LATE CRETACEOUS STABLE CARBON ISOTOPE CHEMOSTRATIGRAPHY, WESTERN INTERIOR BASIN

The Late Cretaceous was one of the warmest periods of deep Earth history for which time control at the scale of orbital cycles (20-400 kyr) has been achieved. As a result, this interval has been studied as a possible analog for a future greenhouse world with anthropogenically elevated pCO₂ levels and increased atmosphere and ocean temperatures. Carbon isotope data has played a fundamental role in the development of our understanding of the Late Cretaceous, both as a window into the behavior of the paleo-carbon cycle, and a tool for regional to global correlation. Publication of Late Cretaceous carbon isotope curves for localities in Europe, Asia, and the deep sea record (ODP) has provided the foundation for development of a contiguous, intercontinental Late Cretaceous chemostratigraphic framework. This study reports a new, high-resolution Cenomanian to Campanian carbon isotope record for the central Western Interior Basin of North America that makes a key contribution to the framework. Major perturbations in the global carbon cycle, such as the Mid-Cenomanian Event, the Cenomanian-Turonian Ocean Anoxic Event 2, and the Late Turonian Event, as well as smaller excursions that are well recognized and correlated in records from Europe and Asia, are clearly represented in this record. This shows that the Western Interior shallow epeiric sea was not an isolated marginal basin – it consistently recorded changes in the global marine carbon cycle during Cenomanian to Campanian time. A critical attribute of the chemostratigraphic dataset produced in this study is its linkage with a revision of the Late Cretaceous time scale that is based on integration of astrochronologic and geochronologic data. The new time scale shifts some stage boundary ages by up to 0.5 myr and significantly reduces age uncertainties. Assignment of revised ages to Western Interior carbon isotope events that are globally expressed will allow export of the improved time scale to localities within and outside the Western Interior where similar carbon isotope records have been generated. This chronostratigraphic tool will allow a number of stratigraphic and geochemical hypotheses to be more rigorously tested.