Modern-like seawater δ18O in the mesoproterozoic: Insights from triple oxygen isotopes of chert

Abstract

Oxygen isotope compositions in ancient seawater (δ¹⁸O_SW and Δ^’17O_SW) are key proxies for deciphering past climate and tectonic regimes. Despite various reconstruction attempts, a consensus on their history remains elusive. Chert, valued for its diagenetic stability, offers a promising archive, particularly through triple oxygen isotope analysis. However, Precambrian cherts frequently exhibit deviations in δ¹⁸O and Δ^’17O from triple oxygen isotope equilibrium, complicating interpretations. Here, we propose that this deviation results from the coprecipitation of oxygen-bearing minerals (detrital silicates, authigenic clays, and barite), each with distinct isotopic signatures. To test this hypothesis, we analyzed Mesoproterozoic chert samples from the Yangzhuang and Wumishan formations in North China. Our results show a significant correlation between chert δ¹⁸O and detrital silicate content, with coexisting oxygen-bearing minerals driving deviations in δ¹⁸O and Δ^’17O from equilibrium. The δ¹⁸O and Δ^’17O values of the purest, most pristine chert samples suggest that Mesoproterozoic seawater had a δ¹⁸O similar to that of modern seawater (i.e. ∼ -1 ‰, ice free world seawater), a conclusion further supported by our oxygen balance model. This study demonstrates that using pure chert or an oxygen balance model is essential for accurately reconstructing ancient δ¹⁸O_SW.