Redox states of Archean surficial environments: The importance of H2,g instead of O2,g for weathering reactions

Abstract

Redox states of the Archean Eon have been constrained by various lines of evidence, including atmospheric, photochemical, and ecological models, mass-independent fractionations of sulfur isotopes, Fe-depletion of paleosols, and preservation of diagnostic detrital minerals. Although these lines of evidence present seemingly consistent upper limits on pO2,g, they are conceptually contradictory about the redox state of Archean surficial environments. Atmospheric, photochemical, and ecological modeling studies suggest weakly reducing environments under redox states represented by moderate H2,g levels. However, current interpretations of Fe-depletion in paleosols and the preservation of detrital minerals are based on low O2,g levels at which the reducing detrital minerals are thermodynamically unstable and survive because of slow kinetics of oxidative weathering.In this study, we show that under the redox state indicated by the Archean pH2,g range, Fe2+ and reducing Fe(II)-minerals are actually thermodynamically stable and have no tendency to be oxidized. We emphasize that pH2,g values are orders of magnitude higher than pO2,g in the Archean atmosphere and that H2,g and O2,g are not in equilibrium. The redox states of Archean surface environments behave as though they were controlled by the more abundant H2 instead of the very low O2, as in modern anoxic basins. Weathering in this case should have involved non-redox acidic dissolution of Fe(II)-species or reductive reaction of Fe(III)-species. Fe(II)-depleted paleosols and the preservation of relatively reduced detrital minerals are natural consequences of their thermodynamic stabilities in the Archean Eon's reducing environments rather than slow kinetics of oxidizing reactions. After the appearance of oxygenic photosynthesis, probably in the middle/late Archean, locally oxygenated environments could have existed, while the atmosphere as a whole remained anoxic. The profile of redox states on the Archean surface seems to be a reverse analogue to the modern Earth. Although oxidizing dissolution of transition metals could happen in O2-oases, quick reduction of oxyanions by abundant reductants, such as aqueous Fe2+, Fe(II)-minerals, and H2, might have restricted riverine transport of oxyanions and potentially complicate the interpretation of signals of O2-whiffs in marine sediments.