LATE WISCONSINAN EOLIAN SAND ACTIVITY IN THE UPPER MISSISSIPPI RIVER BASIN

The timing of eolian sand mobility within the Upper Mississippi River (UMR) basin has the ability to provide important information regarding past surface and atmospheric conditions for areas beyond the Late Wisconsinan limit of the Laurentide Ice Sheet (LIS). Mass wasting deposits (e.g., colluvium, talus), cryogenic features (e.g., ice-wedge casts, ice-wedge polygons), flora and fauna suggest a tundra-like environment with permafrost south of the LIS near and after the last glacial maximum. This study employs optically stimulated luminescence (OSL) dating to determine the age of eolian sand in an attempt to reconstruct past changes in surface sand availibility (linked to ground cover conditions) and atmospheric circulation. We sampled dunes and sand ramps along a north to south transect from the northern Driftless Area in western Wisconsin to east-central Iowa on the Iowan Erosion Surface. Optical ages on dunes and sand ramps from the northern UMR basin range from ~21 to 14 ka, with a large group between 16 and 14 ka. These ages fall within the interval documented by previous research within the study area indicating increased mass wasting between >22 and 14 k cal yr BP based on radiocarbon ages on wood and gastropods. We hypothesize that the largest group of ages between 16 and 14 ka record a increase in sediment availibility due to a change in surface and subsurface hydrology associated with the degradation of permafrost. Factors which allow for the movement of sand, including low ground cover percentage and well drained surface conditions, were maximized immediately after degradation of permafrost. Sources for eolian sand include fill terrace sediments and sandy colluvium. Sand ramps and dunes occur on both valley train terrace deposits (main Mississippi Valley) and non-glaciated tributary terrace deposits (e.g., Root River, MN). Additionally, optical ages on fluvial sediment (fill terraces) document a temporal gap between fluvial deposition and eolian sand accumulation of several thousand years, requiring a period of sand availibility subsequent to abandonment of the terrace. Regional mapping of eolian sand from soil survey data and LiDAR help in identification of the sources of eolian sand, potential transport distances, and morphology of dunes which help aid paleoenvironmental inferences.