Why the atmosphere became oxygenated: A proposal

The increase in the oxidation state of the atmosphere during Earth history is well documented, but not the reasons for this process. The course of atmospheric evolution has been dominated by the effects of photosynthesis and by the composition of volcanic gases. The H2/H2O ratio of volcanic gases has remained essentially constant during most of Earth history. It is here proposed that their CO2/H2O and SO2/H2O ratios have increased with time. A simple model for the degassing and the recycling of CO2, SO2, and H2O can account for many of the major steps in the oxygenation of the atmosphere.