Sulfur isotope rations in oceanic basement samples

Low temperature alteration of oceanic basement rocks is characterized by net gain of sulfur, which commonly yields low d34S values, suggesting involvement of microbial sulfate reduction. In order to test whether secondary sulfide minerals are consistent with a biogenic source, we apply high precision multiple sulfur isotope analysis to bulk rock sulfide and pyrite isolates from two contrasting types of altered oceanic basement rocks, namely serpentinized peridotites and altered basalts. Samples from two peridotite sites (Iberian Margin and Hess Deep) and from a basalt site on the eastern flank of the Juan de Fuca Ridge yield overlapping d34S values ranging from 0 per mil to -44 per mil. In contrast, sulfides in the basalt site are characterized by relatively low D33S values ranging from -0.06 per mil to 0.04 per mil, compared to those from peridotite sites (0.00 per mil to 0.16 per mil). The observed D33S signal is significant considering the analytical precision of 0.014 per mil (2 sigma). We present a batch reaction model that uses observed d34S and D33S relationships to quantify the effect of closed system processes and constrain the isotope enrichment factor intrinsic to sulfate reduction. The estimated enrichment factors as large as 61 per mil and 53 per mil, for peridotite and basalt sites respectively, suggest the involvement of microbial sulfate reduction. The relatively high D33S values in the peridotite sites are due to sulfate reduction in a closed system environment, whereas negative D33S values in the basalt site reflect open system sulfate reduction. A larger extent of sulfate reduction during alteration of peridotite to serpentinite is consistent with its higher H2 production capacity compared to basalt alteration, and further supports in-situ microbial sulfate reduction coupled with H2 production during serpentinization reactions.

Chemistry of sediment profile GeoB4906

Mineralization of organic matter and the subsequent dissolution of calcite were simulated for surface sediments of the upper continental slope off Gabon by using microsensors to measure O2, pH, pCO2 and Ca2+ (in situ), pore-water concentration profiles of NO3-, NH4+, Fe2+, and Mn2+ and SO42- (ex situ), as well as sulfate reduction rates derived from incubation experiments. The transport and reaction model CoTReM was used to simulate the degradation of organic matter by O2, [NO3]-, Fe(OH)3 and [SO4]2-, reoxidation reactions involving Fe2+ and Mn2+, and precipitation of FeS. Model application revealed an overall rate of organic matter mineralization amounting to 50 µmol C cm**-2 yr**-1, of which 77% were due to O2, 17% to [NO3]- and 3% to Fe(OH)3 and 3% to [SO4]2-. The best fit for the pH profile was achieved by adapting three different dissolution rate constants of calcite ranging between 0.01 and 0.5% d-1 and accounting for different calcite phases in the sediment. A reaction order of 4.5 was assumed in the kinetic rate law. A CaCO3 flux to the sediment was estimated to occur at a rate of 42 g m**-2 yr**-1 in the area of equatorial upwelling. The model predicts a redissolution flux of calcite amounting to 36 g m**-2 yr**-1, thus indicating that ~90% of the calcite flux to the sediment is redissolved.

Heterocyst glycolipids in surface sediments and water samples of the Amazon shelf

Marine endosymbiotic heterocystous cyanobacteria make unique heterocyst glycolipids (HGs) containing pentose (C5) moieties. Functionally similar HGs with hexose (C6) moieties found in free-living cyanobacteria occur in the sedimentary record, but C5 HGs have not been documented in the natural environment. Here we developed a high performance liquid chromatography multiple reaction monitoring (MRM) mass spectrometry (HPLC-MS2) method specific for trace analysis of long chain C5HGs and applied it to cultures of Rhizosolenia clevei Ostenfeld and its symbiont Richelia intracellularis which were found to contain C5 HGs and no C6 HGs. The method was then applied to suspended particulate matter (SPM) and surface sediment from the Amazon plume region known to harbor marine diatoms carrying heterocystous cyanobacteria as endosymbionts. C5 HGs were detected in both marine SPM and surface sediments, but not in SPM or surface sediment from freshwater settings in the Amazon basin. Rather, the latter contained C6 HGs, established biomarkers for free-living heterocystous cyanobacteria. Our results indicate that the C5 HGs may be potential biomarkers for marine endosymbiotic heterocystous cyanobacteria.

Snow-pit measurements on Atka Bay landfast sea ice during ANT-Land 2012/2013 season

Up to now, snow cover on Antarctic sea ice and its impact on radar backscatter, particularly after the onset of freeze/thaw processes, are not well understood. Here we present a combined analysis of in situ observations of snow properties from the landfast sea ice in Atka Bay, Antarctica, and high-resolution TerraSAR-X backscatter data, for the transition from austral spring (November 2012) to summer (January 2013). The physical changes in the seasonal snow cover during that time are reflected in the evolution of TerraSAR-X backscatter. We are able to explain 76-93% of the spatio-temporal variability of the TerraSAR-X backscatter signal with up to four snowpack parameters with a root-mean-squared error of 0.87-1.62 dB, using a simple multiple linear model. Over the complete study, and especially after the onset of early-melt processes and freeze/thaw cycles, the majority of variability in the backscatter is influenced by changes in snow/ice interface temperature, snow depth and top-layer grain size. This suggests it may be possible to retrieve snow physical properties over Antarctic sea ice from X-band SAR backscatter.

Ikaite of sediment cores from the Zaire deep-sea fan

Most concentration profiles of sulfate in continental margin sediments show constant or continuously increasing gradients from the benthic boundary layer down to the deep sulfate reduction zone. However, a very marked change in this gradient has been observed several meters below the surface at many locations, which has been attributed to anoxic sulfide oxidation or to non-local transport mechanisms of pore waters. The subject of this study is to investigate whether this feature could be better explained by non-steady state conditions in the pore-water system. To this end, data are presented from two gravity cores recovered from the Zaire deep-sea fan. The sediments at this location can be subdivided into two sections. The upper layer, about 10 m thick, consists of stratified pelagic deposits representing a period of continuous sedimentation over the last 190 kyr. It is underlain by a turbidite sequence measuring several meters in thickness, which contains large crystals of authigenic calcium carbonate (ikaite: CaCO3·6H2O). Ikaite delta13C values are indicative of a methane carbon contribution to the CO2 pool. Radiocarbon ages of these minerals, as well as of the adjacent bulk sediments, provide strong evidence that the pelagic sediments have overthrust the lower section as a coherent block. Therefore, the emplacement of a relatively undisturbed sediment package is postulated. Pore-water profiles show the depth of the sulfate-methane transition zone within the turbiditic sediments. By the adaptation of a simple transport-reaction model, it is shown that the change in the geochemical environmental conditions, resulting from this slide emplacement, and the development towards a new steady state are fully sufficient to explain all features related to the pore-water profiles, particularly, [SO4]2- and dissolved inorganic carbon (DIC). The model shows that the downslope transport took place about 300 yr ago.

Magnesium isotope ratios of pore-fluids and dolomites

Magnesium concentrations in deep-sea sediment pore-fluids typically decrease down core due to net precipitation of dolomite or clay minerals in the sediments or underlying crust. To better characterize and differentiate these processes, we have measured magnesium isotopes in pore-fluids and sediment samples from Ocean Drilling Program sites (1082, 1086, 1012, 984, 1219, and 925) that span a range of oceanographic settings. At all sites, magnesium concentrations decrease with depth. At sites where diagenetic reactions are dominated by the respiration of organic carbon, pore-fluid d26Mg values increase with depth by as much as 2 per mil. Because carbonates preferentially incorporate 24Mg (low d26Mg), the increase in pore-fluid d26Mg values at these sites is consistent with the removal of magnesium in Mg-carbonate (dolomite). In contrast, at sites where the respiration of organic carbon is not important and/or weatherable minerals are abundant, pore-fluid d26Mg values decrease with depth by up to 2 per mil. The decline in pore-fluid d26Mg at these sites is consistent with a magnesium sink that is isotopically enriched relative to the pore-fluid. The identity of this enriched magnesium sink is likely clay minerals. Using a simple 1D diffusion-advection-reaction model of pore-fluid magnesium, we estimate rates of net magnesium uptake/removal and associated net magnesium isotope fractionation factors for sources and sinks at all sites. Independent estimates of magnesium isotope fractionation during dolomite precipitation from measured d26Mg values of dolomite samples from sites 1082 and 1012 are very similar to modeled net fractionation factors at these sites, suggesting that local exchange of magnesium between sediment and pore-fluid at these sites can be neglected. Our results indicate that the magnesium incorporated in dolomite is 2.0-2.7 per mil depleted in d26Mg relative to the precipitating fluid. Assuming local exchange of magnesium is minor at the rest of the studied sites, our results suggest that magnesium incorporated into clay minerals is enriched in d26Mg by 0 per mil to +1.25 per mil relative to the precipitating fluid. This work demonstrates the utility of magnesium isotopes as a tracer for magnesium sources/sinks in low-temperature aqueous systems.

Oxygen and silica flux from 9 multicorer profiles

Pore water profiles from 24 stations in the South Atlantic (located in the Guinea, Angola, Cape, Guyana, and Argentine basins) show good correlations of oxygen and silicon, suggesting microbially mediated dissolution of biogenic silica. We used simple analytical transport and reaction models to show the tight coupling of the reconstructed process kinetics of aerobic respiration and silicon regeneration. A generic transport and reaction model successfully reproduced the majority of Si pore water profiles from aerobic respiration rates, confirming that the dissolution of biogenic silica (BSi) occurs proportionally to O2 consumption. Possibly limited to well-oxygenated sediments poor in BSi, benthic Si fluxes can be inferred from O2 uptake with satisfactory accuracy. Compared to aerobic respiration kinetics, the solubility of BSi emerged as a less influential parameter for silicon regeneration. Understanding the role of bacteria for silicon regeneration requires further investigations, some of which are outlined. The proposed aerobic respiration control of benthic silicon cycling is suitable for benthic-pelagic models. The empirical relation of BSi dissolution to aerobic respiration can be used for regionalization assessments and estimates of the silicon budget to increase the understanding of global primary and export production patterns.

Sulfate and methane concentrations measured in pore water of sediment core GeoB13809-1 and GeoB13849-1

Several previous studies have shown that submarine mass-movements can profoundly impact the shape of pore water profiles. Therefore, pore water geochemistry and diffusion models were proposed as tools for identifying and dating recent (max. several thousands of years old) mass-transport deposits (MTDs). In particular, sulfate profiles evidentially indicate transient pore water conditions generated by submarine landslides. After mass-movements that result in the deposition of sediment packages with distinct pore water signatures, the sulfate profiles can be kink-shaped and evolve into the concave and linear shape with time due to molecular diffusion. Here we present data from the RV METEOR cruise M78/3 along the continental margin off Uruguay and Argentina. Sulfate profiles of 15 gravity cores are compared with the respective acoustic facies recorded by a sediment echosounder system. Our results show that in this very dynamic depositional setting, non-steady state profiles occur often, but are not exclusively associated with mass-movements. Three sites that show acoustic indications for recent MTDs are presented in detail. Where recent MTDs are identified, a geochemical transport/reaction model is used to estimate the time that has elapsed since the perturbation of the pore water system and, thus, the timing of the MTD emplacement. We conclude that geochemical analyses are a powerful complementary tool in the identification of recent MTDs and provide a simple and accurate way of dating such deposits.

Seawater carbonate chemistry, chlorophyll a and phosphate during experiments with Trichodesmium erythraeum IMS101 (CCMP1985), 2010

In this laboratory study, we monitored the buildup of biomass and concomitant shift in seawater carbonate chemistry over the course of a Trichodesmium bloom under different phosphorus (P) availability. During exponential growth, dissolved inorganic carbon (DIC) decreased, while pH increased until maximum cell densities were reached. Once P became depleted, DIC decreased even further and total alkalinity (TA) dropped, accompanied by precipitation of aragonite. Under P-replete conditions, DIC increased and TA remained constant in the postbloom phase. A diffusion-reaction model was employed to estimate changes in carbonate chemistry of the diffusive boundary layer. This study demonstrates that Trichodesmium can induce precipitation of aragonite from seawater and further provides possible explanations about underlying mechanisms.

Theoretical model simulations for variations in thermal reaction norms

Thermal reaction norms for growth rates of six Emiliania huxleyi isolates originating from the central Atlantic (Azores, Portugal) and five isolates from the coastal North Atlantic (Bergen, Norway) were assessed. We used the template mode of variation model to decompose variations in growth rates into modes of biological interest: vertical shift, horizontal shift, and generalist-specialist variation. In line with the actual habitat conditions, isolates from Bergen (Bergen population) grew well at lower temperatures, and isolates from the Azores (Azores population) performed better at higher temperatures. The optimum growth temperature of the Azores population was significantly higher than that of the Bergen population. Neutral genetic differentiation was found between populations by microsatellite analysis. These findings indicate that E. huxleyi populations are adapted to local temperature regimes. Next to between-population variation, we also found variation within populations. Genotype-by-environment interactions resulted in the most pronounced phenotypic differences when isolates were exposed to temperatures outside the range they naturally encounter. Variation in thermal reaction norms between and within populations emphasizes the importance of using more than one isolate when studying the consequences of global change on marine phytoplankton. Phenotypic plasticity and standing genetic variation will be important in determining the potential of natural E. huxleyi populations to cope with global climate change.