What caused the rise of atmospheric O2?

- A self-consistent redox budget for the atmosphere-ocean system is presented.
- Four different hypotheses for the rise of atmospheric O2 are analyzed.
- The evolution of cyanobacteria was, of course, important.
- Continued continental growth was also important.
- Changes in crustal composition, from more to less mafic, were important as well.

Oxygenic photosynthesis appears to have evolved well before O2 levels increased in the atmosphere, at around 2.4 Ga. This has led to numerous suggestions as to what may have kept O2 suppressed and then eventually allowed it to rise. These suggestions include changes in the recycling of carbon and sulfur relative to water (or hydrogen), a switch from dominantly submarine to dominantly subaerial volcanism, gradual oxidation of the continents and a concomitant decrease in reduced metamorphic gases, a decline in deposition of banded iron-formations, a decline in nickel availability, and various proposals to increase the efficiency of photosynthesis. Several of these different mechanisms could have contributed to the rise of O2, although not all of them are equally effective. To be considered successful, any proposed mechanism must make predictions that are consistent with the carbon isotope record in marine carbonates, which shows relatively little change with time, apart from transient (but occasionally spectacular) excursions. The reasons for this constancy are explored here, but are not fully resolved. In the process of making these comparisons, a self-consistent redox balance framework is developed which will hopefully prove useful to others who may work on this problem and to astronomers who may one day try to decipher spectral signatures of oxygen on Earth-like exoplanets.