EVALUATING THERMOHALINE STRATIFICATION IN THE LATE CRETACEOUS MISSISSIPPI EMBAYMENT AND WESTERN INTERIOR SEAWAY USING PHOSPHATE OXYGEN ISOTOPES FROM MARINE VERTEBRATE FOSSILS

Paleotemperature estimates for the North American Western Interior Seaway (WIS) based on invertebrate δ¹⁸O_CO3 analyses often indicate unusual conditions, such as benthic temperatures exceeding 30 °C and an inverse latitudinal temperature gradient. These data imply atypical thermohaline stratification of the water column and support hypothesized WIS reconstructions that include boreal and Tethyan waters forming decoupled surface- and deep- water masses within the seaway and thermohaline circulation involving warm, benthic brines formed along the eastern margin and cool, relatively fresh surface waters originating along the western shore. However, WIS paleoceanic reconstruction remains unresolved, in part, because independent proxies for water temperature and δ¹⁸O_w are often not available; one must be assumed in order to calculate the other.

A new approach utilizing δ¹⁸O_PO4 analysis of coeval marine turtle, fish, and shark fossils independently resolves both water temperature and isotopic composition, which correlates positively with salinity, thus providing new, quantitative data regarding thermohaline stratification within the seaway. Coeval specimens of taxa representing different water depths were sampled from late Coniacian-early Campanian (ca. 87-82 Ma) strata from the Mississippi Embayment (Mississippi and Alabama) and the central seaway (western Kansas). In both areas, δ¹⁸O_w values estimated from pelagic versus benthic taxa are not significantly different, inferring a uniform salinity throughout the water column. Similarly, water temperatures estimated from pelagic versus benthic taxa indicate little thermal stratification within the water column in either area. These data indicate that, during Niobraran time, the water column in each area possessed a homogenous thermohaline structure. These results do not support the presence of decoupled water masses or thermohaline circulation within the central seaway. The estimated temperatures and δ¹⁸O_w values between the two regions are significantly different, implying that the boundary between Tethyan and boreal water masses was located between 35-40 °N paleolatitude.