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

Abstract

The ¹⁸O/¹⁶O ratio of cherts (δ¹⁸O_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 δ¹⁸O (δ¹⁸O_sw). Yet despite decades of work, there is no consensus, leading some to view the δ¹⁸O_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 δ¹⁸O_chert are determined by the kinetics of silica diagenesis. A diagenetic model shows that δ¹⁸O_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 δ¹⁸O_chert, obviating the need for extreme T_SW or δ¹⁸O_sw reconstructions. The seawater oxygen isotope budget is also influenced by solid Earth cooling, with a recent reconstruction placing Archean δ¹⁸O_SW 5 to 10‰ lower than today. Together, this provides an internally consistent view of the δ¹⁸O_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 δ¹⁸O_chert trend.