SOURCES OF SILICA AND CARBON IN ORGANIC-RICH CHERTS FROM THE NEOARCHEAN GAMOHAAN FORMATION, SOUTH AFRICA

The 2.5-billion-year-old organic-rich cherts of the Neoarchean Gamohaan Formation of South Africa and the microbial mat texture and microfossils contained within are important because they formed just prior to the Great Oxygenation Event at ~2.4 billion years ago with variable oxidation environments, and are also potential analogs for extraterrestrial life. While oxygenation of the atmosphere was well underway by the time of deposition, the extent of oxygenation present at the time of deposition was still not equal to modern levels, giving an insight as to how both abiotic and biotic processes changed in response. The chemistry of various chert samples will be analyzed via x-ray fluorescence (XRF) spectroscopy as well as carbon isotope analysis. Samples will be chosen based on their position within the stratigraphic column, with samples higher up in the stratigraphy representing deeper water deposits compared to lower ones, which represent shallower water deposits. XRF analysis will be used to compare differences of major and trace element compositions between samples, the latter of which can represent differing sources of silica that formed the cherts. Discriminating between differing sources of silica could shed light on how silica was being deposited in the oceans during the Great Oxygeneation Event, possibly constraining how changes in oxygen effected the silica cycle. Carbon isotope analysis will be performed in order to constrain the source of carbon found. Differences in carbon isotope values could represent different metabolisms. While it is not possible to necessarily determine exact metabolic pathways, combining evidence of different metabolisms with possible differences in silica sources could be used to better characterize differences in the origin of microbial mat types. The observation of distinct changes in metabolic pathways could offer insight as to how microbial mats responded to the presence of oxygen.