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 ¹⁸O/¹⁶O stable isotope ratio of chemical sediments. We show that the later temperatures had been overestimated because the ¹⁸O/¹⁶O of seawater also changed over time due to intense carbonatization and silicification of the oceanic crust, which consumes heavy ¹⁸O. Because these processes require high fluxes of CO₂, greenhouse warming by a CO₂-rich atmosphere appears most feasible to explain all observations.

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 pCO₂. 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 pCO₂ 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-pCO₂ greenhouse atmosphere before 3 Ga.