MULTI ISOTOPE STUDY OF THE CENOMANIAN- TURONIAN OCEAN ANOXIC EVENT 2 OF THE EAGLE FORD FORMATION OF WEST TEXAS

Ocean anoxic events (OAEs) are periods in the geologic past where the Earth’s global ocean becomes depleted in oxygen. These events resulted in strong biogeochemical changes within the ocean basins and are amongst the largest climatic perturbations in the Phanerozoic. The OAEs resulted in increased ocean productivity, although the mechanisms that sustained such long periods of anoxia (~100- 900 kyr) remain poorly understood. Using a careful dissolution to access the carbonate fractions of core samples from the Late Cretaceous Eagle Ford Group and Buda Limestone of West Texas. This study through the measurement of 87Sr/86Sr, δ88/86Sr, δ44/40Ca and δ53Cr focused on the Cenomanian – Turonian Ocean Anoxic event (OAE 2), one of the most documented and studied OAEs, to understand the biogeochemical mechanisms that were governing the global ocean during this period of anoxia. Results show that seawater 87Sr/86Sr ratios change synchronously with Os isotopes for the onset of massive LIP volcanism, and 60 kyr before the positive C isotope excursion that traditionally marks the onset of OAE 2. Using an ocean Sr box model, it is found that increasing the continental weathering Sr flux by ~1.8-times captures the change in seawater 87Sr/86Sr recorded in the Iona-1 core. The increase in the continental weathering flux is smaller than the threefold increase suggested by studies of seawater Ca and Li isotope changes during OAE 2, suggesting that hydrothermal forcing may have played a larger role in the development of ocean anoxic events than previously considered. The increasing role of hydrothermal forcing resulted in a negative carbonate δ53Cr profile through OAE 2 of the Iona-1 core. Importantly, the decrease in the δ53Cr occurs ~70 kyr after the drop in initial 187Os/188Os that marks the onset of LIP activity, continuing to highlight the significance of hydrothermal activity during this time. The δ44/40Ca on the other hand, shows a positive excursion through OAE 2, punctuated by intervals of high values interpreted to have resulted from syn-depositional diagenesis as a result of nutrient upwelling, thereby highlighting hydrothermal interaction. Overall, the results show that increased LIP activity played a significant role during OAE 2.