Rapidly evolving ocean pH in the early Earth: Insights from global carbon cycle coupled with ocean chemistry

The ocean pH is a fundamental property regulating various aspects of Earth system evolution. However, the early ocean pH remains controversial, with estimates ranging from strongly acidic to alkaline. Here we develop a model integrating global carbon cycling with ocean geochemistry, and incorporating continental growth and mantle thermal evolution. By coupling global carbon cycle with ocean charge balance, and by using solid Earth processes of mantle degassing and crustal evolution to specify the history of volatile distribution and ocean chemistry, we show that a rapid increase in ocean pH is likely during the Hadean to the early Archean, with pH evolving from 5 to neutral by approximately 4.0 Ga. This rapid pH evolution is attributed primarily to elevated rates of both seafloor and continental weathering during the Hadean. This acceleration in weathering rates originates in the unique aspects of Hadean geodynamics, including rapid crust formation, different crustal lithology, and fast plate motion. Earth likely transformed from a hostile state to a habitable one by the end of the Hadean, approximately 4.0 Ga, with important implications for planetary habitability and the origin of life.