Stable isotope analysis of the Cretaceous sulfur cycle

Abstract

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