Stable isotope analysis of the Cretaceous sulfur cycle

We report the sulfur and oxygen isotope composition of sulfate (δ34SSO4 and δ18OSO4, respectively) in coexisting barite and carbonate-associated sulfate (CAS), which we use to explore temporal variability in the marine sulfur cycle through the middle Cretaceous. The δ34SSO4 of marine barite tracks previously reported sulfur isotope data from the tropical Pacific. The δ18OSO4 of marine barite exhibits more rapid and larger isotopic excursions than the δ34SSO4 of marine barite; these excursions temporally coincide with Ocean Anoxic Events (OAEs). Neither the δ34SSO4 nor the δ18OSO4 measured in marine barite resembles the δ34SSO4 or the δ18OSO4 measured in coexisting CAS. Culling our data set for elemental parameters suggestive of carbonate recrystallization (low [Sr] and high Mn/Sr) improves our record of δ18OSO4 in CAS in the Cretaceous. This suggests that the CAS proxy can be impacted by carbonate recrystallization in some marine sediments. A box model is used to explore the response of the δ34SSO4 and δ18OSO4 to different perturbations in the marine biogeochemical sulfur cycle. We conclude that the δ34SSO4 in the middle Cretaceous is likely responding to a change in the isotopic composition of pyrite being buried, coupled possibly with a change in riverine input. On the other hand, the δ18OSO4 is likely responding to rapid changes in the reoxidation pathway of sulfide, which we suggest may be due to anoxic versus euxinic conditions during different OAEs.