Age, origin and paleoclimatic significance of eolian sand within the Green River Lowland, northwestern Illinois.

摘要

The Green River Lowland (GRL) is an area of subdued topography consisting of an extensive, poorly-drained outwash plain and valley train, which served as a major conduit for glacial meltwater from the Laurentide Ice Sheet (US).; The GRL contains a wide variety of parabolic dune forms and sand sheets, which overlie glacial outwash. Geochemical analyses of eolian sand indicate that the sand was likely derived from glaciofluvial sediments, but are depleted in Ca, Sr, K, and Rb relative to the source. Physical comminution and/or syndepositional weathering of carbonates during transport and the lack of micas in the eolian sand likely account for these differences.; A chronology based on 14 optically stimulated luminescence (OSL) ages and three radiocarbon ages indicate that the penultimate period of dune formation occurred from ca. 18-14 ka. Local reactivation of sand sheets occurred during the late Holocene from ca. 4-1.5 ka. Periods of dune reactivation within the Holocene are correlated with evidence for aridity at other localities in the Midwest, suggesting that they may be linked to regional drought.; Dune orientations suggest northwesterly wind direction during formation. This is in agreement with paleowind directions inferred from the regional loess record, but in disagreement with the dominant wind direction produced by GCM's. This discrepancy can be explained by linking dune formation and dust flux to the atmosphere with low frequency, but high intensity cyclonic storms associated with strong north-northwesterly flow along the southern margins of the LIS.; The region is intensively row-cropped today, but dunes remain stable because of relatively high P/PE ratios. However, dunes within the GRL were locally reactivated during the 20th century. This appears to have been linked with breaching of the solum, with subsequent mobilization of unaltered sands beneath. Both intrinsic (i.e. land use) and extrinsic (i.e. precipitation and evapotranspiration rates) factors appear to have played a role in the remobilization events with the latter moderating the periods of activity. This suggests that the dunes within the GRL may be susceptible to changing climatic conditions linked to global warming.