Coupled evolution of Earth's H2-rich proto-atmosphere and oxidized magma ocean: Efficient H2O production through dissolved H2 transport and atmospheric buffering of the magma ocean redox state

Abstract

Giant impacts during Earth's final accretion likely generated global magma oceans, where the disproportionation of Fe^2+ at lower-mantle pressures and segregation of the resulting metallic iron into the core could have oxidized the magma ocean. Earth may nevertheless have had an H2-rich proto-atmosphere in chemical disequilibrium with the magma ocean, but the chemical coupling between this atmosphere and an oxidized magma ocean remains poorly constrained. Here we develop a coupled atmosphere-interior evolution model that includes diffusive transport in the boundary layer between the magma ocean and atmosphere, redox reactions in the melt, partitioning of oxide components and H2O among coexisting phases, and planetary cooling through a proto-atmosphere, with metallic iron produced by Fe^2+ disproportionation removed into the core as it forms. Atmospheric H2 transport into the magma ocean and subsequent H2O production become more efficient with increasingly frequent boundary layer renewal driven by buoyant overturn of dense melt cooled at the surface, allowing the atmosphere and magma ocean to chemically equilibrate within ~10^4 yr when the magma ocean is sufficiently deep and hot to extend into the lower mantle. Redox exchange with the atmosphere buffers the solidification-driven redox evolution of the magma ocean, preventing extreme oxidation or reduction of the coupled system. These results suggest that the coupled evolution could have contributed to early Earth's water inventory while maintaining a reducing H2-rich atmosphere.