Isotopic constraints on ocean redox at the end of the Eocene

A multi-million-year decrease in global temperatures during the Eocene was accompanied by large reorganisations to ocean circulation, ocean chemistry and biological productivity. These changes culminated in the rapid growth of grounded ice on Antarctica during the Eocene–Oligocene climate transition (EOT), ∼34 million years ago. However, while it is likely that environmental perturbations of this magnitude altered the oceanic oxygen inventory, the sign and magnitude of the response is poorly constrained. We show that euxinic, hydrographically restricted conditions developed in the Austrian Molasse Basin during the EOT. The isotopic compositions of molybdenum and uranium captured by sediments accumulating in the Molasse Basin at this time reveal that the global extent of sulfidic conditions during the EOT was not appreciably different to that of the Early Eocene greenhouse world. Our results suggest that the early Cenozoic oceans were buffered against extreme long-term changes in oxygenation.