Reconciling evidence of oxidative weathering and atmospheric anoxia on Archean Earth

Evidence continues to emerge for the production and low-level accumulation of molecular oxygen (O₂) at Earth’s surface before the Great Oxidation Event. Quantifying this early O₂ has proven difficult. Here, we use the distribution and isotopic composition of molybdenum in the ancient sedimentary record to quantify Archean Mo cycling, which allows us to calculate lower limits for atmospheric O₂ partial pressures (PO₂) and O₂ production fluxes during the Archean. We consider two end-member scenarios. First, if O₂ was evenly distributed throughout the atmosphere, then PO₂ > 10^–6.9 present atmospheric level was required for large periods of time during the Archean eon. Alternatively, if O₂ accumulation was instead spatially restricted (e.g., occurring only near the sites of O₂ production), then O₂ production fluxes >0.01 Tmol O₂/year were required. Archean O₂ levels were vanishingly low according to our calculations but substantially above those predicted for an abiotic Earth system.