The paleoenvironmental significance of terrestrial gastropod fossils from the Upper Mississippi Valley in Minnesota and Wisconsin.

摘要

Terrestrial gastropod shells are common fossils within late Pleistocene sediments from the Upper Mississippi Valley (UMV) of southwestern Wisconsin and southeastern Minnesota. This thesis represents the first systematic study of the regional gastropod fauna since Chamberlin and Salisbury (1885). Chapter One outlines the goals of this thesis and important paleoenvironmental questions. Chapter Two provides a sedimentary and stratigraphic context through description of the depositional and architectural relationships of field exposures of Pleistocene deposits. Data presented here support the view that mass wasting and valley aggradation were active between 24,000 and 17,000 cal yr BP. The timing and lithology of structures and sediments related to floodplain incision do not correspond to floods from Glacial Lake Agassiz; incision may have resulted from earlier Lake Superior Basin flood events. Chapter Three presents results of amino acid racemization analyses of 236 terrestrial gastropod shells. Shells that are more weathered in appearance belong to Cordilleran-Boreal taxa and exhibit D/L Asp values characteristic of late Pleistocene age shells. The less-weathered shells of rare Eastern Deciduous Forest taxa exhibit D/L Asp values suggesting that these shells were introduced into the assemblage after its deposition. The small variance in succineid AAR data in conjunction with well-constrained 14C ages indicate time averaging of 1000 years in these deposits. Variance within the data set may result from leaching, heat alteration, or contamination from bacteria. These results have significant implications for future gastropod-based amino acid geochronologic studies. Chapter Four is a paleoecological analysis of collections from Kulas Quarry and Hwy-JJ in Trempealeau County, Wisconsin. Most taxa from these sites have modern Cordilleran-Boreal ranges. Kulas Quarry (floodplain) samples include a larger number of rare species than do samples from Hwy-JJ (upslope). Overall similarities in gastropod rank abundance among samples suggest that the community remained stable between 21,700 and 20,000 cal yr BP. The environment was probably tundra-like, with minor amounts of deciduous vegetation within topographically controlled microhabitats. Chapter Five summarizes all results and provides suggestions for future work. These results reveal the Driftless Area as a temperate refuge in the late Pleistocene, much as it is for cold-adapted taxa today.