LATE CRETACEOUS STABLE CARBON ISOTOPE CHEMOSTRATIGRAPHY, WESTERN INTERIOR BASIN: A NEW TIME-CALIBRATED CARBON ISOTOPE CURVE FOR GLOBAL CORRELATION

The Late Cretaceous has been studied as an analog for the future greenhouse world with elevated pCO₂ and increased atmosphere and ocean temperature. In such studies, carbon isotope data has provided an important window into the behavior of the carbon cycle, as well as a tool for regional to global correlation. Recent revision of the Late Cretaceous time scale, based on integration of new astrochronologic and geochronologic results from the Western Interior Basin of North America, has shifted some stage boundary ages by up to 0.5 myr and significantly reduced age uncertainties. The export of this revised time scale to other global localities that lack in situ radioisotope dates is possible using biostratigraphy, but this approach includes a level of temporal error related to the duration of biozones, as well as issues associated with intercontinental biozone correlation. Carbon isotope chemostratigraphy offers a tool to improve such correlations. The Western Interior has long been recognized for its well-developed molluscan biostratigraphic framework and its radioisotopic contributions to the Cretaceous time scale. Despite the development of δ¹³C records from selected stratigraphic intervals in the basin, a comprehensive δ¹³C record comparable to those developed in Europe and Asia, and from ODP sites, had not yet been completed. This study reports a new, high-resolution δ¹³C curve spanning Early Cenomanian through Early Campanian time. The analyzed samples are from drill cores in which floating astrochronologic frameworks have been developed. Correlation of new radioisotope dates to these stratigraphic sections allows major and minor perturbations in the δ¹³C record to be more precisely dated (since the astrochronology facilitates improved age interpolation). These include the Mid-Cenomanian Event and the Cenomanian-Turonian Ocean Anoxic Event 2, as well as smaller excursions that are recognized and correlated in records from Europe and Asia. The data demonstrate that the Western Interior recorded changes in the global marine carbon cycle during Cenomanian to Campanian time. Using the new chemostratigraphic framework, export of revised ages to other localities within and outside the basin is possible and will contribute to the development of a consistent global chronostratigraphy for the Late Cretaceous.