COMPARATIVE SULFUR ISOTOPE STUDY OF THE MID CENOMANIAN EVENT AND THE CENOMANIAN-TURONIAN OAE2

The Mid-Cenomanian Event (MCE1) is a term applied to a positive carbon isotope (δ¹³C) excursion that precedes the larger magnitude Cenomanian-Turonian Ocean Anoxic Event 2 (OAE2) δ¹³C excursion. The MCE1 is characterized by black shale deposition in some, but not all localities, suggesting at least partially anoxic bottom waters. Given their relative stratigraphic proximity, the MCE1 and OAE2 offer the opportunity for comparative geochemical analysis. Previous work from the U.S. Western Interior Seaway (WIS) has shown that a positive excursion in the stable isotope composition of carbonate-associated sulfate (CAS) coincides with the positive δ¹³C shift at the onset of OAE2. This is interpreted to reflect a global increase in pyrite burial as the net area of anoxic bottom waters expanded. At the same time, δ³⁴S values of pyrite from the WIS shift negatively, then follow the CAS trend to isotopically heavier values. Samples from the MCE1, lower in the studied WIS section, were analyzed for the δ³⁴S composition of pyrite, but lacked sufficient carbonate to allow CAS analysis. The data showed a significant positive shift in δ³⁴S_pyr across the MCE1, suggesting the possibility that a positive shift in sulfate, due to increased pyrite burial, drove the pyrite signal. Such an interpretation would suggest that OAE2 and the MCE1 had similar dynamics, but differed only in magnitude. The purpose of this study is to develop a δ³⁴S record from CAS for the MCE1 in order to test the hypothesis.

Carbonate-rich facies spanning the MCE interval in the Folkestone outcrop of Southern England were analyzed for δ³⁴S_CAS. Preliminary results from the Folkestone carbonates show the largest positive δ¹³C_carb excursion coincides with a negative shift (~-3‰) in δ³⁴S_CAS. Additionally, some extremely negative excursions (<10‰) precede the MCE interval, however, these are likely the result of pyrite oxidation during the CAS extraction process. The principal negative excursion in δ³⁴S_CAS associated with the MCE1 C-isotope shift may be consistent with increased organic matter burial, and a change in the rate of microbial sulfate reduction resulting in a decrease in S-isotope fractionation. This interpretation suggests that the MCE1 was a biogeochemical event similar to OAE2, but it did not exceed a critical threshold to become an OAE.