Fossil microorganisms from the approximately 2800 to 2500 million-year-old Bulawayan stromatolite: Application of ultramicrochemical analyses

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Microfossils, probably representing members of Precambrian photosynthetic communities of bacteria and blue-green algae, have been found in the approximately 2800-2500 million-year-old Bulawayan stromatolites from Rhodesia. Several populations of coccoid and elongate microfossils have been observed in the dark, carbon-rich stromatolite laminae. Some of these elongate forms are morphologically similar to modern bacterial spores. These microfossils were studied in petrographic thin sections and identified by combined scanning electron microscopy-electron microprobe and by analyses of energy dispersive spectra of individual microfossils. The microfossils contain 1-20% organic carbon; some morphotypes contain traces of sulfur and one other, traces of phosphorus. The polymeric nature of the organic carbon was established by analyzing aggregates of microfossils at elevated temperatures in the solid inlet system of an organic mass spectrometer. The coccoid microfossils range in size from 1.2 to 4.3 μm, the elongate microfossils are from 2.4 to 9.8 μm. They are mineralized with dolomite, embedded in a calcite matrix, and are shown to be both indigenous and syngenous with the rock. Identical microfossils also containing organic carbon but mineralized with quartz have been observed in the stromatolites from Belingwe which are part of the Bulawayan Group from Rhodesia. Caution must be used in the interpretation of what these forms are because of their great age and relatively simple morphologies. However, based on morphology and chemical analyses, they represent fossilized bacteria, blue-green algae, or, most likely, both.

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