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

The 18O/16O ratio of cherts (δ18Ochert) increases monotonically by ca. 15 ‰ from the Archean to present. As oxygen isotope fractionation in the silica-water system is temperature-dependent, this trend has often been interpreted as long-term climatic cooling, with a hot Archean ocean at >70°C. Alternative interpretations invoke a low- δ18O Archean ocean or pervasive alteration of the Precambrian chert δ18O record.

There is now increasing recognition that δ18Ochert reflects the temperature and fluid δ18O at the endpoint of the silica diagenetic pathway, in which amorphous opal-A is transformed via dissolution-reprecipitation reactions to microcrystalline quartz via an intermediate opal-CT phase. Here, we investigate how silica diagenesis may have varied over geological time and how this would have impacted the δ18Ochert record. We introduce a 1D silica diagenesis model that tracks the transformation of silica polymorphs in the sediment column during burial heating, revealing the importance of reaction kinetics in controlling the depth, temperature and oxygen isotope fractionations associated with chert formation. Our results show that δ18Ochert is a poor archive of seawater temperatures. Instead, δ18Ochert is sensitive to changes in heat flow through ocean sediments, such that a large fraction of the Archean-Cenozoic difference in δ18Ochert can be explained by reasonable changes in heat flow over Earth history. This removes the need for extremes in either seawater temperature or δ18O, pointing to a temperate to warm Archean with only slightly 18O-depleted oceans, and satisfying all geochemical, palaeoclimatological and biological constraints.