Autotrophic HCO3- fixation may cause calcite precipitation in microbial mats, and cultured cyanobacteria can induce calcite precipitation during HCO3- uptake. However, documentation of calcification in marine mats due to photosynthesis has proven difficult. Also, the evolutionary timing of necessary CO2 concentrating mechanisms (CCM) for HCO3- uptake is unknown. Both are addressed in a study of fenestrate microbialites in the 2521±3 Ma Gamohaan Formation, South Africa, which consist of remnants of vertically oriented microbial supports and draping laminated mat encased in calcite cements. They contain carbon and oxygen isotopic evidence that support microbial communities fixed HCO3- and promoted calcite precipitation. Similar evidence is lacking for laminated mat.
d13C of calcite in the microbialites varies from -0.4 to +0.8%o (PDB). Carbon in calcite associated with supports is enriched up to 1%o relative to calcite that precipitated in voids as little as 250 microns away. In contrast, d13C values associated with laminated mat are identical to void-filling calcite. Enriched carbon is attributed to microbial fixation of CO2 or HCO3- during calcite precipitation due to preferential uptake of 12C species. d18O of calcite in the microbialites varies from -6.4 to -8.6%o (PDB), and d18O values associated with supports are enriched. They are interpreted as primary due to petrographic characteristics and the magnitude of variations over 250 microns (up to 1.7+/-0.1 %o). Enriched d18O values of calcite in the supports require oxygen isotopic disequilibrium between dissolved CO2 and HCO3- and the rapid reaction CO2 + OH- --> HCO3-. (In equilibrium, OH- is significantly enriched in 18O relative to CO2 and H20. For typical seawater pH of about 8, calcite precipitates from HCO3-, oxygen isotopic equilibration time is hours, and the CO2 + OH-reaction is substantial.)
To produce significant carbon and oxygen isotopic disequilibrium, the microbial communities must have rapidly fixed HCO3- to drive the conversion reaction. To preserve disequilibrium in calcite, precipitation must have been contemporaneous with HCO3- uptake. Thus, isotopic evidence suggests that the support microbial communities fixed HCO3- and metabolically induced calcite precipitation. HCO3- uptake implies that a CCM evolved prior to 2.5 Ga, which is significantly earlier than previously suggested.
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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616
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