WHERE SOUTH MEETS NORTH: A PALEOCEANOGRAPHIC FRONT IN THE WESTERN INTERIOR SEAWAY DURING OCEANIC ANOXIC EVENT 2

The Western Interior Seaway (WIS) was a large epicontinental seaway during the Cretaceous, stretching from the Tethyan proto-Gulf of Mexico to the Boreal Arctic. The vast size of the WIS provides a rich landscape to study the effects of global change on shallow water biota across latitude. A large positive carbon isotope excursion prior to the Cenomanian-Turonian boundary defines Ocean Anoxic Event 2 (OAE2), which triggered major biotic changes during the Greenhorn Cycle of transgression and regression. At times of transgression and highstand, the southern part of the WIS was occupied by calcareous foraminifera in warm, saline Tethyan waters, while the northernmost sections of the US WIS and the Canadian prairie provinces were dominated by agglutinated benthic foraminifera due to presence of cooler, less saline Boreal waters. In this study, we explored foraminiferal assemblages from Billings, MT to address how the paleoenvironment was impacted in the northern WIS before, during, and after OAE2.

Billings was in a unique position in the seaway that at times shared foraminiferal traits similar to the southern US WIS, and at other times with the northern Canadian WIS. We attribute the changes in foraminiferal assemblages to shifting positions of an oceanographic front between Tethyan and Boreal waters as sea level changed. Prior to OAE2, the front was near Billings as the region was dominated by agglutinated taxa characteristic of Boreal waters. At the onset of OAE2, a brief pulse of calcareous benthic and planktic foraminifera correlative with the widespread “Benthonic Zone”, a time of increased benthic diversity associated with bottom water ventilation with the rapid incursion of Tethyan waters. At the peak of OAE2, the oceanic front moved northward as agglutinated taxa were again abruptly replaced by calcareous taxa signaling an expansion of Tethyan waters with continued transgression. TheHeterohelixshift, Gavelinella acme, and subsequent change to Neobulimina-dominance are also recognized at Billings, but are delayed relative to sections further south. Canadian sites, meanwhile, contain abundant planktic foraminifera, but are virtually absent of benthic taxa, signaling the most northern edge of the oceanic front. As sea level fell, the front shifted southward, and agglutinated taxa returned to Billings.