ALLUVIAL PALEOSOLS AND THE YOUNGER DRYAS OF THE CENTRAL GREAT PLAINS

The Younger Dryas (ca. 12.9-11.5ka) of the central Great Plains was a time of remarkable soil development in alluvial settings. This episode of pedogenesis produced the first major alluvial soil since that of the Middle Wisconsinan interstade (late MIS 3). Though numerous, subsequent episodes of pedogenesis during the Holocene were more transient. To document Younger Dryas alluvial soil formation, core samples from terrace fills at five localities (one in Nebraska, four in Kansas) were radiocarbon dated and analyzed for their respective stable carbon isotope, magnetic susceptibility, and elemental signals. Site stratigraphy in each instance consisted of basal sand overlain by silt-dominated (loess-derived) sediment intercalated with paleosols, the lowermost of which is the Younger Dryas-age soil. Prior to the Younger Dryas, stable C values of about -23‰ suggest essentially pure C3 vegetation, which is substantiated by dated hardwood charcoal at two of the localities. At the outset of Younger Dryas pedogenesis, vegetation appeared to retain much of the Last Glacial Maximum C3 bias, but, by the termination of pedogenesis, isotopic values had risen to about -15‰, suggesting emergence of a mixed to C4-biased plant community. The early to mid-Holocene isotope record also preserved in these terrace fills varies among sites, with the eastern and northern two sites having a richer C3 signal than those to the west and south. Overall, these data sets document the nature of alluvial soil development and of environmental change at the end of the Late Wisconsinan, and, in doing so, they indicate that the alluvial climatic proxy record compares in sensitivity to that being extracted from the upland loess sequences.