Revisiting the greatness of Earth's great oxidation

The accumulation of free oxygen in the atmosphere ≈2.45 billion years ago was one of the most transformative events in Earth's past. Since its identification, this interval termed the 'Great Oxidation Event' (GOE), has garnered a large amount of attention from a wide array of perspectives, with some suggesting it should define its own geologic era or period. Despite many new tools to interrogate the GOE, today the defendable range of possible atmospheric O₂ levels span orders of magnitude, begging the question, how great was Earth's Great Oxidation? The consequences of such disparate views on oxygenation levels are uncertainties regarding biospheric evolution, interpretations of the sedimentary record, and a limited ability to translate Earth's atmospheric history into insights for exoplanet research. In this review, we revisit the conditions immediately before, during, and after Earth's GOE to explore the key assumptions that underlie differing views on this critical interval of time. We then highlight new discoveries and outline extremely divergent but defendable interpretations of atmospheric oxygen trajectories across the GOE. Reducing such divergent scenarios should be a major target of research progress in the coming years.