Availability of O ₂ and H ₂O ₂ 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 ×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 H ₂O ₂ 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.