THE TEMPERATURE EVOLUTION OF THE WESTERN INTERIOR SEAWAY: ALBIAN TO MAASTRICHTIAN

The strata and fauna of the Cretaceous Western Interior Seaway (WIS) have been extensively studied, but paleoclimate and paleoceanographic studies have been a challenge historically. Early paleotemperature reconstructions using oxygen isotope paleothermometry produced implausibly high temperatures, indicating that oxygen isotopic composition for waters (δ¹⁸O_w) in the seaway likely deviated from the often assumed “ice-free” ocean end-member composition, thus hindering useful application of this method. The more recently developed clumped isotope paleothermometer distinguishes the contributions of temperature and δ¹⁸O_w to the oxygen isotopic composition of carbonate minerals (δ¹⁸O_carb), enabling accurate reconstruction of seaway temperature and δ¹⁸O_w conditions. To date, clumped isotope studies of the WIS have generally focused on single time periods. Here, we present paleotemperature and paleo-δ¹⁸O_w data from fossil mollusk samples dating from the late Albian to Maastrichtian (~103-69 Ma), with the aim of reconstructing the long-term paleoceanographic evolution of the WIS. Measured samples (n > 110) were selected from fossil collections of the U.S. Geological Survey (now housed at the National Museum of Natural History) and the University of Michigan, spanning a wide geographic range within the WIS from outcrops in the Great Plains, Rocky Mountains, and Gulf Coast regions of the United States. The temperatures measured from marine mollusks were compared to those derived from river-dwelling bivalves, but there were challenges disentangling their variability in space and time. Generally, the temperature evolution of the WIS tracks global temperature reconstructions for the Cretaceous, while δ¹⁸O_w values suggest mixing of a saltier Gulf of Mexico water mass with a more brackish Arctic water mass combined with a smaller component of fresh riverine input. This dataset represents a first step towards a broader goal of reconstructing temperature evolution in the Cretaceous WIS through space and time and shows the potential of this method to resolve outstanding issues such as circulation patterns in the WIS.