Contrasting Sulfur Isotope and Iron Speciation Measurements Across Paleoenvironmental Gradients During the Cenomanian-Turonian Oceanic Anoxic Event (OAE2).
Derek D. Adams, Matthew T. Hurtgen, Bradley B. Sageman, and Jason A. Flaum
Department of Earth and Planetary Sciences, Northwestern University, Evanston, IL 60028
The Cenomanian-Turonian oceanic anoxic event is characterized by widespread deposition of organic rich sediments and a positive carbon isotope excursion in both carbonate and organic carbon. The biogeochemical cycles of carbon and sulfur are linked via bacterial sulfate reduction such that enhanced production and delivery of organic carbon to marine bottom waters and sediments should manifest as an increase in sulfate reduction and associated iron sulfide (pyrite) formation, assuming reactive iron is available. Here, we present sulfur isotope (sulfate and sulfide) and iron speciation measurements from two distinct depositional environments that exhibit contrasting depositional redox conditions during Oceanic Anoxic Event 2 in order to evaluate what affect this event had on the biogeochemical cycles of carbon, sulfur, and iron.
Sediments from the Cretaceous Western Interior Seaway of North America are characterized by high Δ34S (defined as the sulfur isotope difference between seawater sulfate and simultaneously deposited sedimentary pyrite) with values ranging from 50-60‰ and low Degree of Pyritization. By contrast, contemporaneously deposited sediments from Demerara Rise (ODP Leg 207, Site 1258) display smaller Δ34S values (30-50‰) and elevated Degree of Pyritization. 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 two localities to differences in the availability of reactive iron, the extent of bacterially-mediated sulfur disproportionation reactions, and pyrite burial efficiency.
AAPG Search and Discover Article #90087 © 2008 AAPG/SEG Student Expo, Houston, Texas