A CO 2 greenhouse efficiently warmed the early Earth and decreased seawater 18 O/ 16 O before the onset of plate tectonics

Significance Due to the lower luminosity of the young Sun, climate modelers struggle to explain why the climate on early Earth was not freezing cold. This “faint young Sun paradox” is in conflict with apparently hot Archean ocean temperatures (∼70 °C) that can be estimated from the 18 O/ 16 O stable isotope ratio of chemical sediments. We show that the later temperatures had been overestimated because the 18 O/ 16 O of seawater also changed over time due to intense carbonatization and silicification of the oceanic crust, which consumes heavy 18 O. Because these processes require high fluxes of CO 2 , greenhouse warming by a CO 2 -rich atmosphere appears most feasible to explain all observations.