Stable isotope analysis of the Cretaceous sulfur cycle

We report the sulfur and oxygen isotope composition of sulfate (delta S-34(SO4) and delta S-18(SO4), 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 delta S-34(SO4) of marine barite tracks previously reported sulfur isotope data from the tropical Pacific. The delta S-18(SO4) of marine barite exhibits more rapid and larger isotopic excursions than the delta S-34(SO4) of marine barite; these excursions temporally coincide with Ocean Anoxic Events (OAEs). Neither the delta S-34(SO4) nor the delta S-18(SO4) measured in marine barite resembles the delta S-34(SO4) or the delta S-18(SO4) 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 delta S-18(SO4) 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 delta S-34(SO4) and delta S-18(SO4) to different perturbations in the marine biogeochemical sulfur cycle. We conclude that the delta S-34(SO4) 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 delta S-18(SO4) 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. (C) 2009 Elsevier B.V. All rights reserved.