The barite record of the past seawater oxygen isotope composition.

The oxygen isotope composition of seawater (δO) is shaped by high- and low-temperature rock-water interactions, reflecting Earth system's dynamics and evolution. The history of δO remains debated due partly to post-depositional imprints to all current mineral proxies. The oxygen atoms in sulfate minerals are among the most inaccessible to later exchange but often not in isotope equilibrium with ambient water. However, the δO may reach a plateau or approach equilibrium with the δO as the corresponding δS increases during microbial sulfate reduction. Here we show 289 paired δO-δS values for sedimentary barite spanning six periods of the Phanerozoic Eon. The δO-δS trajectories point to variable equilibrium δO values for different periods. A ~ 4‰ lower δO value is evident before the Carboniferous than today if assuming the same formation temperature. Utilizing the barite δO-δS trajectory approach, we now have a robust proxy to advance the long-debated issue of seawater δO history.