Evidence for microbial iron reduction in the methanogenic sediments of the oligotrophic SE Mediterranean continental shelf

Abstract. Dissimilatory iron reduction is probably one of the earliest metabolisms, which still participates in important biogeochemical cycles such as carbon and sulfur. Traditionally, this process is thought to be limited to the shallow part of the sediment column, as one of the energetically favorable anaerobic microbial respiration cascade, usually coupled to the oxidation of organic matter. However, in the last decade iron reduction has been observed in the methanogenic depth in many aquatic sediments, suggesting a link between the iron and the methane cycles. Yet, the mechanistic nature of this link has yet to be established, and has not been studied in oligotrophic shallow marine sediments. In this study we present first geochemical and molecular evidences for microbial iron reduction in the methanogenic depth of the oligotrophic Southern Eastern (SE) Mediterranean continental shelf. Geochemical pore-water profiles indicate iron reduction in two zones, the traditional zone in the upper part of the sediment cores and a deeper second zone located in the enhanced methane concentration layer. Results from a slurry incubation experiment indicate that the iron reduction is microbial. The Geochemical data, Spearman correlation between microbial abundance and iron concentration, as well as the qPCR analysis of the mcrA gene point to several potential microorganisms that could be involved in this iron reduction via three potential pathways: H2/organic matter oxidation, an active sulfur cycle or iron driven anaerobic oxidation of methane.

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Evidence for microbial iron reduction in the methanic sediments of the oligotrophic southeastern Mediterranean continental shelf

Biogeosciences ◽

10.5194/bg-16-3165-2019 ◽

2019 ◽

Vol 16 (16) ◽

pp. 3165-3181 ◽

Cited By ~ 3

Author(s):

Hanni Vigderovich ◽

Lewen Liang ◽

Barak Herut ◽

Fengping Wang ◽

Eyal Wurgaft ◽

...

Keyword(s):

Organic Matter ◽

Continental Shelf ◽

Microbial Activity ◽

Iron Reduction ◽

Sulfur Cycle ◽

Anaerobic Oxidation Of Methane ◽

Upward Migration ◽

Oxidation Of Methane ◽

Microbial Iron Reduction ◽

Dissimilatory Iron Reduction

Abstract. Dissimilatory iron reduction is probably one of the oldest types of metabolisms that still participates in important biogeochemical cycles, such as those of carbon and sulfur. It is one of the more energetically favorable anaerobic microbial respiration processes and is usually coupled to the oxidation of organic matter. Traditionally this process is thought to be limited to the shallow part of the sedimentary column in most aquatic systems. However, iron reduction has also been observed in the methanic zone of many marine and freshwater sediments, well below its expected zone and occasionally accompanied by decreases in methane, suggesting a link between the iron and the methane cycles. Nevertheless, the mechanistic nature of this link (competition, redox or other) has yet to be established and has not been studied in oligotrophic shallow marine sediments. In this study we present combined geochemical and molecular evidences for microbial iron reduction in the methanic zone of the oligotrophic southeastern (SE) Mediterranean continental shelf. Geochemical porewater profiles indicate iron reduction in two zones, the uppermost part of the sediment, and the deeper zone, in the layer of high methane concentration. Results from a slurry incubation experiment indicate that the deep methanic iron reduction is microbially mediated. The sedimentary profiles of microbial abundance and quantitative PCR (qPCR) of the mcrA gene, together with Spearman correlation between the microbial data and Fe(II) concentrations in the porewater, suggest types of potential microorganisms that may be involved in the iron reduction via several potential pathways: H2 or organic matter oxidation, an active sulfur cycle, or iron-driven anaerobic oxidation of methane. We suggest that significant upward migration of methane in the sedimentary column and its oxidation by sulfate may fuel the microbial activity in the sulfate methane transition zone (SMTZ). The biomass created by this microbial activity can be used by the iron reducers below, in the methanic zone of the sediments of the SE Mediterranean.

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Glendonites from Mesozoic succession of eastern Barents sea: distribution, genesis and paleoclimatic implications

10.5194/egusphere-egu2020-19076 ◽

2020 ◽

Author(s):

Kseniya Mikhailova ◽

Victoria Ershova ◽

Mikhail Rogov ◽

Boris Pokrovsky ◽

Oleg Vereshchagin

Keyword(s):

Organic Matter ◽

Calcium Carbonate ◽

Sulfate Reduction ◽

Early Cretaceous ◽

Lower Cretaceous ◽

Middle Jurassic ◽

Barents Sea ◽

Upper Jurassic ◽

Anaerobic Oxidation Of Methane ◽

Oxidation Of Methane

Glendonites often used as paleoclimate indicator of cold near-bottom temperature, as these are calcite pseudomorphs of ikaite, a metastable calcium carbonate hexahydrate, precipitates mostly under low temperature (mainly from 0-4oC) and may be stabilized by high phosphate concentrations that occurs due to anaerobic oxidation of methane and/or organic matter; dissolved organic carbon, sulfates and amino acid may contribute ikaite formation as well.&#160; Therefore, glendonites-bearing host rocks frequently include glacial deposits that make them useful as a paleoclimate indicator of near-freezing temperature.Our study is based on material collected from five wells drilled in eastern Barents Sea: Severo-Murmanskaya, Ledovaya &#8211; 1,2; Ludlovskaya &#8211; 1,2. The studied glendonites, mainly represented by relatively small rhombohedral pseudomorphs (0,5-2 cm) and rarely by stellate aggregates, collected from Middle Jurassic to Lower Cretaceous shallow marine clastic deposits. They scattered distributed throughout succession. Totally 18 samples of glendonites were studied. The age of host-bearing rocks were defined by fossils: bivalves or ammonites, microfossils or dinoflagellate. Bajocian-Bathonian glendonites were collected from Ledovaya &#8211; 1 and Ludlovskaya &#8211; 1 and 2 wells; in addition to these occurrences Middle Jurassic glendonites are known also in boreholes drilled at Shtockmanovskoe field. Numerous &#8216;jarrowite-like&#8217; glendonites of the Middle Volgian (~ latest early Tithonian) age were sampled from Severo-Murmanskaya well. Unique Late Barremian glendonites were found in Ledovaya &#8211; 2 well.&#948;18O values of Middle Jurassic glendonite concretions range from &#8211; 5.4 to &#8211;1.7 &#8240; Vienna Pee Dee Belemnite (VPDB); for Upper Jurassic &#8211; Lower Cretaceous &#948;18O values range from &#8211; 4.3 to &#8211;1.6 &#8240; VPDB; for Lower Cretaceous - &#948;18O values range from &#8211; 4.5 to &#8211;3.4 &#8240; VPDB. Carbon isotope composition for Middle Jurassic glendonite concretions &#948;13C values range from &#8211; 33.3 to &#8211;22.6 &#8240; VPDB; for Upper Jurassic &#8211; Lower Cretaceous &#948;13C values range from &#8211; 25.1 to &#8211;18.4 &#8240; VPDB; for Lower Cretaceous - &#948;13C values range from &#8211; 30.1 to &#8211;25.6 &#8240; VPDB.Based on &#948;18O data we supposed that seawater had a strong influence on ikaite-derived calcite precipitation. Received data coincide with &#948;18O values reported from other Mesozoic glendonites and Quaternary glendonites formed in cold environments. Values of &#948;13C of glendonites are close to bacterial sulfate reduction and/or anaerobic oxidation of methane or organic matter. Glendonites consist of carbonates forming a number of phases which different in phosphorus and magnesium content. Mg-bearing calcium carbonate and dolomite both include framboidal pyrite, which can indicate (1) lack of strong rock transformations activity and (2) presence of sulfate-reduction bacteria in sediments.To conclude, Mesozoic climate was generally warm and studied concretions indicate cold climate excursion in Middle Jurassic, Upper Jurassic-Early Cretaceous and Early Cretaceous.&#160;The study was supported by RFBR, project number 20-35-70012.

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Sedimentary Transport Influences on Diagenetic Processes at the Amazon Continental Shelf, Brazil

Journal of Geography Environment and Earth Science International ◽

10.9734/jgeesi/2019/v20i430113 ◽

2019 ◽

pp. 1-16

Author(s):

Fabio Aprile ◽

Gilmar W. Siqueira ◽

Assad Darwich ◽

Georg Irion

Keyword(s):

Trace Elements ◽

Organic Matter ◽

Continental Shelf ◽

Trace Element ◽

Pore Water ◽

Coastal Zone ◽

Iron Reduction ◽

Net Production ◽

Diagenetic Processes ◽

Physical Chemical

This research aimed to correlate the sedimentary transport with the diagenetic processes in the coastal zone and Amazon Continental Shelf (ACS). Physical and physical-chemical parameters, trace element contents (Cr, Pb, Ni, Zn and Hg), and O2, CO2 and iron flux were determined in sediment and pore water. Sedimentary incubation (96 hours) and algorithms were applied to determine the variation of the activity coefficient (ΔI) and ionic strength (Fi) of the predominant chemical species, and to estimate the net production and mineralization of the organic matter (ΔCO2T) in the system. There are not many studies applying incubation tests to identify the diagenetic processes, especially in fluvial-marines sediments. The results showed a strong zonation associated to the transport and deposition processes, influenced mainly by the grain-size and texture of sediment and fluvial streams. The distribution of trace elements followed the trend of the sedimentary pattern, with higher levels of metals in the deposits of clay minerals and organic matter. A factor of weight (Fw), calculated to establish the degree of importance of each parameter under the distribution and mobility of trace elements, suggests that the mobility of Cr, Ni and Zn is controlled by depth, clay and organic compounds contents, and concentration of dissolved oxygen. The vertical flow of O2 and CO2 and the Fe2+/Fe3+ ratio in the pore water suggest a predominance of organic matter oxidation in the sedimentary layer between 0.0 and 0.2 m, with partially anaerobic mineralization of the sediments below 0.4 m. Increases in trace element concentrations were observed in iron reduction zones, indicating processes of desorption of oxides and hydroxides of Fe and mineralization of organic matter. The extrapolation of the results of the incubation test to the studied system allowed to establish three hypotheses related to the diagenetic processes: 1) the flow of marine currents may be allowing the aerobic oxidation in the sandy sediments, with the nitrification route more accentuated than the ammonification route; 2) in the region of the coastal zone and inner continental shelf the routes of oxidation and reduction may be alternating according to the physical-chemical factors and seasonality; 3) in the coastal zone and inner shelf the net mineralization rate exceeded the net production rate of the organic matter (ΔCO2T >0).

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Efficiency and adaptability of the benthic methane filter at Quepos Slide cold seeps, offshore of Costa Rica

Biogeosciences ◽

10.5194/bg-12-6687-2015 ◽

2015 ◽

Vol 12 (22) ◽

pp. 6687-6706 ◽

Cited By ~ 2

Author(s):

P. Steeb ◽

S. Krause ◽

P. Linke ◽

C. Hensen ◽

A. W. Dale ◽

...

Keyword(s):

Fluid Flow ◽

Costa Rica ◽

Sediment Cores ◽

Methane Flux ◽

Sediment Surface ◽

Anaerobic Oxidation Of Methane ◽

Cold Seeps ◽

Osa Peninsula ◽

Oxidation Of Methane

Abstract. Large amounts of methane are delivered by fluids through the erosive forearc of the convergent margin offshore of Costa Rica and lead to the formation of cold seeps at the sediment surface. Besides mud extrusion, numerous cold seeps are created by landslides induced by seamount subduction or fluid migration along major faults. Most of the dissolved methane migrating through the sediments of cold seeps is oxidized within the benthic microbial methane filter by anaerobic oxidation of methane (AOM). Measurements of AOM and sulfate reduction as well as numerical modeling of porewater profiles revealed a highly active and efficient benthic methane filter at the Quepos Slide site, a landslide on the continental slope between the Nicoya and Osa Peninsula. Integrated areal rates of AOM ranged from 12.9 ± 6.0 to 45.2 ± 11.5 mmol m−2 d−1, with only 1 to 2.5 % of the upward methane flux being released into the water column. Additionally, two parallel sediment cores from Quepos Slide were used for in vitro experiments in a recently developed sediment-flow-through (SLOT) system to simulate an increased fluid and methane flux from the bottom of the sediment core. The benthic methane filter revealed a high adaptability whereby the methane oxidation efficiency responded to the increased fluid flow within ca. 170 d. To our knowledge, this study provides the first estimation of the natural biogeochemical response of seep sediments to changes in fluid flow.

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Early Cretaceous glendonites of Arctic realm: distributions and their paleoclimate implication

10.5194/egusphere-egu21-5021 ◽

2021 ◽

Author(s):

Kseniya Mikhailova ◽

Victoria Ershova ◽

Mikhail Rogov

Keyword(s):

Organic Matter ◽

Early Cretaceous ◽

Isotope Composition ◽

Cold Climate ◽

The Arctic ◽

Anaerobic Oxidation Of Methane ◽

Anaerobic Oxidation ◽

Arctic Canada ◽

Oxidation Of Methane ◽

Pee Dee

In the middle of 20th century glendonites were purposed as an indicator of cold climate. There is no doubt that unique morphology and sizes of pseudomorphs occurring through Precambrian to Quaternary succession indicate uncommon geochemical environment. Here, we present an overview of Early Cretaceous glendonites distribution across Arctic which widely distributed here despite generally greenhouse climate conditions in Early Cretaceous.Late Berriasian pseudomorphs are known on northeastern Siberia and Arctic Canada. Valanginian glendonites are the widest ones are described from the Northern and Western Siberia, Spitsbergen and the Arctic Canada. Late Hauterivian concretions were studied on Svalbard. Barremian and lower Aptian glendonites are unknown in this area due to wide distributed continental succession, but late Barremian glendonites were reported from the wells drilled on the Barents Sea shelf. Middle and Upper Aptian glendonites are found on Svalbard, &#160;North Greenland, the Arctic Canada and North-East Russia. Lower Albian glendonites are found on Svalbard, islands of Arctic Canada and the Koryak Uplands.Nowadays it is reliable known that the precursor of glendonites is an ikaite - metastable calcium carbonate hexahydrate, forming in a narrow temperature range from 0-4oC, mainly in near-bottom conditions. Besides low temperature, high phosphate concentrations that occurs due to anaerobic oxidation of methane and/or organic matter; dissolved organic carbon, sulfates and amino acid may favor to ikaite formation as well. However, glendonites associated with terrigenious rocks, often including glacial deposits, that allow to use them as a paleoclimate indicator.Glendonites show a wide variability in form and size: from single crystal blades to stellate aggregates and rosettes, usually ranged from a few mm to dozens of cm. Mineralogical composition of pseudomorph is represented mainly by three calcite phases determining by CL-light. Both &#948;18O and &#948;13C of glendonites are characterized by a broad range of values. Oxygen isotope composition ranges from -14 to -0 &#8240; Vienna Pee Dee Belemnite (VPDB), whilst &#160;carbon isotope composition ranges from -52.4 to &#8211; 14 &#8240; Vienna Pee Dee Belemnite (VPDB).Based on received data we suggest that &#948;18O reflects the complex processes involved in ikaite-glendonite transformation, supposing mixing depleted fluids with seawater. Nevertheless, received data coincide with &#948;18O values reported from Paleozoic-Quaternary glendonites formed in near-freezing environments. Values of &#948;13C of glendonites is the result of both mixing seawater inorganic carbon and sedimentary organic diagenesis and close to bacterial sulfate reduction and/or anaerobic oxidation of methane or organic matter.To conclude,&#160; Early Cretaceous climate was warm generally, however studied pseudomorphs point to cold episodes in Late Berriasian, Valanginian, Late Hauterivian, Middle-Late Aptian and Early Albian.The study was supported by RFBR, project number 20-35-70012.

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Biogeochemical Evidence that Thermophilic Archaea Mediate the Anaerobic Oxidation of Methane

Applied and Environmental Microbiology ◽

10.1128/aem.69.3.1680-1686.2003 ◽

2003 ◽

Vol 69 (3) ◽

pp. 1680-1686 ◽

Cited By ~ 91

Author(s):

Stefan Schouten ◽

Stuart G. Wakeham ◽

Ellen C. Hopmans ◽

Jaap S. Sinninghe Damsté

Keyword(s):

Active Site ◽

Sediment Cores ◽

Anaerobic Oxidation Of Methane ◽

Cold Seeps ◽

Anaerobic Oxidation ◽

Membrane Composition ◽

Oxidation Of Methane ◽

Isotopic Analyses ◽

Tetraether Lipids ◽

Environmental Temperatures

ABSTRACT Distributions and isotopic analyses of lipids from sediment cores at a hydrothermally active site in the Guaymas Basin with a steep sedimentary temperature gradient revealed the presence of archaea that oxidize methane anaerobically. The presence of strongly 13C-depleted lipids at greater depths in the sediments suggests that microbes involved in anaerobic oxidation of methane are present and presumably active at environmental temperatures of >30°C, indicating that this process can occur not only at cold seeps but also at hydrothermal sites. The distribution of the membrane tetraether lipids of the methanotrophic archaea shows that these organisms have adapted their membrane composition to these high environmental temperatures.

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Vivianite formation in methane-rich deep-sea sediments from the South China Sea

10.5194/bg-2018-340 ◽

2018 ◽

Author(s):

Jiarui Liu ◽

Jiasheng Wang ◽

Gareth Izon ◽

Gilad Antler ◽

Zhou Wang ◽

...

Keyword(s):

South China Sea ◽

South China ◽

Iron Reduction ◽

Northern South China Sea ◽

Iron Phosphate ◽

Anaerobic Oxidation Of Methane ◽

Apatite Formation ◽

China Sea ◽

Reactive Iron ◽

Oxidation Of Methane

Abstract. Phosphorus is often invoked as the ultimate limiting nutrient, modulating primary productivity on geological timescales. Consequently, along with nitrogen, phosphorus bioavailability exerts a fundamental control on organic carbon production, linking all the biogeochemical cycles across the Earth system. Unlike nitrogen that can be microbially fixed from an essentially infinite atmospheric reservoir, phosphorus availability is dictated by the interplay between its sources and sinks. While authigenic apatite formation has received considerable attention as the dominant sedimentary phosphorus sink, the quantitative importance of reduced iron-phosphate minerals, such as vivianite, has only recently been acknowledged and their importance remains under-explored. Combining microscopic and spectroscopic analyses of handpicked mineral aggregates with sediment geochemical profiles we characterize the distribution and mineralogy of iron-phosphate minerals present in methane-rich sediments recovered from the northern South China Sea. Here, we demonstrate that vivianite authigenesis is pervasive in the iron oxide-rich sediments below the sulfate-methane transition zone (SMTZ). We hypothesize that the downward migration of the SMTZ concentrated vivianite formation below the current SMTZ. Our observations support recent findings from non-steady state post-glacial coastal sedimentary successions, suggesting that iron reduction below the SMTZ, probably driven by iron-mediated anaerobic oxidation of methane (Fe-AOM), is coupled to phosphorus cycling on a much greater spatial scale than previously assumed. Calculations reveal that vivianite acts as an important burial phase for both iron and phosphorus below the SMTZ, sequestering approximately half of the total reactive iron pool. By extension, sedimentary vivianite formation could serve as a mineralogical marker of Fe-AOM, signalling a low-sulfate availability against methanogenic and ferruginous backdrop. Given that similar conditions were likely present throughout vast swaths of Earth history, it is possible that Fe-AOM may have modulated phosphorus and methane availability on the early Earth.

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Different impacts of an electron shuttle on nitrate- and nitrite-dependent anaerobic oxidation of methane in paddy soil

Plant Soil and Environment ◽

10.17221/579/2020-pse ◽

2021 ◽

Vol 67 (No. 5) ◽

pp. 264-269

Author(s):

Yaohong Zhang ◽

Fangyuan Wang

Keyword(s):

Organic Matter ◽

Paddy Soil ◽

Anaerobic Oxidation Of Methane ◽

Electron Shuttle ◽

Anaerobic Oxidation ◽

Terminal Electron Acceptor ◽

Electron Shuttles ◽

Oxidation Of Methane ◽

Nitrate And Nitrite ◽

C Assimilation

Quinones, redox-active functional groups in soil organic matter, can act as electron shuttles for microbial anaerobic transformation. Here, we used 13CH4 to trace 13C conversion (13C-CO2 + 13C-SOC) to investigate the influence of an artificial electron shuttle (anthraquinone-2,6-disulfonate, AQDS) on denitrifying anaerobic methane oxidation (DAMO) in paddy soil. The results showed that AQDS could act as the terminal electron acceptor for the anaerobic oxidation of methane (AOM) in the paddy field. Moreover, AQDS significantly enhanced nitrate-dependent AOM rates and the amount of 13C-CH4 assimilation to soil organic carbon (SOC), whereas it was remarkably reduced nitrite-dependent AOM rates and 13C assimilation. Ultimately, AQDS notably increased the total DAMO rates and 13C assimilation to SOC. However, the electron shuttle did not change the percentage of 13C-SOC in total 13C-CH4 conversion. These results suggest that electron shuttles in the natural organic matter might be able to offset methane emission by facilitating AOM coupled with the denitrification process.

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Sedimentation rate and organic matter dynamics shape microbiomes across a continental margin

10.5194/bg-2021-25 ◽

2021 ◽

Author(s):

Sabyasachi Bhattacharya ◽

Tarunendu Mapder ◽

Svetlana Fernandes ◽

Chayan Roy ◽

Jagannath Sarkar ◽

...

Keyword(s):

Fatty Acids ◽

Organic Matter ◽

Organic Carbon ◽

Sedimentation Rate ◽

Bottom Water ◽

Sediment Cores ◽

Sediment Surface ◽

Geochemical Data ◽

Sediment Layer ◽

Shelf Slope

Abstract. Marine sedimentation rate and bottom-water O2 concentration control the remineralization/sequestration of organic carbon across continental margins; but whether/how they shape microbiome architecture (the ultimate effector of all biogeochemical phenomena), across shelf/slope sediments, is unknown. Here we reveal distinct microbiome structures and functions, amidst comparable pore fluid chemistries, along 300 cm sediment horizons underlying the seasonal (shallow coastal) and perennial (deep sea) oxygen minimum zones (OMZs) of the Arabian Sea, situated across the western-Indian margin (water-depths: 31 m and, 530 and 580 m, respectively). The sedimentary geomicrobiology was elucidated by analyzing metagenomes, metatranscriptomes, and enrichment cultures, and also sedimentation rates measured by radiocarbon and lead excess (210Pbxs); the findings were then evaluated in the light of the other geochemical data available for the cores investigated. Along the perennial- and seasonal-OMZ sediment cores, microbial communities were dominated by Gammaproteobacteria and Alphaproteobacteria, and Euryarchaeota and Firmicutes, respectively. As a perennial-OMZ signature, a cryptic methane production-consumption cycle was found to operate near the sediment-surface (within the sulfate reduction zone); overall diversity, as well as the relative abundances of simple-fatty-acids-requiring anaerobes (methanogens, anaerobic methane-oxidizers, sulfate-reducers and acetogens), peaked in the topmost sediment-layer and then declined via synchronized fluctuations until the sulfate-methane transition zone was reached. The entire microbiome profile was reverse in the seasonal-OMZ sediment horizon. In the perennial-OMZ sediments organic carbon deposited was higher in concentration and marine components-rich, so it potentially degraded readily to simple fatty acids; lower sedimentation rate afforded higher O2 exposure time for organic matter degradation despite perennial hypoxia in the bottom-water; thus, the resultant abundance of reduced carbon substrates sustained multiple inter-competing microbial processes in the upper sediment-layers. Remarkably, the whole geomicrobial scenario was opposite in the sediments of the seasonal/shallow-water OMZ. Our findings create a microbiological baseline for understanding carbon-sulfur cycling across distinct marine depositional settings and water-column oxygenation regimes.

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Biogeochemistry of a low-activity cold seep in the Larsen B area, western Weddell Sea, Antarctica

Biogeosciences Discussions ◽

10.5194/bgd-6-5741-2009 ◽

2009 ◽

Vol 6 (3) ◽

pp. 5741-5769 ◽

Cited By ~ 3

Author(s):

H. Niemann ◽

D. Fischer ◽

D. Graffe ◽

K. Knittel ◽

A. Montiel ◽

...

Keyword(s):

Surface Sediments ◽

Sediment Cores ◽

Weddell Sea ◽

Cold Seep ◽

Anaerobic Oxidation Of Methane ◽

Ice Shelf ◽

Sediment Depth ◽

Oxidation Of Methane ◽

Low Activity ◽

Thermogenic Methane

Abstract. 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, chemo syntheticclam 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 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 (Domack et al., 2005; EOS 86, 269–276).

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