The sulfur isotope ratio (δS) recorded in marine barite is commonly used to estimate secular changes in the sulfur isotopic composition of seawater sulfate (δS) throughout Earth's history. The Cenozoic record demonstrates a drastic 5‰ increase in seawater δS during the Early Eocene. However, the gradient of this excursion is based on only a few data points. Taking advantage of a much improved biostratigraphic framework, we redefine the Eocene δS data with a new high-resolution barite based δS record between 60 and 30 Ma. Our results show that the rise of δS starts about 3 million years later (~53 Ma) and lasts about 9 million years longer (until 38 Ma) than previously depicted. As such, the gradient of the δS curve is ~0.4‰/Myr. The radiogenic strontium isotopic ratio in barite suggests that some of the samples used for the original barite record are affected by diagenetic alteration.

We thank H. Li for her support with the S-isotope isotope analysis, and Dr. Guillaume Paris and one anonymous reviewer for their comments which greatly improved this manuscript. This research was supported the Discovery Grant of the Natural Sciences and Engineering Research Council of Canada (NSERC) to U.G.W., the National Science Foundation (NSF) CAREER grant (OCE-0449732) to A.P., and internal funds from Ohio State University to E.M.G. Project CGL2017- 92600-EXP is also acknowledged.