6.7 - The Great Oxidation Event Transition

Abstract

The Great Oxidation Event (GOE) was an increase in atmospheric oxygen levels from less than 1ppm to 0.2–2% by volume 2.4–2.2 billion years ago. In atmospheric chemistry, hydrogen-bearing reduced gases, such as methane and hydrogen, are adversaries of O2. When the concentration of hydrogen-bearing reduced gases goes up, O2 declines, and vice versa. Thus, the pre-GOE atmospheric redox chemistry should have been dominated by methane and hydrogen. Before the GOE, oxygen was driven to trace levels by reactions with volcanic and metamorphic reductants, including dissolved cations (e.g., Fe2+) in surface waters and reducing gases (H2, CH4, CO, SO2, and H2S). Rapid escape of hydrogen to space from such an atmosphere would have slowly oxidized the Earth. A ‘tipping point’ was reached when the flux of O2 associated with the burial of organic carbon exceeded O2 losses. Oxidative weathering then became significant. Models suggest that methane level fell before the GOE and such loss of greenhouse gases plausibly caused global cooling. Multiple glaciations during 2.45–2.22Ga hint that the climate and atmospheric composition oscillated until permanent oxygenation was established. Subsequent levels of O2 were sufficient to protect the Earth's surface from harmful ultraviolet with a stratospheric ozone layer.