MAMMALIAN ENAMEL AND FLOODPLAIN CARBONATE STABLE CARBON AND OXYGEN ISOTOPE RECORDS OF MIDDLE PALEOCENE PALEOCLIMATE, WESTERN INTERIOR OF NORTH AMERICA

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 mammalian orders 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 mid-Paleocene based on the carbon and oxygen isotope composition of mammalian tooth enamel and floodplain limestones from the Crazy Mountains Basin (CMB), Montana and the San Juan Basin (SJB), New Mexico. Superposed faunas in the CMB and SJB span from the Torrejonian (To) to the Tiffanian (Ti) North American Land Mammal Ages (~63-58 Ma). Mean d¹⁸O and d¹³C values for CMB tooth enamel from five different localities range from 27.5‰ to 35.9‰ (VSMOW) and -8.9‰ to -5.0‰ (VPDB), respectively, and mean values for CMB floodplain carbonates are 17.1±2.3‰ and -6.1±3.1‰, respectively. Surprisingly high d¹⁸O values of tooth enamel likely result from diagenetic alteration. Variability in d¹³C values of floodplain carbonates can be explained by varying degrees of methanogenesis in the sediment at different times. Covariation of CMB enamel d¹⁸O and d¹³C values is consistent with a Positive Linear Covariant (PLC) diagenetic trend, while values for CMB floodplain carbonates may represent a Meteoric Calcite Line (MCL) diagenetic trend. Interpretations will be augmented by additional data from the SJB and the relationship between d¹⁸O of tooth enamel and latitude will be discussed.