Sulfur isotope ratios for carbonate-hosted sulfate show a rapid increase (+6 parts per thousand to +9 parts per thousand/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 hla). 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 mag have significantly increased the flux of oxygen to the atmosphere (9-27 x 10(18) mol/m.y.). Furthermore, removal of the large amount of iron associated,vith this process might have had a profound effect on iron cycle.