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Title: Highly isotopically depleted isoprenoids: Molecular markers for ancient methane venting

The authors 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 C{sub 20} isoprenoid 2,6,11,15-tetramethylhexadecane (crocetane), a C{sub 25} 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 {sup 13}C-depleted isotope compositions ({delta}{sup 13}C {approximately} {minus}108 to {minus}115.6% PDB) suggest that the respective archaea have directly or indirectly introduced isotopically depleted, methane-derived carbon into their biomass. The authors postulate that a second major cluster of biomarkers showing heavier isotope values ({delta}{sup 13}C {approximately} {minus}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 CO{sub 2} and H{sub 2}; and (2) SRB that consume the resulting H{sub 2}. Furthermore, the respective organic moleculesmore » 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 antigenic carbonates.« less
Authors:
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Publication Date:
OSTI Identifier:
20006267
Resource Type:
Journal Article
Resource Relation:
Journal Name: Geochimica et Cosmochimica Acta; Journal Volume: 63; Journal Issue: 23-24; Other Information: PBD: Dec 1999
Research Org:
Univ. Hamburg (DE)
Country of Publication:
United States
Language:
English
Subject:
03 NATURAL GAS; 54 ENVIRONMENTAL SCIENCES; NATURAL GAS DEPOSITS; METHANE; SEEPS; ITALY; GEOCHEMISTRY; ORGANIC COMPOUNDS; BIOLOGICAL MARKERS; SEDIMENTS; DEGASSING