Chert oxygen isotope ratios are driven by Earth's thermal evolution.

The 18 O/ 16 O ratio of cherts (δ 18 O chert ) increases nearly monotonically by ~15‰ from the Archean to present. Two end-member explanations have emerged: cooling seawater temperature (T SW ) and increasing seawater δ 18 O (δ 18 O sw ). Yet despite decades of work, there is no consensus, leading some to view the δ 18 O chert record as pervasively altered. Here, we demonstrate that cherts are a robust archive of diagenetic temperatures, despite metamorphism and exposure to meteoric fluids, and show that the timing and temperature of quartz precipitation and thus δ 18 O chert are determined by the kinetics of silica diagenesis. A diagenetic model shows that δ 18 O chert is influenced by heat flow through the sediment column. Heat flow has decreased over time as planetary heat is dissipated, and reasonable Archean-modern heat flow changes account for ~5‰ of the increase in δ 18 O chert , obviating the need for extreme T SW or δ 18 O sw reconstructions. The seawater oxygen isotope budget is also influenced by solid Earth cooling, with a recent reconstruction placing Archean δ 18 O SW 5 to 10‰ lower than today. Together, this provides an internally consistent view of the δ 18 O chert record as driven by solid Earth cooling over billion-year timescales that is compatible with Precambrian glaciations and biological constraints and satisfyingly accounts for the monotonic nature of the δ 18 O chert trend.