MICROFOSSILS OF THE 2.5-BILLION-YEAR-OLD GAMOHAAN FORMATION OF SOUTH AFRICA AND INTERPRETATION OF THEIR DEPOSITIONAL ENVIRONMENT AND METABOLIC PATHWAYS

Here, is reported the presence of ~15–20 µm-wide filamentous structures, interpreted as probable microfossils, in outcrop and drill core samples collected from the 2.5-billion-year-old Tsineng Member of the Gamohaan Formation in the Kaapvaal Craton of South Africa. A limited number of similar structures were previously identified from a drill core sample of the same unit and were interpreted as fossil cyanobacteria based on their morphology. Specimens for this study were located using transmitted light microscopy and their biologic morphology was confirmed using confocal laser scanning microscopy. Additionally, the kerogenous nature of the suspect microfossils was studied using Raman spectroscopy/imagery.

The identified microfossils are now interpreted to have been deposited in a deep water environment, below the photic zone, based on a recent sedimentological re-interpretation of the geologic unit. This interpretation precludes that of a phototrophic origin for the fossil microorganisms as was previously thought. Morphological comparisons suggest these structures are possibly fossils of sulfur oxidizing bacteria. This hypothesis will be tested by means of additional geochemical and isotopic analyses of the microfossils. These data could have implications for our understanding of the evolutionary history of microbial metabolisms and ocean-atmosphere cycling of biologically important elements, particularly oxygen and sulfur.