A CO2 greenhouse efficiently warmed the early Earth and decreased seawater O-18/O-16 before the onset of plate tectonics

The low O-18/O-16 stable isotope ratios (delta O-18) of ancient chemical sediments imply similar to 70 degrees C Archean oceans if the oxygen isotopic composition of seawater (sw) was similar to modern values. Models suggesting lower delta O-18(sw) of Archean seawater due to intense continental weathering and/or low degrees of hydrothermal alteration are inconsistent with the triple oxygen isotope composition (Delta'O-17) of Precambrian cherts. We show that high CO2 sequestration fluxes into the oceanic crust, associated with extensive silicification, lowered the delta O-18(sw) of seawater on the early Earth without affecting the Delta'O-17. Hence, the controversial long-term trend of increasing delta O-18 in chemical sediments over Earth's history partly reflects increasing delta O-18(sw) due to decreasing atmospheric pCO(2). We suggest that delta O-18(sw) increased from about -5 parts per thousand at 3.2 Ga to a new steady-state value close to -2 parts per thousand at 2.6 Ga, coinciding with a profound drop in pCO(2) that has been suggested for this time interval. Using the moderately low delta O-18(sw) values, a warm but not hot climate can be inferred from the delta O-18 of the most pristine chemical sediments. Our results are most consistent with a model in which the faint young Sun was efficiently counterbalanced by a high-pCO(2) greenhouse atmosphere before 3 Ga