Biogeochemical sulfur cycling during Cretaceous oceanic anoxic events: A comparison of OAE1a and OAE2

Biogeochemical sulfur cycling has varied widely over geologic time, mainly in response to changes in primary productivity and organic carbon burial, volcanism, weathering, and evaporite deposition. Several of these processes are explicitly linked to discreet (<1.2 Ma) intervals of widespread organic carbon burial, termed oceanic anoxic events (OAEs). During the Cretaceous, there is a highly distinctive ~4‰ negative excursion in the sulfur isotope composition of seawater sulfate (δ³⁴S_SO4) that is bracketed by the two most prominent OAEs (OAE1a and OAE2). This excursion lasted for ~25 Ma and has been variously attributed to enhanced volcanism, changes in weathering, evaporite burial, and/or changes in modes of organic carbon remineralization. We present new high-resolution carbon and sulfur isotope records from carbonate-associated sulfate and pyrite for OAE1a and OAE2. OAE1a is characterized by a monotonic decrease in δ³⁴S_SO4 values. Both negative and positive δ³⁴S_SO4 excursions are associated with OAE2. To refine hypotheses for the observed changes in biogeochemical sulfur cycling associated with these events, we use a simple sulfur isotope box model. Both empirical and modeling results indicate that δ³⁴S_SO4 variability was dominated by input fluxes during OAE1a, whereas enhanced volcanism, weathering, and pyrite burial controlled δ³⁴S_SO4 records during OAE2. Our analysis supports the conclusion that Cretaceous marine sulfate concentrations were much lower than modern concentrations and indicates that increases in marine sulfate occurred at the onset of both events. We conclude that increases in marine sulfate from low background concentrations, in conjunction with other environmental characteristics, contributed to the development of OAEs.