Chemical composition and stable sulfur isotope record of Black Sea sediments

The main terminal processes of organic matter mineralization in anoxic Black Sea sediments underlying the sulfidic water column are sulfate reduction in the upper 2-4 m and methanogenesis below the sulfate zone. The modern marine deposits comprise a ca. 1-m-deep layer of coccolith ooze and underlying sapropel, below which sea water ions penetrate deep down into the limnic Pleistocene deposits from >9000 years BP. Sulfate reduction rates have a subsurface maximum at the SO4[2-]-CH4 transition where H2S reaches maximum concentration. Because of an excess of reactive iron in the deep limnic deposits, most of the methane-derived H2S is drawn downward to a sulfidization front where it reacts with Fe(III) and with Fe2+ diffusing up from below. The H2S-Fe2+ transition is marked by a black band of amorphous iron sulfide above which distinct horizons of greigite and pyrite formation occur. The pore water gradients respond dynamically to environmental changes in the Black Sea with relatively short time constants of ca. 500 yr for SO4[2-] and 10 yr for H2S, whereas the FeS in the black band has taken ca. 3000 yr to accumulate. The dual diffusion interfaces of SO4[2-]-CH4 and H2S-Fe2+ cause the trapping of isotopically heavy iron sulfide with delta34S = +15 to +33 per mil at the sulfidization front. A diffusion model for sulfur isotopes shows that the SO4[2-] diffusing downward into the SO4[2-]-CH4 transition has an isotopic composition of +19 per mil, close to the +23 per mil of H2S diffusing upward. These isotopic compositions are, however, very different from the porewater SO4[2-] (+43 per mil) and H2S (-15 per mil) at the same depth. The model explains how methane-driven sulfate reduction combined with a deep H2S sink leads to isotopically heavy pyrite in a sediment open to diffusion. These results have general implications for the marine sulfur cycle and for the interpretation of sulfur isotopic data in modern sediments and in sedimentary rocks throughout earth's history.

Biogeochemical investigation of sediments at the Hakon Mosby Mud Volcano (Barents Sea)

We have investigated if in a cold seep methane or sulfide is used for chemosynthetic primary production and if significant amounts of the sulfide produced by anaerobic oxidation of methane are oxidized geochemically and hence are not available for chemosynthetic production. Geochemically controlled redox reactions and biological turnover were compared in different habitats of the Håkon Mosby Mud Volcano. The center of the mud volcano is characterized by the highest fluid flow, and most primary production by the microbial community depends on oxidation of methane. The small amount of sulfide produced is oxidized geochemically with oxygen or is precipitated with dissolved iron. In the medium flow peripheral Beggiatoa habitat sulfide is largely oxidized biologically. The oxygen and nitrate supply is high enough that Beggiatoa can oxidize the sulfide completely, and chemical sulfide oxidation or precipitation is not important. An internally stored nitrate reservoir with average concentrations of 110 mmol L-1 enables the Beggiatoa to oxidize sulfide anaerobically. The pH profile indicates sequential sulfide oxidation with elemental sulfur as intermediate. Gray thiotrophic mats associated with perturbed sediments showed a high heterogeneity in sulfate turnover and high sulfide fluxes, balanced by the opposing oxygen and nitrate fluxes so that biological oxidation dominates over geochemical sulfide removal processes. The three habitats indicate substantial small-scale variability in carbon fixation pathways either through direct biological use of methane or through indirect carbon fixation of methane-derived carbon dioxide by chemolithotrophic sulfide oxidation.

Microbial community composition in sediments of the Hydrate Ridge (Cascadian Margin) collected during RV SONNE cruises SO143 (1999) and SO148-1 (2000)

Cold seep environments such as sediments above outcropping hydrate at Hydrate Ridge (Cascadia margin off Oregon) are characterized by methane venting, high sulfide fluxes caused by the anaerobic oxidation of methane, and the presence of chemosynthetic communities. This investigation deals with the diversity and distribution of sulfate-reducing bacteria, some of which are directly involved in the anaerobic oxidation of methane as syntrophic partners of the methanotrophic archaea. The composition and activity of the microbial communities at methane vented and nonvented sediments are compared by quantitative methods including total cell counts, fluorescence in situ hybridization (FISH). Bacteria involved in the degradation of particulate organic carbon (POC) are as active and diverse as at other productive margin sites of similar water depths. The availability of methane supports a two orders of magnitude higher microbial biomass (up to 9.6×10**10cells/cm**3). Sediment samples were obtained during RV SONNE cruises SO143-2 and SO148-1 at the crest of southern Hydrate Ridge at the Cascadia convergent margin off the coast of Oregon. Sediment cores of 20 - 40 cm length were obtained using a video-guided multiple corer from gas hydrate bearing sediments and from reference sites not enriched in methane in the surface sediments. Samples for total cell counts were obtained from 1 cm core slices, fixed with 2% formaldehyde and stored cold (4°C) and the quantification of aggregates was done via epifluorescence microscopy after staining the sediments with Acridine Orange Direct Counts (AODC) according to the method of Meyer- Reil (1983, doi:10.1007/BF00395813). Total cell counts were defined as the sum of single cells plus the aggregated cells in the syntrophic consortia. DAPI staining was used to measure ANME2/DSS aggregate sizes via epifluorescence microscopy of FISH-treated samples. For FISH, subsamples of sediment cores were sliced into 1 cm intervals and fixed for 2-3 h with 3% formaldehyde (final concentration), washed twice with 1×PBS (10 mM sodium phosphate; 130 mM NaCl), and finally stored in 1×PBS/EtOH (1:1) at -20°C.

Concentration of archaeal intact polar lipids (IPLs) of ROV sediment core GeoB12315-9

Here we report the amount of archaeal cardiolipins analogues in a cold seep off Pakistan (Makran accretionary prism). The published data (Yoshinaga et al., 2012) describes a series of tentatively identified cardiolipin analogues (dimeric phospholipids or bisphosphatidylglycerol), which represented 0.5% to 5% of total archaeal intact polar lipids.

The SUGAR Toolbox - Version 7

The SUGAR Toolbox contains scripts coded in MATLAB for calculating various thermodynamic, kinetic, and geologic properties of substances occurring in the marine environment, particularly gas hydrate and seep systems. Brief descriptions of the toolbox scripts and some notes on the underlying basic theory as well as tables of additional property values can be found in the accompanying documentation.

High-resolution pore-water and solid-phase analyses of sediment cores GeoB6520-3 (Hydrate Hole) and GeoB6521-2 (Worm Hole) of the northern Congo Fan

The geochemical cycling of barium was investigated in sediments of pockmarks of the northern Congo Fan, characterized by surface and subsurface gas hydrates, chemosynthetic fauna, and authigenic carbonates. Two gravity cores retrieved from the so-called Hydrate Hole and Worm Hole pockmarks were examined using high-resolution pore-water and solid-phase analyses. The results indicate that, although gas hydrates in the study area are stable with respect to pressure and temperature, they are and have been subject to dissolution due to methane-undersaturated pore waters. The process significantly driving dissolution is the anaerobic oxidation of methane (AOM) above the shallowest hydrate-bearing sediment layer. It is suggested that episodic seep events temporarily increase the upward flux of methane, and induce hydrate formation close to the sediment surface. AOM establishes at a sediment depth where the upward flux of methane from the uppermost hydrate layer counterbalances the downward flux of seawater sulfate. After seepage ceases, AOM continues to consume methane at the sulfate/methane transition (SMT) above the hydrates, thereby driving the progressive dissolution of the hydrates "from above". As a result the SMT migrates downward, leaving behind enrichments of authigenic barite and carbonates that typically precipitate at this biogeochemical reaction front. Calculation of the time needed to produce the observed solid-phase barium enrichments above the present-day depths of the SMT served to track the net downward migration of the SMT and to estimate the total time of hydrate dissolution in the recovered sediments. Methane fluxes were higher, and the SMT was located closer to the sediment surface in the past at both sites. Active seepage and hydrate formation are inferred to have occurred only a few thousands of years ago at the Hydrate Hole site. By contrast, AOM-driven hydrate dissolution as a consequence of an overall net decrease in upward methane flux seems to have persisted for a considerably longer time at the Worm Hole site, amounting to a few tens of thousands of years.