Biology – Quantitative Biology – Genomics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p54a..02e&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P54A-02
Biology
Quantitative Biology
Genomics
0424 Biosignatures And Proxies, 0428 Carbon Cycling (4806), 0454 Isotopic Composition And Chemistry (1041, 4870), 0465 Microbiology: Ecology, Physiology And Genomics (4840), 1055 Organic And Biogenic Geochemistry
Scientific paper
Molecular fossils are particularly valuable ancient biosignatures that can provide key insight about microbial sources and ecology in early Earth studies. In particular, hopanes carrying 2-methyl or 3-methyl substituents are proposed to be derived from cyanobacteria and oxygen-respiring methanotrophs, respectively, based on both their modern occurrences and their Proterozoic and Phanerozoic sedimentary distributions. Here we report the distribution of methylhopanes in 2.72-2.56 billion-year-old rocks from the Hamersley Province, Western Australia. The relative abundance of C31-3beta-methylhopane, but not that of C31-2alpha-methylhopane, shows a strong correlation to the stable carbon isotopic composition of co- occurring kerogen (insoluble particulate organic matter). This relationship was unanticipated. The correlation provides evidence for a shallow-water locus of carbon cycling through aerobic oxidation of methane and, coincidentally, a means to demonstrate biomarker syngenicity. 2alpha-Methylhopane is abundant in both shale and carbonate from shallow-water sediments. In contrast, 2alpha-methylhopane abundances are variable, but generally lower in deeper water facies. A cyanobacterial source is consistent with these observations. Cyanobacteria were likely key organisms of shallow-water microbial ecosystems providing molecular oxygen, fixed carbon, and possibly fixed nitrogen. This study demonstrates that distributions of organic molecules preserved in ancient sediments can be particularly revealing about organic sources and associated (bio)geochemical processes when closely tied to other geochemical and sedimentary observations. When these processes involve biology, the relationships uncovered by the matrix of sedimentary observations can potentially reveal ecological signatures, i.e. 'ecosignatures'. A similar approach may be appropriate for organic (bio)geochemical investigations of early Mars sedimentary records.
Eigenbrode Jennifer
Freeman Katherine H.
Summons Roger E.
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