OXYGEN ISOTOPES IN SULFATE ACROSS CRETACEOUS OCEAN ANOXIC EVENTS

We present initial results of the d¹⁸O_SO4 of carbonate associated sulfate (CAS) and trace barite for the late Albian, Cenomanian, and early Turonian from the pelagic limestones of the Scaglia Bianca formation in the Umbria-Marche Apennines of Italy. The oxygen isotopic composition of sulfate of CAS and trace barite show wide variability over the interval, with excursions of over 5‰. This may be further evidence of lower marine sulfate concentrations in the Cretaceous. While the d¹⁸O_SO4 of CAS and trace barite track each other over the section, the d¹⁸O_SO4of trace barite is, on average, 1 to 3‰ lighter. The sulfur and oxygen isotopic composition of sulfate in the Cretaceous is distinct from the Cenozoic; the d34S is on average lower and the d¹⁸O_SO4 is on average higher. Reevaluation of the sulfur cycle suggests that these differences may be explained through changes in the dynamics of sulfur cycling in warm shallow oceans consistent with conditions in the Cretaceous.

High-resolution isotopic and elemental measurements were made around the latest Cenomanian Bonarelli shale horizon (representative of global Oceanic Anoxic Event -2, OAE-2), which lies near the top of the section. Oxygen isotopes in sulfate are a sensitive indicator of changes in redox conditions in organic rich sediments because they track subtle changes in the sulfide reoxidation pathway. Around the Bonarelli, the d¹⁸O_SO4 rises by 6‰, and remains enriched for several meters above the shale. The rise precedes the shale by a few meters, indicating sulfate oxygen may highlight processes causal to the OAE.