HOLOCENE POLLEN AND PLANT MACROFOSSIL RECORD OF COLDWATER LAKE, NORTH DAKOTA

Paleobotanical data from Coldwater Lake (46°01'N, 99°05'W) provide a high-resolution record of Holocene vegetation changes that have occurred in southeastern North Dakota in response to changes in climate and hydrology. Detailed fossil pollen and plant macrofossil analyses of a 19-m long sediment core collected from this small (0.5 km²) kettle lake reconstruct three paleovegetation assemblages that have occupied the Missouri Coteau upland since deglaciation. Twelve AMS ¹⁴C ages document the timing of these paleoenvironmental changes.

Open white spruce parkland composed of Picea glauca and upland and lowland prairie herbs became established in southeastern North Dakota by 10,800 ¹⁴C yr B.P. These herbs currently exist in the area, whereas spruce trees do not, which suggest that the terminal Pleistocene climate of North Dakota was probably only slightly cooler than present. By about 10,600 ¹⁴C yr B.P., the spruce trees were replaced by deciduous trees, such as Betula papyrifera (paper birch), Ulmus (elm), Quercus (oak), and Populus tremuloides-type (aspen poplar). Between 10,200 and 9100 ¹⁴C yr B.P., the deciduous trees gradually disappeared and upland herbs expanded, forming grassland. The onset of maximum aridity occurred at 8100 ¹⁴C yr B.P., as indicated by greater abundance of Ambrosia-type (ragweed) pollen and higher seed counts of taxa adapted to brackish and saline habitats (e.g., Chenopodium rubrum [red goosefoot] and Zannichellia palustris [horned pondweed]). Lake levels periodically fluctuated during the remainder of the Holocene.

This paleovegetation succession indicates a warming and drying trend during the early and mid-Holocene, which was probably the result of greater summer insolation and reduced winter precipitation. The results from this study agrees with that of previously published reports for Coldwater Lake, based on other paleoenvironmental indicators (e.g., geochemistry, diatoms, and charcoal) from the same core.