Late-Wisconsin Pollen Stratigraphy in East-Central Minnesota

Abstract

Reconstruction of the landscape bared by the melting Grantsburg sublobe is aided by pollen diagrams from three sites. At Horseshoe Lake and Cedar Bog Lake, on the Anoka Sand Plain, the pollen spectra above a thin basal layer of plant detritus are dominated by Picea, but other conifers (except Pinus), deciduous trees (especially Populus, Fraxinus, Betula, and Quercus), and herbs (chiefly Cyperaceae, Artemisia, and Ambrosia-type) are well represented. Comparison of the pollen flora of this late-Wisconsin zone with modern floristic provinces emphasizes its complexity. Although plants now common to the Boreal and Great Lakes provinces contributed about one-half of the pollen rain, 35 of the 104 taxa identified belong to deciduous elements, and a prairie element is represented by 6 taxa. Secondary redeposition of part of the late-Wisconsin pollen flora cannot explain its singularity. At Andree Bog, on the Lake Grantsburg plain, the basal spectra are contaminated by secondary microfossils largely Cretaceous in age, and their subtraction has little effect on the pollen flora. At Horseshoe Lake and Cedar Bog Lake the amount of secondary pollen is negligible. The available radiocarbon dates indicate that some lakes on the Anoka Sand Plain were open as early as 12,500 radiocarbon years ago but that melting of buried ice may have continued for as long as 1000 years. On this unstable surface a mosaic of forest and openings is visualized, with Picea, Fraxinus, Populus, and other deciduous trees on moist sites and prairie species in more xeric habitats.