A YOUNGER DRYAS CARBON ISOTOPE RECORD FROM SOIL ORGANIC MATTER ON THE SOUTHERN MARGIN OF THE PRAIRIE PENINSULA

Stable carbon isotope studies of soil organic matter from the deeply stratified Big Eddy archaeological site (23CE426) and nearby localities in the Sac River valley in southwest Missouri are producing a detailed record of late glacial and Holocene vegetation change along the southern margin of the Prairie Peninsula. The Younger Dryas (YD) appears as an abrupt millennial-scale event during the early deglacial period of the North Atlantic region when the climate returned to the cooler and drier conditions that characterized the previous glacial. Several studies have suggested that the event was global in extent and that other parts of North America witnessed climatic variability corresponding to the YD. However, there is little documented evidence for the YD event in the continental interior between the Rocky Mountains and the Great Lakes. Detailed and well-dated carbon isotope profiles of soil organic matter from a buried land surface show a prominent increase in δ¹³C values between 13,210 and 11,940 cal yr B.P., a period that closely corresponds to the YD event in the GRIP and GISP2 ice cores. During this period at Big Eddy, δ¹³C values increase between 4 and 5 ‰, reflecting a 40% increase in C4 vegetation. Although the increase in the relative abundance of C4 plants is often attributed to higher temperatures, it is the east-west precipitation gradient that determines the position of the southern prairie-forest boundary. Therefore, the isotopic trend documented at Big Eddy most likely reflects vegetation response to increased aridity rather than elevated temperatures, and suggests that the YD along the southern Prairie Peninsula was drier (and possibly cooler) than the periods immediately preceding and postdating it.