The barite record of the past seawater oxygen isotope composition

The oxygen isotope composition of seawater (δ¹⁸O_seawater) is shaped by high- and low-temperature rock-water interactions, reflecting Earth system's dynamics and evolution. The history of δ¹⁸O_seawater 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 δ¹⁸O_sulfate may reach a plateau or approach equilibrium with the δ¹⁸O_seawater as the corresponding δ³⁴S_sulfate increases during microbial sulfate reduction. Here we show 289 paired δ¹⁸O-δ³⁴S values for sedimentary barite spanning six periods of the Phanerozoic Eon. The δ¹⁸O-δ³⁴S trajectories point to variable equilibrium δ¹⁸O_barite values for different periods. A ~ 4‰ lower δ¹⁸O_seawater value is evident before the Carboniferous than today if assuming the same formation temperature. Utilizing the barite δ¹⁸O-δ³⁴S trajectory approach, we now have a robust proxy to advance the long-debated issue of seawater δ¹⁸O history.