Contributors: Laboratoire d’Océanologie et de Géosciences (LOG) - UMR 8187 (LOG); Institut national des sciences de l'Univers (INSU - CNRS)-Université du Littoral Côte d'Opale (ULCO)-Université de Lille-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD France-Nord ); Kochi Institute for Core Sample Research; Japan Agency for Marine-Earth Science and Technology (JAMSTEC); NASA Astrobiology Institute (NAI); Géosciences Montpellier; Institut national des sciences de l'Univers (INSU - CNRS)-Université de Montpellier (UM)-Centre National de la Recherche Scientifique (CNRS)-Université des Antilles (UA); Institut de Physique du Globe de Paris (IPGP (UMR_7154)); Institut national des sciences de l'Univers (INSU - CNRS)-Université de La Réunion (UR)-Institut de Physique du Globe de Paris (IPG Paris)-Centre National de la Recherche Scientifique (CNRS)-Université Paris Cité (UPCité); Biogéosciences UMR 6282 (BGS); Université de Bourgogne (UB)-Centre National de la Recherche Scientifique (CNRS); Institut de minéralogie, de physique des matériaux et de cosmochimie (IMPMC); Muséum national d'Histoire naturelle (MNHN)-Institut de recherche pour le développement IRD : UR206-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS); Funding provided by the NASA Astrobiology Institute (NNA13AA94A), the São Paulo Research Foundation (FAPESP grant 2015/16235-2), the National Science Foundation, Division of Earth Sciences (1053466, 1355590, 1658823) and the University of Wisconsin – Madison; ANR-10-BLAN-0602,eLIFE2,Environnements de la vie primitive terrestre - 2(2010); ANR-15-CE31-0003,M6fossils,Identification moléculaire, minéralogique, morphologique et isotopique des micro- et macrofossiles aux échelles micro et nano.(2015)
نبذة مختصرة : International audience ; The extreme 13C-depletions recorded in Neoarchean organic matter (OM) have commonly been interpreted as markers of methanotrophy. This methane oxidation metabolism could have been performed using oxidants such as dioxygen, sulfate, nitrite/nitrate, and/or Fe3+- and Mn4+-minerals. Acetogenesis using acetyl-CoA metabolism may produce similar 13C-depletions. We investigated δ13C and S/C values of OM in 2.72 Ga old lacustrine stromatolites of the Tumbiana Formation (Australia) using Secondary Ion Mass Spectrometry (SIMS), coupled with X-ray spectroscopy. Type-A OM is embedded in quartz and/or chlorite. We show that mixtures of chlorite bias negatively the δ13Corg of associated OM measured by SIMS, likely through recombination of hydrogen from chlorite with carbon from OM during analysis. Type-A OM, associated with quartz or interleaved with chlorites (but remote enough to avoid H-recombination), displays δ13Corg between −56.1 and −50.6‰ VPDB (mean: −53.5‰, SD ± 1.8‰). The intimate association of Type-A OM with Fe-rich chlorite and its main occurrence in pyritic layers coupled with 13C-depletions is most parsimoniously interpreted as anaerobic methanotrophy using Fe3+-minerals and/or oxidized S. Type-B OM comprises <2-µm organic globules embedded in calcite. A mass balance correction using the δ13C values measured on Type-B globules mixed with calcite and those measured on pure calcite yielded corrected δ13C∗org values for the globules ranging between −65.2 and −52.5‰ (mean −58.8‰, SD ± 3.6‰). In a context where Fe and S reduction could outcompete acetogenesis for a fraction of any available H2, these extremely low δ13Corg values are difficult to explain with chemoautotrophic acetogenesis. Atomic S/C ratios in Type-B globules reach up to 0.042, i.e. up to tenfold the values (0.003–0.004) in Type A-OM. These organic S concentrations, combined with the extremely low δ13C∗org values, support anaerobic oxidation of methane coupled to sulfur oxidation. Finally, Type-C OM, interpreted as ...
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