Biogenic Methane, Hydrogen Escape, and the Irreversible Oxidation of Early Earth

Abstract

The low O₂ content of the Archean atmosphere implies that methane should have been present at levels ∼10² to 10³ parts per million volume (ppmv) (compared with 1.7 ppmv today) given a plausible biogenic source. CH₄ is favored as the greenhouse gas that countered the lower luminosity of the early Sun. But abundant CH₄ implies that hydrogen escapes to space (↑space) orders of magnitude faster than today. Such reductant loss oxidizes the Earth. Photosynthesis splits water into O₂ and H, and methanogenesis transfers the H into CH₄. Hydrogen escape after CH₄ photolysis, therefore, causes a net gain of oxygen [CO₂ + 2H₂O → CH₄ + 2O₂ → CO₂ + O₂ + 4H(↑space)]. Expected irreversible oxidation (∼10¹² to 10¹³ moles oxygen per year) may help explain how Earth's surface environment became irreversibly oxidized.