Carbon and Sulfur Isotopes of Microfossils and Pyrites from Neoarchean Cherts: Microbial Diversity in an Ancient Deep-Marine Environment.

The 2.52 Ga Gamohaan Formation in South Africa offers critical insights into microbial ecosystems that inhabited deep-marine environments during the late Archean. Large spheroidal microfossils and pyrite grains preserved in finely laminated black cherts reveal evidence of diverse metabolic processes and a complex deep-marine sulfur cycle. This study combines optical microscopy, scanning electron microscopy, and Raman spectroscopy to characterize fossil kerogen and sedimentary pyrites, along with secondary ion mass spectrometry to analyze their in situ carbon (δ13C) and sulfur (δ34S) isotope compositions, respectively. Raman spectroscopy established the kerogenous composition and thermal maturity of the microfossils. In situ δ13Corg values (-41.3‰ to -32.2‰) are lower on average than bulk organic matter and define two statistically distinct δ13Corg populations, with large spheroidal fossils exhibiting systematically lower values than small spheroids. This size-correlated pattern is consistent with differences in carbon sources, fixation pathways, or ecological setting. Secondary ion mass spectrometry δ34S analyses of pyrite grains (-0.7‰ to +6.3‰) show variability corresponding to differences among pyrite morphologies, consistent with microbial sulfate reduction and further sulfur cycling under sulfate-limited conditions in this deep-marine environment. These isotope data provide an independent geochemical framework for evaluating sulfur-based chemotrophic interpretations of the microfossil assemblage. Collectively, the carbon and sulfur isotope signatures support the presence of a complex benthic sulfur cycle and highlight the ecological and metabolic complexity of offshore marine ecosystems in the Neoarchean.