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1.  Availability of O2 and H2O2 on Pre-Photosynthetic Earth 
Astrobiology  2011;11(4):293-302.
Old arguments that free O2 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 O2 of about 4 × 10−8 bar. We used numerical models to study whether such O2 concentrations might have been provided by atmospheric photochemistry. Results show that disproportionation of H2O2 near the surface might have yielded enough O2 to satisfy this constraint. Alternatively, poleward transport of O2 from the equatorial stratosphere into the polar night region, followed by downward transport in the polar vortex, may have brought O2 directly to the surface. Thus, our calculations indicate that this “early respiration” hypothesis might be physically reasonable. Key Words: Early Earth—Oxygen—Respiration—Tracer transport—General circulation. Astrobiology 11, 293–302.
PMCID: PMC3097080  PMID: 21545266

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