The Earth's early hydrosphere

The date at which the Earth's hydrosphere came into existence is unknown. Although all current geological models for the very early Precambrian predict or suppose a hydrosphere^1-4, there are difficulties in reconciling this with geological and astronomical theories of planetary formation. Furthermore the effect of a substantial hydrosphere on the Precambrian climatic environment is difficult to determine. Recent developments make it possible to synthesize a picture of this environment during the 700 Myr preceding the deposition of the oldest known sediments⁵. We attempt such a synthesis here, drawing together work on the dynamics of accretion from the proto-planetary nebula ^6,7 and of planetary differentiation⁷, on the role of volatiles in impact cratering^8-10, and on the composition ^11-22, isotope systematics^23-25 and radiation climatology^26-29 of the early hydrosphere. We can reconcile these diverse contributions if the hydrosphere - The inventory of excess volatiles¹ - Is allowed to differentiate during accretion, with most of the H₂O and CO₂ going rapidly into oceans and sediments. Both gradual outgassing³⁰ and a massive CO₂ atmosphere² are unlikely, and a reduced mantle beneath a neutral hydrosphere is not paradoxical^1,3,17. Results from both a one-dimensional radiative-convective (1D RC) model and a general circulation model suggest that self-regulating mechanisms are important in the climate of this early hydrosphere.