EVIDENCE FOR RAPID RATES OF OXYGENIC PHOTOSYNTHESIS WITHIN NEOARCHEAN STROMATOLITES

The Great Oxidation Event (GOE) ~2.4 Ga altered the trajectory of Earth’s geobiology, but the origins of oxygenic photosynthesis remain unclear. Gas production in microbial mats produces bubble textures which are preserved within stromatolites as fenestral fabrics, but additional constraints are required to distinguish which metabolisms formed ancient fenestrae. This study examines pre-GOE microbial gas production within 2.7 Ga lacustrine stromatolites from the Hartbeesfontein Basin (Ventersdorp Supergroup, South Africa). Hartbeesfontein textures resemble modern stromatolites from Yellowstone National Park, where oxygen bubbles produced by cyanobacteria are preserved as fenestrae on a near-daily basis. Bubble lithification rates from Yellowstone were coupled with Hartbeesfontein fenestral measurements to estimate ancient rates of gas production. Calculated gas-production rates were then compared with modern in-situ mat studies to determine candidate metabolisms. Additionally, petrography, XRF, and Raman analysis were performed on iron oxides within fenestrae in both locations, which are specifically associated with oxygen production in Yellowstone. Oxygenic photosynthesis is considered the most likely metabolism to have formed Hartbeesfontein fenestrae, because 1) oxygen production rates best match calculated values for Archean mats; 2) iron oxide chemistry and petrography within fenestrae implies primary, local oxidation of reduced iron by oxygen within Archean mats, and 3) other gas-producing metabolisms (e.g., sulfate reduction, fermentation, methanogenesis) were likely limited in Archean environments due to lower sulfate and organic carbon concentrations. Abundant fenestrae within 2.7 Ga lacustrine stromatolites support the presence of oxygenic photosynthesis and localized oxygen oases 300 million years before the GOE, and supports previous hypotheses for non-marine origins of cyanobacteria and oxygen production.