Highly isotopically depleted isoprenoids: Molecular markers for ancient methane venting

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 C-20 isoprenoid 2,6,11,1 5-tetramethylhexadecane (crocetane), a C-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 C-13-depleted isotope compositions (delta(13)C approximate to -108 to -115.6 parts per thousand 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 (delta(13)C approximate to -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 H-2; and (2) SRB that consume the resulting H-2. 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. Copyright (C) 1999 Elsevier Science Ltd.