THE NATURE OF THE PLEISTOCENE/HOLOCENE TRANSITION IN THE UPPER MIDWEST, USA: GEOARCHAEOLOGICAL IMPLICATIONS

In the Upper Midwest the Pleistocene/Holocene transition witnessed dramatic changes in biotic communities, biogeomorphic response to those changes, and consequent effects on human populations. The transition is marked by the replacement of terminal Pleistocene conifer-hardwood communities by mesic deciduous communities in the east and savanna and grassland communities in the west. These biotic changes were driven by climatic change that was diachronous across the region. In addition, the sensitivity of extant biota to climatic change varied across the region as well as locally across various elements of the landscape. The rapidly retreating Laurentide ice sheet strongly influenced the position of the jet stream and produced a strong N-S climatic gradient across the region. The region's diverse landscapes harbored a wide range of microhabitats at the Pleistocene/Holocene transition that supported a variety of communities, many without modern analogs. Large valleys linked the diverse landscapes and served as avenues of N-S dispersion for both thermophilous and boreal species. Valley corridors harbored early Holocene boreal relics whose main populations had already shifted to the north. General landscape stability characterized the Pleistocene/Holocene transition in the Upper Midwest. Soils formed on floodplains in both large and small valleys, and late glacial dune fields and valley slopes were also stable. This period of stability was short lived and by 9.5-8.5 ka floodplains and alluvial fans began to aggrade as sediment delivery from valley slopes increased. The environmental diversity at the Pleistocene/Holocene transition provided incredible resource diversity for human populations. Interpreting the archaeological record of human adaptations to this nonanalog environment is complicated by Holocene land cape changes that have strongly influenced the preservation and visibility of late glacial and early Holocene landscapes and archaeological deposits.