Earth's First Redox Revolution

Abstract

The rise of molecular oxygen (O 2 ) in the atmosphere and oceans was one of the most consequential changes in Earth's history. While most research focuses on the Great Oxidation Event (GOE) near the start of the Proterozoic Eon—after which O 2 became irreversibly greater than 0.1% of the atmosphere—many lines of evidence indicate a smaller oxygenation event before this time, at the end of the Archean Eon (2.5 billion years ago). Additional evidence of mild environmental oxidation—probably by O 2 —is found throughout the Archean. This emerging evidence suggests that the GOE might be best regarded as the climax of a broader First Redox Revolution (FRR) of the Earth system characterized by two or more earlier Archean Oxidation Events (AOEs). Understanding the timing and tempo of this revolution is key to unraveling the drivers of Earth's evolution as an inhabited world—and has implications for the search for life on worlds beyond our own. ▪ Many inorganic geochemical proxies suggest that biological O 2 production preceded Earth's GOE by perhaps more than 1 billion years. ▪ Early O 2 accumulation may have been dynamic, with at least two AOEs predating the GOE. If so, the GOE was the climax of an extended period of environmental redox instability. ▪ We should broaden our focus to examine and understand the entirety of Earth's FRR.