Geological history of oxygen

Although oxygen is the most abundant element in Earth's crust, due to its high reactivity it mostly exists in compound (oxide) forms such as water, carbon dioxide, iron oxides and silicates. Before photosynthesis evolved, Earth's atmosphere had no free diatomic elemental oxygen (O₂).^[2] Small quantities of oxygen were released by geological^[3] and biological processes, but did not build up in the reducing atmosphere due to reactions with then-abundant reducing gases such as atmospheric methane and hydrogen sulfide and surface reductants such as ferrous iron.

Oxygen began building up in the prebiotic atmosphere at approximately 1.85 Ga during the Neoarchean-Paleoproterozoic boundary, a paleogeological event known as the Great Oxygenation Event (GOE). At current rates of primary production, today's concentration of oxygen could be produced by photosynthetic organisms in 2,000 years.^[4] In the absence of plants, the rate of oxygen production by photosynthesis was slower in the Precambrian, and the concentrations of O₂ attained were less than 10% of today's and probably fluctuated greatly.

The increase in oxygen concentrations had wide ranging and significant impacts on Earth's biosphere. Most significantly, the rise of oxygen and the oxidative depletion of greenhouse gases (especially atmospheric methane) due to the GOE led to an icehouse Earth that caused a mass extinction of anaerobic microbes, but paved the way for the evolution of eukaryotes and later the rise of complex lifeforms.