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

A clear 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.