LATE GLACIAL PALEOWIND DIRECTIONS AS INFERRED FROM SAND DUNES WITHIN THE GREEN RIVER LOWLAND, NORTH-CENTRAL ILLINOIS: A TEST CASE FOR ATMOSPHERIC GENERAL CIRCULATION MODELS(AGCMS)

Wind transported (eolian) sediments can serve as proxies of atmospheric phenomena such as wind direction and speed. Landforms composed of eolian deposits can therefore provide climatic information for periods of the past. The Green River Lowland (GRL) is an area of subdued topography consisting of an extensive, poorly-drained outwash plain and valley train. This region served as a major conduit for glacial meltwater when the Late Wisconsin Lake Michigan Lobe of the Laurentide Ice sheet was at the ice front marked by the Bloomington Moraine in north-central Illinois.

Extensive wind reworking of outwash within the GRL has produced a widespread cover of eolian sand. A wide variety of eolian landforms are present within the region including well-defined parabolic, compound parabolic, linear and dome-like dunes and sand sheets. Orientation studies of the parabolic dunes within the region indicate a northwesterly to westerly paleowind direction when the dunes were forming. Preliminary OSL (optically stimulated luminescence) age estimates indicate that the last major period of dune activation was ca. 18,000 to 14,000 years ago. This period of time corresponds with the late glacial, a period of critical importance to climate modelers. Dunes within the GRL are adjacent to the Late Wisconsin ice front in Illinois, and are therefore uniquely situated to test Atmospheric General Circulation Models (AGCMs).

A feature common to many AGCMs is the presence of a strong glacial anticyclone over the interior of North America during the last glacial maximum. Although weaker and not as extensive, the feature is still present 14,000 years ago. According to these models, winds generated from this high pressure cell would have been easterly or northeasterly to the south of the Laurentide ice sheet. The contrast between modeled paleowind direction and those indicated by dune orientations within the GRL is striking. This discrepancy has also been noted in the loess record, suggesting that the AGCMs need to be modified, or that eolian deposition south of the of the Laurentide ice margin were the result of surface northwesterly winds unrelated to the presence of the glacial anticyclone.