Isotopic signature of specific biomarkers derived from anaerobic methanotrophic archaea (ANME groups) and sulphate-reducing bacteria (SRB) at different cold seep provinces

Anaerobic methane-oxidizing microbial communities in sediments at cold methane seeps are important factors in controlling methane emission to the ocean and atmosphere. Here, we investigated the distribution and carbon isotopic signature of specific biomarkers derived from anaerobic methanotrophic archaea (ANME groups) and sulphate-reducing bacteria (SRB) responsible for the anaerobic oxidation of methane (AOM) at different cold seep provinces of Hydrate Ridge, Cascadia margin. The special focus was on their relation to in situ cell abundances and methane turnover. In general, maxima in biomarker abundances and minima in carbon isotope signatures correlated with maxima in AOM and sulphate reduction as well as with consortium biomass. We found ANME-2a/DSS aggregates associated with high abundances of sn-2,3-di-O-isoprenoidal glycerol ethers (archaeol, sn-2-hydroxyarchaeol) and specific bacterial fatty acids (C16:1omega5c, cyC17:0omega5,6) as well as with high methane fluxes (Beggiatoa site). The low to medium flux site (Calyptogena field) was dominated by ANME-2c/DSS aggregates and contained less of both compound classes but more of AOM-related glycerol dialkyl glycerol tetraethers (GDGTs). ANME-1 archaea dominated deeper sediment horizons at the Calyptogena field where sn-1,2-di-O-alkyl glycerol ethers (DAGEs), archaeol, methyl-branched fatty acids (ai-C15:0, i-C16:0, ai-C17:0), and diagnostic GDGTs were prevailing. AOM-specific bacterial and archaeal biomarkers in these sediment strata generally revealed very similar d13C-values of around -100 per mill. In ANME-2-dominated sediment sections, archaeal biomarkers were even more 13C-depleted (down to -120 per mill), whereas bacterial biomarkers were found to be likewise 13C-depleted as in ANME-1-dominated sediment layers (d13C: -100 per mill). The zero flux site (Acharax field), containing only a few numbers of ANME-2/DSS aggregates, however, provided no specific biomarker pattern. Deeper sediment sections (below 20 cm sediment depth) from Beggiatoa covered areas which included solid layers of methane gas hydrates contained ANME-2/DSS typical biomarkers showing subsurface peaks combined with negative shifts in carbon isotopic compositions. The maxima were detected just above the hydrate layers, indicating that methane stored in the hydrates may be available for the microbial community. The observed variations in biomarker abundances and 13C-depletions are indicative of multiple environmental and physiological factors selecting for different AOM consortia (ANME-2a/DSS, ANME-2c/DSS, ANME-1) along horizontal and vertical gradients of cold seep settings.

Pore water methane characteristics and sulphate reduction rates of ODP Leg 164 sediments

Anaerobic methane oxidation (AMO) was characterized in sediment cores from the Blake Ridge collected during Ocean Drilling Program (ODP) Leg 164. Three independent lines of evidence support the occurrence and scale of AMO at Sites 994 and 995. First, concentration depth profiles of methane from Hole 995B exhibit a region of upward concavity suggestive of methane consumption. Diagenetic modeling of the concentration profile indicates a 1.85-m-thick zone of AMO centered at 21.22 mbsf, with a peak rate of 12.4 nM/d. Second, subsurface maxima in tracer-based sulfate reduction rates from Holes 994B and 995B were observed at depths that coincide with the model-predicted AMO zone. The subsurface zone of sulfate reduction was 2 m thick and had a depth integrated rate that compared favorably to that of AMO (1.3 vs. 1.1 nmol/cm**2/d, respectively). These features suggest close coupling of AMO and sulfate reduction in the Blake Ridge sediments. Third, measured d13CH4 values are lightest at the point of peak model-predicted methane oxidation and become increasingly 13C-enriched with decreasing sediment depth, consistent with kinetic isotope fractionation during bacterially mediated methane oxidation. The isotopic data predict a somewhat (60 cm) shallower maximum depth of methane oxidation than do the model and sulfate reduction data.

The sulphate component in aerosol samples over the Noth Atlantic Ocean (Tab. 1)

The size distribution of sulphate containing particles over the North Atlantic was determined for particles with radii larger than 0.18 µm. It is compared with the size distributions of the total aerosol, the hygroscopic and the insoluble parts in the maritime aerosol. From mass concentrations of sulphate and sodium, it can be concluded that only a fraction of sulphate originates from the ocean. The sulphate mass distribution separated into its sea salt sulphate and excess sulphate components is compared with a continental distribution.

Lead, sulphate, and aluminium concentrations and lead crustal enrichment factors in Mt. Logan ice core during pre-anthropogenic to modern periods

A high-resolution, 8000 year-long ice core record from the Mt. Logan summit plateau (5300 m asl) reveals the initiation of trans-Pacific lead (Pb) pollution by ca. 1730, and a >10-fold increase in Pb concentration (1981-1998 mean = 68.9 ng/l) above natural background (5.6 ng/l) attributed to rising anthropogenic Pb emissions from Asia. The largest rise in North Pacific Pb pollution from 1970-1998 (end of record) is contemporaneous with a decrease in Eurasian and North American Pb pollution as documented in ice core records from Greenland, Devon Island, and the European Alps. The distinct Pb pollution history in the North Pacific is interpreted to result from the later industrialization and less stringent abatement measures in Asia compared to North America and Eurasia. The Mt. Logan record shows evidence for both a rising Pb emissions signal from Asia and a trans-Pacific transport efficiency signal related to the strength of the Aleutian Low.

Anaerobic oxidation of methane and sulphate reduction rates, hydrocarbons, sulphate and sulphide concentrations of sediment cores from the Larsen B embayment at the eastern site of the Antarctic Peninsula

First videographic indication of an Antarctic cold seep ecosystem was recently obtained from the collapsed Larsen B ice shelf, western Weddell Sea (Domack et al., 2005). Within the framework of the R/V Polarstern expedition ANTXXIII-8, we revisited this area for geochemical, microbiological and further videographical examinations. During two dives with ROV Cherokee (MARUM, Bremen), several bivalve shell agglomerations of the seep-associated, chemosynthetic clam Calyptogena sp. were found in the trough of the Crane and Evans glacier. The absence of living clam specimens indicates that the flux of sulphide and hence the seepage activity is diminished at present. This impression was further substantiated by our geochemical observations. Concentrations of thermogenic methane were moderately elevated with 2 µM in surface sediments of a clam patch, increasing up to 9 µM at a sediment depth of about 1 m in the bottom sections of the sediment cores. This correlated with a moderate decrease in sulphate from about 28 mM at the surface down to 23.4 mM, an increase in sulphide to up to 1.43 mM and elevated rates of the anaerobic oxidation of methane (AOM) of up to 600 pmol cm**-3 d**-1 at about 1 m below the seafloor. Molecular analyses indicate that methanotrophic archaea related to ANME-3 are the most likely candidates mediating AOM in sediments of the Larsen B seep.

Sulphate and d34S in pore waters and d13C, d18O and mineral composition of carbonate nodules from ODP Leg 188 sites

Carbon and oxygen isotopic compositions of authigenic carbonate nodules or layers reflect the diagenetic conditions at the time of nodule growth. The shallowest samples of carbonate nodules and dissolved inorganic carbon of pore water samples beneath the sulfate reduction zone (0-160 meters below seafloor [mbsf]) at Site 1165 have extremely negative d13C values (-50 per mil and -62 per mil, respectively). These negative d13C values indicate nodule formation in association with anaerobic methane oxidation coupled with sulfate reduction. The 34S of residual sulfate at Site 1165 shows only minor 34S enrichment (+6 per mil), even with complete sulfate reduction. This small degree of apparent 34S enrichment is due to extreme "open-system" sulfate reduction, with sulfate abundantly resupplied by diffusion from overlying seawater. Ten calcite nodules from Site 1165 contain minor quartz and feldspar and have d13C values ranging from -49.7 per mil to -8.2 per mil. The nodules with the most negative d13C values currently are at depths of 273 to 350 mbsf and must have precipitated from carbonate largely derived from subsurface anaerobic methane oxidation. The processes of sulfate reduction coupled with methane oxidation in sediments of Hole 1165B are indicated by characteristic concentration and isotopic (d34S and d13C) profiles of dissolved sulfate and bicarbonate. Three siderite nodules from Site 1166 contain feldspar and mica and one has significant carbonate-apatite. The siderite has d13C values ranging from -15.3 per mil to -7.6 per mil. These siderite nodules probably represent early diagenetic carbonate precipitation during microbial methanogenesis.

Sulphate concentrations and sulphur isotopic composition in pore fluids from ODP Leg 168 sites

A numerical model of sulfate reduction and isotopic fractionation has been applied to pore fluid SO4**2- and d34S data from four sites drilled during Ocean Drilling Program (ODP) Leg 168 in the Cascadia Basin at 48°N, where basement temperatures reach up to 62°C. There is a source of sulfate both at the top and the bottom of the sediment column due to the presence of basement fluid flow, which promotes bacterial sulfate reduction below the sulfate minimum zone at elevated temperatures. Pore fluid d34S data show the highest values (135 per mil) yet found in the marine environment. The bacterial sulfur isotopic fractionation factor, a, is severely underestimated if the pore fluids of anoxic marine sediments are assumed to be closed systems and Rayleigh fractionation plots yield erroneous values for a by as much as 15 per mil in diffusive and advective pore fluid regimes. Model results are consistent with a = 1.077+/-0.007 with no temperature effect over the range 1.8 to 62°C and no effect of sulfate reduction rate over the range 2 to 10 pmol/ccm/day. The reason for this large isotopic fractionation is unknown, but one difference with previous studies is the very low sulfate reduction rates recorded, about two orders of magnitude lower than literature values that are in the range of µmol/ccm/day to tens of nmol/ccm/day. In general, the greatest 34S depletions are associated with the lowest sulfate reduction rates and vice versa, and it is possible that such extreme fractionation is a characteristic of open systems with low sulfate reduction rates.