Stable isotopic record of climatic and environmental change in continental settings.

Abstract

This research focuses on climate variations present within continental settings because these represent regions of the greatest change in climate and have the potential to provide the most detailed representation of ancient conditions. Seasonal distribution of temperature is an important parameter which influences biota, atmospheric and oceanic circulation and geochemical cycles. This dissertation establishes a fish-otolith paleothermometer and then applies it to decipher temporal patterns of climate change in North America. The temperature-fractionation relation was determined using $\delta\sp{18}$O values of otoliths from fish raised in well-constrained natural environments and laboratory-controlled conditions coupled with $\delta\sp{18}$O values of environmental water. Microsampling of otoliths revealed that the seasonal signal of temperature stored as $\delta\sp{18}$O variation provides an accurate means of determining seasonal temperature variation in the past. Long-term variation of $\delta\sp{13}$C values of ancient carbonate, which is assumed to reflect the isotopic composition of global seawater, has been used as a paleoenvironmental indicator. Examination of the isotopic chemistry of seawater from modern carbonate platforms (analogs to Phanerozoic seaways) displays distinctive spatial and temporal variations in $\delta\sp{13}$C values which is not accounted for in models of ancient environments. The distinctive chemistry of these platform waters may serve as an isotopic tracer for thermohaline circulation in ancient oceans. Through a combination of isotopic data from shallow marine platforms and from fish-otoliths obtained from archeological sites, it was possible to investigate the secular increase in seasonality and decrease in mean annual temperature in western North America. Seasonality in temperature and precipitation was low in the Eocene and Miocene, gradually increasing through the Pleistocene in agreement with independent faunal data. Seasonality during the Paleocene and Oligocene was augmented by mountain uplift and increased rainshadow effects. Globally, the latitudinal temperature gradient increased through the Tertiary as a result of diminished thermohaline circulation. A similar approach was used to reconstruct climate variation over the last millennium. Using $\delta\sp{18}$O and $\delta\sp{13}$C values of environmental water in conjunction with derived temperatures, it is possible to estimate ancient atmospheric and oceanic circulation as well as precipitation patterns. A more complete picture of global climate change can be obtained by reconstruction of global circulation patterns, as well as seasonality of temperature and precipitation.