Rapid timescale for an oxic transition during the Great Oxidation Event and the instability of low atmospheric O 2 .

The Great Oxidation Event (GOE), arguably the most important event to occur on Earth since the origin of life, marks the time when an oxygen-rich atmosphere first appeared. However, it is not known whether the change was abrupt and permanent or fitful and drawn out over tens or hundreds of millions of years. Here, we developed a one-dimensional time-dependent photochemical model to resolve time-dependent behavior of the chemically unstable transitional atmosphere as it responded to changes in biogenic forcing. When forced with step-wise changes in biogenic fluxes, transitions between anoxic and oxic atmospheres take between only 10 2 and 10 5 y. Results also suggest that O 2 between [Formula: see text] and [Formula: see text] mixing ratio is unstable to plausible atmospheric perturbations. For example, when atmospheres with these O 2 concentrations experience fractional variations in the surface CH 4 flux comparable to those caused by modern Milankovich cycling, oxygen fluctuates between anoxic ([Formula: see text]) and oxic ([Formula: see text]) mixing ratios. Overall, our simulations are consistent with possible geologic evidence of unstable atmospheric O 2 , after initial oxygenation, which could occasionally collapse from changes in biospheric or volcanic fluxes. Additionally, modeling favors mid-Proterozoic O 2 exceeding [Formula: see text] to [Formula: see text] mixing ratio; otherwise, O 2 would periodically fall below [Formula: see text] mixing ratio, which would be inconsistent with post-GOE absence of sulfur isotope mass-independent fractionation.