HR: 17:00h
AN: B54C-05    [Abstracts]
TI: Lipid Biomarkers Indicating Aerobic Methanotrophy at Ancient Marine Methane- Seeps
AU: * Birgel, D
EM: dbirgel@uni-bremen.de
AF: Bremen University, DFG-Forschungszentrum Ozeanraender, Bremen, 28334, Germany
AU: Peckmann, J
EM: peckmann@uni-bremen.de
AF: Bremen University, DFG-Forschungszentrum Ozeanraender, Bremen, 28334, Germany
AB: The inventory of lipid biomarkers of a number of ancient methane-seep limestones has been studied over the last decade. The molecular fingerprints of the chemosynthesis-based microbial communities tend to be extremely well-preserved in these limestones. The key process at seeps is the anaerobic oxidation of methane, performed by consortia of sulfate-reducing bacteria and methanotrophic archaea. Compounds preserved within modern and ancient seep settings comprise C-13-depleted lipid biomarkers. Besides the occurrence of C-13- depleted isoprenoids (archaea) and n-alkyl-chains (bacteria), C-13-depleted hopanoids have been reported in seep limestones. Here, lipid biomarker data are presented from three ancient methane-seep limestones embedded in Miocene and Campanian strata. These examples provide strong evidence that methane was not solely oxidized by an anaerobic process. In a Miocene limestone, 3-beta-methylated hopanoids were found (delta C-13: -100 per mil). Most likely, 3-beta-methylated hopanepolyols, prevailing in aerobic methanotrophs were the precursor lipids. In another Miocene limestone, a series of C-13-depleted 4-methylated steranes (lanostanes; -80 to -70 per mil) is derived from aerobic methanotrophs. Lanosterol is the most likely precursor of lanostanes, known to be produced by aerobic methanotrophs, some of which are outstanding among bacteria in having the capacity to produce steroids. In a Campanian seep limestone a suite of conspicuous secohexahydrobenzohopanes (-110 to -107 per mil) is found. These hopanoids probably represent early degradation products of seep-endemic aerobic methanotrophs. This interpretation is supported by the presence of "regular" hopanoids that can be discriminated from the unusual secohexahydrobenzohopanes by only moderately low delta C-13 values (-49 to -42 per mil). Structural and carbon isotope data reveal that aerobic methanotrophy is more common at ancient methane- seeps than previously noticed. Our data indicate that anaerobic and aerobic oxidation of methane at ancient seeps occurred in the same setting, probably in close proximity to each other.
DE: 0420 Biomolecular and chemical tracers
DE: 0424 Biosignatures and proxies
DE: 0428 Carbon cycling (4806)
DE: 0448 Geomicrobiology
DE: 0456 Life in extreme environments
SC: Biogeosciences [B]
MN: 2007 Fall Meeting