WAITING FOR O2: THE ROLE OF HYDROGEN ESCAPE IN THE OXIDATION OF EARTH

Oxygenic photosynthesis appears to be a necessary condition for the creation of an oxygen-rich atmosphere like Earth's. But available geological and geochemical evidence suggest that at least 200 Myr, and possibly as much as 800 Myr, elapsed between the advent of oxygenic photosynthesis and the establishment of an oxygen atmosphere. The interregnum implies that at least one other necessary condition for O2 needed to be met. Here we argue that the second condition was the oxidation of the surface and crust to the point where free O2 became more stable than competing reduced gases such as CH4, and that the cause of earth's surface oxidation is the same cause as it is for other planets with oxidized surfaces: hydrogen escape to space. The duration of the interregnum was determined by the rate of hydrogen escape and by the size of the reduced reservoir that needed to be oxidized before O2 became favored.