Highly isotopically depleted isoprenoids: molecular markers for ancient methane venting |
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| Institution: | 1. Shanghai Engineering Research Center of Hadal Science and Technology, College of Marine Sciences, Shanghai Ocean University, Shanghai 201306, China;2. Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266061, China;3. U.S. Geological Survey, Woods Hole Coastal & Marine Science Center, Woods Hole, MA 02543, USA;4. Institute for Geology, Center for Earth System Research and Sustainability, Universität Hamburg, 20146 Hamburg, Germany;5. Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Innovation Academy of South China Sea Ecology and Environmental Engineering, Chinese Academy of Sciences, Guangzhou 510301, China;6. Coastal Studies Institute, College of the Coastal and Environment, Louisiana State University, Baton Rouge, LA 70803, USA;1. Department of Geodynamics and Sedimentology, Universität Wien, 1090 Vienna, Austria;2. Institute for Geology, Center for Earth System Research and Sustainability, Universität Hamburg, 20146 Hamburg, Germany;3. Department of Geosciences and MARUM, Center for Marine and Environmental Sciences, 28359 Bremen, Germany;4. Institute of Earth Sciences, Karl-Franzens Universität Graz, 8010 Graz, Austria;5. Department of Geology, Lund University, 22362 Lund, Sweden;6. Archaea Biology and Ecogenomics Division, Department of Ecogenomics and Systems Biology, Universität Wien, 1090 Vienna, Austria;7. Akdeniz University, Department of Geological Engineering, Antalya, Turkey;1. LOG, UMR CNRS 8187, Université de Lille, Bât. SN5, 59655 Villeneuve d''Ascq cedex, France;2. GéoRessources, UMR 7359, CNRS, Université de Lorraine, BP 70239, F–54506 Vandoeuvre–lès–Nancy, France |
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| Abstract: | We propose that organic compounds found in a Miocene limestone from Marmorito (Northern Italy) are source markers for organic matter present in ancient methane vent systems (cold seeps). The limestone contains high concentrations of the tail-to-tail linked, acyclic C20 isoprenoid 2,6,11,15-tetramethylhexadecane (crocetane), a C25 homolog 2,6,10,15,19-pentamethylicosane (PME), and a distinctive glycerol ether lipid containing 3,7,11,15-tetramethylhexadecyl (phytanyl-) moieties. The chemical structures of these biomarkers indicate a common origin from archaea. Their extremely 13C-depleted isotope compositions (δ13C ≈ ?108 to ?115.6‰ PDB) suggest that the respective archaea have directly or indirectly introduced isotopically depleted, methane-derived carbon into their biomass. We postulate that a second major cluster of biomarkers showing heavier isotope values (δ13C ≈ ?88‰) is derived from sulfate-reducing bacteria (SRB). The observed biomarkers sustain the idea that methanogenic bacteria, in a syntrophic community with SRB, are responsible for the anaerobic oxidation of methane in marine sediments. Marmorito may thus represent a conceivable ancient scenario for methane consumption performed by a defined, two-membered bacterial consortium: (1) archaea that perform reversed methanogenesis by oxidizing methane and producing CO2 and H2; and (2) SRB that consume the resulting H2. Furthermore, the respective organic molecules are, unlike other compounds, tightly bound to the crystalline carbonate phase. The Marmorito carbonates can thus be regarded as “cold seep microbialites” rather than mere “authigenic” carbonates. |
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