A Theory of Archean Oxygen Stability

English Abstract:

Though much attention has been given to the Great Oxidation Event of ~2.4 Ga, there is still no mechanistic model for what controls the steady-state levels of atmospheric O₂ in the late Archean or the early Proterozoic. Here we discuss the major controls on pO₂ during various stages of the Archean, using a simple model that describes the biogeochemical of oxygen, hydrogen, carbon, iron, sulfur and phosphorus in the ocean and atmosphere. In the prebiotic era, the redox balance was dominated by the input of reduced species derived from the mantle and from water-crust interactions, compensated by the escape of hydrogen to space. Based on a simple energy balance model of the thermosphere, we find the escape rate may have been slower than the diffusion-limited rate often assumed, leading to elevated hydrogen concentrations. The appearance of chemoautotrophic life altered this balance by adding an additional sink for hydrogen, drawing down the concentration of H₂-bearing species in the atmosphere. In our model the drop in hydrogen concentrations can be as large as several orders of magnitude, with a similarly large increase in pO₂ due to slowed reaction rates between atmospheric reductants and photochemically-derived oxygen. Such large changes in the composition of the atmosphere may have implications for detecting the origin of life, at least as a geochemically significant force, in the rock record. Our model results show that an atmosphere with pO₂ of 10^-10 PAL would have been stable even after the evolution of oxygenic photosynthesis, provided that the rates of outgassing and serpentinization were sufficiently large in the Archean relative to the organic carbon burial, which was likely limited by alternative sinks for phosphorous.