USING THE OXYGEN ISOTOPE RATIOS OF BISON BONE PHOSPHATE AS A PALEOCLIMATIC PROXY

Analyses of the oxygen isotope ratios of fossil and sub-fossil mammalian bones and teeth may serve as a proxy for the oxygen isotope ratios of past precipitation, which in turn may yield paleoclimatic information about storm tracks and/or paleotemperatures. However, vital effects influence the relationship between an animal's oxygen isotope ratios of and those of local waters, and other factors, such as relative humidity, are important in some taxa. These effect must be accounted for before the oxygen isotope ratios of a fossil can accurately interpreted. The isotopes of bison have excellent potential for use as a paleoenvironmental proxy because: 1) Bison are one of the few large fossil herbivores in North America to have survived into modern times, and thus the isotopic fractionation between individuals and their environment can be directly measured. 2) Bison occur in high abundance in fossil and archaeological deposits. 3) Prehistoric bison ranged from Alaska to Mexico, so they have the potential for providing information about paleoenvironmental conditions in across much of North America

We compare the oxygen isotope ratios of bone phosphate from modern bison (Bison bison) to variations in mean annual temperature, mean annual precipitation, relative humidity, and the isotope ratios of local surface waters. Study sites were chosen to represent a range of climatic and ecological conditions; we sampled free ranging bison from populations in California, Kansas, Montana, Oklahoma, Nebraska, North Dakota, South Dakota, Utah, and Wyoming. The mean oxygen isotope values of bones and teeth are well correlated with the oxygen isotope values of local surface waters. This demonstrates that analyses of the oxygen isotope values of bison have good potential to serve as a proxy for reconstructing the oxygen isotope ratios of surface waters and/or precipitation.