EVIDENCE FOR THE YOUNGER DRYAS IN OKLAHOMA BASED ON THE STABLE CARBON, OXYGEN, AND NITROGEN ISOTOPE COMPOSITION OF BISON BONES AND TEETH

The Younger Dryas (12,800 to 11, 500 cal B.P.) is a globally widespread period of brief cooler, drier climatic conditions during the Late Pleistocene/Early Holocene. Attempts have been made to document this abrupt climatic shift based on the stable isotope composition of fossil herbivores (e.g. Connin et al. 1998). In the present study, bison bones and teeth were recovered from nine kill sites in Oklahoma and one site in Texas. The sites range in age from 13,350 to 1,700 cal B.P., with two sites occurring within the Younger Dryas. The distribution and stereochemistry of the amino acid constituents of collagen isolated from the samples were used to assess preservation. In addition to determining the stable carbon and nitrogen isotope compositions of collagen, the stable carbon and oxygen isotope compositions were determined for carbonate associated with bone and tooth apatite (CAA). The δ¹³C and δ¹⁵N values for collagen from well-preserved petrous (inner ear) bones indicate cooler conditions at the start of the Younger Dryas, reflecting a predominance of C₃ vegetation in the bison diet. Toward the end of the Younger Dryas, the δ¹³C and δ¹⁵N values became more enriched in ¹³C and ¹⁵N, indicating a transition to warmer, more arid conditions and a higher percentage of C₄ grasses comprising the bison diet. The δ¹³C values for CAA showed the same trends as the collagen values. The depletion in δ¹⁸O values for the CAA during the Younger Dryas is also indicative of a cooling trend during this time interval.