Abstract
The low (18)O/(16)O stable isotope ratios (δ(18)O) 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 δ(18)O(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'(17)O) of Precambrian cherts. We show that high CO(2) sequestration fluxes into the oceanic crust, associated with extensive silicification, lowered the δ(18)O(sw) of seawater on the early Earth without affecting the Δ'(17)O. Hence, the controversial long-term trend of increasing δ(18)O in chemical sediments over Earth's history partly reflects increasing δ(18)O(sw) due to decreasing atmospheric pCO(2) We suggest that δ(18)O(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 pCO(2) that has been suggested for this time interval. Using the moderately low δ(18)O(sw) values, a warm but not hot climate can be inferred from the δ(18)O 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.