Autotrophic Geochemical Signatures in 2.5 Billion-year-old Microbial Communities

Dawn Y. Sumner

Third Astrobiology Science Conference, NASA Ames, Abstracts (CD rom)

2.5 billion-year-old microbial communities, Campbellrand platform, South Africa, produced complex structures that varied with depositional environment1. In environments where calcite precipitation was rapid and sediment influx low, complex microbial structures with open, void-rich frameworks are preserved as organic inclusions in calcite cements. Cross cutting relationships demonstrate that specific communities enhanced local calcite precipitation; this calcite is 13C-enriched relative to neighboring calcite, which suggests that the communities were autotrophic. Autotrophic carbon isotopic signatures have been identified in fresh water carbonates2, but are rare in marine systems, possibly due to production of 12CO2 during sulfate reduction and organic carbon oxidation. Sulfate reduction also enhances calcite precipitation rates3. An equal or greater rate of calcite precipitation during sulfate reduction than peak autotrophic CO2 removal would mask autotrophic isotopic signatures. However, during late Archean time, sulfate concentrations may have been low, as is true of most fresh waters, allowing preservation of autotrophic carbon isotopic signatures in calcite.

It is unclear whether the autotrophic CO2 removal was due to photosynthesis or chemosynthesis. New stratigraphic correlations suggest that the microbial communities grew in water depths ranging from a few 10's of meters, where photosynthesis is reasonable, to at least 100 m, where the photon flux may have been too low to support substantial photosynthetic CO2 fixation. Isotopic signatures have not been studied along depth gradients to date. Calcite precipitation rate decreased with depth, and deeper microbial structures are more commonly compacted. Thus, it may be difficult to find deep structures containing calcite that precipitated contemporaneously with microbial activity and likely to preserve evidence of metabolic processes.

1Sumner, DY, 1997, Palaios, 12:300-316; 2000, in Microbial Sediments (eds. Riding, RE., Awramik, SM.; Springer Verlag), 307-314.
2Guo, L, Andrews, J, Riding, R, Dennis, P, Dresser, Q, 1996, J. Sedimentary Research, 66:468-472.
3Visscher, PT, Reid, RP, Bebout, BM, 2000, Geology, 28:919-922.

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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616