![]() 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. 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 ( ~ 10 − 8 ) and oxic ( ~ 10 − 4 ) mixing ratios. Results also suggest that O 2 between ~ 10 − 8 and ~ 10 − 4 mixing ratio is unstable to plausible atmospheric perturbations. When forced with step-wise changes in biogenic fluxes, transitions between anoxic and oxic atmospheres take between only 10 ² and 10 ⁵ y. 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. 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. 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.
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