T182 Quaternary Landscape Dynamics Beyond the Ice Margin in the Upper Mississippi Valley (Posters): A Tribute to the Career of James C. Knox

Tuesday, 11 October 2011
9:00 AM - 6:00 PM
Hall C

LATE QUATERNARY EOLIAN AND HILLSLOPE PROCESSES AND THE DISTRIBUTION OF XERIC PLANT COMMUNITIES IN THE UPPER MISSISSIPPI VALLEY

Paper number: T182-5
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

The legacy of late Quaternary paleoenvironments in the modern landscape of the Upper Mississippi Valley is a recurrent theme in the research of Jim Knox and his students. The effects of Late Pleistocene eolian and hillslope processes are especially evident in the loess and colluvium that mantle much of the UMV outside Late Pleistocene glacial limits. A more subtle legacy involves patches of the landscape where loess is absent or is mixed with sand or rock fragments, diluting its capacity for moisture and nutrient retention. These patches were often the setting for distinctive xeric, fire-dependent plant communities, preserved to varying degrees in the modern landscape. In river valleys of southeastern Minnesota and along the Upper Mississippi in northwestern Illinois, Late Pleistocene eolian sand formed ramps climbing to adjacent ridgetops, usually at locations where there is a long down- or cross-valley fetch for westerly or northwesterly winds. At these sites, weakly developed sandy soils, with much lower capacity for moisture and nutrient retention than well-developed Alfisols in adjacent loess-mantled areas, hosted a variety of distinctive sand barrens and other xeric plant communities. At a very different spatial scale, the extensive areas of xeric vegetation in west-central Wisconsin are at least in part related to widespread eolian sand transport and the resulting lack of loess cover.

Hillslope processes also influenced the vegetation mosaic in the bedrock-controlled landscape of southeastern Minnesota. Steep upper slopes are mantled by sheets of blocky colluvium with a loess-derived matrix that developed mainly in the Late Pleistocene. Where this colluvial mantle extends downslope over Cambrian sandstone, it is thin or absent and soils are often formed partly or completely in weathered sandstone or sandy hillslope sediment. Field measurements indicate that soil moisture is more rapidly depleted in this zone of sandstone-influenced soils than in thick loess or silt-rich colluvium, potentially favoring xeric vegetation. “Goat prairies” are still present today on some south-facing slopes with a thin patchy colluvial mantle over sandstone, and a variety of evidence including carbon isotopes in soil organic matter suggests that open oak savanna was common in this geomorphic setting in the past.