Great Oxygenation Event

Timescale O2 build-up in the Earth's atmosphere. Red and green lines represent the range of the estimates while time is measured in billions of years ago (Ga).

Stage 1 (3.85–2.45 Ga): Practically no O2 in the atmosphere. The oceans were also largely anoxic—with the possible exception of O2 in the shallow oceans. Stage 2 (2.45–1.85 Ga): O2 produced, rising to values of 0.02 and 0.04 atm, but absorbed in oceans and seabed rock. (Great Oxidation Event) Stage 3 (1.85–0.85 Ga): O2 starts to gas out of the oceans, but is absorbed by land surfaces. No significant change in oxygen level. Stages 4 and 5 (0.85 Ga – present): Other O2 reservoirs filled; gas accumulates in atmosphere. Stage 4 is known as the neoproterozoic oxygenation event.