Availability of O2 and H2O2 on Pre-Photosynthetic Earth

Old arguments that free O-2 must have been available at Earth's surface prior to the origin of photosynthesis have been revived by a new study that shows that aerobic respiration can occur at dissolved oxygen concentrations much lower than had previously been thought, perhaps as low as 0.05 nM, which corresponds to a partial pressure for O-2 of about 4 x 10(-8) bar. We used numerical models to study whether such O-2 concentrations might have been provided by atmospheric photochemistry. Results show that disproportionation of H2O2 near the surface might have yielded enough O-2 to satisfy this constraint. Alternatively, poleward transport of O-2 from the equatorial stratosphere into the polar night region, followed by downward transport in the polar vortex, may have brought O-2 directly to the surface. Thus, our calculations indicate that this ``early respiration'' hypothesis might be physically reasonable.