Global Change: A Record of Late Eocene to Miocene Continental Paleoclimate: Stable Isotope Geochemistry of Equid Enamel-Phosphate

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We will reconstruct late eocene to Miocene continental climate from the oxygen isotopic composition of fossel equid enamel phosphate. This interval is an important period of global climatic deterioration inferred from paleoceanographic data with scant quantitative geochemical information from continents. The 18O mammalian apatite precipitates in equilibrium with body water, whcih reflects ingested water. Body temperature of mammals is constant and known so 18OPO4 records the 18O of local water sources which are dominated by the global hydrologic cycle. The 18OPO4 of fossil enamel may record continetal paleoclimate. Fossil equids are ideal because they are geographically widespread, common from the Eocene to late Pleistocne, and have modern analogues. We will determine the isotopic fractionation between teeth, body water, and ingested water in modern equids to quantitatively reconstruct 18O of ingested water for fossil equids. We will determine if a continental climatic signal is preserved in fossil tooth enamal; if continental climate reflects the marine record in magnitude, direction, and timing; and if equid evolutionary change occurred concurrently with continetal climatic change. We will analyze fossils from Wyoming, South Dakota, and Nebraska, where fossil samples are available, regional stratigraphy and chronology are well known, and location near the center of the continent is advantageous for climate studies. Our preliminary data indicate a primary signal is retained by most enamel samples and diagenesis can be detected. We will further develop criteria for evaluating diagenesis with infrared, XRD, and elemental analyses. If diagenesis is negligible, we will develop a quantitative record of late Eocene to Miocene paleoclimate from 18O of ancient precipitation.