Helium isotope ratios and pore gases in deep-sea sediments of the Japan Sea

We have measured the 3He/4He and 4He/20Ne ratios and chemical compositions of gases exsolved from deep-sea sediments at two sites (798 and 799) in the Japan Sea. The 3He/4He and 4He/20Ne ratios vary from 0.642 Ratm (where Ratm is the atmospheric 3He/4He ratio of 1.393*10**-6) to 0.840 Ratm, and from 0.41 to 4.5, respectively. Helium in the samples can be explained by the mixing between atmospheric helium dissolved in bottom water of the Japan Sea and crustal helium in the sediment. The sedimentary helium is enriched in mantle-derived 3He compared with those from the Japan Trench and the Nankai Trough. This suggests that the basement of the Japan Sea has relatively large remnants of mantle-derived helium compared with that of the Pacific. Major chemical compositions of the samples are methane and nitrogen. There is a positive correlation between methane content and helium content corrected for air component. Based on the 3He/4He-Sum C/3He diagram, the major part of methane can be attributed to crustal and/or organic origin.

3He/4He ratios of pore gases in pore water and chemical composition of sediments at DSDP Sites 87-583 and 87-584

We have measured the 3He/4He and 20Ne/4He ratios of gases dissolved in the pore water in sediments at two sites in the Nankai Trough (Site 583) and the Japan Trench (Site 584). The 3He/4He and 20Ne/4He ratios vary from 0.215 * 10**-6 to 1.23 * 10**-6 and from 50 * 10**-3 to 2700 * 10**-3, respectively. These values can be explained by mixing two components, one from the atmosphere and one with a 3He/4He ratio of (0.2 to 0.3) * 10**-6. The latter component may be derived from the ocean crust near the subduction zone.

Chemical and isotopic compositions of water, particulates, plankton, gases, sediments, and minerals from the Kara Sea and lower Ob and Yenisei Rivers

The Kara Sea is an area uniquely suitable for studying processes in the river-sea system. This is a shallow sea, into which two great Siberian rivers, Yenisei and Ob, flow. From 1995 to 2003, the sea was studied by six international expeditions onboard the R/V Akademik Boris Petrov. This publication summarizes the results obtained, within the framework of this project, at the Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences. Various hydrogeochemical parameters, concentrations and isotopic composition of organic and carbonate carbon of the sediments, plankton, particulate organic matter, hydrocarbons, and dissolved CO2 were examined throughout the whole sea area at more than 200 sites. The d13C varies from -22 and -24 per mil where Atlantic waters enter the Kara Sea and in the north-eastern part of the water area to -27 per mil in the Yenisei and Ob estuaries. The value of d13C of the plankton is only weakly correlated with the d13C of the organic matter from the sediments and is lower by as much as 3-4 per mil. The paper presents the results obtained from a number of meridional river-sea profiles. It was determined from the relations between the isotopic compositions of plankton and particulate matter that the river waters carry material consisting of 70% detrital-humus matter and 30% planktonogenic material in the river part, and the material contained in the offshore waters consists of 30% terrigenous components, with the contribution of bioproducers amounting to 70%. The carbon isotopic composition of the plankton ranges from -29 to -35 per mil in the riverine part, from -28 to -27 per mil in the estuaries, and from -27.0 to -25 per mil in the marine part. The relative lightness of the carbon isotopic composition of plankton in Arctic waters is explained by the temperature effect, elevated CO2 concentrations, and long-distance CO2 supply to the sea with river waters. The data obtained on the isotopic composition of CO2 in the surface waters of the Kara Sea were used to map the distribution of d13C. The complex of hydrocarbon gases extracted from the waters included methane, C2-C5, and unsaturated C2=-C4= hydrocarbons, for which variations in the concentrations in the waters were studied along river-estuary-sea profiles. The geochemistry of hydrocarbon gases in surface fresh waters is characterized by comparable concentrations of methane (0.3-5 µl/l) and heavier hydrocarbons, including unsaturated ones. Microbiological methane with d13C from -105 to -90 per mil first occurs in the sediments at depths of 40-200 cm. The sediments practically everywhere display traces of methane oxidation in the form of a shift of the d13C of methane toward higher values and the occurrence of autogenic carbonate material, including ikaite, enriched in the light isotope. Ikaite (d13C from -25 to -60 per mil) was found and examined in several profiles. The redox conditions in the sediments varied from normal in the southern part of the sea to highly oxidized along the Novaya Zemlya Trough. Vertical sections through the sediments of the latter exemplify the complete suppression of the biochemical activity of microorganisms. Our data provide insight into the biogeochemistry of the Kara Sea and make it possible to specify the background values needed for ecological control during the future exploration operations and extraction of hydrocarbons in the Kara Sea.

Analyses of hydrocarbon gases and sulphate in sediments from the King George Basin, Antarctica

Thermogenic hydrocarbons, formed by the thermal alteration of organic matter, are encountered in several piston core stations in the King George Basin, Anatarctica. These hemipelagic sediments are being deposited in an area of active hydrothermalism, associated with the back-arc spreading in the Bransfield Strait. The lateral extent of sediments infiltrated by the hydrothermally influenced interstitial fluids is characterized by basalt diapiric intrusions and is delineated by an acoustically turbid zone in the sediments of the eastern part of the basin. Iron-sulphide-bearing veins and fractures cut across the sediment in several cores; they appear to be conduits for flow of hydrothermally altered fluids. These zones have the highest C2+ and ethene contents. The thermogenic hydrocarbons have molecular C1/(C2 + C3) ratios typically < 50 and delta13CH4 values between -38? and -48?, indicating an organic source which has undergone strong thermal stress. Several sediment cores also have mixed gas signatures, which indicate the presence of substantial amounts of bacterial gas, predominantly methane. Hydrocarbon generation in the King George Basin is thought to be a local phenomenon, resulting from submarine volcanism with temperatures in the range 70-150°C. There are no apparent seepages of hydrocarbons into the water column, and it is not believed that significant accumulation of thermogenic hydrocarbons reside in the basin.

Composition of noble gases in sediments and metasediments

Solar-type helium (He) and neon (Ne) in the Earth's mantle were suggested to be the result of solarwind loaded extraterrestrial dust that accumulated in deep-sea sediments and was subducted into the Earth's mantle. To obtain additional constraints on this hypothesis, we analysed He, Ne and argon (Ar) in high pressure-low temperature metamorphic rocks representing equivalents of former pelagic clays and cherts from Andros (Cyclades, Greece) and Laytonville (California, USA). While the metasediments contain significant amounts of 4He, 21Ne and 40Ar due to U, Th and K decay, no solar-type primordial noble gases were observed. Most of these were obviously lost during metamorphism preceding 30 km subduction depth. We also analysed magnetic fines from two Pacific ODP drillcore samples, which contain solar-type He and Ne dominated by solar energetic particles (SEP). The existing noble gas isotope data of deep-sea floor magnetic fines and interplanetary dust particles demonstrate that a considerable fraction of the extraterrestrial dust reaching the Earth has lost solar wind (SW) ions implanted at low energies, leading to a preferential occurrence of deeply implanted SEP He and Ne, fractionated He/Ne ratios and measurable traces of spallogenic isotopes. This effect is most probably caused by larger particles, as these suffer more severe atmospheric entry heating and surface ablation. Only sufficiently fine-grained dust may retain the original unfractionated solar composition that is characteristic for the Earth's mantle He and Ne. Hence, in addition to the problem of metamorphic loss of solar noble gases during subduction, the isotopic and elemental fractionation during atmospheric entry heating is a further restriction for possible subduction hypotheses.

Underway measurements of dissolved gases along POLARSTERN cruise track ANT-XXVII/2

We present measurements of pCO2, O2 concentration, biological oxygen saturation (Delta O2/Ar) and N2 saturation (Delta N2) in Southern Ocean surface waters during austral summer, 2010-2011. Phytoplankton biomass varied strongly across distinct hydrographic zones, with high chlorophyll a (Chla) concentrations in regions of frontal mixing and sea-ice melt. pCO2 and Delta O2 /Ar exhibited large spatial gradients (range 90 to 450 µatm and -10 to 60%, respectively) and co-varied strongly with Chla. However, the ratio of biological O2 accumulation to dissolved inorganic carbon (DIC) drawdown was significantly lower than expected from photosynthetic stoichiometry, reflecting the differential time-scales of O2 and CO2 air-sea equilibration. We measured significant oceanic CO2 uptake, with a mean air-sea flux (~ -20 mmol m-2 d-1) that significantly exceeded regional climatological values. N2 was mostly supersaturated in surface waters (mean Delta N2 of +2.5 %), while physical processes resulted in both supersaturation and undersaturation of mixed layer O2 (mean Delta O2phys = 2.1 %). Box model calculations were able to reproduce much of the spatial variability of Delta N2 and Delta O2phys along the cruise track, demonstrating significant effects of air-sea exchange processes (e.g. atmospheric pressure changes and bubble injection) and mixed layer entrainment on surface gas disequilibria. Net community production (NCP) derived from entrainment-corrected surface Delta O2 /Ar data, ranged from ~ -40 to > 300 mmol O2 m-2 d-1 and showed good coherence with independent NCP estimates based on seasonal mixed layer DIC deficits. Elevated NCP was observed in hydrographic frontal zones and regions of sea-ice melt with shallow mixed layer depths, reflecting the importance of mixing in controlling surface water light and nutrient availability.