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.