Earth's first redox revolution

The rise of molecular oxygen (Obinf2einf) 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 Eonmdashafter which Obinf2einf became irreversibly greater than 0.1% of the atmospheremdashmany 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 oxidationmdashprobably by Obinf2einfmdashis 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 worldmdashand has implications for the search for life on worlds beyond our own. squfensp Many inorganic geochemical proxies suggest that biological Obinf2einf production preceded Earth's GOE by perhaps more than 1 billion years. squfensp Early Obinf2einf 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. squfensp We should broaden our focus to examine and understand the entirety of Earth's FRR.