Oxygen isotope ratios of biogenic phosphate as indicators of change in continental climate variables at different time scales.

Abstract

It has been suggested that the oxygen isotope composition ($\rm\delta\sp{18}O$ value) of biogenic phosphate has the potential to provide quantitative records of seasonal and long-term climate change in continental settings by acting as a proxy for the $\rm\delta\sp{18}O$ value of local precipitation which is in turn a proxy for continental climate variables. In this dissertation, I tested this hypothesis (1) by investigating the present-day spatial correlations between climate variables and $\rm\delta\sp{18}O$ values of precipitation, and determining when and where they can be used to infer climate change in the past, (2) by analyzing the tooth enamel of modern herbivores that lived under known climatic conditions, and (3) by analyzing the fossil remains of vertebrates from several known episodes of climate change. In chapter II it is demonstrated that the $\rm\delta\sp{18}O$ value of precipitation is strongly correlated with temperature and specific humidity at mid- to high-latitudes, and that proxy records for precipitation do have the potential to provide information on past climate. In chapters III and IV, the potential of $\rm\delta\sp{18}O$ values of mammalian tooth enamel phosphate as such a proxy is investigated by analyzing teeth from a wide geographical range. The results indicate that both seasonal and mean annual temperatures, and the amount of precipitation, can be inferred by taking multiple samples from single teeth. $\rm\delta\sp{18}O$ values of biogenic phosphate are then used to investigate Little Ice age cooling in the north Atlantic region (chapter V), and the Latest Paleocene Thermal Maximum in Wyoming (chapter VI). From the isotopic analyses of human teeth from Greenland and Denmark, it is possible to resolve changes in mean annual temperature over time and space, although interpretations are complicated by uncertainties regarding human behavior and the limits of the technique in resolving small changes in climate. The intense episode of global warming that occurred during the early Cenozoic was investigated by analyzing both herbivore teeth and fish scales, demonstrating an unmistakable shift to higher $\rm\delta\sp{18}O$ values of phosphate is consistent with the increase an temperature. This research reveals the potential, and demonstrate the applicability, of $\rm\delta\sp{18}O$ values of biogenic phosphates as climatic indicators and provides a foundation for future research of continental paleoenvironments.