Planktic foraminiferal porosity: a water mass proxy for latest Cenomanian paleoceanography, Greenhorn Sea, Western Interior USA and Canada

Planktic foraminiferal porosity analyses can be used as a water mass proxy and were conducted on samples from above the latest Cenomanian Neocardioceras or B bentonite from the Western Interior Seaway of North America. This time slice provides a snapshot of water mass characteristics in this vast epicontinental sea during the early phase of Oceanic Anoxic Event 2. Mean sample porosity decreases northward and is interpreted as northward decrease in water temperature at the depth Hedbergella delrioensis (Carsey) calcified. Four water masses are defined by porosity, their boundaries are extremely similar to water mass boundaries previously identified by others using the distributions of macrofossils, microfossils and lithology. The boundary between the Subtropical–Tropical Water Mass and Central Subtropical Water Mass was located in southern Colorado. The boundary between the Central Subtropical Water Mass and the Northern Temperate Water Mass lay at approximately 48°N latitude. This boundary is displaced northward approximately 8° latitude as compared to the Holocene planktic foraminiferal temperate ocean province. Within-sample porosity variation suggests all water masses except the Temperate Water Mass were thermally stratified. Samples from the south indicate that the Subtropical–Tropical Water Mass was the most stratified. The porosity data support a previously published data-based paleoceanographic circulation of the southwestern seaway.