Three-billion-year Secular Evolution of Oxygen Isotopes in Seawater Inferred from Chert δ18O and Δ17O

The surface temperature of the Earth in the deep past has been a topic of debate for decades. Modeling of Solar output for a young sun indicates that Earth's surface should be cool and yet estimates of Earth surface temperatures have ranged from 45-80°C in the Archean. The primary basis for these estimates is the light oxygen isotope compositions of Archean marine carbonates and cherts. Early modeling efforts suggested that the oxygen isotope composition has remained within 1 to 2‰ of the modern composition as a result of buffering by balanced high and low temperature weathering fluxes. However, more recent work has suggested that these fluxes may change through Earth's history and much lighter δ¹⁸O values may be accessible. This study aims to constrain the δ'¹⁸O of seawater throughout Earth's history using the ¹⁸O-¹⁷O-¹⁶O composition of chert, a ubiquitous marine sediment. It is found that marine cherts older than 600Ma are incompatible with having last recrystallized in a water with a composition similar to that of modern seawater. The findings of this study suggest that seawater has remained isotopically unchanged over the Phanerozoic. It is estimated that the δ'¹⁸O of seawater was < -11.1‰ in the Paleoarchean. This δ'¹⁸O estimate indicates that the surface temperature during the Paleoarchean was no higher than 34°C.