Probable Cold and Alkaline Surface Environment of the Hadean Earth Caused by Impact Ejecta Weathering

Abstract

Abstract Constraining the surface environment of the early Earth is essential for understanding the origin and evolution of life. The release of cations from silicate weathering depends on climatic temperature and pCO2, and such cations sequester CO2 into carbonate minerals in or on the seafloor, providing a stabilizing feedback on climate. Previous studies have suggested that this carbonate‐silicate cycle can keep the early Earth's surface temperature moderate by increasing pCO2 to compensate for the faint young Sun. However, the Hadean Earth experienced a high meteorite impactor flux, which produced ejecta that is easily weathered by carbonic acid. In this study, we estimated the histories of surface temperature and ocean pH during the Hadean and early Archean using a new model that includes the weathering of impact ejecta, empirically justified seafloor weathering, and ocean carbonate chemistry. We find that relatively low pCO2 and surface temperatures are probable during the Hadean, for example, at 4.3 Ga, log10(pCO2) (in bar) is −2.21−2.54+3.01 [2σ] and temperature is 259.2−14.4+84.1 [2σ] K. Such a low pCO2 would result in a circumneutral to basic pH of seawater, for example, 7.90−1.69+1.21 [2σ] at 4.3 Ga. A probably cold and alkaline marine environment is associated with a high impact flux. Hence, if there was an interval of an enhanced impact flux, that is, Late Heavy Bombardment, similar conditions may have existed in the early Archean. Therefore, if the origin of life occurred in the Hadean, life likely emerged in a cold global environment and probably spread into an alkaline ocean.