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

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