A FRESH LOOK: SALINITY AND TEMPERATURE OF THE WESTERN INTERIOR SEAWAY USING THE CLUMPED ISOTOPE PALEOTHERMOMETER

The Western Interior Seaway covered much of the central US during the Cretaceous and was shallower than any modern ocean its size, reaching only a few hundred meters at its deepest. The evolution of its coastlines through the Cretaceous has been reconstructed by geologic mapping, but questions still remain about its salinity, evaporative balance, and its connectivity to the open ocean. Some suggest it was fully marine, while others propose that it was semi-restricted and therefore more saline due to increased evaporation. The oxygen isotopic composition of seawater (δ¹⁸O_w) is strongly related to salinity in the modern ocean, and can be determined for past oceans using a combination of the new clumped isotope paleothermometer and the traditional oxygen isotope paleothermometer. We use this approach on invertebrates from the Western Interior Seaway to reconstruct temperature and δ¹⁸O_w, and through that investigate salinity. We find highly depleted δ¹⁸O_w values relative to expected marine values (-3 to -7‰ VSMOW), indicating significant influence of continental runoff. We compare climate and water conditions in the Maastrichtian and the Cenomanian periods, which represent the cold and warm end members of Cretaceous climate and atmospheric CO₂ levels. We also compare samples from the Western Interior Seaway and the coastal Atlantic Ocean at similar latitudes to understand broader mid-latitude climate at this time. New clumped isotope data are compared to CESM model runs under paleoclimate boundary conditions (paleo-topography, atmospheric CO₂ of 560-1120ppm) and to other published proxy data.