Full Citation: Kadoya, S., & Catling, D. C. (2019). Constraints On Hydrogen Levels In The Archean Atmosphere Based On Detrital Magnetite. Geochimica et Cosmochimica Acta, 262, 207219. https://doi.org/10.1016/j.gca.2019.07.041.
Abstract: The partial pressure of atmospheric hydrogen (pH2) on the early Earth is important because it has been proposed that high pH2 warmed the planet or allowed prebiotic chemistry in the early atmosphere. However, such hypotheses lack observational constraints on pH2 . Here, we use the existence of detrital magnetites in (? 3.0 Ga) Archean riverbeds to constrain pH2 . Under the condition of high pH2 , magnetite should disappear via reductive dissolution. We investigated the timescale for a magnetite particle in a river to dissolve, which depends on pH2 and pCO2 . Using published estimates of Archean pCO2 and assuming the presence of Fe(III)-reducing microbes, the survival timescale is ? 1 kyr when pH2 is ?10-2 bar , and decreases as pH2 increases. Considering that the residence time of a particle in a short river (< 1000 km) is ? 1 kyr , the existence of detrital magnetite particles in Archean riverbeds likely indicates that pH2 was below ?10-2 bar . Such a level would preclude H2 as a greenhouse gas or a strongly reducing Archean atmosphere. It is also consistent with limits imposed on H2 by consumption by methanogens because conversion to CH4 is thermodynamically favored. URL: https://doi.org/10.1016/j.gca.2019.07.041