A 17,500-YEAR POLLEN RECORD FROM SILVER LAKE IN NORTHERN LOWER MICHIGAN, USA SUGGESTS DEGLACIATION AND PLANT COLONIZATION MILLENNIA EARLIER THAN PREVIOUS ESTIMATES

We report on the oldest postglacial pollen record in Michigan, which is, surprisingly, located in the northern portion of the Lower Peninsula. Here, a basal age of 17,540 cal yr BP (14,390 +/- 100 ¹⁴C yr BP) was recently obtained for Silver Lake in Cheybogan County. This date documents deglaciation ~4000 cal yr BP earlier than previously determined for this area, and agrees with recently obtained luminescence dates on nearby kame deltas, which indicate that much of northern Lower Michigan was ice-free by 23 ka ago. Presented here are pollen data from analyses of a 520-cm sediment core extracted from the bottom of Silver Lake (45°16.3’N, 84°38.1’W; 251 m ASL; 30 hectares).

The basal age of Silver Lake was obtained from a ¹⁴C date on Dryas integrifolia leaves and Picea needle/stem fragments. These remains came from tundra-boreal plants that probably grew in the glacial till overlying a stagnant ice block; they may later have been deposited as a “trash layer” upon kettle lake formation. Pollen analysis of this lowest level indicates a boreal forest dominated by Picea glauca (white spruce), P. mariana (black spruce) and Abies spp. (fir). By 13,100 cal yr BP, Pinus, Picea mariana and Cupressaceae dominated the flora, which also included Betula papyrifera (paper birch, identified by seeds) and a few other deciduous trees. Thus, during the Bolling-Allerod the vegetation shifted from that of a boreal forest to a northern mixed hardwood forest, although certain key species (Quercus (oak)), Tsuga (hemlock)) did not arrive until the early and mid-Holocene. After 4080 cal yr BP, Pinus declined in abundance on the drier upland areas, and the northern mixed hardwood forest shifted to taxa characteristic of mesic and hydric habitats (e.g. Cupressaceae), indicating a cooler and moister trend for the Late Holocene. In summary, although the Silver Lake pollen record is not particularly sensitive to Holocene climate changes, partly due to its location within the lake effect zone of the Great Lakes, it does provide the oldest paleovegetation record for the Upper Midwest and offers insights into the patterns of plant colonization and succession after deglaciation. Equally importantly, the basal date for the lake core suggests that glacial ice had withdrawn from the region thousands of years prior to the chronology of conventional deglaciation models.