The timing of early evolution and life's chemical influence on Earth can be constrained by characterizing the interactions between microbial communities and mineral precipitation. For example, microbial communities can influence carbonate precipitation through changes in microenvironmental chemistry or mineral nucleation rates. d13C signatures in calcite that precipitated contemporaneously with microbial growth can record metabolic influences on microchemistry. If the communities fixed abundant CO2, the calcite should have a heavier d13C due to preferential biological uptake of 12C. In contrast, if substantial concentrations of CO2 were derived from microbial respiration, calcite precipitating with the microbial communities may have low d13C. However, if only a small fraction of total CO2 was involved in metabolism, an isotopic shift would not be expected, and it is less likely that the microbial communities affected calcite precipitation by changing carbonate chemistry.
Complex microbial structures from the 2520 Ma Gamohaan Formation, South Africa, contain two microbial communities that affected carbonate precipitation differently. Morphological characterization of these microbialites documented that the timing of calcite precipitation was different on vertically oriented microbial communities (supports) than on draping laminated mat; the earliest generation of fibrous calcite is concentrated along the supports. This relationship suggests that either the supports enhanced calcite precipitation or the laminated mat inhibited precipitation. Preliminary d13C analyses suggest the presence of microbially produced isotopic shifts. Calcite encasing well developed supports appears to be 13C-enriched relative to calcite hundreds to thousands of microns away, implying abundant CO2 fixation by the supports. Calcite associated with laminated mat has yet to show an isotopic shift relative to calcite lacking organic inclusions. These relationships may imply that the supports metabolically promoted calcite precipitation, whereas the laminated mat did not. An alternative explanation is that organic decay affected the isotopic values of carbonate farther from the supports. Additional analyses at strategic locations will distinguish between these two interpretations.
These results are the first step towards evaluating the metabolism and behavior of very complex microbial communities that evolved early in Earth's history. By constraining the metabolic processes in Late Archean microbial communities, firmer constraints on early evolution can be established. In addition, by characterizing the conditions under which microbial communities influence the geochemistry of contemporaneous minerals, better approaches to searching for evidence of life in extraterrestrial samples can be developed.
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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616