Barbara M.A. Teichert

Relationship of pore water freshening to accretionary processes in the Cascadia margin: Fluid sources and gas hydrate abundance

[1] Drilling in the Cascadia accretionary complex enable us to evaluate the contribution of dehydration reactions and gas hydrate dissociation to pore water freshening. The observed freshening with depth and distance from the prism toe is consistent with enhanced conversion of smectite to illite, driven by increase in temperature and age of accreted sediments. Although they contain gas hydrate -as evidenced by discrete low chloride spikes- the westernmost sites drilled on Hydrate Ridge show no freshening trend

Clathrites: Archives of near-seafloor pore-fluid evolution (δ44/40Ca, δ13C, δ18O) in gas hydrate environments

Aragonitic clathrites are methane-derived precipitates that are found at sites of massive near-seafloor gas hydrate (clathrate) accumulations at the summit of southern Hydrate Ridge, Cascadia margin. These platy carbonate precipitates form inside or in proximity to gas hydrate, which in our study site currently coexists with a fluid that is highly enriched in dissolved ions as salts are excluded during gas hydrate formation. The clathrites record the preferential incorporation of 18O into the hydrate structure and hence the enrichment of 16O in the surrounding brine. We measured δ18O values as high as 2.27‰ relative to Peedee belemnite that correspond to a fluid composition of −1.18‰ relative to standard mean ocean water. The same trend can be observed in Ca isotopes. Ongoing clathrite precipitation causes enrichment of the 44Ca in the fluid and hence in the carbonates. Carbon isotopes confirm a methane source for the carbonates. Our triple stable isotope approach that uses the three main components of carbonates (Ca, C, O) provides insight into multiple parameters influencing the isotopic composition of the pore water and hence the isotopic composition of the clathrites. This approach provides a tool to monitor the geochemical processes during clathrate and clathrite formation, thus recording the evolution of the geochemical environment of gas hydrate systems.

Quantification of Microbial Communities in Forearc Sediment Basins off Sumatra

Sediments in the Indian Ocean off the coast of the Indonesian island Sumatra were sampled at 25 stations in high resolution near the sediment surface and at three stations up to a maximum depth of 12 meter below seafloor (mbsf) for a quantitative microbial community analysis. Total cell counts were determined applying two different protocols including SYBR Green II as fluorescent staining dye. Total cell counts without detaching cells from sediment particles were 10 9 cells/ mL sediments at the sediment surface with little variation between all stations. They decreased to 10 8 cells/ mL at 0.2 to 0.4 mbsf and to 10 7 cells/ mL below 6 mbsf. The total cell counts after detaching cells from sediment particles were up to one order of magnitude lower above 6 mbsf and showed similar values below. This difference for the two protocols can be explained by a loss of cells during the detachment procedure and/or counting of unspecific signals without detaching cells from sediment particles. Particular phylogenetic and physiological prokaryotic groups were quantified by quantitative, real-time PCR (Q-PCR) targeting 16S rRNA and functional genes. Archaea and Bacteria were found overall in similar 16S rRNA gene copy numbers in the range of the total cell counts at all sediment depths, thus, neither Archaea nor Bacteria could be considered as dominant. The eukaryotic 18S rRNA gene occurred in two orders of magnitude lower numbers than prokaryotic 16S rRNA genes. Fe(III)- and Mn(IV)-reducing bacteria (16S rRNA gene of Geobacteraceae) and sulfate-reducing bacteria (functional gene dsrA) were detected in variable (up to 10 8 gene copies/ mL sediment) but in always significantly lower numbers than total Bacteria. The proportion of sulfate reducers on the prokaryotic community was between 0.2 and 19%. Calculated aereal sulfate reduction rates were overall low with values between 0.002 and 0.027 mmol m − 2 a − 1 , resulting in sulfate reduction rates per cell of 0.0007 and 0.81 fmol cell − 1 a − 1 , similar to published data for other deeply buried marine sediments. Methanogenesis did not seem to play a big role since methane was detected only below 6.5 mbsf, and the functional gene of methanogens and anaerobic methanotrophs mcrA could not be detected in any sample.

Anaerobic Oxidation of Methane at a Marine Methane Seep in a Forearc Sediment Basin off Sumatra, Indian Ocean

A cold methane seep was discovered in a forearc sediment basin off the island Sumatra, exhibiting a methane-seep adapted microbial community. A defined seep center of activity, like in mud volcanoes, was not discovered. The seep area was rather characterized by a patchy distribution of active spots. The relevance of anaerobic oxidation of methane (AOM) was reflected by 13C-depleted isotopic signatures of dissolved inorganic carbon. The anaerobic conversion of methane to CO2 was confirmed in a 13C-labeling experiment. Methane fueled a vital microbial community with cell numbers of up to 4 × 109 cells cm−3 sediment. The microbial community was analyzed by total cell counting, catalyzed reporter deposition–fluorescence in situ hybridization (CARD–FISH), quantitative real-time PCR (qPCR), and denaturing gradient gel electrophoresis (DGGE). CARD–FISH cell counts and qPCR measurements showed the presence of Bacteria and Archaea, but only small numbers of Eukarya. The archaeal community comprised largely members of ANME-1 and ANME-2. Furthermore, members of the Crenarchaeota were frequently detected in the DGGE analysis. Three major bacterial phylogenetic groups (δ-Proteobacteria, candidate division OP9, and Anaerolineaceae) were abundant across the study area. Several of these sequences were closely related to the genus Desulfococcus of the family Desulfobacteraceae, which is in good agreement with previously described AOM sites. In conclusion, the majority of the microbial community at the seep consisted of AOM-related microorganisms, while the relevance of higher hydrocarbons as microbial substrates was negligible.

Gas hydrate transect across Northern Cascadia Margin

Gas hydrate is a solid compound mainly comprised of methane and water that is stable under low temperature and high pressure conditions. Usually found in offshore environments with water depths exceeding about 500 meters and in arctic regions associated with permafrost, gas hydrates form an efficient storage system for natural gas. Hence, they may represent an important future energy resource [e.g., Kvenvolden, 1988]. Gas hydrates also form a natural geo-hazard, and may play a significant role in global climate change [e.g., Dillon et al., 2001].

Sulphur diagenesis in the sediments of the Kiel Bight, SW Baltic Sea, as reflected by multiple stable sulphur isotopes

In this work, the biogeochemistry of marine sediments from the Kiel Bight, coastal SW Baltic Sea, is studied based on the abundance and isotopic composition of organic carbon and different forms of sedimentary sulphur. Active bacterial sulphate reduction, partly under sulphate-limiting conditions, is evident from paired δ34S and δ18O values of pore water sulphate. The resulting pore water sulphide is partly precipitated as acid-volatile iron sulphide and subsequently forms sedimentary pyrite, partly serves in later diagenetic sulphurisation of organic matter, or remains dissolved in the pore water, all evident from the respective δ34S values. Microbial sulphate turnover is associated with an apparent isotopic fractionation between dissolved sulphate and dissolved sulphide (Δ34S) that varies between 46 and 66‰.

Multiple sulphur and oxygen isotopes reveal microbial sulphur cycling in spring waters in the Lower Engadin, Switzerland

Highly mineralized springs in the Scuol-Tarasp area of the Lower Engadin and in the Albula Valley near Alvaneu, Switzerland, display distinct differences with respect to the source and fate of their dissolved sulphur species. High sulphate concentrations and positive sulphur (δ34S) and oxygen (δ18O) isotopic compositions argue for the subsurface dissolution of Mesozoic evaporitic sulphate. In contrast, low sulphate concentrations and less positive or even negative δ34S and δ18O values indicate a substantial contribution of sulphate sulphur from the oxidation of sulphides in the crystalline basement rocks or the Jurassic sedimentary cover rocks. Furthermore, multiple sulphur (δ34S, Δ33S) isotopes support the identification of microbial sulphate reduction and sulphide oxidation in the subsurface, the latter is also evident through the presence of thick aggregates of sulphide-oxidizing Thiothrix bacteria.

Calcium–ammonium exchange experiments on clay minerals using a 45Ca tracer technique in marine pore water

Understanding cation exchange processes is important for evaluating early diagenetic and synsedimentary processes taking place in marine sediments. To quantify calcium (Ca) exchange and Ca–ammonium exchange in a seawater environment, we performed experiments with a radioactive 45Ca tracer on clay mineral standards (Fithian illite, montmorillonite and kaolinite) and marine sediments from the North Atlantic Integrated Ocean Drilling Program Site U1306A in artificial seawater (ASW). The results show that equilibrium during the initial attachment of Ca as well as the exchange of Ca by is attained in less than 2 min. On average 8–20% of the exchangeable sites of the clay minerals were occupied by Ca in a seawater medium. The conditional selectivity coefficient, describing the exchange in ASW is mineral specific and it was determined to be 0.07 for montmorillonite, 0.05 for a natural marine sediment and 0.013 for Fithian illite.

Preparation of Authigenic Pyrite from Methane-bearing Sediments for In Situ Sulfur Isotope Analysis Using SIMS.

Different sulfur isotope compositions of authigenic pyrite typically result from the sulfate-driven anaerobic oxidation of methane (SO4-AOM) and organiclastic sulfate reduction (OSR) in marine sediments. However, unravelling the complex pyritization sequence is a challenge because of the coexistence of different sequentially formed pyrite phases. This manuscript describes a sample preparation procedure that enables the use of secondary ion mass spectroscopy (SIMS) to obtain in situ δ34S values of various pyrite generations. This allows researchers to constrain how SO4-AOM affects pyritization in methane-bearing sediments. SIMS analysis revealed an extreme range in δ34S values, spanning from -41.6 to +114.8‰, which is much wider than the range of δ34S values obtained by the traditional bulk sulfur isotope analysis of the same samples. Pyrite in the shallow sediment mainly consists of 34S-depleted framboids, suggesting early diagenetic formation by OSR. Deeper in the sediment, more pyrite occurs as overgrowths and euhedral crystals, which display much higher SIMS δ34S values than the framboids. Such 34S-enriched pyrite is related to enhanced SO4-AOM at the sulfate-methane transition zone, postdating OSR. High-resolution in situ SIMS sulfur isotope analyses allow for the reconstruction of the pyritization processes, which cannot be resolved by bulk sulfur isotope analysis.