Highly isotopically depleted isoprenoids: molecular markers for ancient methane venting

摘要: Abstract We propose that organic compounds found in a Miocene limestone from Marmorito (Northern Italy) are source markers for matter present ancient methane vent systems (cold seeps). The contains high concentrations of the tail-to-tail linked, acyclic C20 isoprenoid 2,6,11,15-tetramethylhexadecane (crocetane), C25 homolog 2,6,10,15,19-pentamethylicosane (PME), and distinctive glycerol ether lipid containing 3,7,11,15-tetramethylhexadecyl (phytanyl-) moieties. chemical structures these biomarkers indicate common origin archaea. Their extremely 13C-depleted isotope compositions (δ13C ≈ −108 to −115.6‰ PDB) suggest respective archaea have directly or indirectly introduced isotopically depleted, methane-derived carbon into their biomass. postulate second major cluster showing heavier values −88‰) is derived sulfate-reducing bacteria (SRB). observed sustain idea methanogenic bacteria, syntrophic community with SRB, responsible anaerobic oxidation marine sediments. may thus represent conceivable scenario consumption performed by defined, two-membered bacterial consortium: (1) perform reversed methanogenesis oxidizing producing CO2 H2; (2) SRB consume resulting H2. Furthermore, molecules are, unlike other compounds, tightly bound crystalline carbonate phase. carbonates can be regarded as “cold seep microbialites” rather than mere “authigenic” carbonates.