Fig. 1. Evidence for the GOE.

Multiple lines of geologic and geochemical evidence support the view that oxygen gas first became a permanent component of Earth’s atmosphere and surface ocean ca. 2.4 billion years ago. Sedimentary iron formation (a), which requires transport of ferrous iron through the ocean, is abundant in successions that predate the GOE but uncommon afterward (c, with resurgences around 1900–1850 and 715–660 Ma). Similarly, redox-sensitive minerals such as pyrite (FeS₂) occur in detrital facies before the GOE (b) but not afterward. In contrast, red beds (d) and sulfate salts (e), which bespeak O₂ in surface environments, have the opposite time distribution, gaining prominence only after the GOE. It is estimated that atmospheric pO₂ increased from <10⁻⁵ to 1–10% of PAL at this time (c). (Data on iron formations in (c) are taken from Bekker et al.⁴. The blue shaded region denoting atmospheric O₂ levels is only notional, as it is possible that atmospheric pO₂ dropped below 1% of PAL during the Proterozoic^1,2.).