Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.b54b..05d&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #B54B-05
Other
0424 Biosignatures And Proxies, 0450 Hydrothermal Systems (1034, 3017, 3616, 4832, 8135, 8424), 5215 Origin Of Life, 9330 Australia, 9619 Precambrian
Scientific paper
Robust detection of carbonaceous biosignatures is essential in the search for life elsewhere in the universe. However, the biogenicity of ancient kerogen-like residues on our own planet is suspect in some cases, such as microbe-like features within 3.5 Ga Apex cherts, potentially the oldest evidence of life on Earth, and stromatolite-like features within the slightly younger Strelley Pool Chert. We analyzed carbonaceous material in situ within these cherts using TEM, EELS, XANES, and SIMS. Samples were compared with Gunflint kerogen, graphite, amorphous carbon, and two abiotic Fischer-Tropsch-type (FTT) carbons. The two Archean samples are similar to Gunflint kerogen. Carbonaceous material is localized along quartz grain boundaries and at triple junctions, and when mechanically separated from chert particles it appears as long strings or crisscrossing networks. EELS spectra indicate a complex, kerogen-like substance distinct from both amorphous carbon and graphite, containing significant domains of polyaromatic carbon. The higher energy resolution of corresponding XANES spectra allows the resolution of a spectral peak consistent with carbonyl functional groups and a minor peak attributable to phenol groups. Minor concentrations of N, S, and P are detected with SIMS. These results strongly suggest that the ancient carbonaceous materials preserved within the Apex and Strelley Pool Cherts are indeed kerogen, and that the respective microbe-like and stromatolite- like features are also biogenic. FTT carbon is visually and spectrally distinct from ancient kerogen-like matter, but contains some similar characteristics. For example, EELS and XANES spectra of FTT material are similar except for much larger concentrations of carbonyl. We present one possible model for geologic processing of FTT products within an ancient hydrothermal vent. Hydrophobic FTT molecules may aggregate and precipitate on vent walls, protected by precipitated chert. During diagenesis, FTT material would lose volatile groups and condense to form polyaromatic domains, significantly reducing the intensity of the original carbonyl peak. Afterwards, the structure and bonding of this mature FTT material may be virtually identical to kerogen. This model depends on several assumptions: (1) the original FTT products are complex enough to withstand typical abiotic degradation processes during diagenesis; (2) N, S, and P can be easily incorporated into FTT products; (3) carbonyl groups on hydrocarbon chains can be efficiently transformed into hydroxyl groups on polyaromatic domains during condensation.
de Gregorio Bradley T.
Flynn George James
Sharp Thomas G.
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