Paleoenvironment of the Western Interior Seaway inferred from δ18O and δ13C values of molluscs from the Cretaceous Bearpaw marine cyclothem

δ18O and δ13C values of ammonites and inoceramids provide information on the paleoenvironmental conditions in the Western Interior Seaway (WIS) during the Campanian–Maastrichtian Bearpaw marine cycle, the last major Transgression–Regression (T–R) cycle to affect the seaway during the Cretaceous Period. Ammonites and inoceramids exhibit distinct stable-isotope values: inoceramids have higher δ13C values (0.6‰ to 5.0‰) and lower δ18O values (−4.0‰ to −2.8‰), whereas coeval baculitid ammonites have lower δ13C values (−4.8‰ to −0.3‰) and higher δ18O values (−2.3‰ to 0.0‰), with other ammonites having stable-isotope values ranging between the baculitids and inoceramids. The isotopic composition of the inoceramids and ammonites are unlikely due to kinetic or vital effects because species from open ocean environments have values expected from marine water and there is no correlation between the δ13C and δ18O values of the inceramids as has been reported for modern foraminifers and calcareous algae suspected of exerting vital effects. The heterogeneity in stable-isotope values of coeval molluscs implies that the Bearpaw Sea was isotopically stratified. Isotopic exchange between the water and sediments, and the formation of 18O-depleted deepwater mass by mixing of Tethyan and Boreal waters in the WIS, may have caused such isotopic stratification. r relationship exists between the δ18O values of Bearpaw zonal baculitids and their biostratigraphic sequence. Baculitids from zones during peak transgression have the lowest average δ18O values (−2.3‰ to −0.7‰), whereas those from the underlying and overlying zones have higher δ18O values (−0.8‰ to 0.2‰). This pattern of δ18O values can be explained by fluctuations in temperature rather than variations in freshwater influx, this influx probably having been reduced by lower precipitation and run-off under drier, warmer, climatic conditions. The Bearpaw Sea was not brackish and other paleoenvironmental factors likely account for the character of the Bearpaw fauna that is dominated by ammonites and inoceramids and lacks many of the taxa prevalent in the contemporaneous open oceans and seas.