COLDER OR DRIER? THE LATE CENOMANIAN PLENUS EVENT IN NORTH AMERICA

The Plenus Cold Event is a geologically brief interval during the Cenomanian-Turonian Ocean Anoxic Event 2 that was recognized and defined in Europe based on positive excursions in d¹⁸O_carb and the appearance of Boreal faunas. The occurrence of this event is somewhat enigmatic given evidence that OAE2 was associated with a significant increase in LIP volcanism and significant climatic warming. Interestingly, the positive d¹⁸O_carb excursion was recognized by prior workers in the Western Interior, but is not associated with paleontological evidence for cooling in the seaway. In contrast, the shift occurs within an interval characterized by a significant northward migration of warm water Tethyan taxa. In this study we investigated the geochemical record further by measuring d¹⁸O_carb values in inocermid shell and bulk foraminiferal shell material at a number of locations in the basin. The results confirm the presence of the d¹⁸O_carb shift to positive values, as well as its association with faunal evidence for warmer, not cooler conditions. An alternate explanation for the isotopic data is an increase in salinity within the basin. It has long been recognized that the basin was a shallow epeiric sea with significant fresh water and terrigenous input, and recent Os isotopic measurements support this conclusion. Perhaps the best explanation of the d¹⁸O_carb data is a decrease in fresh water input related to drier climate conditions following the onset of OAE2. In this study, plant and soil biomarkers in addition to PAHs, which help constrain variations in terrigenous organic matter and paleofires linked to hydrology on land, will be used to support interpretations of the d¹⁸O_carb data.