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

Science  03 Aug 2001:
Vol. 293, Issue 5531, pp. 839-843
DOI: 10.1126/science.1061976

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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.

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