CHRONOLOGY AND PALEOENVIRONMENTAL SIGNIFICANCE OF EOLIAN SAND ACTIVITY SOUTH OF THE LAURENTIDE ICE SHEET DURING THE LAST GLACIAL PERIOD, UPPER MISSISSIPPI RIVER BASIN

Eolian sediments (sand and loess) of the North American midcontinent provide important information regarding past climate and terrestrial processes. An accurate chronology of eolian activity is necessary to compare with other records of paleoenvironmental change, including fluctuations of the Laurentide Ice Sheet and paleoecological records of vegetation change. Optically stimulated luminescence dating was used to determine the depositional age of eolian sand within the Upper Mississippi River basin to reconstruct the timing of past changes in surface sand availability and patterns of atmospheric circulation. Regional mapping of eolian sand from soil survey data and LiDAR DEMs beyond the Late Wisconsin limit of the Laurentide Ice Sheet in western Wisconsin, southeastern Minnesota and eastern Iowa provides key insight into the sources for eolian sand, potential transport distances, and morphology of dunes. Dunes were sampled along a north to south transect from the northern Driftless Area in western Wisconsin to the Iowan Erosion Surface in northeastern Iowa. Twenty nine optical ages on eolian sand range from ca. 20 to 13 ka, with the majority of ages clustered around 16 ka. The bulk of ages fall near the end of an interval documented by previous research within the study area, indicating accelerated mass wasting between ca. 23 and 14 ka based on calibrated radiocarbon ages on wood and gastropods from colluvium. One possibility is that the cluster of ages ca. 16 ka records an increase in sediment availability owing to a change in surface and subsurface hydrology associated with the degradation of permafrost, a time during which factors that allow for the movement of sand, including low ground cover percentage and well-drained surface conditions, were maximized. Significant retreat of the Lake Michigan and Des Moines Lobes also occurred after 16 ka, and the final record of tundra-like vegetation and permafrost occurs near 16 ka in the study area. Optical ages on eolian sand sourced from several distinct geomorphic settings, including fluvial (fill terrace) sediments from both glaciated and unglaciated fluvial catchments and colluvium derived from Cambrian sandstones, overlap within error at ca. 16 ka, suggesting a regional eolian response rather than localized responses due to changes in sediment supply.