ECOSYSTEM INSTABILITY DURING THE RISE OF DINOSAURS: EVIDENCE FROM THE LATE TRIASSIC IN NEW MEXICO AND ARIZONA

During the Late Triassic, major evolutionary events, including the origin of mammals and the early diversification of dinosaurs, took place against a paleoenvironmental background that is only known at a very coarse level for terrestrial environments. We sought to better illuminate the responses of terrestrial ecosystems to major perturbations such as long-term climate change and bolide impacts by establishing the first bulk organic carbon isotope (δ¹³C_org) record from the Late Triassic Chinle Formation of New Mexico and Arizona. At Ghost Ranch, New Mexico, we obtained a fine-scale carbon isotopic record from bulk organic material, wood, and charcoal collected from three fossiliferous paleochannel deposits dated to about 215 Ma. Our data document extensive short-term variability and multiple δ¹³C_org isotopic excursions on the order of 2-3 per mil. This interval occurs shortly after a biotic turnover event in vertebrates and plants recorded in the Chinle Formation, and is approximately coeval with the Manicouagan impact in Quebec. We also sampled a longer stratigraphic section at Ghost Ranch containing these and other fossiliferous paleochannel and overbank deposits that span up to 10-15 million years of deposition, from the mid-Norian through Rhaetian. These δ¹³C_org records and those from a partly time-equivalent section at Petrified Forest National Park, Arizona, display similar variability, documenting long-term climatic and ecosystem instability throughout the mid- to late Norian.