The 2520 Ma Gamohaan Formation, South Africa, contains complex microbial structures encased in calcite cements. These microbial structures show diverse morphological features demonstrating the segregation of two distinct microbial communities that affected calcite nucleation differently (Sumner, 1997, Palaios, v. 12, p. 300). Stable isotopic data presented here demonstrate that the microbial communities did not affect the isotopic composition of the calcite even though they affected the location of crystal nucleation.
The analyzed cements infill abundant primary voids in the microbial structures. The first generation of cement consists of herringbone calcite, a fibrous Mg-calcite marine cement. Microbial communities affected the location of herringbone calcite nucleation as shown by a correlation between microbial community morphology and the presence or absence of herringbone calcite. Herringbone calcite is followed by bladed and then blocky low Mg-calcite. In sample DK20, the final herringbone calcite contains concentrated organic inclusions at the tops of some voids. This organic matter is interpreted as early hydrocarbons from decay of associated organics. Following introduction of hydrocarbons, this sample was fractured slightly due to burial compaction. Blocky calcite precipitated after fracturing, filling both the voids and the fractures.
Oxygen and carbon isotopic analyses suggest that all of the cements precipitated from marine-like waters with little microbial influence. Delta 13C values range from -0.5 to +0.3%o PDB and show no systematic variations among herringbone, bladed, and blocky cements. Delta 18O values range from -8.7 to -6.2%o PDB. Herringbone calcite d18O values average about 1%o lighter than those of bladed and blocky calcite from the same sample. This variation probably is due to more extensive oxygen exchange between burial fluids and the fibrous herringbone calcite than with larger bladed and blocky crystals.
The absence of a trend in d13C in the cements suggests that neither metabolic processes nor organic decay affected d13C. Sample DK20 contains evidence for the production of mobile organic matter prior to blocky calcite precipitation. However, d13C values from the earliest through the latest generations of cement are well within the variability seen within each component (+/- 0.2%o). Thus, it appears that neither microbial metabolism nor early decay of organic carbon influenced the isotopic composition of the carbonates.
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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616