ISOTOPIC EXPRESSION OF THE YOUNGER DRYAS EVENT IN THE CENTRAL GREAT PLAINS

Stable carbon (δ¹³C) assays 0n bulk soil organic carbon (SOC) from both alluvial and loess-mantled upland sites within Kansas, Nebraska, and Colorado have yielded chemostratigraphic time series that document a dramatic shift in climate during the Younger Dryas (YD) Event. The Younger Dryas Chronozone (YDC) is represented by a well defined paleosol or pedocomplex in most landscape positions within the Central Great Plains. Sites were samples from exposure profiles, cores, or both, and were selected to provide a spatial sense of environmental change within the YDC both north-south and east-west within the region. AMS and conventional ¹⁴C and OSL dating provide numerical control and temporal definition of the YDC. Based on isotopic and limited micro- and macrobotanical fossil data, pre-YD conditions were characterized by C₃ plants (tree, shrubs, cool-season grasses), but by the end of the YD most sites experienced a pronounced C₄ grass environment. Magnitude and timing of environmental change differ somewhat between upland and alluvial environments in that alluvial site data reflect somewhat more conservative and retarded environmental shifts than the uplands. Combining spatial and temporal attributes of the data set produces an impression of ecotonal shift during the course of the YD.