussed. Finally, the limitations of this new model are critically reviewed.

Highlights

► We present a new model for the heat conductivity of porous dust layers in vacuum. ► The model was tested by Laboratory experiments, which are also introduced in this paper. ► The implications of this new model on the modeling of cometary activity are discussed. ► Furthermore, we present the temperature and grain size dependence of the Hertz factor.

Thermal regeneration of sulfuric acid hydrates after irradiation

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 219, Issue 2

Mark J. Loeffler, Reggie L. Hudson

In an attempt to more completely understand the surface chemistry of the jovian icy satellites, we have investigated the effect of heating on two irradiated crystalline sulfuric acid hydrates, H2SO4·4H2O and H2SO4·H2O. At temperatures relevant to Europa and the warmer jovian satellites, post-irradiation heating recrystallized the amorphized samples and increased the intensities of the remaining hydrate’s infrared absorptions. This thermal regeneration of the original hydrates was nearly 100% efficient, indicating that over geological times, thermally-induced phase transitions enhanced by temperature fluctuations will reform a large fraction of crystalline hydrated sulfuric acid that is destroyed by radiation processing. The work described is the first demonstration of the competition between radiation-induced amorphization and thermally-induced recrystallization in icy ionic solids relevant to the outer Solar System.

Highlights

► We investigated the effect of heating irradiated H2SO4·4H2O and H2SO4·H2O. ► Post-irradiation heating recrystallizes the samples and regenerates the original hydrates. ► Over geological times, the destroyed acid hydrates will be regenerated on the icy satellites. ► This is the first report of recrystallization in ionic solids relevant to the outer solar system.

Constraints on Saturn’s tropospheric general circulation from Cassini ISS images

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 219, Issue 2

Anthony D. Del Genio, John M. Barbara

An automated cloud tracking algorithm is applied to Cassini Imaging Science Subsystem high-resolution apoapsis images of Saturn from 2005 and 2007 and moderate resolution images from 2011 and 2012 to define the near-global distribution of zonal winds and eddy momentum fluxes at the middle troposphere cloud level and in the upper troposphere haze. Improvements in the tracking algorithm combined with the greater feature contrast in the northern hemisphere during the approach to spring equinox allow for better rejection of erroneous wind vectors, a more objective assessment at any latitude of the quality of the mean zonal wind, and a population of winds comparable in size to that available for the much higher contrast atmosphere of Jupiter. Zonal winds at cloud level changed little between 2005 and 2007 at all latitudes sampled. Upper troposphere zonal winds derived from methane band images are ∼10ms⁻¹ weaker than cloud level winds in the cores of eastward jets and ∼5ms⁻¹ stronger on either side of the jet core, i.e., eastward jets appear to broaden with increasing altitude. In westward jet regions winds are approximately the same at both altitudes. Lateral eddy momentum fluxes are directed into eastward jet cores, including the strong equatorial jet, and away from westward jet cores and weaken with increasing altitude on the flanks of the eastward jets, consistent with the upward broadening of these jets. The conversion rate of eddy to mean zonal kinetic energy at the visible cloud level is larger in eastward jet regions (5.2×10⁻⁵ m² s⁻³) and smaller in westward jet regions (1.6×10⁻⁵ m² s⁻³) than the global mean value (4.1×10⁻⁵ m² s⁻³). Overall the results are consistent with theories that suggest that the jets and the overturning meridional circulation at cloud level on Saturn are maintained at least in part by eddies due to instabilities of the large-scale flow near and/or below the cloud level.

Highlights

► Saturn eastward jets broaden with increasing altitude. ► Eddy momentum flux into eastward jets, decreases upward near eastward jets. ► Implies jets are driven by processes acting within the middle troposphere clouds.

Tidally-induced melting events as the origin of south-pole activity on Enceladus

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 219, Issue 2

Marie Běhounková, Gabriel Tobie, Gaël Choblet, Ondřej Čadek

The intense activity at the south pole of Enceladus hints at an internal water reservoir. However, there is no direct evidence of liquid water at present and its long-term stability in the interior remains problematic. By modeling heat production and transfer in the ice shell in a spherical geometry, we show that tidal heating naturally leads to a concentration of convective hot upwellings in the south polar region, favoring the preservation of liquid water at depth. We show that large volumes of water are produced within the ice shell at the south pole during periods of elevated orbital eccentricity (3–5 times the present-day value). Strong lateral variations in the melt production and crystallization rates result in stress concentration in the south polar region, thus providing an explanation for the tectonic activity observed today. We predict that an internal ocean may be sustained over the long term as the consequence of repeated periods with elevated orbital eccentricity, leading to episodic melting and resurfacing events.

Highlights

► We model heat production and transfer in Enceladus’ ice shell in a full spherical geometry. ► Internal oceans with varying horizontal extension are considered. ► Tidal heating leads to ice melting at high latitudes for eccentricity enhanced by a factor of 3. ► Large volumes of water are produced during tidally-induced melting events. ► Stress due to water accumulation should break the ice lithosphere and trigger tectonic activities.

The O2 nightglow in the martian atmosphere by SPICAM onboard of Mars-Express

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 219, Issue 2

A.A. Fedorova, F. Lefèvre, S. Guslyakova, O. Korablev, J.-L. Bertaux, F. Montmessin, A. Reberac, B. Gondet

We present observations of the O2(a¹Δg) nightglow at 1.27μm on Mars using the SPICAM IR spectrometer onboard of the Mars Express orbiter. In contrast to the O2(a¹Δg) dayglow that results from the ozone photodissociation, the O2(a¹Δg) nightglow is a product of the recombination of O atoms formed by CO2 photolysis on the dayside at altitudes higher than 80km and transported downward above the winter pole by the Hadley circulation. The first detections of the O2(a¹Δg) nightglow in 2010 indicate that it is about two order of magnitude less intense than the dayglow (Bertaux, J.-L., Gondet, B., Bibring, J.-P., Montmessin, F., Lefèvre, F. [2010]. Bull. Am. Astron. Soc. 42, 1040; Clancy et al. [2010]. Bull. Am. Astron. Soc. 42, 1041). SPICAM IR sounds the martian atmosphere in the near-IR range (1–1.7μm) with the spectral resolution of 3.5cm⁻¹ in nadir, limb and solar occultation modes. In 2010 the vertical profiles of the O2(a¹Δg) nightside emission have been obtained near the South Pole at latitudes of 82–83°S for two sequences of observations: Ls =111–120° and Ls =152–165°. The altitude of the emission maximum varied from 45km on Ls =111–120° to 38–49km on Ls =152–165°. Averaged vertically integrated intensity of the emission at these latitudes has shown an increase from 0.22 to 0.35 MR. Those values of total vertical emission rate are consistent with the OMEGA observations on Mars-Express in 2010. The estimated density of oxygen atoms at altitudes from 50 to 65km varies from 1.5×10¹¹ to 2.5×10¹¹ cm⁻³. Comparison with the LMD general circulation model with photochemistry (Lefèvre, F., Lebonnois, S., Montmessin, F., Forget, F. [2004]. J. Geophys. Res. 109, E07004; Lefèvre et al. [2008]. Nature 454, 971–975) shows that the model reproduces fairly well the O2(a¹Δg) emission layer observed by SPICAM when the large field of view (>20km on the limb) of the instrument is taken into account.

Highlights

► The vertical profiles of the O2 nightside emission near the South Pole at Ls =111–165° are presented. ► Integrated intensity of the emission has shown an increase from 0.22 to 0.35 MR. ► The integrated values consist with previous observations by OMEGA/MEX. ► The estimated density of oxygen atoms at altitudes from 50 to 65km varies from 1.5×10¹¹ to 2.5×10¹¹ cm⁻³.

Structural control of scarps in the Rembrandt region of Mercury

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 219, Issue 2

Javier Ruiz, Valle López, James M. Dohm, Carlos Fernández

Lobate scarps, thought to be the surface expression of large thrust faults, are the most spectacular contractional tectonic features visible on Mercury. Most lobate scarps follow a general and relatively simple pattern, with a roughly arcuate or linear form in plan view, and an asymmetric cross section characterized by a steeply rising scarp face and a gently declining back scarp. In this work, we study two peculiar and complex scarps in the Rembrandt region of Mercury through MESSENGER imagery. On the one hand, the formation of these scarps resulted in the deformation of features such as impact craters, fractures, extensional faults, and volcanic plains, while on the other hand, the deformed features partly influenced the formation of the scarps. Evidence for structural control on the formation of the scarps includes their orientation, segmentation, bifurcation, change in structural trend and dip orientation, and transition into high-relief ridges or wrinkle ridge morphologies in some cases. Thus, these two lobate scarps provide examples of complex geological relations among other features, expanding the recognized richness of mercurian geology. Also, the southern scarp records a complex history of contraction, suggesting that the development of some mercurian lobate scarps may be more complex than usually thought.

Highlights

► Lobate scarps are remarkable contractional tectonic structures on Mercurio. ► Here we study two remarkable and complex scarps in the Rembrandt region of Mercury. ► These scarps show evidences of structural control by pre-existing landforms. ► The development of some mercurian scarps may be more complex than usually thought.

N-body simulations of cohesion in dense planetary rings: A study of cohesion parameters

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 219, Issue 2

Randall P. Perrine, Derek C. Richardson

We present results from a large suite of simulations of Saturn’s dense A and B rings using a new model of particle sticking in local simulations (Perrine, R.P., Richardson, D.C., Scheeres, D.J. [2011]. Icarus 212, 719–735). In this model, colliding particles can be incorporated into or help fragment rigid aggregations on the basis of certain user-specified parameters that can represent van der Waals forces or interlocking surface frost layers. Our investigation is motivated by laboratory results that show that interpenetration of surface layers can allow impacting frost-covered ice spheres to stick together. In these experiments, cohesion only occurs below specific impact speeds, which happen to be characteristic of impact speeds in Saturn’s rings. Our goal is to determine if weak bonding is consistent with ring observations, to constrain cohesion parameters in light of existing ring observations, to make predictions about particle populations throughout the rings, and to discover other diagnostics that may constrain bonding parameters. We considered the effects of five parameters on the equilibrium characteristics of our ring simulations: speed-based merge and fragmentation limits, bond strength, ring surface density, and patch orbital distance (i.e., the A or B ring), some with both monodisperse and polydisperse comparison cases. In total, we present data from 95 simulations. We find that weak cohesion is consistent with observations of the A and B rings (e.g., French, R.G., Nicholson, P.D. [2000]. Icarus 145, 502–523), and we present a range of simulation parameters that reproduce the observed size distribution and maximum particle size. It turns out that the parameters that match observations differ between the A and B rings, and we discuss the potential implications of this result. We also comment on other observable consequences of cohesion for the rings, such as optical depth and scale height effects, and discuss whether very large objects (e.g., “propeller” source objects) are grown bottom-up from cohesion of smaller ring particles.

Highlights

► We present 95 planetary ring simulations with merging and fragmentation. ► The new method uniquely uses semi-rigid aggregates to represent cohesion. ► We verify that particle cohesion in Saturn’s rings is consistent with observations. ► We find plausible ranges of parameters that match observations of the rings.

Physical properties of trans-neptunian binaries (120347) Salacia–Actaea and (42355) Typhon–Echidna

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 219, Issue 2

J.A. Stansberry, W.M. Grundy, M. Mueller, S.D. Benecchi, G.H. Rieke, K.S. Noll, M.W. Buie, H.F. Levison, S.B. Porter, H.G. Roe

We report new Hubble Space Telescope and Spitzer Space Telescope results concerning the physical properties of the trans-neptunian object (TNO) binaries (120347) Salacia–Actaea (formerly 2004 SB60), and (42355) Typhon–Echidna (formerly 2002 CR46). The mass of the (120347) Salacia–Actaea system is 4.66±0.22×10²⁰ kg. The semi-major axis, period, and eccentricity of the binary orbit are a =5619±87km, P =5.49380±0.00016days, and e =0.0084±0.0076, respectively. In terms of the ratio of the semimajor axis to the radius of the Hill sphere, a/r H , (120347) Salacia–Actaea is the tightest TNO binary system with a known orbit. Based on hybrid Standard Thermal Model (hybrid-STM) fits to the data, the effective diameter and V-band geometric albedo of the system are D =954±109km (making it one of the largest known TNOs), and
$p_{V} = {3.57}_{- 0.72}^{+ 1.03} %$

. Thermophysical models for (120347) Salacia suggest that it probably has a thermal inertia ⩽5Jm⁻² s^−1/2 K⁻¹, although we cannot rule out values as high as 30Jm⁻² s^−1/2 K⁻¹. Based on the magnitude difference between Salacia and Actaea, δ =2.37±0.06, we estimate their individual diameters to be d 1 =905±103km and d 2 =303±35km. The mass density of the components is
$ρ = {1.16}_{- 0.36}^{+ 0.59}$

g/cm³. Hybrid-STM fits to new Spitzer data for Typhon–Echidna give an effective diameter and V-band geometric albedo for the system of D =157±34km, and
$p_{V} = {6.00}_{- 2.08}^{+ 4.10} %$

. Thermophysical models for (42355) Typhon suggest somewhat lower albedos (probably no higher than about 8.2%, as compared to the hybrid-STM upper limit of 10.1%). Taken together with the previously reported mass, this diameter indicates a density of
$ρ = {0.60}_{- 0.29}^{+ 0.72} g / {cm}^{3}$

, consistent with the very low densities of most other TNOs smaller than 500km diameter. Both objects must have significant amounts of void space in their interiors, particularly if they contain silicates as well as water–ice (as is expected). The ensemble of binary-TNO densities suggests a trend of increasing density with size, with objects smaller than 400km diameter all having densities less than 1g/cm³, and those with diameters greater than 800km all having densities greater than 1g/cm³. If the eccentricity of the binary orbit of (42355) Typhon–Echidna is not due to recent perturbations, considerations of tidal evolution suggest that (42355) Typhon–Echidna must have a rigidity close to that of solid water ice, otherwise the orbital eccentricity of the system would have been damped by now.

Highlights

► TNO (120347) 2004 SB60 is over 900km in diameter. ► It has the lowest albedo and density of any TNO that big. ► TNO Typhon is confirmed to have a density <0.7g/cc.

Identification of known objects in Solar System surveys

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 220, Issue 1

Andrea Milani, Zoran Knežević, Davide Farnocchia, Fabrizio Bernardi, Robert Jedicke, Larry Denneau, Richard J. Wainscoat, William Burgett, Tommy Grav, Nick Kaiser, Eugene Magnier, Paul A. Price

The discovery of new objects in modern wide-field asteroid and comet surveys can be enhanced by first identifying observations belonging to known Solar System objects. The assignation of new observations to a known object is an attribution problem that occurs when a least squares orbit already exists for the object but a separate fit is not possible to just the set of new observations. In this work we explore the strongly asymmetric attribution problem in which the existing least squares orbit is very well constrained and the new data are sparse. We describe an attribution algorithm that introduces new quality control metrics in the presence of strong biases in the astrometric residuals. The main biases arise from the stellar catalogs used in the reduction of asteroid observations and we show that a simple debiasing with measured regional catalog biases significantly improves the results. We tested the attribution algorithm using data from the PS1 survey that used the 2MASS star catalog for the astrometric reduction. We found small but statistically significant biases in the data of up to 0.1arcsec that are relevant only when the observations reach the level of accuracy made possible by instruments like PS1. The false attribution rate was measured to be <1/1000 with a simple additional condition that can reduce it to zero while the attribution efficiency is consistent with 100%.

Highlights

► Algorithm to attribute new high quality data to well constrained asteroid orbits. ► Application of the latest astrometric debiasing techniques. ► Measured efficiency and false attribution rate using PAN-STARRS PS1 data. ► Derived indication for a PAN-STARRS PS1 observation error model.

Investigating magnetospheric interaction effects on Titan’s ionosphere with the Cassini orbiter Ion Neutral Mass Spectrometer, Langmuir Probe and magnetometer observations during targeted flybys

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 219, Issue 2

J.G. Luhmann, D. Ulusen, S.A. Ledvina, K. Mandt, B. Magee, J.H. Waite, J. Westlake, T.E. Cravens, I. Robertson, N. Edberg, K. Agren, J.-E. Wahlund, Y.-J. Ma, H. Wei, C.T. Russell, M.K. Dougherty

In the ∼6years since the Cassini spacecraft went into orbit around Saturn in 2004, roughly a dozen Titan flybys have occurred for which the Ion Neutral Mass Spectrometer (INMS) measured that moon’s ionospheric density and composition. For these, and for the majority of the ∼60 close flybys probing to altitudes down to ∼950km, Langmuir Probe electron densities were also obtained. These were all complemented by Cassini magnetometer observations of the magnetic fields affected by the Titan plasma interaction. Titan’s ionosphere was expected to differ from those of other unmagnetized planetary bodies because of significant contributions from particle impact due to its magnetospheric environment. However, previous analyses of these data clearly showed the dominance of the solar photon source, with the possible exception of the nightside. This paper describes the collected ionospheric data obtained in the period between Cassini’s Saturn Orbit Insertion in 2004 and 2009, and examines some of their basic characteristics with the goal of searching for magnetospheric influences. These influences might include effects on the altitude profiles of impact ionization by magnetospheric particles at the Titan orbit location, or by locally produced pickup ions freshly created in Titan’s upper atmosphere. The effects of forces on the ionosphere associated with both the draped and penetrating external magnetic fields might also be discernable. A number of challenges arise in such investigations given both the observed order of magnitude variations in the magnetospheric particle sources and the unsteadiness of the magnetospheric magnetic field and plasma flows at Titan’s (∼20Rs (Saturn Radius)) orbit. Transterminator flow of ionospheric plasma from the dayside may also supply some of the nightside ionosphere, complicating determination of the magnetospheric contribution. Moreover, we are limited by the sparse sampling of the ionosphere during the mission as the Titan interaction also depends on Saturn Local Time as well as possible intrinsic asymmetries and variations of Titan’s neutral atmosphere. We use organizations of the data by key coordinate systems of the plasma interaction with Titan’s ionosphere to help interpret the observations. The present analysis does not find clear characteristics of the magnetosphere’s role in defining Titan’s ionosphere. The observations confirm the presence of an ionosphere produced mainly by sunlight, and an absence of expected ionospheric field signatures in the data. Further investigation of the latter, in particular, may benefit from numerical experiments on the inner boundary conditions of 3D models including the plasma interaction and features such as neutral winds.

Highlights

► We examined the collected Cassini flyby ionospheric profiles for signs of magnetospheric influences. ► The previously found solar control dominates the ionospheric densities. ► The ionosphere does not appear sensitive to variable magnetic fields in and around it. ► The net influences of the magnetosphere are either minimal or our sampling too sparse to detect them.

Patterns of accumulation and flow of ice in the mid-latitudes of Mars during the Amazonian

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 219, Issue 2

James L. Dickson, James W. Head, Caleb I. Fassett

Evidence has accumulated that non-polar portions of Mars have undergone significant periods of glaciation during the Amazonian Period. This evidence includes tropical mountain glacial deposits, lobate debris aprons, lineated valley fill, concentric crater fill, pedestal craters, and related landforms, some of which suggest that ice thicknesses exceeded a kilometer in many places. In some places, several lines of evidence suggest that ice is still preserved today in the form of relict debris-coved glaciers. The vast majority of deposit morphologies are analogous to those seen in cold-based glacial deposits on Earth, suggesting that little melting has taken place. Although these features have been broadly recognized, and their modes of ice accumulation and flow analyzed at several scales, they have not been analyzed and well-characterized globally despite their significance for understanding the evolution of the martian climate. A major outstanding question is the global extent of accumulation and flow of ice during periods of non-polar glaciation: As a mechanism to address this question, we outline two end-member scenarios to provide a framework for further discussion and analysis: (1) ice accumulation was mainly focused within individual craters and valleys and flow was largely local to regional in scale, and (2) ice accumulation was dominated by global latitudinal scale cold-based ice sheets, similar in scale to the Laurentide continental ice sheets on Earth. In order to assess these end members, we conducted a survey of ice-related features seen in Context Camera (CTX) images in each hemisphere and mapped evidence for flow directions within well-preserved craters in an effort to decipher orientation preferences that could help distinguish between these two hypotheses: regional/hemispheric glaciation or local accumulation and flow. These new crater data reveal a latitudinal-dependence on flow direction: at low latitudes in each hemisphere (<40–45°) cold, pole-facing slopes are strongly preferred sites for ice accumulation, while at higher latitudes (>40–45°), slopes of all orientations show signs of ice accumulation and ice-related flow. This latitudinal onset of concentric flow of ice within craters in each hemisphere correlates directly with the lowest latitudes at which typical pedestal craters have been mapped. Taken together, these observations demarcate an important latitudinal boundary that partitions each hemisphere into two zones: (1) poleward of ∼45°, where net accumulation of ice is interpreted to have occurred on all surfaces, and (2) equatorward of ∼45°, where net accumulation of ice occurred predominantly on pole-facing slopes. These results provide important constraints for deciphering the climatic conditions that characterized Mars during periods of extensive Amazonian non-polar glaciation.

Highlights

► The orientation of ice-related flow features on Mars is latitude dependent. ► Ice-related flow almost always trends towards the pole at mid-latitudes. ► Flow occurs evenly at all orientations poleward of 45°. ► Concentric flow of ice matches the distribution of pedestal craters. ► We can now map where ice has been capable of accumulating in the Amazonian.

Effects of space weathering on diagnostic spectral features: Results from He+ irradiation experiments

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 219, Issue 2

Xiaohui Fu, Yongliao Zou, Yongchun Zheng, Ziyuan Ouyang

We performed ion irradiation of mineral samples with 50keV He⁺, aimed to investigate ion irradiation effects on diagnostic spectral features. Reflectance spectra of samples in 0.375–2.5μm are measured before and after ion irradiation. Silicates, including Luobusha olivine, plagioclase and basaltic glass, have shown reddening and darkening of reflectance spectra at the VIS–NIR range. Olivine is more sensitive to ion irradiation than plagioclase and basaltic glass. Irradiated Panzhihua ilmenite exhibits higher reflectance and stronger absorption features, which is totally different from lunar soil and analog silicate materials in other experiments. Using continuum removal and MGM fit, we extracted and compared absorption features of olivine spectra before and after irradiation. Ion irradiation can induce band strength decrease of olivine but negligible band centers shift. We estimate band centers shift caused by ion irradiation are quite limited, even less than variations due to chemical composition in silicates. It provides one possible explanation for no systematic shift in band positions in lunar soil. Irradiated Luobusha olivine spectrum matches spectra of olivine-dominated asteroids. Our results suggest space weathering should be new clues to explain the subtle difference between A-type asteroid spectra and laboratory spectra of olivine.

Highlights

► Minerals show different sensitivity under the same conditions of ion irradiation. ► Ion irradiation induces band strength decrease but negligible band center shift. ► He⁺ Irradiated olivine spectrum matches spectra of olivine-dominated asteroids. ► Irradiated ilmenite exhibits higher reflectance and stronger absorption features.

Physical constraints on impact melt properties from Lunar Reconnaissance Orbiter Camera images

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 219, Issue 2

Brett W. Denevi, Steven D. Koeber, Mark S. Robinson, W. Brent Garry, B. Ray Hawke, Thanh N. Tran, Samuel J. Lawrence, Laszlo P. Keszthelyi, Olivier S. Barnouin, Carolyn M. Ernst, Livio L. Tornabene

Impact melt flows exterior to Copernican-age craters are observed in high spatial resolution (0.5m/pixel) images acquired by the Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC). Impact melt is mapped in detail around 15 craters ranging in diameter from 2.4 to 32.5km. This survey supports previous observations suggesting melt flows often occur at craters whose shape is influenced by topographic variation at the pre-impact site. Impact melt flows are observed around craters as small as 2.4km in diameter, and preliminary estimates of melt volume suggest melt production at small craters can significantly exceed model predictions. Digital terrain models produced from targeted NAC stereo images are used to examine the three-dimensional properties of flow features and emplacement setting, enabling physical modeling of flow parameters. Qualitative and quantitative observations are consistent with low-viscosity melts heated above their liquidii (superheated) with limited amounts of entrained solids.

Highlights

► Many Copernican lunar craters have well preserved impact melt flows. ► Meter-scale digital terrain models allow quantitative analysis of impact melt flow deposits. ► Observations of impact melt deposits show that they were highly fluid and emplaced at high temperatures (superheated). ► Melt volume per impact event can vary significantly above model predictions.

Outer Main Belt asteroids: Identification and distribution of four 3-μm spectral groups

admin — Sun, 20 May 2012 09:52:03 +0000

Publication year: 2012
Source:Icarus, Volume 219, Issue 2

Driss Takir, Joshua P. Emery

This paper examines the distribution and the abundance of hydrated minerals (any mineral that contains H2O or OH) on outer Main Belt asteroids spanning the 2.5< a <4.0AU region. The hypothesis we are testing is whether planetesimals that accreted closer to the Sun experienced a higher degree of aqueous alteration. We would expect then to see a gradual decline of the abundance of hydrated minerals among the outer Main Belt asteroids with increasing heliocentric distance (2.5< a <4.0AU). We measured spectra (0.8–2.5μm and 1.9–4.1μm) of 28 outer Main Belt asteroids using the SpeX spectrograph/imager at the NASA Infrared Telescope Facility (IRTF). We identified four groups on the basis of the shape and the band center of the 3-μm feature. The first group, which we call “sharp”, exhibits a sharp 3-μm feature, attributed to hydrated minerals (phyllosilicates). Most asteroids in this group are located in the 2.5< a <3.3AU region. The second group, which we call “Ceres-like”, consists of 10 Hygiea and 324 Bamberga. Like Asteroid Ceres, these asteroids exhibit a 3-μm feature with a band center of 3.05±0.01μm that is superimposed on a broader absorption feature from ∼2.8 to 3.7μm. The third group, which we call “Europa-like”, includes 52 Europa, 31 Euphrosyne, and 451 Patientia. Objects in this group exhibit a 3-μm feature with a band center of 3.15±0.01μm. Both the Ceres-like and Europa-like groups are concentrated in the 2.5< a <3.3AU region. The fourth group, which we call “rounded”, is concentrated in the 3.4< a <4.0AU region. Asteroids in this group are characterized by a rounded 3-μm feature, attributed to H2O ice. A similar rounded 3-μm feature was also identified in 24 Themis and 65 Cybele. Unlike the sharp group, the rounded group did not experience aqueous alteration. Of the asteroids observed in this study, 140 Siwa, a P-type, is the only one that does not exhibit a 3-μm feature. These results are important to constrain the nature and the degree of aqueous alteration in outer Main Belt asteroids.

Highlights

► We measured NIR spectra of 28 Outer Main Belt asteroids using SpeX at the NASA IRTF. ► We identified four groups on the basis of the shape and the band center of the 3-μm feature. ► These results provide important constraints on the nature of aqueous alteration in asteroids.

The distinct nitrogen isotopic compositions of low and high molecular weight marine DON

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry

Angela N. Knapp, Daniel M. Sigman, Adam B. Kustka, Sergio A. Sañudo-Wilhelmy, Douglas G. Capone

To constrain the sources and cycling of bulk and size-fractionated marine dissolved organic nitrogen (DON), samples were collected for concentration and isotopic (δ¹⁵N) analysis from the western tropical North Atlantic Ocean as well as over the North Australian shelf. Bulk DON concentrations were typically between 4 and 6μM, with low molecular weight (LMW, i.e., <1000 Dalton) DON accounting for 60 to 70% of total DON. The δ¹⁵N of both bulk and high molecular weight (HMW, i.e., >1000 Dalton to <0.2μm) DON from the western tropical North Atlantic are similar to previous measurements, as well as to bulk and HMW DON collected off the North Australian Shelf, ~4‰. Here we report the first measurement of marine LMW DON δ¹⁵N, and a coherent pattern emerges where LMW DON δ¹⁵N<HMW DON δ¹⁵N. An analytical concern is that the bulk DON δ¹⁵N as calculated from our coupled HMW and LMW measurements is often lower than that measured directly for the bulk DON. Despite this discrepancy, the self-consistency of the data in other regards argues for the robustness of the basic observation of lower δ¹⁵N in LMW relative to HMW DON. One explanation for this isotopic difference is considered most likely, based on the model that DON δ¹⁵N elevation relative to PON is fundamentally due to DON production from PON without isotopic fractionation, coupled with DON destruction occurring with fractionation. In this model, HMW DON loss must occur predominantly through the breakage of N-containing bonds, for which isotope fractionation should be substantial, whereas LMW DON loss is partly due to direct assimilation by phytoplankton and other microbes, which appears to occur with only minor isotopic fractionation. However, an alternative hypothesis exists: LMW and HMW DON may have distinct sources, for example, prokaryotic and eukaryotic plankton, respectively. In either case, it appears that most LMW DON has not passed through the HMW DON pool, as this would require an unrealistically large isotope fractionation at the HMW to LMW conversion. Finally, neither the concentration nor the δ¹⁵N of the bulk or either size fraction of DON varied with in situ N2 fixation rates measured at the time of sample collection. However, the δ¹⁵N of both bulk and LMW DON from the western tropical North Atlantic show significant differences between samples collected six months apart, which may be due to seasonally variable stimulation of primary production by the Amazon River.

Highlights

► First report of LMW DON δ¹⁵N from the ocean; LMW DON δ¹⁵N<HMW DON δ¹⁵N. ► Isotopic analysis provides constraints on source of LMW DON, suggests LMW DON comes from PON degradation. ► Significant variation in LMW DON δ¹⁵N collected six months apart in western tropical North Atlantic attributed to stimulation of productivity by Amazon plume.

Effect of pressure on silica solubility of diatom frustules in the oceans: Results from long-term laboratory and field incubations

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry

Socratis Loucaides, Erica Koning, Philippe Van Cappellen

The oceanic cycle of silicon (Si) has been studied extensively due to its close coupling to the oceanic carbon cycle and the biological CO2 pump. The oceanic Si cycle is dominated by the uptake of dissolved silicate (dSi) by planktonic organisms, predominantly diatoms, which use it to synthesize siliceous frustules. As oceanic waters are undersaturated with respect to biogenic silica (bSiO2) the frustules dissolve after death of the organisms, thereby regenerating dSi. Because the dissolution rate of bSiO2 depends on the degree of undersaturation, the thermodynamic solubility of bSiO2 is a key parameter controlling the recycling efficiency of nutrient Si in the water column and sediments. While an extensive body of data exists describing the dependence of bSiO2 solubility on temperature, the effect of pressure on the solubility of natural diatom frustules has never been measured directly. In this study, we conducted long-term (up to 22months) laboratory and field equilibration experiments to determine the solubility of cleaned frustules of a cultured marine diatom (Thalassiosira punctigera) in seawater, for pressures between 1 and 700bar, and temperatures between 2 and 21°C. According to our results, the solubility of the frustules decreases by about 10% when pressure increases from 1 to ~200bar. From 200bar on, the pressure dependence reverses, and at 700bar the solubility is about 15% higher than at atmospheric pressure. Integrated over an average oceanic water depth of 4000m, a drop in temperature of 15-20°C has a far more significant effect on the solubility of bSiO2 than a corresponding 400bar increase in pressure.

Highlights

► We examine the effect of pressure on diatom frustule solubility. ► Solubility of diatom frustules decreases from 1 to ~200bar. ► At 200bar, the pressure dependence of diatom frustule solubility reverses. ► At 700bar, the solubility is about 15% higher than at atmospheric pressure.

Distribution of anaerobic ammonia-oxidizing bacteria in a subterranean estuary

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry

James P. Sáenz, Ellen C. Hopmans, Dan Rogers, Paul B. Henderson, Matthew A. Charette, Karen L. Casciotti, Stefan Schouten, Jaap S. Sinninghe Damsté, Timothy I. Eglinton

The traditional paradigm that rivers and terrestrial run-off are the major contributors of nutrients to coastal waters has been challenged by observations that nutrient fluxes originating from coastal aquifer subterranean estuaries can equal or even exceed that of other terrestrial sources. Within a coastal aquifer where organic carbon is scarce and ammonium is abundant, bacteria capable of anaerobic ammonium oxidation (anammox bacteria) may play a role in the removal of fixed nitrogen. We investigated the presence of anammox bacteria in a coastal groundwater system (Waquoit Bay, MA USA) using lipid biomarkers. From the distribution of sediment-bound ladderane phospholipids, biomarkers for viable anammox bacteria, we demonstrate the presence of these organisms in association with aqueous chemical transition zones within the aquifer. The distribution of ladderane fatty acids in contrast, provided insight into the historical distribution of anammox bacteria and temporal stability of that distribution. The results suggest that anammox communities have been present over a broad range of depths, most likely determined by changes in the depths of the redox transition zones over time, but that they are more prevalent in the upper portion of the subterranean estuary where ammonium and nitrate coexist.

Highlights

► The distribution of anaerobic ammonium oxidizing (anammox) bacteria is investigated in a subterranean estuary ► Ladderane lipids, biomarkers for anammox bacteria, were detected at depths corresponding to redox transition zones ► Distribution of ladderanes core lipids indicates that the distribution of anammox has been persistent feature of the subterranean estuary.

Two-dimensional dissolved ferrous iron distributions in marine sediments as revealed by a novel planar optical sensor

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry

Qingzhi Zhu, Robert C. Aller

A new, single use planar optical sensor was developed for measuring high resolution, two-dimensional Fe²⁺ distributions in marine sediments. Ferrozine (3-(2-Pyridyl)-5, 6-diphenyl-1, 2, 4-triazine-p, p′-disulfonic acid monosodium salt hydrate) was used as the Fe²⁺ indicator, and was covalently immobilized onto a transparent poly(vinyl alcohol) membrane through a water soluble poly(N-isopropylacryl amide) polymer chain, backed by a polyester sheet. When the colorless and transparent sensor membranes are deployed in Fe²⁺ solutions or natural marine sediment samples, a violet-red color with maximum absorption wavelength at 562nm develops due to the formation of ferrozine-Fe²⁺ complex in the sensor membrane. The absorbance of the sensor at 562nm shows excellent linear relationships versus Fe²⁺ concentrations in the range of 0 – 200 μM, with a lower detection limit of 4.5 μM. The response time of the sensor film varies with Fe²⁺ concentration, following Langmuir kinetics, with a typical practical range of 10 – 30min at room temperature (~22º C). The absorbance attained is largely independent of temperature, oxygen exposure before or after development, salinity and pH changes. No interferences from other major components and trace metal ions in seawater have been observed. The sensor is simple, stable (irreversible after removal), and precise, and has been successfully used to measure virtually continuous two-dimensional Fe²⁺ distributions in intertidal flat and subtidal sediment samples. Images are readily obtained from the membrane by using a scanner or inexpensive LED excitation and commercial grade digital cameras, with typical pixel resolution of~50×50 μm over areas>150cm². The complex heterogeneous distribution patterns of Fe²⁺ associated with both inhabited and abandoned biogenic structures and other natural diagenetic heterogeneity, are readily revealed and can be related directly to the corresponding visible images of sedimentary features. These patterns demonstrate directly that the average concentrations typically measured in traditional vertical profiles can be a misleading indicator of the microenvironmental Fe²⁺ concentrations controlling sediment – water fluxes and authigenic mineral formation – dissolution.

Highlights

► Two-dimensional measurements of dissolved ferrous iron in marine sediments using a new planar optical sensor. ► High spatial resolution and stability. ► In-situ, high resolution dissolved iron mapping in bioturbated sediments.

Brominated methane compounds and isoprene in surface seawater of Sagami Bay: Concentrations, fluxes, and relationships with phytoplankton assemblages

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 134–135

Michiko Kurihara, Motoko Iseda, Teru Ioriya, Naho Horimoto, Jota Kanda, Takashi Ishimaru, Yukuya Yamaguchi, Shinya Hashimoto

Concentrations of bromodichloromethane (CHBrCl2), chlorodibromomethane (CHBr2Cl), bromoform (CHBr3) and isoprene were measured along with cell counts of phytoplankton in the surface waters of Sagami Bay, from April 2001 to December 2001, to evaluate their biological sources and their relative contributions to Br in the marine boundary layer. The mean (range) chlorophyll a (Chl. a) concentrations were 0.46 (0.23–1.0) μg L⁻¹ from April to December. Observations of temporal variations in cell volume showed that diatoms were predominant in the April, July, and December samples, and that dinoflagellates were dominant in the June and September samples. In the May, August, October and November samples, coccolithophorids were relatively more abundant when diatoms were absent. The mean concentrations of trace gases were 7.4pmol L⁻¹ CHBrCl2, 8.4pmol L⁻¹ CHBr2Cl, 33.8pmol L⁻¹ CHBr3, and 7.3pmol L⁻¹ isoprene. The mean sea-to-air fluxes of trace gases were 7.4nmolm⁻² d⁻¹ CHBrCl2, 8.7nmolm⁻² d⁻¹ CHBr2Cl, 32.2nmolm⁻² d⁻¹ CHBr3, and 5.9nmolm⁻² d⁻¹ isoprene. A significant positive correlation between CHBrCl2 and CHBr2Cl was observed in this study (r ² =0.79, n =48, p <0.001). Bromoform, however, was not correlated with CHBrCl2 or CHBr2Cl. These results suggest that the production and fate of CHBrCl2 and CHBr2Cl in the surface seawater were similar, and that the production and fate of CHBr2Cl and CHBrCl2 were different from those of CHBr3. A significant positive correlation was observed between the isoprene and Chl. a concentrations (r ² =0.49, n =8, p =0.05). No significant correlation was found between Chl. a and any of brominated methanes (i.e., CHBrCl2, CHBr2Cl, and CHBr3). No individual or group of phytoplankton could be attributed as the source of CHBr3 (or CHBrCl2, CHBr2Cl) in this study. The results thus highlighted the difficulty of estimating production of brominated methanes in seawater using Chl. a data or phytoplankton assemblage analysis and the necessity of determining the essential factors that control production of brominated methanes. The bromine fluxes of short-lived brominated methanes from the ocean to the atmosphere were estimated at 7.4nmolBrm⁻² d⁻¹ for CHBrCl2, 17.4nmolBrm⁻² d⁻¹ for CHBr2Cl, and 96.5nmolBrm⁻² d⁻¹ for CHBr3. The relative contribution of CHBr3 to bromine in the marine boundary layer from surface ocean was calculated to be from 5.2±0.6 (November) to 27±5.5 (September) for BrCHBr3/BrCHBrCl2. Our results underscore the importance of CHBr3 as a source of atmospheric bromine from the ocean.

Highlights

► A positive correlation was observed between CHBrCl2 and CHBr2Cl in surface seawater. ► No source of brominated methanes was identified by phytoplankton assemblage analysis. ► Relative contribution of CHBr3 to Br in marine boundary layer varied over 9months.

Dissolved oxygen and suspended particles regulate the benthic flux of iron from continental margins

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 134–135

William B. Homoky, Silke Severmann, James McManus, William M. Berelson, Timothy E. Riedel, Peter J. Statham, Rachel A. Mills

We present ex situ sediment incubation results from the California and Oregon shelves and compare the calculated benthic flux of dissolved Fe with those from in situ incubations and pore water concentration profiles. We also examine the influence of oxygen depletion and sediment re-suspension on benthic Fe exchange. Ex situ incubation of the California and Oregon shelf sites yielded average benthic Fe fluxes of 3.5 and 8.3μmolm⁻² day⁻¹, respectively, compared to 17 and 55μmolm⁻² day⁻¹ from the in situ Lander determinations, and 73 and 103μmolm⁻² day⁻¹ from modeling of pore water concentration profiles. Differences between benthic Fe flux estimates are primarily accounted for by [1] differences in Fe (II) oxidation kinetics, which result from distinct oxygen consumption rates between incubation methods, and the absence of kinetic considerations in the overlying bottom water in pore water flux calculations, and [2] the effects of biological sediment irrigation that are best represented by in situ incubations due to their sampling area and preservation of bottom water conditions. Bottom water oxygen concentrations were higher at the California shelf site than that at the Oregon shelf site, and probably accounted for the greater discrepancy between methods used to determine benthic Fe flux. The comparison of techniques used to determine benthic Fe flux indicates that the concentration of bottom water oxygen exerts a principle control over the fate of dissolved Fe entering the overlying bottom water — supporting the view that the expansion of coastal hypoxia has the potential to enhance the benthic supply of Fe (II) to shelf waters. An episode of surface sediment re-suspension during ex situ incubation led to a rapid removal of 76–89% of dissolved Fe from seawater, followed by a steady return towards initial seawater concentrations during particle settling, indicating that diffusive inputs of dissolved Fe from sediment pore water are rapidly adsorbed and desorbed by particles during periods of benthic re-suspension. The findings suggest that dissolved Fe concentrations in bottom waters may reflect an equilibrium concentration of non-stabilized aqueous Fe and particle-adsorbed Fe phases — where the addition of suspended particles to bottom waters leads to scavenging of dissolved Fe into labile particulate Fe phases. Thus we suggest that suspended particles are a significant buffer of dissolved Fe released from shelf sediments, an important transport mechanism for benthic Fe inputs, and a regulator of dissolved Fe concentrations in seawater.

Highlights

► We compare ex situ incubations to in situ methods for benthic Fe flux determination. ► Seawater deoxygenation increased benthic Fe flux from shelf sediments. ► Particle re-suspension scavenged dissolved Fe above ligand concentrations. ► Oxygen and suspended particles co-regulate Fe flux and transport from shelf sediments.

DMS flux over the Antarctic sea ice zone

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 134–135

Anne Trevena, Graham Jones

This study presents concentrations of dimethylsulphide (DMS) and its precursor compound dimethylsulphoniopropionate (DMSP), and chlorophyll a in a variety of sea ice and seawater habitats from pack and fast ice in the Antarctic sea ice zone (ASIZ) of eastern Antarctica. Estimated “hot spot” seawater DMS concentrations released during sea ice melting in October and derived from a total DMSP (DMSPt)-sea ice depth relationship and application of a DMSPt:DMS conversion ratio, predict that sea ice derived DMS of the order of 20–50nM is associated with DMSPt released from melting sea ice of 0.4m thickness. In November and December, melting of large areas of ice less than 0.6m thick could release larger amounts of DMS around 50–80nM and 150–270nM, respectively. In addition, DMSPt released from 1.0 to 1.2m thick ice in December could result in “hot spot” concentrations of DMS of around 100nM. DMS flux in November and mid to late December was highest in pack ice>fast ice>ice edge (pack ice=54μmolm⁻² d⁻¹; (range 1–325); fast ice=16μmolm⁻² d⁻¹; (range 10–50); ice edge=12 (range 1.2–26) and 23μmolm⁻² d⁻¹). In ice-free seawater in the Davis area from January to February, although dissolved DMS concentrations were low, DMS fluxes were high due to high wind speeds (DMS=3nM; DMS flux=27μmolm⁻² d⁻¹ (range 1–101)). DMS concentrations and flux from a fast ice tide crack (DMS=12nM; DMS flux 6–81μmolm⁻² d⁻¹), and two days following fast ice breakout from the Davis region DMS concentrations (12nM) and DMS flux (45–84μmolm⁻² d⁻¹) also indicated that fast ice was a significant source of DMS to the atmosphere of this region. In contrast DMS flux from a fast ice melt-pool was low (DMS flux 0.5–7μmolm⁻² d⁻¹). These measurements support the suggestion that during sea ice melting during late spring to early summer (November–December), the ASIZ is an area of high DMS (P) production from various sources, leading to very high and variable fluxes of DMS to the atmosphere of this region. As sea ice melts in late November voids in sea ice become larger enabling sea ice to vent DMS directly to the atmosphere, as well as being released from sea ice melting. At most sea ice sites DMS often displayed non-detectable to elevated concentrations of DMS (mean 11nM) in the surface 0–0.1m. Using average DMS concentrations from a range of studies we estimate that DMS flux from ASIZ in November is about 23μmolm⁻² d⁻¹, much lower than what we have estimated from pack ice using a DMSPt:DMS conversion ratio technique (54μmolm⁻² d⁻¹). The difference (31μmolm⁻² d⁻¹) could reflect venting or degassing of DMS directly from pack ice to the Antarctic atmosphere. Depth-integrated sea ice DMS concentrations varied with time of day, with elevated DMS concentrations (18–30nM) in the cores during the later part of the day (i.e. 16.00–20.00h), whilst close to mid-day (11.00–12.00h) concentrations were much lower (2–9nM). The lower depth-integrated DMS concentrations found in pack ice in November could suggest that large amounts of DMS are vented from pack ice close to mid-day when temperatures and wind speeds often increase. Varying DMSPt–chlorophyll a correlations in the sea ice highlight how varying biological activity affects DMSP concentrations, and suggest that variations in biological activity in sea ice also influence DMS flux.

Highlights

► High and variable DMS flux values are reported for Antarctic pack and fast ice. ► In spring to summer the Antarctic atmosphere receives large amounts of atmospheric DMS as sea ice melts. ► Sea ice DMS concentrations vary with time of day suggesting venting of DMS to the Antarctic atmosphere.

Editorial Board

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 132–133

Distinct trends in dissolved Fe speciation between shallow and deep waters in the Pacific Ocean

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 134–135

Yoshiko Kondo, Shigenobu Takeda, Ken Furuya

The chemical speciation of dissolved Fe was investigated in the Pacific Ocean to elucidate the regional variability of organic ligands. The concentration and conditional stability constants (K′ Fe′L , with respect to inorganic Fe) of organic ligands were analyzed by a Competitive Ligand Equilibration-Adsorptive Cathodic Stripping Voltammetric (CLE-ACSV) method using 2-(2-thiazolylazo)-p-cresol (TAC). In the surface waters of the Pacific Ocean, a low concentration of dissolved Fe was observed (0.20±0.14nM, n = with an excess of organic ligands (0.27±0.14Eq of nM Fe, n =7); thus, most of the dissolved Fe was estimated to be complexed with these ligands. A 2D cross-section diagram along 160°W and 170°W showed a roughly increasing trend in the concentration of organic ligands toward the north of the deep waters, suggesting the accumulation of ligands in the biogeochemical cycle of deep water. In the equatorial Pacific and central subtropical North Pacific, the concentration of organic ligands was observed to exceed that of dissolved Fe throughout the water column. On the contrary, the concentration of dissolved Fe at depths ranging from 1000 to 3000m was higher than that of organic ligands in the western North Pacific, suggesting that the excess dissolved Fe can exist as colloidal Fe and/or organic/inorganic complexes with the ligands that were not detectable by our method. A greater concentration of excess dissolved Fe relative to the concentration of organic ligands was also observed in the deep (below 1000m) waters of the subtropical South Pacific. These differences regarding the saturation state of organic ligands ([L]/[D–Fe]) between the surface and deep waters probably reflect different sink and source mechanisms of organic ligands within the water column. The extent of the organic complexation of Fe varies with the biogeochemical processes in the ocean interior and plays a key role in oceanic Fe cycling.

Highlights

► We investigate the speciation of dissolved Fe in the Pacific Ocean. ► Most of the dissolved Fe was complexed with organic ligands in the surface waters. ► Excess dissolved Fe with respect to the organic ligand concentration was found in the deep waters. ► The organic ligands tended to be more saturated with Fe in the deep waters. ► Biogeochemical processes affect the extent of organic Fe complexation in seawater.

Nitrogen cycling within suboxic and anoxic sediments from the continental margin of Western North America

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 128–129

Lauren S. Chong, Maria G. Prokopenko, William M. Berelson, Amy Townsend-Small, James McManus

Here we report rates of benthic nitrogen (N) cycling and assess controls on biological NO3 ⁻ sequestration and transport in sediments underlying oxygen deficient regions of the ocean ranging from anoxic/suboxic ([O2] of 0–2μM) to more oxic (57μM [O2]) conditions. N mass balances were constructed based on benthic fluxes (N2, NH4 ⁺ and NO3 ⁻) and pore water profiles (N2, NO3 ⁻, NO2, N2O, Fe and HS⁻) at sites in the Southern California Borderland and the Mexican Margin. Fluxes across the sediment-water interface for N2, NO3 ⁻ and NH4 ⁺ were determined directly by whole core incubations, and fluxes of N2 were also determined by modeling mm-scale pore water profiles. Estimates of the N2 flux by these two methods agree to ±50%, thereby establishing a range of net N2 production in these settings. The average N2 efflux was four times larger at a site with high pore water H2S concentrations (Soledad Basin 3.14±1.10mmolNm⁻² day⁻¹) compared to an iron-rich site (Santa Monica Basin 0.74±0.21mmolNm⁻² day⁻¹) despite both sites having comparable NO3 ⁻ uptake fluxes (−0.93±0.14 vs. −0.82±0.08mmolNm⁻² day⁻¹ respectively). Pore water profiles from both sulfidic and iron-rich sites reveal subsurface maxima in NO3 ⁻, NO2 ⁻, and N2O that are likely caused by the presence of NO3 ⁻ sequestered by infaunal microbiota. In Soledad Basin, the sequestered pool of microbial NO3 ⁻ contributes to NH4 ⁺ production via DNRA resulting in an NH4 ⁺ efflux (2.66±0.52mmolNm⁻² day⁻¹) to the overlying water. This flux exceeds the rate of NH4 ⁺ production by Corg degradation by 10 times. At the suboxic sites, a total N balance can only be achieved if the flux of NO3 ⁻ into sediments is composed of two components: diffusive and biologically mediated transport. The more oxic site shows no evidence of a sequestered microbial NO3 ⁻ pool and diffusive fluxes can account for all N transformations. Core incubations do not capture the total amount of NO3 ⁻ uptake where biological transport is important as they do not account for NO3 ⁻ sequestered prior to the start of the incubation. Pore water N2O concentrations of up to 500nM in sub-surface sediments greatly exceed the background concentration (7nM) and are likely generated by the metabolic reduction of the intracellular nitrate pool, however, there was no measurable efflux of N2O from sediments to the overlying water. Biological NO3 ⁻ transport is a ubiquitous process in suboxic and anoxic sediments, however the magnitude of its importance appears to be linked to the presence of dissolved iron or sulfide in the pore waters.

Highlights

► Porewater profiles in reducing sediments have maxima caused by biological storage and use of NO3 ⁻. ► The total NO3 ⁻ flux into reducing sediments is the sum of diffusive and biological transport. ► NO3 ⁻ storage and transport appears to be ubiquitous in reducing sediments. ► More N leaves sediments in sulfidic systems than combined NO3- and Corg burial fluxes can account for.

Quasi-Lagrangian drifter studies of iron speciation and cycling off Point Conception, California

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 128–129

Andrew L. King, Kristen N. Buck, Katherine A. Barbeau

The distribution and speciation of dissolved Fe (dFe) were measured during four quasi-Lagrangian drogued drifter studies (~4d duration each) that were conducted in the southern California Current System in May 2006 and April 2007. Three of the four drifter studies were within the coastal upwelling regime and one drifter study was in a warm-core anticyclonic eddy. Incubation bottle experiments were also conducted to determine the degree of phytoplankton Fe limitation and to assess changes in the concentration of Fe-binding ligands. In the coastal upwelling drifter studies, in situ dFe (1.4–1.8nM) and macronutrients were initially high and declined over time. Fe addition incubation experiments indicated that the phytoplankton community was not Fe limited at the beginning of the coastal upwelling drifter experiments (when μM nitrate:nM dFe ratios were ~7–8). By the end of two of the three drifter studies (when μM nitrate:nM dFe ratios were ~12–19), Fe addition resulted in larger nitrate and silicic acid drawdown, and larger accumulations in chlorophyll a, particulate organic carbon and nitrogen, and diatom and dinoflagellate-specific carotenoid pigments. Fe speciation was measured in situ in three of the four drifter studies with stronger L1-type ligands found to be present in excess of dFe in all samples. In Fe speciation incubation experiments, L1-type ligand production was observed in conjunction with phytoplankton growth under Fe-limiting conditions. The results presented here support and add a quasi-Lagrangian perspective to previous observations of dFe and macronutrient cycling over space and time within the California coastal upwelling regime, including Fe limitation within the phytoplankton community in this region and the biological production of Fe-binding ligands concomitant with Fe limitation.

Highlights

► We report changes in dissolved Fe (dFe) during quasi-Lagrangian drifter studies. ► In two of four drifter studies, dFe decreased more quickly than macronutrients. ► Low Fe availability resulted in Fe limitation within the phytoplankton community. ► Strong L1-type ligands, when measured, were in excess of dissolved Fe. ► Fe stress resulted in apparent in vitro biological production of L1-type ligands.

Editorial Board

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 128–129

Dissolved organic carbon release from surface sand of a high energy beach along the Southeastern Coast of North Carolina, USA

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 132–133

G. Brooks Avery, Robert J. Kieber, Kelly J. Taylor, Jennifer L. Dixon

The purpose of this study was to determine the flux of dissolved organic carbon (DOC) from high-energy beach sand to the coastal ocean. The DOC concentration of seawater was measured before and after exposure to the beach sand in laboratory experiments. Increases were observed in all experiments (n=36) conducted on seawater and high-energy beach sand collected on a monthly basis for one year. Final dissolved organic carbon in seawater after exposure to surface sands were positively correlated with water column DOC concentrations measured in the surf zone at the same time of sand collection for laboratory experiments. Increases in DOC during laboratory experiments displayed marked seasonality with larger increases observed during summer with a corresponding low DOC carbon to nitrogen ratio (C:N) suggesting phytoplankton as the source of DOC during this time of year. Higher DOC C:N was observed during winter indicating a more aged organic material as the source of DOC during this time of low flux. Sandy beach sediments release over an order of magnitude more DOC per meter squared compared to coastal shelf sediments and rainwater suggesting that in locations where there are no direct river inputs these beach environments play a significant, previously unrecognized role as an external source of DOC to the immediate coastal ocean. It is likely that DOC released from these environments has a significant effect on surf zone biogeochemistry potentially affecting oxygen concentrations in coastal regions.

Highlights

► High energy beach sands release DOC upon exposure to seawater. ► High energy beach sand DOC flux on a per meter squared basis is higher than other coastal sediments. ► DOC release from sandy sediments represents approximately 10% of total carbon remineralized in these systems. ► Sources of high-energy beach sand DOC vary seasonally with larger fluxes during warmer months. ► Sources of high-energy beach sand DOC are phytoplankton during the warmer months and more degraded material during the winter months.

Deep water circulation and composition in the Arctic Ocean by dissolved barium, aluminium and silicate

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 132–133

T. Roeske, M. Rutgers vd Loeff, R. Middag, K. Bakker

As part of the ARK-XXII/2 Polarstern expedition in summer 2007, dissolved Ba was analyzed in the Eurasian Basins and the Makarov Basin including the Alpha and Mendeleyev Ridges as well as on the adjacent shelves. The data was compared with data of dissolved Al and Si from the same cruise. Superimposed on the gradual increase of concentration with depth by dissolution of the particle rain, we observe different flow patterns in intermediate waters along the track. In the Atlantic and Intermediate Depth Water (AIDW) in the Amundsen Basin the influence from Eurasian shelf water can be seen in slightly enhanced concentrations of dissolved Ba compared with Al and Si. At the same time Al concentrations decrease with distance from the Eurasian shelves. Source waters to the Atlantic Layer Water (ALW) in the Makarov Basin have the same background Ba concentrations as the Nansen AIDW. We describe the distributions of the elements in the Deep Eurasian and Bottom Water (DEBW) by deep shelf convection as well as diffusion from sediments controlling concentrations in the Nansen DEBW while in the Amundsen DEBW diffusion from sediments appears to be more important. In the Makarov Basin inflow from the Canadian Basin and overflow from the Amundsen Basin at 2000m depth at the Lomonosov Ridge are required to explain the composition of bottom waters.

Highlights

► Profiles of dissolved Ba on ARK-XXII/2 Polarstern expedition in summer 2007. ► Different flow patterns of Ba, Al and Si in intermediate and deep waters. ► Deep shelf convection and diffusion from sediments important in the Nansen Basin. ► In the Amundsen Basin diffusion from sediments into deep water important. ► Inflow from the Canadian and overflow from the Amundsen Basin into the Makarov Basin.

pH and biogeochemical processes in the Gotland Basin of the Baltic Sea

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2011
Source:Marine Chemistry, Volume 127, Issues 1–4

Adam Ulfsbo, Stefan Hulth, Leif G. Anderson

The Baltic Sea is a semi-enclosed sea characterized by a limited water exchange, large input of river runoff and a low salinity. Data collected in 1995 and 2008 in the Gotland Deep have been evaluated with the objective to assess how pH couples to biogeochemical processes. One result of the density stratification between 60 and ~100m, is the difference in dominating biogeochemical reactions in the surface layer compared to those in the bottom water. In the surface layer, distributions of seawater constituents were mainly determined by the seasonal balance between primary production and respiration. Chemical composition in the bottom water was governed by the availability of oxygen and the frequency of bottom-water intrusions. In 1995, oxygen was present throughout the water column and, as a consequence, nitrate concentrations were high (~10μmol/kg) also in the bottom water. In contrast, there was no oxygen or nitrate in the bottom water in 2008, while hydrogen sulfide was detected at significant concentrations (up to ~100μmol/kg). The largest decrease in pH was observed in the layer where oxygen decreased from ~300μmol/kg to about zero. When oxygen was low or hydrogen sulfide was present pH was nearly constant both in 1995 and 2008 at a low level of ~7.2. The distributions of chemical constituents with depth in the water column showed that a major fraction of organic matter mineralization was caused by oxygen and sulfate as electron acceptors, while nitrate played a minor role in the low oxygen or anoxic environment. The small variability in pH under conditions of low oxygen concentrations indicated that Fe- and Mn-oxides were of minor importance as electron acceptors during organic matter mineralization at this site. In oxic environments, observed data were consistent with mineralization of the model substance (CH2O)45(CH2)44(NHCH2CO)16(CHPO4Me). Under anoxic conditions, however, the observed changes in pH and DIC could be explained by mineralization of organic matter with significantly lower lipid content, e.g. material of terrestrial origin. In parallel, denitrification of preformed nitrate in the bottom layer may have contributed to changes in pH and DIC.

Highlights

► Coupling between biogeochemical processes and pH distributions in the Gotland Deep. ► Major fraction of organic matter mineralization was caused by oxygen and sulfate respiration. ► The largest change in pH coincided with the gradient in oxygen concentration. ► Bottom water with low oxygen or anoxic conditions had low and constant pH of ~7.2. ► Fe- and Mn-oxides were of minor importance as electron acceptors during OM mineralization.

Copper and nickel in ultrafiltered brackish water: Labile or non-labile?

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 132–133

Heléne Österlund, Johan Gelting, Fredrik Nordblad, Douglas C. Baxter, Johan Ingri

Copper and nickel were sampled at three stations in the Baltic Sea using diffusive gradients in thin film (DGT) passive samplers and ultrafiltration (<1kDa). Two versions of DGT devices were used, the normal open pore (OP) and a restricted pore (RP). The OP DGT and RP DGT concentrations closely followed each other both in depth profiles and time series. The lack of significant difference between OP and RP DGT suggests that the labile complexes were smaller than the pore size of the RP gel (approximately 1nm). These data, together with OP DGT measurements at the same location in two different years, clearly demonstrate that the DGT method is robust and indicates reproducible results during routine field conditions. Between 50 and 80% of the ultrafiltered fractions for Ni and Cu could not be detected by the DGT method, using standard procedures. This suggests the presence of complexing ligands for Cu and Ni. Assuming 100% complexation of Ni to fulvic acid ligand gave DGT concentrations similar to ultrafiltered Ni concentrations. The equivalent calculation for Cu indicates that up to 75% of the ultrafiltered Cu fraction is non-labile. The non-labile Cu complexes are proposed to be produced at sea since the fraction increases with decreasing terrestrial influence.

Highlights

► Ni complexes in the ultra-filtered fraction (<1kDa) are suggested to be completely DGT-labile. ► Up to 75% of Cu complexes in the ultra-filtered fraction were non-labile. ► Non-labile Cu complexes are proposed to be produced at sea since the fraction increases with decreasing terrestrial influence. ► Open pore and restricted pore DGT passive samplers gave similar results regarding both Cu and Ni measurements.

Lipids in the sedimentary record as markers of the sources and deposition of organic matter in a tropical Brazilian estuarine–lagoon system

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2011
Source:Marine Chemistry, Volume 127, Issues 1–4

Renato S. Carreira, Michelle P. Araújo, Talitha L.F. Costa, Gertrud Spörl, Bastiaan A. Knoppers

Lipid biomarkers [fatty acids (FAs), sterols and alcohols] and bulk properties [total organic carbon (TOC), C/N ratio and δ ¹³C] were determined in sediments of four cores (maximum length=48cm; three in the system’s proper and one at the river end-member) to characterize the local sedimentation patterns of organic matter (OM) in the tropical Mundaú–Manguaba estuarine–lagoon system (MMELS), NE-Brazil. The range of concentrations of fatty acids (60.1 to 644.2μgg⁻¹), sterols (0.5 to 82.0μgg⁻¹) and alcohols (0.8 to 39.0μgg⁻¹), including phytol, increased from the bottom toward the upper sediment layers of the lagoon cores. Assignments of lipid biomarkers to inputs from phyto- and zooplankton, terrestrial/riverine and bacterial sources of OM were confirmed by principal component analysis (PCA). An increase in the relative contribution of autochthonous OM in the more recent sediment layers was identified in all the 3 cores collected in the lagoon’s proper, suggesting that the cultural eutrophication symptoms observed over the last 3 decades could be related to intensive sugar-cane agricultural practices implemented since the 1970s in the watersheds and, likely also, to effluent inputs to the lower portion of Mundaú lagoon. These results confirmed the applicability of the lipid biomarker approach in the studies of OM geochemistry in complex tropical ecosystems. Moreover, the lipid biomarkers and total organic carbon isotopic data showed the presence of distinct estuarine/lagoon and fluvial compartments, and thus it was possible to test the validity of a quantitative model based on lipid biomarkers proposed by Volkman et al. (2008) to evaluate the contribution of terrestrial source to the pool of sedimentary OM. The best results were obtained using C26 n-alkanol, although with distance from the river source other compounds, like C28 n-alkanol and C26 and C28 FA could be applied as well.

Highlights

► Lipid biomarkers indicated changes of organic matter sources to a tropical system. ► Eutrophication caused an increase in the accumulation of autochthonous organic matter. ► A qualitative model were considered to evaluate the input of terrestrial organic matter.

Corrigendum to “Recent sudden decrease of lead in Adriatic coastal seawater during the years 2000–2004 in parallel with the phasing out of leaded gasoline in Italy” [Marine Chemistry 113 (2009) 238–249]

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 130–131

A. Annibaldi, C. Truzzi, S. Illuminati, G. Scarponi

The organic matrix of pelagic mucilaginous aggregates in the Tyrrhenian Sea (Mediterranean Sea)

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 132–133

Michele Giani, Paola Sist, Daniela Berto, Gian Paolo Serrazanetti, Vittoria Ventrella, Ranieri Urbani

In the last fifteen years the presence and distribution of large aggregates in the Mediterranean Sea have begun to be studied suggesting that these particles could play a relevant role in the carbon biogeochemistry of the basin. However the massive formation is a sporadic event and it is often difficult to detect. During the autumn of 2000 high densities of mucilage aggregates were observed in the major part of the coastal zone of the Tyrrhenian Sea (western Mediterranean Sea). The chemical composition and origin of these aggregates were evaluated. The carbohydrates, proteins, lipids and the humic and fulvic acid fractions constitute 45–68% of the organic carbon (OC). Marine origin was suggested by UV spectra and C/N ratios of humic substances extracted from mucilage samples. The polysaccharide component was isolated and purified by extraction of four fractions of different solubility and the saccharide carbon percentage ranged from 13 to 20% of the OC. By comparison the qualitative and quantitative monosaccharide, fatty acid and sterol compositions were very similar to those obtained from Adriatic pelagic mucilages, while on the other hand the results showed remarkable differences in comparison to benthic aggregates. Scanning electron microscopy (SEM) on native samples showed a complex morphology with the presence of fibrillar structures forming a highly branched tri-dimensional network similar to those found in Adriatic pelagic samples.

Highlights

► The organic components of mucilage samples of the Tyrrhenian Sea was studied. ► Fatty acid, carbohydrates, humic fractions, proteins were analyzed. ► The results showed several similarities with samples of the Northern Adriatic Sea. ► The results showed remarkable differences in comparison to benthic aggregates.

Seasonal and spatial dynamics of iron availability in the Scotia Sea

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 130–131

Maria C. Nielsdóttir, Thomas S. Bibby, C. Mark Moore, Daria J. Hinz, Richard Sanders, Michael Whitehouse, Rebecca Korb, Eric P. Achterberg

The Southern Ocean is the world’s largest high nutrient low chlorophyll (HNLC) region. However, satellite images highlight several areas associated with island chains and shallow topographic features which display high phytoplankton biomass. Here we present the first study of seasonal variations in phytoplankton biomass and iron availability in the Scotia Sea over both austral spring and summer seasons. Based on dissolved iron (dFe) and Chlorophyll a (Chl a) concentrations, the study area is be divided into three regions: North of South Georgia, south of South Georgia and the vicinity of South Orkney Islands. The Scotia Sea to the south of South Georgia exhibited low dFe concentrations (<0.027–0.05nM) in surface waters during both the spring and summer seasons. Nevertheless, nitrate concentrations were considerably lower in spring compared to summer (difference ~8μM). Summer Chl a concentrations were ~1.4mgm⁻³ and in situ phytoplankton populations displayed evidence of iron stress, suggesting the development of seasonal iron limitation. Surface water dFe concentrations in the South Georgia bloom waters (north of the islands) were elevated and slightly lower during spring than summer (0.20nM compared to 0.31nM, P>0.05). Nitrate concentrations were 16μM lower in summer compared to spring, whilst Chl a standing stocks remained high. Enhanced dFe (~0.25nM) and Chl a concentrations were furthermore observed in the vicinity of the South Orkney Islands, located in the southern Scotia Sea. Iron addition experiments showed that in situ phytoplankton were iron replete spring and summer north of South Georgia and in the vicinity of South Orkney Islands during summer. We thus suggest that increased iron supply in high productivity areas including the area north of South Georgia and the South Orkney Islands, was sustained by a continuous benthic supply from their shelf systems, with a potential additional input from seasonally retreating sea ice in the South Orkney system.

Highlights

► High dissolved Fe concentrations are observed on the South Georgia shelf. ► Enhance dissolved Fe concentrations are observed in the South Georgia bloom. ► Fe limitation only develops during summer in the central Scotia Sea. ► The seasonal cycle of Fe availability varies in sub-regions of the Scotia Sea.

Chemical Biomarkers in Aquatic Systems, T. Bianchi and E. Canuel

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 132–133

Ronald Benner

An evaluation of the efficacy of radium isotopes as tracers of submarine groundwater discharge to southern Rhode Island’s coastal ponds

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 130–131

Scott L. Stachelhaus, S. Bradley Moran, Roger P. Kelly

Submarine groundwater discharge (SGD) represents a significant flux of water and chemical constituents to coastal regions. Because ²²⁶Ra and ²²⁸Ra are highly concentrated in groundwater relative to surface water, and their signal is well-integrated throughout coastal surface waters, these isotopes have been increasingly utilized to quantify SGD over the past several decades. In this study, a combined radium and salt balance is used to yield estimates of fresh and saline SGD to four of Rhode Island’s coastal ponds (Winnapaug, Quonochontaug, Ninigret, and Point Judith Ponds). Fresh SGD rates determined for each pond range between 0 and 17Lm⁻² d⁻¹. These values largely fall in line with previous estimates made using hydrologic models. The uncertainty of saline SGD estimates for each pond is high; minimum and maximum saline SGD estimates differ by as much as three orders of magnitude. These results echo the discrepancy between two previous radium-based studies from this location. The variability of dissolved radium in the Rhode Island coastal aquifer plays a large role in the uncertainty of these saline SGD estimates. Groundwater dissolved radium activity is dependent upon many different factors, including groundwater salinity and redox conditions, sediment thorium and radium activities, and the amount of time to which the sediment has been exposed to saline groundwater. Due to the heterogeneous nature of glacial outwash sediments, none of these parameters are likely consistent throughout the aquifer in this location. Groundwater ²²⁶Ra and ²²⁸Ra activities in the coastal ponds vary significantly both laterally and with sediment depth. For example, groundwater samples collected in Ninigret Pond separated by a vertical distance of only 5cm differ by as much as 3×10³ dpm100L⁻¹ for both ²²⁶Ra and ²²⁸Ra. The results from this study indicate that a single groundwater sample or suite of samples may not accurately reflect the average radium activity of the groundwater actually discharging into each pond. Targeted sampling in areas where SGD is qualitatively observed can potentially remedy this problem.

Highlights

► Salt balance-based fresh SGD values for RI coastal ponds range from 0 to 17Lm⁻² d⁻¹. ► Ra isotope-based SGD estimates are highly variable in Rhode Island’s coastal ponds. ► High uncertainty in SGD values is attributed to variability in groundwater Ra. ► Minimum and maximum groundwater Ra activities differ by as much as 3×10⁴ dpm100L⁻¹ ► An average groundwater sample may not accurately represent discharging water.

Chemical speciation of iron in Antarctic waters surrounding free-drifting icebergs

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 128–129

Hai Lin, Benjamin S. Twining

Free-drifting icebergs in the Southern Ocean may serve as an important source of iron (Fe) to surrounding waters. We studied the chemical speciation of Fe in the waters around several Antarctic icebergs during cruises to the Scotia and Weddell Seas in June 2008 and March 2009. Iron(II) was measured by flow injection chemiluminescence, and Fe-binding ligands were determined following the cruises via competitive ligand equilibrium–cathodic stripping voltammetry. Iron(II) concentrations were significantly higher within 1km of the nearest iceberg (146±11pM; 12±0.8% of total dissolved Fe (DFe)) compared to farther stations (66±12pM; 4±1% of DFe). Iron(II) concentrations were significantly positively correlated with incident light levels, indicating a photochemical source. However spatial Fe(II) gradients were statistically significant even after accounting for light effects and were stronger than gradients in DFe or phytoplankton biomass. Iron(II) was also higher at deeper depths along the iceberg face, suggesting Fe(II) is directly released from ice-rafted minerals. Total Fe-binding ligand concentrations were significantly higher closer to icebergs in the fall but not in the winter, when plankton biomass and grazing activity were substantially lower. Ligands were always present in excess of DFe, but ligands were more fully titrated with DFe in the fall. Ligand/DFe ratios were relatively low (1.2–1.7) compared to other Southern Ocean studies, suggesting that ligands were mostly titrated and additional Fe inputs would be prone to scavenging. Conditional stability constants of ligands (K′, with respect to Fe′) ranged from 10^11.9 to 10^12.9. Macrozooplankton grazing pressure was also elevated near icebergs and may have served as a source of Fe(II) and ligands. Icebergs appear to affect, directly and indirectly, the speciation of Fe in surrounding waters with potential implications for bioavailability and scavenging behavior.

Highlights

► We measured the speciation of iron in waters surrounding free-drifting icebergs. ► Iron(II) concentrations were higher in close proximity to icebergs. ► Iron(II) concentrations were higher closer to the bottom of icebergs. ► Iron-binding ligand concentrations were higher in proximity to icebergs. ► Iron-binding ligands may be produced by elevated grazing activity near the icebergs.

Influences of two air–sea exchange schemes on the distribution and storage of bomb radiocarbon in the Pacific Ocean

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 130–131

Yangchun Li, Yongfu Xu

A basin-wide ocean general circulation model of the Pacific Ocean is employed to study how the air–sea exchange coefficient affects the distribution and storage of bomb ¹⁴C in the ocean. Two numerical experiments are designed, including a constant exchange coefficient (RUN1) and a wind-dependent exchange coefficient (RUN2). RUN1 greatly overestimates the observed surface bomb ¹⁴C concentration in 1973, while RUN2 reduces this overestimate due to a decreased flux in the tropical region. The difference of column bomb ¹⁴C inventories between the two simulations in the North Pacific in 1973 is mainly controlled by the distribution of bomb ¹⁴C in the upper ocean, because the simulated penetration is shallower than the observations. This indicates that the simulated transport of the North Pacific Intermediate water (NPIW) is not strong enough. The surface bomb ¹⁴C concentration in 1994 from the two simulations is obviously decreased relative to that in 1973. The inventory distribution characteristic with high-west and low-east in the subtropical Pacific is more distinct in 1994 than in 1973, and the simulated results in 1994 are closer to the observations than those in 1973, especially in RUN2. Regardless of the surface or section distributions of bomb ¹⁴C concentrations, the results from RUN2 are closer to the observations than those from RUN1. Therefore, it can be concluded that the wind-dependent exchange scheme provides a more accurate bomb ¹⁴C concentration estimate. In addition, a comparison of these two simulations indicates that the simulation of NPIW transport can affect the accurate parameterization comparison used in the simulation of passive tracers in the North Pacific.

Highlights

► Two runs including constant exchange rate and wind-dependent exchange rate. ► Wind-dependent case reduces overestimates of surface bomb ¹⁴C concentrations. ► As the flux decreases with time, main storage areas from the two runs become close. ► Transport of passive tracers along the pathway of NPIW is significant.

Absorption and fluorescence of dissolved organic matter in submarine hydrothermal vents off NE Taiwan

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 128–129

Liyang Yang, Huasheng Hong, Weidong Guo, Chen-Tung Arthur Chen, Pei-I Pan, Chun-Chin Feng

The role of hydrothermal vents as either a source or a sink for chromophoric and fluorescent dissolved organic matter (CDOM and FDOM) in the oceans is unknown, since DOM absorption and fluorescence have not been reported for submarine hydrothermal vents. Water samples were collected from two shallow submarine hydrothermal vents off NE Taiwan, the white vent and the yellow vent, during two cruises in August, 2010. Absorption and excitation-emission-matrix fluorescence spectroscopy were used to characterize the optical properties of DOM from such extremely special environments. The absorption coefficients at wavelength 300nm (a 300) were much higher at the white vent mouth and 1m below it (2.52±0.88m^-1) than in the background (0.34±0.12m^-1). This indicated that the white vent was a source of CDOM for seawater. Three fluorescent components were identified using parallel factor analysis: humic-like C1, tyrosine-like C3, and C2 as a combination of tryptophan-like and marine humic-like components. Both C1 and C3 (but not C2) had their highest fluorescence intensity at the white vent mouth and 1m below it, suggesting the role of the vent as a source for both humic-like and tyrosine-like DOM. Samples from the yellow vent mouth also had a higher a 300 than the ambient seawater in our first cruise, but had fluorescence intensities of C(1–3) similar to the ambient seawater. Overall, the low humification index (HIX: 1.40±0.30) and the high autochthonous index (BIX: 1.27±0.63) indicated that the DOM likely had low humic contents and was mainly autochthonous of biological or bacterial origin in the study area. A biplot of HIX and BIX showed that DOM from the hydrothermal vents had a characteristic similar to terrestrial cave and spring waters, but was distinct from isolated humics.

Highlights

► The white hydrothermal vent off NE Taiwan is a source of CDOM for seawater. ► Fluorescent components are identified using EEM-PARAFAC. ► HIX and BIX suggested the DOM was mainly autochthonous with low humic contents. ► This is the first time CDOM in submarine hydrothermal vents is studied.

Conservative behavior of uranium vs. salinity in Arctic sea ice and brine

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 130–131

Christelle Not, Kristina Brown, Bassam Ghaleb, Claude Hillaire-Marcel

The conservative behavior of uranium (U) with respect to salinity in open ocean waters is widely accepted. However, in low salinity oceanic environments, such as estuaries, its behavior seems more variable and information on its distribution over broad salinity scales still needs to be further documented. Sea ice formation in polar oceans constitutes a natural mechanism concentrating dissolved seawater constituents into high salinity brine through the distillation of “pure” water into sea ice. Here we present relative U-concentration data in fifteen samples from the Arctic Ocean. They include low-salinity sea ice, underlying surface seawater and sea ice brine covering a salinity range of ~0 to 135. Results suggest that U vs. salinity exhibits conservative behavior over the whole range of salinity investigated. In addition, δ²³⁴U measurements agree well with the mean oceanic ²³⁴U/²³⁸U ratio, suggesting that the processes of sea ice formation and melt do not affect the oceanic relationship between the ²³⁴U and ²³⁸U.

Highlights

► Uranium concentrations are conservative across a large range of salinity. ► Arctic sea ice is a good environment to investigate the salinity effect. ► All measured uranium concentrations are equivalent to the mean ocean value if normalized to a salinity of 35. ► The sea ice formation process does not influence the ²³⁴U/²³⁸U equilibrium.

The dynamics of absorption coefficients of CDOM and particles in the St. Lawrence estuarine system: Biogeochemical and physical implications

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 128–129

Huixiang Xie, Cyril Aubry, Simon Bélanger, Guisheng Song

Absorption spectra of chromophoric dissolved organic matter (CDOM) and particles were obtained in May 2007 in the St. Lawrence estuary (SLE, Canada), the northwestern Gulf of St. Lawrence (NWG), and the Saguenay Fjord (CDOM only), the main tributary of the SLE. CDOM absorption generally decreased downstream and with depth and showed an inverse relationship to tidal cycles. Phytoplankton absorption in surface water of the SLE increased seaward while non-algal particle absorption trended oppositely; both variables declined with depth. Surface water CDOM absorption surpassed particle absorption in the SLE while phytoplankton absorption dominated in the NWG. Elevated non-algal and CDOM absorption were found in the turbidity maximum zone near the head of the SLE. Enriched CDOM absorption also occurred in the bottom water of the lower SLE and NWG. The spectral slope ratio of CDOM absorption, defined as the ratio of the spectral slope between 275 and 295nm to that between 350 and 400nm, was confirmed to be an indicator of the source and molecular weight of CDOM. This surrogate functionality, however, failed for absorption spectra exhibiting shoulders in short ultraviolet wavelengths observed in deep waters of the SLE and NWG. CDOM absorption mainly displayed conservative mixing behavior in both the SLE and the Saguenay Fjord. CDOM was employed to trace the source identity of the Fjord’s deepwater. It was found that the marine end member of the Fjord’s deepwater possessed a salinity of 32.92 and a temperature of ca. 1°C and originated from the intermediate cold layer of the lower SLE. The marine end member contributed 94% of the deepwater by volume while freshwater mainly flown from the Saguenay River supplied the remaining 6%. Implications of our results for remote sensing-based assessments of primary productivity, surface water circulation, and water column photochemistry in the SLE are also discussed.

Highlights

► CDOM absorption behaves conservatively during estuarine mixing. ► CDOM as a tracer for deepwater formation of the Saguenay Fjord. ► Spectral slope ratio as indicator of CDOM sources and molecular weight. ► Particulate absorption behaves non-conservatively. ► Applications of satellite-based ocean color imaging to the St. Lawrence estuary.

Removal of nitrite interference in the Winkler determination of dissolved oxygen in seawater

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 130–131

George T.F. Wong

Nitrite is a well recognized interference in iodometry. However, while steps for its removal are part of the routine practice in the standard methods for the determination of dissolved oxygen in freshwater and wastewater when the iodometric Winkler method is used, they are not included in any of the widely used standard procedures for seawater analyses. In de-ionized reagent-grade water, artificial seawater and natural surface seawater, the concentration of apparent oxygen increased linearly with increasing concentrations of added nitrite, indicating that the presence of nitrite does lead to an over-estimation. The average ratio of increase in apparent oxygen to nitrite added was 0.4±0.1mol mol⁻¹. At the widely claimed accuracy and precision in the determination of dissolved oxygen in seawater of ±0.1 to ±0.3%, the interference of nitrite may be safely ignored only when the concentration of nitrite is below about 0.3μM. At higher concentrations, such as those found at the nitrite maxima and in coastal waters, it should be taken into account. Since nitrite can be easily and readily removed by the addition of azide, such a step should be incorporated into the standard procedures for the determination of dissolved oxygen in seawater.

Highlights

► Nitrite interferes in the Winkler determination of dissolved oxygen in seawater. ► Each mole of nitrite results in an apparent oxygen of 0.4±0.1mol. ► Nitrite interference may be safely ignored only when its concentration is <0.3μM. ► The interference of nitrite can be removed by the addition of azide

High air–sea CO2 uptake rates in nearshore and shelf areas of Southern Greenland: Temporal and spatial variability

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 128–129

S. Rysgaard, J. Mortensen, T. Juul-Pedersen, L.L. Sørensen, K. Lennert, D.H. Søgaard, K.E. Arendt, M.E. Blicher, M.K. Sejr, J. Bendtsen

The present study is based on hourly samplings of wind speed, monthly sampling sessions of temperature, salinity, dissolved inorganic carbon, alkalinity, nutrients, primary productivity and vertical export in the outer sill region (station GF3) of a sub-arctic SW Greenland fjord (Godthåbsfjord) through 2005–2010. Air–sea CO2 fluxes varied at GF3 from c. −20gCm⁻² month⁻¹ (uptake from the atmosphere) to 25gCm⁻² month⁻¹ (release to the atmosphere) during 2005–10. The average annual air–sea CO2 flux of −83 to −108gCm⁻² yr⁻¹ was within the range of the local gross annual primary productivity of 76–106gCm⁻² yr⁻¹. Furthermore, the estimated vertical export of phytoplankton carbon to depths below 60m of 38–89gCm⁻² suggests that a large fraction of the mineralization (release of CO2) occurs in deeper waters in the outer sill region of the fjord. However, there was no statistically significant correlation between average annual gross primary production and annual air–sea flux during 2005–2010, which suggests that regulation of pCO2 in the fjord is more complex. Despite three confined periods with supersaturated pCO2 conditions in surface waters during 2005–2010, Godthåbsfjord can be considered as a strong sink (7.2tons C month⁻¹ km⁻²) for atmospheric CO2. In addition, measurements from Godthåbsfjord during the summer season showed that mixing between glacial meltwater and coastal water could explain a large part of the low pCO2-values observed in the innermost part of the fjord. Finally, a larger survey confirmed the existence of very low pCO2 conditions in nearshore and shelf waters around Southern Greenland.

Highlights

► Comprehensive study of ocean pCO2 conditions in Greenland waters. ► Seasonal and inter annual variations in pCO2. ► Coastal areas of Greenland constitute a much larger CO2 sink than anticipated.

In situ determination of iron(II) in the anoxic zone of the central Baltic Sea using ferene as spectrophotometric reagent

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 130–131

David Meyer, Ralf D. Prien, Olaf Dellwig, Douglas P. Connelly, Detlef E. Schulz-Bull

Continuous flow analysis for soluble Fe(II) species in seawater was performed with a colorimetric method using ferene as a spectrophotometric reagent. The method is based on the measurement of absorbance of the [Fe^II(fer)3]^4--complex at 594nm. No preconcentration-steps are required. Samples prepared in the laboratory in line with external calibration experiments have been successfully analyzed in the nanomolar range (R² =0.959). A detection limit of 20nM was obtained. Furthermore, investigations were performed in situ with a wet chemical analyzer in the water column of the central Baltic Sea. Finally the results were consistent with measurements made by an independent laboratory based method (ICP-OES).

Highlights

►Ferene was used for the in situ determination of Fe(II). ►Calibration experiments have been carried out in the nanomolar range (R² =0.959). ►A detection limit of 20nM was obtained. ►Investigations were performed in situ with a wet chemical analyzer. ►Results were consistent with measurements made by ICP-OES.

Analysis of Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb in seawater using the Nobias-chelate PA1 resin and magnetic sector inductively coupled plasma mass spectrometry (ICP-MS)

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 130–131

Dondra V. Biller, Kenneth W. Bruland

A method was developed using the chelating resin Nobias-chelate PA1 in an off-line pre-concentration manifold with magnetic sector inductively coupled plasma mass spectrometry (ICP-MS) detection for analysis of Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb. This multi-element method allows for the simultaneous extraction of eight samples in a closed column manifold. Standard additions to seawater demonstrated quantitative recovery and eliminated the need to use isotope dilution to account for non-quantitative recoveries of the metals on the resin. UV-oxidation of seawater samples before extraction was necessary to provide accurate total dissolved concentrations of Co and Cu. Samples (40mL) were loaded onto the chelating resin column at a pH of 6.2±0.3 after pH adjustment with a basic ammonium acetate (NH4Ac) solution. The columns were rinsed after sample loading with a weak (0.05M) NH4Ac solution to remove the bulk of the major ions in seawater. The trace metals were eluted with 3 to 4mL of 1N Q-HNO3 and analyzed on a magnetic sector ICP-MS. This multi-element method is ideal for high sample throughput and quantification over a broad range of element concentrations with low blanks and detection limits. We report results here from the SAFe and GEOTRACES reference samples as well as the U.S. GEOTRACES Atlantic and Pacific Baseline profiles.

Graphical abstract

Highlights

► Offline pre-concentration of trace metals in seawater with Nobias-chelate PA1 resin. ► ICP-MS detection of Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb. ► UV-oxidation is necessary for accurate determination of Cu and Co. ► Standard additions to seawater showed quantitative recovery of all metals. ► Vertical profiles from the U.S. GEOTRACES Baseline inter-calibration stations in both the Atlantic and Pacific.

Tidal cycling of mercury and methylmercury between sediments and water column in the Venice Lagoon (Italy)

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 130–131

S. Guédron, L. Huguet, D.A.L. Vignati, B. Liu, F. Gimbert, B.J.D. Ferrari, R. Zonta, J. Dominik

The sediment of Venice Lagoon regularly undergoes complex redistribution due to tidal forcing, which affects the cycling of contaminants such as mercury (Hg) between the sediment and the water column. We examined the distribution of total Hg (THg) and monomethylmercury (MMHg) in the water column, sediment and pore-water at two sites: VE1 (located in a depositional area adjacent to salt marshes) and VE2 corresponding to a moderately erosive, open area. We obtained instantaneous (using cores and micro-needle samplers) and time-integrated (using peepers) concentrations of the two mercury species in both dissolved and particulate forms. THg and MMHg concentrations were higher in the sediments at site VE1 (621.9±213.7ngg⁻¹ and 1.25±0.63ngg⁻¹ for THg and MMHg, respectively) than in those of the site VE2 (386.9±92.7ngg⁻¹ and 0.53±0.30ngg⁻¹). Hg concentrations in sediments were positively correlated with silts and organic matter content. Over two tidal cycles, the concentrations of THg and MMHg varied with the evolution of the tides. During the tidal flooding, both THg and MMHg peaked at the sediment–water interface and a moderate increase of dissolved MMHg was also observed in the water column. These fluctuations were observed during both tides and are suggestively related to advection of mercury species from surficial sediment pore-water to the water column and to desorption from suspended particles. The short-term increase in MMHg concentrations can result from in situ production, release from organic matter degradation, or from oxidative dissolution of redox-sensitive sulfide minerals and iron oxide reduction by micro-organisms; the two latter mechanisms being favored by redox oscillations in the surface sediment layers due to the tidal forcing. The decrease of both dissolved THg and MMHg concentrations at the sediment–water interface after high tide was attributed to a rapid adsorption onto particles. THg concentrations on suspended particles showed little variations during the tidal cycle with a minor peak at tide maximum, while MMHg concentrations on suspended particles slightly increased during ebb tide. MMHg concentrations on suspended particles were double than those in surface sediments, suggesting that tidal flushing may enhance dispersal of particle bound MMHg throughout the lagoon.

Highlights

► Mercury and methylmercury were investigated during two tidal cycles in Venice lagoon. ► Both species peaked at the sediment-water interface at late tidal flooding. ► Fluctuations were attributed to advection from sediment to water column and to desorption. ► Suspended particles may enhance dispersal of methylmercury during tidal flushing. ► Tide-driven mobilization of methylmercury is an important factor controlling mercury cycling.

Re-evaluating the 238U-salinity relationship in seawater: Implications for the 238U–234Th disequilibrium method

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2011
Source:Marine Chemistry, Volume 127, Issues 1–4

S.A. Owens, K.O. Buesseler, K.W.W. Sims

The concentration of ²³⁸U in seawater is an important parameter required for applications of uranium decay-series radionuclides used to understand particle export and cycling in marine environments. Using modern mass spectrometer techniques, we re-evaluated the relationship between ²³⁸U and salinity in the open ocean. The new ²³⁸U-salinity relationship determined here is based on a larger sample set and a wider salinity range than previous work in the open ocean. Four samples from 500 to 1000m in the subtropical Atlantic deviated significantly from their concentration predicted from salinity; these low concentrations are hypothesized to be the result of a remote removal process rather than analytical bias or local removal of uranium. We also bring attention to unique deficits of ²³⁴Th in the mesopelagic zone of the subtropical Atlantic and encourage future applications of ²³⁴Th to delve into the cause of these features. Determining the concentration of ²³⁸U in the open ocean is critical for minimizing uncertainty in the ²³⁸U-²³⁴Th disequilibrium method, which is a key tool for understanding particle flux to the deep ocean.

Highlights

►A new ²³⁸U-salinity relationship has been derived from a large sample set with a wide salinity range. ►Low ²³⁸U concentrations were observed in the subtropical Atlantic in 2007 between 500 and 1000m. ►Unique deficits of ²³⁴Th relative to ²³⁸U in the mesopelagic were identified.

Editorial Board

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 130–131

Editorial Board

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2011
Source:Marine Chemistry, Volume 127, Issues 1–4

Influence of trace metal release from volcanic ash on growth of Thalassiosira pseudonana and Emiliania huxleyi

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 132–133

L.J. Hoffmann, E. Breitbarth, M.V. Ardelan, S. Duggen, N. Olgun, M. Hassellöv, S.-Å. Wängberg

Recent studies demonstrate that volcanic ash has the potential to increase phytoplankton biomass in the open ocean. However, besides fertilizing trace metals such as Fe, volcanic ash contains a variety of potentially toxic metals such as Cd, Cu, Pb, and Zn. Especially in coastal regions closer to the volcanic eruption, where ash depositions can be very high, toxic effects are possible. Here we present the first results from laboratory experiments, showing that trace metal release from different volcanic ashes can have both fertilizing and toxic effects on marine phytoplankton in natural coastal seawater. The diatom Thalassiosira pseudonana generally showed higher growth rates in seawater that was in short contact with volcanic ash compared to the controls without ash addition. In contrast to that, the addition of volcanic ash had either no effect or significantly decreased the growth rate of the coccolithophoride Emiliania huxleyi. It was not possible to attribute the effects to single trace metals, however, our results suggest that Mn plays an important role in regulating the antagonistic and synergistic effects of the different trace metals. This study shows that volcanic ash can lead to changes in the phytoplankton species composition in the high fall-out area of the surface ocean.

Highlights

► We tested the effect of volcanic ash on growth of T. pseudonana and E. huxleyi ► Volcanic ash increased growth of T. pseudonana but not of E. huxleyi ► Mn seems important to regulate the effects of different trace metals from the ash ► Volcanic eruptions have the potential to change phytoplankton community structures

First measurements of methane and its carbon isotope ratio in the Japan Sea (East Sea)

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 128–129

Toshitaka Gamo, Urumu Tsunogai, Akinari Hirota, Noriko Nakayama, Dong-Jin Kang, Kyung-Ryul Kim

We measured dissolved methane (CH4) and its carbon isotope ratio (¹³C/¹²C or δ ¹³C) from surface to bottom seawaters (maximum depth: 3574m) for the first time in three major offshore areas of the Japan Sea (East Sea): the eastern Japan Basin, Yamato Basin, and Ulleung Basin. Seawater samples were taken during the KT-07-24 cruise of the R/V Tansei Maru in 2007 as part of a Japan–Korea collaborative study. The CH4 concentration and its δ ¹³CPDB were obtained by continuous flow isotope ratio mass spectrometry. While the CH4 concentrations of surface seawater were 2.6–3.8nmolkg⁻¹, which was 43(±22)% supersaturated compared to the atmospheric equilibrium values, those in the subsurface (D=30–150m) showed much higher values up to 9.7nmolkg⁻¹ (~300% supersaturation). This subsurface CH4 had the minimum δ ¹³C value of −54.3‰ (less than the atmospheric CH4 value of −47‰), implying a contribution from in situ microbial CH4 production in some anoxic microenvironments. The CH4 concentration decreased with depth toward minimum values of ~1nmolkg⁻¹ (δ ¹³C: −30 to −40‰) at 1500–2000m depth. Below a depth of 2000m, we found a clear difference in CH4 profiles among the three basins; in the eastern Japan Basin the CH4 concentration remained almost constant down to the seafloor, while it gradually increased with depth up to >2.0nmolkg⁻¹ in the Yamato and Ulleung basins. This CH4 increase toward the bottom was accompanied by a decrease of δ ¹³C–CH4 to less than −50‰, suggesting a microbial CH4 supply from bottom sediment in the Yamato and Ulleung basins. We observed a local CH4 plume over a deep channel east of the Yamato Basin, indicating unknown submarine CH4 seepage with a microbial δ ¹³C value of about −60‰.

Highlights

► We measured methane and its carbon isotope ratio in a marginal sea (Japan Sea). ► Subsurface waters showed up to ~300% supersaturation and microbial δ ¹³C signatures. ► We noted CH4 and its δ ¹³C are useful as transient tracers for deep convection. ► We found a CH4 plume probably derived from sub-seafloor CH4 gas hydrates.

Regression-based estimates of the rate of accumulation of anthropogenic CO2 in the ocean: A fresh look

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 132–133

William Carlisle Thacker

Regression-based methods used for estimating the rate of increase of anthropogenic CO2 in the ocean are reviewed and guidelines for improvement are presented. Following these guidelines leads to a local two-regression method, the first regression accounting for changes in oceanic carbon due to natural variability and the second regression associating the remaining systematic temporal variability with the anthropogenic signal to quantify the rate of accumulation. While a formal measure of the accumulation rate’s uncertainty is provided by the standard error of the second regression’s slope parameter, both the available data’s limited ability to characterize carbon’s natural variability in the absence of any anthropogenic contribution and the choice of regressors to account for that variability present significant uncertainties that are less easily quantified. An attractive feature of the method is its applicability to data other than those from repeated hydrographic surveys, such as might be provided by appropriately instrumented profiling floats.

Highlights

► This study discusses problems with MLR and eMLR methodologies and provides guidelines for improvement. ► It suggests a local two-regression method. ► It discusses adequacy of data, overfitting, and uncertainties of resulting estimates.

Pasteurization: A reliable method for preservation of nutrient in seawater samples for inter-laboratory and field applications

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 128–129

Anne Daniel, Roger Kérouel, Alain Aminot

Following previous work, the production of reference material for nutrients in seawater, using pasteurization as a preservation method, was carried out seven times between 2006 and 2010 in the framework of inter-laboratory exercises. The preparation of samples from natural seawater allowed to become depleted in nutrients then spiked, bottled and pasteurized, is described. Five main nutrients are involved in this study: ammonium, nitrite, nitrate, phosphate and silicate. Bottles are in glass for the ammonium samples and in plastic for the other nutrients. Pasteurization was performed at 80±3°C for 2h. Samples were controlled for homogeneity and stability at 3–5month interval, before and after the period allocated for each exercise and a third time between 8 to 40months storage. Homogeneity, not altered by sample aging, remains within a few nanomoles per litre for nitrite (<1.4μmol/L) and phosphate (<3μmol/L) and within a few tens of nanomoles per litre for ammonium (<4.5μmol/L), nitrate (<35μmol/L) and silicate (<30μmol/L). Except for ammonium, stability data are close to homogeneity data at the low level and within 0.3–0.5% at the high level. Ammonium shows a slight drift due to atmospheric contamination through the plastic cap, in the order of 0.1μmol/L per year, which allows nevertheless an acceptable performance over an 1–2month analytical delay. Except for ammonium, pasteurization does not produce detectable concentration changes, thus it can be applied to field samples for long term storage at ambient temperature.

Highlights

► Pasteurization to prepare nutrients reference material and to preserve field samples. ► Homogeneity and stability studies better than a few ten nanomoles. ► Production of samples for inter-laboratory exercises as a robust method validation.

Vertical methylmercury distribution in the subtropical North Pacific Ocean

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 132–133

Chad R. Hammerschmidt, Katlin L. Bowman

Humans are exposed to toxic and bioaccumulative monomethylmercury (MMHg) principally by consuming seafood. However, knowledge of the sources of MMHg to surface-dwelling marine organisms has been hampered by a paucity of information on its vertical distribution in the open ocean. Here, we report the first complete high-resolution profile of MMHg, from sea surface to bottom water, in the Pacific Ocean. Filtered water and suspended particles were sampled at the SAFe station (140°W, 30°N) during the U.S. GEOTRACES Intercalibration. Distributions of MMHg and dimethylmercury suggest that both are synthesized in low-oxygen and oxic strata of the water column and that deep-sea sediments are not an important source. Scaling estimates imply that a majority of MMHg in phytoplankton and, by extension, the pelagic food web at this location results from production in the mixed layer, which is impacted by anthropogenic mercury inputs and thus may be affected by future changes in emissions to the atmosphere.

Graphical abstract

Highlights

► We measured full-depth profiles of monomethylmercury (MMHg) and dimethylmercury (DMHg). ► Vertical distributions suggest that MMHg and DMHg are formed in low-oxygen and oxic layers. ► Deep-sea sediments do not appear to be an important source. ► Scaling estimates imply that most of the MMHg in open-ocean plankton is produced in the mixed layer.

Utility of dissolved barium in distinguishing North American from Eurasian runoff in the Arctic Ocean

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 132–133

T. Roeske, D. Bauch, M. Rutgers V.D. Loeff, B. Rabe

Dissolved barium has been shown to have the potential to distinguish Eurasian from North American (NA) river runoff. As part of the ARK-XXII/2 Polarstern expedition in summer 2007, Ba was analyzed in the Barents, Kara, Laptev seas, and the Eurasian Basins as well as the Makarov Basin up to the Alpha and Mendeleyev Ridges. By combining salinity, δ¹⁸O and initial phosphate corrected for mineralization with oxygen (PO4*) or N/P ratios we identified the water mass fractions of meteoric water, sea ice meltwater, and marine waters of Atlantic as well as Pacific origin in the upper water column. In all basins inside the lower halocline layer and the Arctic intermediate waters we find Ba concentrations close to those of the Fram Strait branch of the lower halocline (41–45nM), reflecting the composition of the incoming Atlantic water. A layer of upper halocline water (UHW) with higher Ba concentrations (45–55nM) is identified in the Makarov Basin. Atop of the UHW, the Surface Mixed Layer (SML), including the summer and winter mixed layers, has high concentrations of Ba (58–67nM). In the SML of the investigated area of the central Arctic the meteoric fraction can be identified by assuming a conservative behavior of Ba to be primarily of Eurasian river origin. However, in productive coastal regions biological removal compromises the use of Ba to distinguish between Eurasian and NA rivers. As a consequence, the NA river water fraction is underestimated in productive surface waters or waters that have passed a productive region, whereas this fraction is overestimated in subsurface waters containing remineralised Ba, particularly when these waters have passed productive shelf regions. Especially in the Laptev Sea and small regions in the Barents Sea, Ba concentrations are low in surface waters. In the Laptev Sea exceptionally high Ba concentrations in shelf bottom waters indicate that Ba is removed from surface waters to deep waters by biological activity enhanced by increasing ice-free conditions as well as by scavenging by organic matter of terrestrial origin. We interpret high Ba concentrations in the UHW of the Makarov Basin to result from enrichment by remineralisation in bottom waters on the shelf of the Chukchi Sea and therefore the calculated NA runoff is an artefact. We conclude that no NA runoff can be demonstrated unequivocally anywhere during our expedition with the set of tracers considered here. Small contributions of NA runoff may have been masked by Ba depletion and could only be resolved by supportive tracers on the uptake history. We thus suggest that Ba has to be used with care as it can put limits but not yield quantitative water mass distributions. Only if the extra Ba inputs exceed the cumulative biological uptake the signal can be unequivocally attributed to NA runoff.

Highlights

► Dissolved Ba in surface waters on ARK-XXII/2 Polarstern expedition in summer 2007. ► Distinguish Eurasian from North American river runo_ in the Arctic Ocean. ► Ba is removed from surface waters to deep waters by biological activity. ► North American river water fraction is underestimated in productive surface waters. ► High Ba in upper halocline water result of remineralization in the Chukchi Sea.

A model of Saharan dust deposition to the eastern Gulf of Mexico

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 134–135

J.M. Lenes, J.M. Prospero, W.M. Landing, J.I. Virmani, J.J. Walsh

The regional deposition of mineral dust aerosols was simulated from May–October 1999 using a simple two-dimensional model developed specifically to estimate high-resolution (kilometer scale) iron fertilization within the eastern Gulf of Mexico. In this model, daily atmospheric dust concentrations were assumed to be homogeneous over the model domain, where measurements at Miami served as the forcing function. Spatial deposition was driven by spatial variation in precipitation, i.e. wet deposition processes. A daily accumulation index (DAI) was formulated in order to provide high-resolution rainfall rates over time and space. The model calculated that 0.9–2.2gdustm⁻² (28–78mgFem⁻²) was deposited over the model domain during the 6-month simulation. Wet deposition was responsible for 82–93% of total dust deposition. The highest concentrations of dust (iron) were deposited along the coast of west central Florida between 26° and 29°N latitude (Tampa Bay to Charlotte Harbor). This region saw the largest amount of precipitation during periods of elevated mineral dust, with 44–64% of total annual deposition occurring during 5 wet deposition events.

Highlights

► We model daily dust and iron deposition to the eastern Gulf of Mexico at kilometer scale resolution. ► Total deposition was calculated using wet and dry deposition. ► Wet deposition dominate annual deposition.

Changes in spectral slopes of colored dissolved organic matter absorption with mixing and removal in a terrestrially dominated marine system (Hudson Bay, Canada)

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 134–135

Mats A. Granskog

We investigated the changes in absorption and spectral slopes of colored dissolved organic matter (CDOM) using a data set of salinity, δ¹⁸O and CDOM absorption in Hudson Bay. Following the fraction of river water (determined with salinity and δ¹⁸O tracers) one can track the changes in terrestrial CDOM optical properties with mixing and removal, as salinity cannot alone be used in waters with significant influence from sea-ice melt. CDOM in Hudson Bay was controlled by terrestrial inputs, in contrast to adjacent Hudson Strait. CDOM absorption was removed significantly, likely due to photobleaching. There was no or negligible indication of absorption removal during initial estuarine mixing, in agreement with earlier studies. Of the many absorption spectral slope (S) parameters that have been used as proxies for CDOM dynamics, the ones at shorter wavelengths proved the best indicators for absorption removal by photobleaching. Increase in absorption spectral slopes at 275 to 295 (S275–295) and 290 to 350nm (S290–350) are strongly correlated with the apparent removal of CDOM absorption. S275–295 and S290–350 in combination with spectral slopes and ratios at other wavelength intervals, which are sensitive to other processes and sources, can potentially reveal more information about CDOM origin and dynamics than a single slope alone.

Highlights

► In Arctic assessing conservative behavior against salinity is not necessarily valid. ► CDOM absorption can be almost completely removed in Hudson Bay. ► CDOM absorption slopes at short wavelengths increase with absorption removal. ► A combination of spectral slopes rather than a single one provides more information.

Fluorometric detection of total dissolved zinc in the southern Indian Ocean

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 132–133

Kathleen J. Gosnell, William M. Landing, Angela Milne

Zinc acts as a micronutrient in the ocean, capable of influencing and potentially controlling phytoplankton productivity and community structure. Thus, it is important to quantify the distribution of dissolved Zn in the oceans, in addition to understanding the biogeochemical behavior of this important element. Meeting this objective has been elusive since dissolved Zn concentrations in the upper water column can be extremely low, and it is difficult to eliminate contamination during sample collection and analysis. Our approach to this problem was to utilize a Flow Injection Analysis (FIA) method initially described by Nowicki et al. (1994), and collecting uncontaminated seawater using a trace-metal clean rosette system (Measures et al., 2008). Samples for total dissolved Zn analysis were during the 2009 CLIVAR I5 cruise across the southern Indian Ocean (from Cape Town, South Africa to Fremantle, Australia). Dissolved Zn concentrations have not been previously reported for this region. Extremely low dissolved Zn concentrations (0.02nM) were observed in surface waters of the central Indian Ocean gyre, documenting the extreme biological depletion of Zn typical of the open ocean. Concentrations of Zn and Si both increased with depth. The highest concentrations measured for dissolved Zn (>3.5nM) were collected at 1300m off western Australian. Total dissolved Zn concentrations were observed to be oceanographically consistent, and well correlated with dissolved silicate across the transect. The linear regression of total dissolved Zn vs. Si for all of the data yielded a slope of 0.059±0.003 (nM Zn/μM Si), which is consistent with the values reported for the north Pacific and thus support the previously reported nutrient-type Zn–silicate relationship. The zonal section of the dissolved Zn/Si ratios also exhibit broad maxima and minima, consistent with variable sources for Zn and different recycling rates for Zn vs. Si.

Highlights

► First measured and reported values of dissolved Zn for this region of the southern Indian Ocean. ► Total Zn measured using a flow-injection utilizing fluorescence detection. ► Lowest Zn concentrations (0.02nM) observed in surface waters throughout the transect. ► Here we report 42 profiles of total dissolved Zn for the CLIVAR I5 transect. ► Linear regression of Zn vs. Si for all data yielded a slope of 0.059±0.04 (nM Zn/μM Si).

Metal composition and fluxes of sinking particles and post-depositional transformation in a ria coastal system (NW Iberian Peninsula)

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 134–135

J. Santos-Echeandía, R. Prego, A. Cobelo-García, M. Caetano

The intra annual variation of the quantity and composition of the sinking particles in a ria coastal system (Vigo Ria, Galicia, NW Iberian Peninsula) has been studied. Levels of particulate metals (Al, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Ti, U, V and Zn) were determined both in the labile (1M HCl extraction) and in the total fraction. The levels and type of metals found in different seasons of the year depended upon oceanographic, physical, chemical and biological conditions but, in general, lithogenic type metals (Al, Fe, Ti, U and V) were higher during winter rainy campaigns when high quantities of SPM were recovered in the trap. However, the more biogenic metals (Cu, Pb and Zn) followed a scattered variation. Vertical fluxes values varied within the following intervals: 0.4–6.1gm⁻² d⁻¹ for Al, 0.7–7.6μgm⁻² d⁻¹ for Cd, 0.1–0.6mgm⁻² d⁻¹ for Co, 0.8–6.4mgm⁻² d⁻¹ for Cr, 1.2–10.9mgm⁻² d⁻¹ for Cu, 0.3–2.7gm⁻² d⁻¹ for Fe, 1.8–19.8mgm⁻² d⁻¹ for Mn, 0.5–2.9mgm⁻² d⁻¹ for Ni, 1.7–7.9mgm⁻² d⁻¹ for Pb, 0.1–0.2gm⁻² d⁻¹ for Ti, 0.1–0.6mgm⁻² d⁻¹ for U, 0.6–7.2mgm⁻² d⁻¹ for V and 0.8–39.8mgm⁻² d⁻¹ for Zn. In special, the deposited labile fraction is subject to postdepositional diagenetic processes with subsequent benthic fluxes of dissolved Cr, Cu, Mn, Ni, Pb and Zn form of the metals to the water column. For Co and Cd, however, their enrichment in the sediments compared to the traps suggests their association with a more refractory fraction upon postdepositional remineralization. This work represents one of the first studies about labile trace metal concentrations and intra annual variation in the particulate matter recovered from a sediment trap in a ria system.

Highlights

► Seasonal dependence of trap metal composition driven by biological, physical and chemical processes. ► Terrestrial material during winter changing to marine-like towards spring and summer. ► Terrestrial inputs control Al/Ti/U/V; organic matter Cu/Pb/Zn/Cd; others depend upon both factors. ► Vertical fluxes of Cu/Pb/Zn increase with organic matter while continental inputs does for Al/Ti/U/V. ► Degradation of sedimentary organic matter releases metals (except Cd and Co) to the dissolved phase.

The analysis of dimethylsulfide and dimethylsulfoniopropionate in sea i Dry-crushing and melting using stable isotope additions

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 128–129

Jacqueline Stefels, Gauthier Carnat, John W.H. Dacey, Thomas Goossens, J. Theo M. Elzenga, Jean-Louis Tison

Sea ice is thought to be an important source of the climate-active gas dimethylsulfide (DMS), since extremely high concentrations of its precursor dimethylsulfoniopropionate (DMSP) have been found associated with high algal biomass. Accurate measurements of DMS and associated compounds in sea ice were until now not possible due to difficulties associated with the unavoidable melting process before analysis. Here we present and evaluate two methods to analyze DMS and DMSP in sea-ice cores accurately. The first, describes the dry-crushing method, which has its focus on the volatile compound DMS. A sub-sample of deeply frozen (<−30°C) ice is crushed in a stainless steel vessel and the released gas phase is analyzed directly for DMS. The remaining ice is subsequently analyzed for its total DMSP content. With this method, DMS and DMSP profiles can be resolved even in ice cores stored deeply frozen for two years. The second method, involves a melting procedure, during which the conversion of compounds is monitored by adding differently deuterated isotopes of DMS and DMSP. Natural concentrations and stable isotopes of DMS and DMSP are simultaneously analyzed on a Proton-Transfer-Reaction Mass Spectrometer (PTR-MS). Loss and conversion rates of the artificial isotopes are used to reconstruct the original concentrations of DMS and DMSP in ice and give important information on potential dynamical processes in sea-ice communities. It is concluded that in stored cores, the dry-crushing method provides the best results when the aim of the study is to differentiate between DMS and DMSP. When direct processing and analysis of the samples is possible, the isotope-addition method has the potential of providing concentrations of all S-compounds, including dissolved and particulate fractions. Moreover, it is suitable for the determination of process rates within the S-cycle.

Highlights

► Two new methods to measure sulfur compounds in sea ice are developed and compared. ► The dry-crushing (DC) method focuses on the volatile compound dimethylsulfide (DMS). ► The stable-isotope addition (SIA) method measures dissolved and particulate S-compounds. ► The DC method can resolve DMS and DMSPt profiles in ice cores stored for years. ► The SIA method is the first to measure conversion processes and concentrations simultaneously.

The geochemical characteristics and Fe(II) oxidation kinetics of hydrothermal plumes at the Southwest Indian Ridge

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 134–135

Hu Wang, Qunhui Yang, Fuwu Ji, Marvin D. Lilley, Huaiyang Zhou

The Southwest Indian Ridge (SWIR) is one of the world’s slowest spreading ridges with a full spreading rate of ~14mm a⁻¹. Due to its low thermal budget, high-temperature hydrothermal activity along the SWIR was once considered to be impossible. The Chinese cruise DY115-19 on board R/V Dayang Yihao successfully discovered the first SWIR active hydrothermal field at 37°47′S 49°39′E, located at a magmatically robust spreading segment. Here, the geochemical characteristics of hydrothermal plumes from the hydrothermal field are first reported, and water column anomalies of light transmission, Fe, Mn, Al, both dissolved and particulate, are discussed. The total Fe and dissolved Mn concentrations in the plumes varied from 13.7 to 277.4 nM and 0.47 to 10.41 nM, respectively. The composition of Fe-Mn-Al implied that particles in the plumes were mainly hydrothermal in origin, also included small contributions of resuspended sediments or background particles. Dissolved Fe constituted a considerable fraction of the total Fe, more than 80% in plume samples from station CTD 4. High Fe concentrations might be sustained in the dissolved phase because of the existence of organic complexes and nanoparticles. On board incubation experiments verified the Fe(II) oxidation half-lives for plumes of CTD 4 and CTD 13 were 1.8 and 1.6h, respectively, which are much longer than the calculated value of ~0.5h based on the deep water pH and oxygen concentration.

Highlights

► The evidences for the existence of hydrothermal plumes at the SWIR are first reported. ► The Fe, Mn and Al concentrations, both particulate and dissolved, are measured. ► The particle samples in plumes are mainly of hydrothermal origin. ► Dissolved Fe constitute a considerable part of total Fe in samples. ► The Fe(II) half-lives in plumes were measured by shipboard incubation experiments.

Large fluxes of rare earth elements through submarine groundwater discharge (SGD) from a volcanic island, Jeju, Korea

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2011
Source:Marine Chemistry, Volume 127, Issues 1–4

Intae Kim, Guebuem Kim

To evaluate the role of submarine groundwater discharge (SGD) as a source of rare earth elements (REEs) in the coastal ocean, we estimated the SGD associated discharge of REEs into two semi-enclosed coastal bays off a volcanic island, Jeju, Korea. The coastal brackish groundwater showed pronounced enrichments of middle REEs (MREE) relative to light REEs (LREE) and heavy REEs (HREE) when normalized against the upper continental crust (UCC), whereas seawater samples outside the bays showed a HREE enrichment pattern. The enrichment of both MREE and HREE was clearly identified in bay waters, resulting from mixing between groundwater and offshore seawater. The mass balances of REEs demonstrated that the REE fluxes through SGD were two to three orders of magnitude higher than those that occurred through the other sources, such as diffusion from bottom sediments and atmospheric dust fallout. The SGD-driven Nd flux from the entire Jeju Island during this summer was approximately 120±60mold⁻¹, which is comparable to the Nd fluxes from major rivers (i.e., Mississippi River). Our results imply that highly permeable oceanic islands are particularly important for REE fluxes to the ocean.

Highlights

► SGD-driven REE fluxes were measured in coastal water off a volcanic island. ► Coastal groundwater showed relatively high MREE enrichment in this region. ► SGD-driven Nd fluxes from Jeju island rival those from major rivers. ► SGD is the dominant source of REEs in coastal seawater off Jeju Island. ► Significant removal of LREE and MREE was observed in bay seawater.

Reduced river discharge intensifies phytoplankton bloom in Godavari estuary, India

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 132–133

T. Acharyya, V.V.S.S. Sarma, B. Sridevi, V. Venkataramana, M.D. Bharathi, S.A. Naidu, B.S.K. Kumar, V.R. Prasad, D. Bandyopadhyay, N.P.C. Reddy, M.D. Kumar

Changes in river discharge alter material load and the consequent estuarine and coastal biogeochemical process. Evidence for biogeochemical response to variable rainfall over catchment area or changes in river discharges due to dam regulations is sparse. Inter-annual variability in monsoon rainfall and the consequent river discharges, either dam regulated or otherwise, in India are the best suited to test the influence of altered discharges on estuarine biogeochemistry. Our experiments in Godavari river estuary over three years revealed that a decrease in precipitation over the Indian subcontinent from 2007 to 2009 resulted in the lowering of mean annual discharge from 748.63m³ s^-1 in 2007 to 218.40m³ s^-1 in 2009. The reduced water discharge, during the peak discharge period, slowed the flushing of the estuary from 1.2days to 6.3days, respectively. The consequent increase in stability of water column and reduced suspended material load gave rise to intense phytoplankton blooms (Chl a 18μgl^-1 in 2007 to 28μgl^-1 in 2009). Resilience towards the unwanted phytoplankton bloom and overall health of the Indian estuaries is thus tuned by the variability in monsoon rainfall and dam regulated discharge.

Highlights Capsule

► Indian estuaries receive huge nutrients from land drainage and agricultural fields ► Light limitation and fast flushing prevents potentially intense phytoplankton bloom ► Reduced discharge promotes intense bloom by enhancing stability and stratification ► Changing climate and more dam construction may potentially reduce discharge ►Monsoonal estuaries are highly sensitive towards eutrophication in the context of reduced water availability due to dam construction and ‘monsoon vagaries’.

The abiotic formation of TEP under different ocean acidification scenarios

admin — Sun, 20 May 2012 09:39:13 +0000

Publication year: 2012
Source:Marine Chemistry, Volumes 128–129

Uta Passow

In view of rising atmospheric CO2 concentrations, the question if the marine biological carbon pump will increase or decrease in efficiency as ocean acidification progresses becomes central for predictions of future atmospheric pCO2. Aggregation and sinking of aggregates contributes significantly to the flux of carbon to depths and changes in aggregation behavior will have far reaching consequences for the biological pump. The abundance and characteristics of transparent exopolymer particles, TEP, are central in regulating aggregation. We investigated the impact of ocean acidification on the abiotic formation of TEP from their precursors. Our results demonstrate that, contrary to earlier suggestions, ocean acidification as expected in the future ocean has no impact on the equilibrium conditions between TEP and their precursors. However, if the carbonate system is altered by adding acid, which does not simulate the future ocean carbonate system correctly, TEP concentration increases with decreasing pH, presumably due to changes in total alkalinity (TA). This implies that abiotic TEP formation is sensitive to changes in TA, but not pH. The discrepancy in results caused by different experimental approaches emphasizes the fact that acidification experiments do not mimic future conditions adequately and may even be misleading.

Highlights

► Transparent exopolymer particles (TEP) regulate the efficiency of the biological carbon pump. ► The abiotic formation of TEP is NOT sensitive to ocean acidification. ► This equilibrium appears, however, sensitive to changes in total alkalinity. ► Experiments simulating ocean acidification by acid addition give misleading results.