The Continuing Puzzle of the Great Oxidation Event

The rise of atmospheric O[subscript 2] was a milestone in the history of life. Although O[subscript 2] itself is not a climate-active gas, its appearance would have removed a methane greenhouse present on the early Earth and potentially led to dramatic cooling. Moreover, by fundamentally altering the biogeochemical cycles of C, N, S and Fe, its rise first in the atmosphere and later in the oceans would also have had important indirect effects on Earth's climate. Here, we summarize major lines of evidence from the geological literature that pertain to when and how O[subscript 2] first appeared in significant amounts in the atmosphere. On the early Earth, atmospheric O[subscript 2] would initially have been very low, probably