Contrasting Sulfur Isotope and Iron Speciation Measurements across Paleoenvironmental Gradients during the Cenomanian-Turonian Oceanic Anoxic Event (OAE2)

The Cenomanian-Turonian Oceanic Anoxic Event (OAE2), which is characterized by widespread deposition of organic rich sediments and a positive carbon isotope excursion in both carbonate and organic carbon, represents a significant, albeit short-lived (<1 m.y.) perturbation to the exogenic carbon cycle. The biogeochemical cycles of carbon and sulfur are linked via bacterial sulfate reduction (BSR) such that enhanced production and delivery of organic carbon to marine bottom waters and sediments should manifest as an increase in BSR and associated iron sulfide (pyrite) formation. Here, we present sulfur isotope (sulfate and sulfide) and iron speciation measurements from two distinct depositional environments that exhibit contrasting depositional redox conditions during OAE2 in order to evaluate what affect this event had on the global sulfur cycle. Sediments from the Cretaceous Western Interior Seaway (WIS) of North America are characterized by relatively high Δ³⁴S values (defined as the sulfur isotope difference between seawater sulfate and simultaneously deposited sedimentary pyrite) with values ranging from 50 to 60‰ and lower highly reactive iron to total iron ratios (FeHR/FeT). By contrast, contemporaneously deposited sediments from Demerara Rise (ODP Leg 207, Site 1258) display smaller Δ³⁴S values (20 to 30‰) and elevated FeHR/FeT ratios. We hypothesize that reactive iron concentration gradients may have been driven by the “iron shuttle” where highly reactive iron is remobilized via redox reactions from the oxic shelf to euxinic environments where it is sequestered during pyrite formation. Furthermore, we attribute the contrasting Δ³⁴S values between the WIS and Demerara Rise to differences in the availability of reactive iron, the extent of bacterially-mediated sulfur disproportionation reactions, and pyrite burial efficiency.