Interstitial water chemistry of DSDP Leg 85 sites

The concentration changes in pore waters of dissolved calcium, magnesium, sulfate, strontium, and silica and of alkalinity are controlled by diagenetic reactions occurring within the biogenic sediments of DSDP Sites 572, 573, and 574. Downcore increases in dissolved Sr2 + indicate recrystallization of calcite, and increases in dissolved SiO2 reflect dissolution of amorphous silica. Minor gradients in dissolved Ca(2+) and Mg(2+) suggest little if any influence from reactions involving volcanic sediments or basalt. Differences in interstitial water profiles showing the downhole trends of these chemical species mark variations in carbonate and silica diagenesis, sediment compositions, and sedimentation rate histories among the sites. The location and extent of carbonate diagenesis in these sediments are determined from Sr/Ca distributions between the interstitial waters and the bulk carbonate samples. Pore water strontium increases in the upper 100 to 250 m of sediment are assumed to reflect diffusion from underlying zones where calcite recrystallization has occurred. On the basis of calculations of dissolved strontium production and comparisons between observed and calculated "equilibrium" Sr/Ca ratios of the solids, approximately 30 to 50% of the carbonate has recrystallized in these deeper intervals. These estimates agree with the observed amounts of chalk at these sites. Variations in Sr/Ca ratios of these carbonates reflect differences in calcareous microfossil content, in diagenetic history, and, possibly, in changes in seawater Sr/Ca with time. Samples of porcelanite recovered below 300 m at Site 572 suggest formation at temperatures 20 to 30° C greater than ones estimated assuming oceanic geothermal gradients from sedimentary sections similar to those recovered on Leg 85. The higher temperatures may partially account for higher Sr/Ca ratios determined for recrystallized carbonates from this site.

Stable oxygen, hydrogene and chlorine isotopes of mid-Atlantic abyssal peridotites

Serpentinized abyssal peridotites sampled by the Ocean Drilling Program Leg 209 along the mid-Atlantic Ridge near the 15°20'N Fracture Zone have been analyzed for oxygen, hydrogen, and chlorine isotope compositions in order to determine isotopic behavior under a wide range of serpentinization conditions and place constraints on fluid history. Oxygen and hydrogen thermometry suggests peak serpentinization temperatures of 300-500°C. Serpentine separates have low deltaD values possibly due to a magmatic fluid component or low-temperature exchange during seafloor weathering. Chlorine geochemistry focused on three holes: 1274A and 1272A (serpentinized peridotites) and 1268A (serpentinite locally altered to talc). Concentrations of both, water-soluble chloride (WSC) and structurally bound chloride (SBC) are significantly lower at Hole 1268A compared to Holes 1274A and 1272A. The delta37Cl values for WSC and SBC of serpentinites in Holes 1274A and 1272A are slightly positive (avg. WSC = 0.20 per mil, n = 22 and avg. SBC = 0.35 per mil, n = 22), representing typical seawater-hydration conditions commonly determined for abyssal peridotite. The SBC of serpentinites from Hole 1268A are also positive (avg. = 0.63 per mil); whereas, the SBC in talc-dominated samples is negative (avg. = -1.22 per mil). The WSC of both talc- and serpentine-dominated samples are also negative (avg. = -0.15 per mil). We interpret the chlorine isotope data to preserve a record of multiple fluid events. As seawater hydrated the peridotite, 37Cl was preferentially incorporated into the forming serpentine and water-soluble salts, yielding similar delta37Cl values on a regional scale as sampled by Holes 1268A, 1274A and 1272A. The resultant pore fluid was left depleted in 37Cl. Locally (Hole 1268A), this evolved fluid was remobilized possibly due to the initiation of hydrothermal circulation in response to emplacement of a mafic magma body. The low delta37Cl pore fluids attained elevated SiO2 and sulfur concentrations due to interaction with the gabbroic intrusion and, when ascending through the surrounding serpentinite, caused formation of isotopically negative talc. This secondary fluid also flushed the preserved serpentinite of its previously formed salts, resulting in negative delta37Cl WSC values. The delta37Cl SBC values of the serpentinite samples remained unmodified by reaction with the secondary fluid.

Grain size composition of sediment cores from the Weddell Sea, Antarctica

Sedimentary processes in the southeastern Weddell Sea are influenced by glacial-interglacial ice-shelf dynamics and the cyclonic circulation of the Weddell Gyre, which affects all water masses down to the sea floor. Significantly increased sedimentation rates occur during glacial stages, when ice sheets advance to the shelf edge and trigger gravitational sediment transport to the deep sea. Downslope transport on the Crary Fan and off Dronning Maud and Coats Land is channelized into three huge channel systems, which originate on the eastern-, the central and the western Crary Fan. They gradually turn from a northerly direction eastward until they follow a course parallel to the continental slope. All channels show strongly asymmetric cross sections with well-developed levees on their northwestern sides, forming wedge-shaped sediment bodies. They level off very gently. Levees on the southeastern sides are small, if present at all. This characteristic morphology likely results from the process of combined turbidite-contourite deposition. Strong thermohaline currents of the Weddell Gyre entrain particles from turbidity-current suspensions, which flow down the channels, and carry them westward out of the channel where they settle on a surface gently dipping away from the channel. These sediments are intercalated with overbank deposits of high-energy and high-volume turbidity currents, which preferentially flood the left of the channels (looking downchannel) as a result of Coriolis force. In the distal setting of the easternmost channel-levee complex, where thermohaline currents are directed northeastward as a result of a recirculation of water masses from the Enderby Basin, the setting and the internal structures of a wedge-shaped sediment body indicate a contourite drift rather than a channel levee. Dating of the sediments reveals that the levees in their present form started to develop with a late Miocene cooling event, which caused an expansion of the East Antarctic Ice Sheet and an invigoration of thermohaline current activity.

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.

Geomicrobiology, bulk sediment characteristics and pore water chemistry of sediment cores from Laguna Potrok Aike, southern Patagonia

Authigenic minerals can form in the water column and sediments of lakes, either abiotically or mediated by biological activity. Such minerals have been used as paleosalinity and paleoproductivity indicators and reflect trophic state and early diagenetic conditions. They are also considered potential indicators of past and perhaps ongoing microbial activity within sediments. Authigenic concretions, including vivianite, were described in late glacial sediments of Laguna Potrok Aike, a maar lake in southernmost Argentina. Occurrence of iron phosphate implies specific phosphorus sorption behavior and a reducing environment, with methane present. Because organic matter content in these sediments was generally low during glacial times, there must have been alternative sources of phosphorus and biogenic methane. Identifying these sources can help define past trophic state of the lake and diagenetic processes in the sediments. We used scanning electron microscopy, phosphorus speciation in bulk sediment, pore water analyses, in situ ATP measurements, microbial cell counts, and measurements of methane content and its carbon isotope composition (d13C CH4) to identify components of and processes in the sediment. The multiple approaches indicated that volcanic materials in the catchment are important suppliers of iron, sulfur and phosphorus. These elements influence primary productivity and play a role in microbial metabolism during early diagenesis. Authigenic processes led to the formation of pyrite framboids and revealed sulfate reduction. Anaerobic oxidation of methane and shifts in pore water ion concentration indicated microbial influence with depth. This study documents the presence of active microbes within the sediments and their relationship to changing environmental conditions. It also illustrates the substantial role played by microbes in the formation of Laguna Potrok Aike concretions. Thus, authigenic minerals can be used as biosignatures in these late Pleistocene maar sediments.

Radionuclide measurements in sediments and water of the Kara Sea

In this report, the results of a 2000-2001 radiogeoecological investigation are presented for the region of the Ob and Yenisei estuaries and the adjacent Kara Sea. In order to study the behaviour and migration of Cs, Sr and Pu radionuclides in a river - sea system experimental research on the distribution of these radionuclides in the water column and surface sediments has been carried out. In addition, the role of suspended and dissolved organic matter on the behaviour of radionuclides in water solutions has been studied. The 137Cs and 239,240Pu concentrations in the upper 0-2cm layer of the sediments varied between 1,4 and 50,0 Bq/kg, with a mean of 12,4 Bq/kg, and between 0,065-1,96 Bq/kg, with a mean of 0,62 Bq/kg, respectively. There is a direct relationship of a specific radioactivity of 137Cs and 239,240Pu in the sediments and the content of clay fraction. The 137Cs, 90Sr and 239,240Pu concentrations in the water samples varied between 0,4 and 7,0 Bq/m**3 (mean of 3,6 Bq/m**3), 0,4 and 9,7 Bq/m**3 (mean of 3,3 Bq/m**3), and 0,01-0,3 Bq/m**3 (mean of 0,02 Bq/m**3), respectively. In the water samples the concentration of the water-soluble species l37Cs increases with increasing salinity, whereas the concentration of the 90Sr-radionuclide decreases with increasing salinity. This may be related to the physico-chemical behaviour of these radionuclides in water solutions and the influence of several sources on radioactive pollution in this basin. In sea water the suspended matter may absorb up to 10% 137Cs, 90Sr and 239,240Pu, in river water samples these values may reach 15-30%. More than 50% 90Sr and 239,240Pu is able to form complexes with dissolved organic matter. This effect is smaller in saline water. The comparison of the data of 137Cs radioactivity in the surface sediments in 1995 and 2000-2001 showed that the level of radioactivity has decreased.

Seawater carbonate chemistry, sample density and Strongylocentrotus purpuratus size, filtering and respiration rate during experiments, 2011

Anthropogenic CO2 emissions are acidifying the world's oceans. A growing body of evidence is showing that ocean acidification impacts growth and developmental rates of marine invertebrates. Here we test the impact of elevated seawater pCO2 (129 Pa, 1271 µatm) on early development, larval metabolic and feeding rates in a marine model organism, the sea urchin Strongylocentrotus purpuratus. Growth and development was assessed by measuring total body length, body rod length, postoral rod length and posterolateral rod length. Comparing these parameters between treatments suggests that larvae suffer from a developmental delay (by ca. 8%) rather than from the previously postulated reductions in size at comparable developmental stages. Further, we found maximum increases in respiration rates of + 100 % under elevated pCO2, while body length corrected feeding rates did not differ between larvae from both treatments. Calculating scope for growth illustrates that larvae raised under high pCO2 spent an average of 39 to 45% of the available energy for somatic growth, while control larvae could allocate between 78 and 80% of the available energy into growth processes. Our results highlight the importance of defining a standard frame of reference when comparing a given parameter between treatments, as observed differences can be easily due to comparison of different larval ages with their specific set of biological characters.