MAMMALIAN ENAMEL AND FLOODPLAIN CARBONATE OXYGEN ISOTOPE RECORDS OF MIDDLE PALEOCENE PALEOCLIMATE, CRAZY MOUNTAINS BASIN, MONTANA

The Paleocene epoch represents a critical period in the evolution and diversification of mammals after the extinction of non-avian dinosaurs at the end of the Cretaceous and before the radiation of various modern mammals in the early Eocene. The climatic regime in North America during the Paleocene is expected to be different from today: the Paleocene was an ice-free interval between known greenhouse periods (Late Cretaceous, early Eocene) and before the onset of the Cenozoic icehouse state in the earliest Oligocene. Although much work has focused on paleoclimate across the Paleocene-Eocene boundary and associated mammalian faunal turnover during that interval, less work has been done earlier in the Paleocene during the initial evolutionary radiation of mammals. Isotopic data from marine benthic foraminifera are sparse for the Paleocene compared to later intervals and do not adequately constrain regional climate in continental North America. Terrestrial climate reconstructions for the Paleocene of North America are based on a limited range of proxies, primarily interpretations of floras.

We present new paleoclimatic information for the middle Paleocene based on the oxygen isotope composition of mammalian tooth enamel and floodplain limestones from the Crazy Mountains Basin (CMB), Montana. Mammalian biostratigraphy in the CMB indicates that superposed faunas span from the Torrejonian (To) to the Tiffanian (Ti) North American Land Mammal Ages (~63-58 Ma), allowing us to examine climate change in the Western Interior during an interval of possible faunal turnover. Mammalian hydroxyapatite forms in isotopic equilibrium with body water at constant body temperature. So, for medium- to large-bodied herbivorous mammals, variation in δ18O_ha reflects that in the δ18O of body water, which primarily reflects ingested surface waters and water in food. Surface water, in turn, reflects local meteoric water δ18O_mw, which correlates with air temperature, and plant water is sensitive to both δ18O_mw and relative humidity. Thus, δ18O of both mammal teeth and floodplain limestones is a proxy for local climate shifts and is being used to test proposed connections between climate change and faunal turnover across the To-Ti boundary.