ISOTOPIC EVIDENCE FOR LATE CRETACEOUS ECOSYSTEM VARIABILITY IN CAMPANIAN CONTINENTAL SEDIMENTS OF SOUTHERN UTAH

The Late Cretaceous hothouse experienced pronounced secular variations in global temperature and latitudinal temperature gradients. Marine proxy data indicate an earliest Campanian temperature maximum followed by a long-term cooling trend through the rest of the stage. However, corresponding continental records from sections preserving terrestrial faunal and floral changes are lacking. Of particular interest are records of the North American Western Interior, home to a diverse but isolated fauna that displayed pronounced latitudinal zonation during the Campanian, a pattern proposed to have resulted from the paleoclimatic conditions of the time. Here we present the first organic carbon stable isotopic record from the Wahweap Formation, a fossiliferous alluvial sequence deposited ~ 81-76 Ma in what is now southern Utah. We sampled the middle member of the Wahweap Formation at sub-meter intervals for analysis of organic carbon and nitrogen content and isotopic ratios. Fluctuations in total organic carbon content (TOC) and carbon to nitrogen ratios (C:N) reflect variation in local ecological and depositional conditions, whereas the stable isotopic ratio of organic carbon (∂¹³C_org) may reflect both global and local environmental change. By comparing these records, we demonstrate that the sampled Wahweap sequence experienced significant local environmental variation, reflected by shifts in TOC, C:N, and ∂¹³C_org values, as well as at least two negative ∂¹³C_org excursions that likely resulted from larger-scale forcings such as changing atmospheric gas inventories, climatic conditions, or large-scale secular change in organic carbon source(s). An overall ∂¹³C_org enrichment upsection also appears to have resulted from larger-scale ∂¹³C forcings rather than local ecological or preservational effects. The interrelationships between local and global paleoenvironmental data recorded in the Wahweap Formation thus reveal a dynamic early Campanian paleoclimate that may have structured ecosystems throughout the Western Interior and brought about its distinctive biotic history.