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Putative fossils of chemotrophic microbes preserved in seep carbonates from Vestnesa Ridge, off northwest Svalbard, Norway

Authors

Himmler,  Tobias
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Crémière,  Antoine
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Birgel,  Daniel
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Wirth,  R.
3.5 Interface Geochemistry, 3.0 Geochemistry, Departments, GFZ Publication Database, Deutsches GeoForschungsZentrum;

Orphan,  Victoria J.
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Kirsimäe,  Kalle
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Knies,  Jochen
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Peckmann,  Jörn
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Lepland,  Aivo
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Citation

Himmler, T., Crémière, A., Birgel, D., Wirth, R., Orphan, V. J., Kirsimäe, K., Knies, J., Peckmann, J., Lepland, A. (2022): Putative fossils of chemotrophic microbes preserved in seep carbonates from Vestnesa Ridge, off northwest Svalbard, Norway. - Geology, 50, 2, 169-173.
https://doi.org/10.1130/G49620.1


Cite as: https://gfzpublic.gfz-potsdam.de/pubman/item/item_5009054
Abstract
The microbial key players at methane seeps are methanotrophic archaea and sulfate-reducing bacteria. They form spherical aggregates and jointly mediate the sulfate-dependent anaerobic oxidation of methane (SD–AOM: CH4 + SO42– → HCO3– + HS– + H2O), thereby inducing the precipitation of authigenic seep carbonates. While seep carbonates constitute valuable archives for molecular fossils of SD–AOM-mediating microbes, no microfossils have been identified as AOM aggregates to date. We report clustered spherical microstructures engulfed in 13C-depleted aragonite cement (δ13C values as low as –33‰) of Pleistocene seep carbonates. The clusters comprise Mg-calcite spheres between ~5 μm (single spheres) and ~30 μm (clusters) in diameter. Scanning and transmission electron microscopy revealed a porous nanocrystalline fabric in the core area of the spheres surrounded by one or two concentric layers of Mg-calcite crystals. In situ measured sphere δ13C values as low as –42‰ indicate that methane-derived carbon is the dominant carbon source. The size and concentric layering of the spheres resembles mineralized aggregates of natural anaerobic methanotrophic archaea (ANME) of the ANME-2 group surrounded by one or two layers of sulfate-reducing bacteria. Abundant carbonate-bound 13C-depleted lipid biomarkers of archaea and bacteria indicative of the ANME-2-Desulfosarcina/Desulfococcus consortium agree with SD–AOM-mediating microbes as critical agents of carbonate precipitation. Given the morphological resemblance, in concert with negative in situ δ13C values and abundant SD–AOM-diagnostic biomarkers, the clustered spheres likely represent fossils of SD–AOM-mediating microbes.