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

Abstract

The low ¹⁸O/¹⁶O stable isotope ratios (δ¹⁸O) 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 δ¹⁸O_sw of Archean seawater due to intense continental weathering and/or low degrees of hydrothermal alteration are inconsistent with the triple oxygen isotope composition (Δ'¹⁷O) of Precambrian cherts. We show that high CO₂ sequestration fluxes into the oceanic crust, associated with extensive silicification, lowered the δ¹⁸O_sw of seawater on the early Earth without affecting the Δ'¹⁷O. Hence, the controversial long-term trend of increasing δ¹⁸O in chemical sediments over Earth's history partly reflects increasing δ¹⁸O_sw due to decreasing atmospheric <i>p</i>CO₂ We suggest that δ¹⁸O_sw 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 <i>p</i>CO₂ that has been suggested for this time interval. Using the moderately low δ¹⁸O_sw values, a warm but not hot climate can be inferred from the δ¹⁸O 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-<i>p</i>CO₂ greenhouse atmosphere before 3 Ga.