We present new data of oxygen isotopes in marine sulfate (δ18OSO4) in pore fluid profiles through organic-rich deep-sea sediments from 11 ODP sites around the world. In almost all sites studied sulfate is depleted with depth, through both organic matter oxidation and anaerobic methane oxidation. The δ18OSO4 increases rapidly near the top of the sediments, from seawater values of 9 to maxima between 22 and 25, and remains isotopically heavy and constant at these values with depth. The δ18OSO4 in these pore fluid profiles is decoupled from variations in sulfur isotopes measured on the same sulfate samples (δ34SSO4); the δ34SSO4 increases continuously with depth and exhibits a shallower isotopic increase. This isotopic decoupling between the δ34SSO4 and the δ18OSO4 is hard to reconcile with the traditional understanding of bacterial sulfate reduction in sediments. Our data support the idea that sulfate or sulfite and water isotopically exchange during sulfate reduction and that some of the isotopically altered sulfur pool returns to the environment. We calculate that the rapid increase in the δ18OSO4 in the upper part of these sediments requires rates of this oxygen isotope exchange that are several orders of magnitude higher than the rates of net sulfate reduction calculated from the sulfate concentration profiles and supported by the δ34SSO4. We suggest several mechanisms by which this may occur, including ‘net-zero’ sulfur cycling, as well as further experiments through which we can test and resolve these processes.