Sulfur isotope ratios for carbonate-hosted sulfate show a rapid increase (+6‰ to +9‰/m.y.) across the Livello Bonarelli black shale (Marche-Umbrian Apennines of Italy), which was deposited at the Cenomanian-Turonian boundary in the middle Cretaceous (93.5 Ma). The increase suggests that rates of pyrite burial increased substantially over the event, resulting in a decrease of 9%–27% in the oceanic reservoir of SO42−. The enhanced pyrite burial may have significantly increased the flux of oxygen to the atmosphere (9–27 × 1018 mol/m.y.). Furthermore, removal of the large amount of iron associated with this process might have had a profound effect on iron cycle.