Late Pleistocene palaeoenvironments of the southern Lake Agassiz basin, USA

Catherine H. Yansa and Alan C. Ashworth

    Macroscopic plant remains, pollen, insect and mollusc fossils recovered from a cut bank on the Red River in North Dakota, U.S.A., provide evidence that an extensive wetland occupied the southern basin of Lake Agassiz from 10,230 to 9,900 14C yr B.P. Marsh-dwelling plants and invertebrates had colonized the surface of a prograding delta during the low-water Moorhead Phase of Lake Agassiz. A species of Salix (willow) was abundant along distributary channels, and stands of Populus tremuloides (aspen), Ulmus sp. (elm), Betula sp. (birch), and Picea sp. (spruce) grew on the better-drained sand bars and beach ridges. Most of the species of plants, insects, and molluscs represented as fossils are within their existing geographic ranges. Based on a few species with more northerly distributions, mean summer temperature may have been about 1-2C lower than the present day. No change in species composition occurred in the transition from the Younger Dryas to Preboreal. At the time that the wetland existed, Lake Agassiz was draining either eastward to the North Atlantic Ocean or northwestward to the Arctic Ocean. The wetland was drowned during the Emerson Phase transgression that resulted in meltwater draining southward to the Gulf of Mexico after 9,900 14C yr B.P.