via Stable Isotopes of Fossil Turtles and Fish Testing Paleoenvironmental Models of the Cretaceous Western Interior Seaway via Stable Isotopes of Fossil Turtles and Fish Matthew J. Kohn; University of South Carolina and Boise State University Past ocean circulation patterns are key for understanding how fossil fuel resources are distributed and preserved. Yet one major source of fossil fuels, the Cretaceous Western Interior Seaway (c. 100 million years old), is as yet poorly characterized with respect to these circulation patterns. To better constrain effects of evaporation and freshwater input, we have calibrated a novel isotopic method, oxygen isotopes in turtle bone phosphate, that can be used in concert with isotope compositions of fossil fish. Our main results from fossils indicate major isotopic (left figure) and temperature gradients (right figure) from south to north along the seaway. These data indicate minimal northward transport of heat and mass, so that northern waters were substantially cooler and less saline than southern waters. These data further explain why southern and northern biota differ - they reflect differing temperature or salinity tolerances.