TRACING MIGRATION PATTERNS AND CLIMATE CHANGE USING ISOTOPE ANALYSES OF PREHISTORIC BISON BONES AND TEETH

Several of the different stable isotope ratios that can be measured in the teeth, bones, and organic molecules of prehistoric bison have good potential for use as proxies for reconstructing paleoecological and paleoclimatic conditions. 1) The oxygen isotope ratios of modern bison have been shown to track closely the oxygen isotope ratios of local surface waters reflecting mean annual temperatures and regional hydrology. 2) Bison carbon isotope ratios correlate with the composition of grassland vegetation (i.e., the abundance of C-4 versus C-3 grasses) and with mean annual temperatures. 3) The nitrogen isotope values of modern bison vary geographically with changes in precipitation and soil type. However, a complicating factor in interpreting the environmental significance of any isotopic analyses of ancient bison is the possible presence of individuals that migrated into the local area from other environments.

Here we present analyses of the stable oxygen, carbon, and nitrogen isotope values of samples of tooth enamel carbonate, bone phosphate, and bone collagen from Late Prehistoric bison jumps in Wyoming. Values for prehistoric bison were compared with corresponding analyses of modern bison from across the Great Plains. We also have analyzed the strontium isotope ratios of a subset of the archaeological samples; the strontium isotope ratios of individuals reflects soil values in their habitat which, in turn, vary depending on the type and age of the bedrock that weathered to form the soil.

The majority of bison at each archaeological site displayed oxygen and carbon isotope ratios that were similar to those of modern bison from the same region (the northern Great Plains). However, a few individual at each site displayed oxygen and/or carbon isotope ratios that differed by up to 5 per mil from the values for modern bison from the same region. Archaeological bison with extreme oxygen and carbon isotope ratios also displayed extreme strontium and nitrogen isotope ratios that differed from the mean values for these proxies at each location. Overall, our results suggest that corresponding analyses of multiple isotopic systems can be used to distinguish between migrant and resident animals, and that 10 to 15% of prehistoric bison in the Northern Great Plain region migrated long distances across different environments.