The Earth's early hydrosphere

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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 hydrosphere1–4, 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 nebula6,7 and of planetary differentiation7, on the role of volatiles in impact cratering8–10, and on the composition11–22, isotope systematics23–25 and radiation climatology26–29 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 paradoxical1,3,17. 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.