BIOGENIC METHANE, HYDROGEN ESCAPE, AND THE IRREVERSIBLE OXIDATION OF EARLY EARTH

The redox history of the Earth is poorly understood. For Mars and Venus, escape of hydrogen to space is widely believed to have led to their present highly oxidized states. Hydrogen escape has usually been ignored for Archean Earth, because water vapor is cold-trapped at the tropopause to a few ppmv, rendering hydrogen escape negligible. However, biogenic methane is not cold trapped. Methane’s expected abundance in the Archean, given low oxygen levels, would have been ~100-1000 times present, resulting in significant hydrogen escape and irreversible oxidation.

 

After accounting for back-reaction of photosynthetic organic matter with O₂, the net result of photosynthesis and methanogenesis is CO₂+2H₂O ® CH₄+2O₂. In the Archean, the kinetic fates of O₂ and CH₄ would be reversed compared to today. Inert CH₄ would have a residence time ~10⁴ years, whereas reactive O₂ would be lost in ~days to reduced metamorphic/volcanic gases, and oceanic cations like Fe²⁺. Thus before the rise of atmospheric O₂ on early Earth, biogenic methane is widely postulated to have been the greenhouse gas that countered 20-30% lower solar luminosity [e.g., Pavlov et al. (2000) JGR v105, 11981-11990; Rye et al (1995) Nature 378, 603-605].

 

Calculated Archean CH₄ levels would be ~200-3000 ppmv, if the biogenic source were 0.1-1 times present, producing hydrogen escape to space orders of magnitude faster than today. Such hydrogen escape is coupled to photosynthesis and oxidizes the Earth. Photosynthesis splits water into O₂ and hydrogen, hydrogen is transferred to CH₄ by methanogenesis, and hydrogen escape from the upper atmosphere after CH₄ photolysis results in irreversible gain of oxygen (CO₂+2H₂O ® CH₄+2O₂ ® CO₂ + O₂ +4H(­)). Irreversible oxidation  is significant (~10¹²-10¹³ mol O₂ yr^-1) and would result in cumulative oxidation of the Earth’s crust and associated metamorphic gases. We suggest that this process should not be neglected in attempting to understand the rise of O₂.