NEOARCHEAN MICROORGANISMS TOOK ADVANTAGE OF A SUB-WAVEBASE O2 GRADIENT APPROXIMATELY 100-200 MILLION YEARS PRIOR TO THE GOE

The Great Oxygenation Event (GOE) is arguably history’s most consequential example of the co-evolution of Earth and life. However, knowledge of the life forms that existed prior to and during the GOE is limited. Here we present the results of optical microscopic, petrographic, Raman spectroscopic, and confocal laser scanning microscopic analyses of Neoarchean microfossils preserved in a sub-wavebase environment approximately 100-200 million years before atmospheric oxygen reached 10^-5 of the present atmospheric level. The microfossils are small (10 to 15 µm) spheroids, three-dimensionally preserved in early diagenetic chert, composed of kerogen, and feature frequent intracellular microcrystals. We observe the microfossils in thin section in tightly packed layers situated tens to hundreds of microns above organic-rich layers containing ambient mineral inclusion trails that terminate with microcrystalline pyrite grains. Previous workers have suggested that similar ambient inclusion trails in early diagenetic chert indicate the presence of hydrogen sulfide produced by organotrophic sulfate reducing microorganisms that lived in the sediment prior to silicification. We suggest that the fossilized microorganisms situated in layers tens to hundreds of microns above organic material that is being broken down by sulfate reducing microorganisms could have been behaving similarly to extant microaerophilic sulfur oxidizing microorganisms, which form veil-like structures at the oxic-sulfidic interface above decaying organic matter. This interpretation would indicate that deep-sea marine microorganisms evolved metabolic strategies to take advantage of microoxic conditions early in the history of Earth’s oxygenation.