A NEW GLACIAL VARVE CHRONOLOGY FROM NORTHERN MINNESOTA AND NORTHWESTERN ONTARIO

Correlated sequences of varved sediment from lakes at the margins of former ice sheets provide extraordinary records of ice sheet dynamics. Here we report on a new varve chronology from glacial lakes that fronted the Rainy Lobe of the Laurentide Ice Sheet. The varve thickness data are developed from 14 lake-sediment cores, plus two varve series by Rittenhouse (1933). There are currently two separate, but related chronologies: an older, ~1700-year series from the Rainy basin of glacial Lakes Norwood and Agassiz, and a younger, ~1800-year series from the Wabigoon basin of Lake Agassiz (near Dryden, Ontario.) Tying these two chronologies together is work in progress, but the older chronology ends before the creation of the Eagle-Finlayson moraine, and the younger chronology begins just prior to the creation of the same moraine. Radiocarbon dates establish the absolute chronology of the younger series from 11,000 to 12,800 cal ybp, but the older chronology is not directly dated. Collectively these records offer remarkable insights into ice sheet processes and glacial lake paleohydrology. These include: rates of ice sheet retreat, which vary from 130 to 30 m/yr, with only a minor advance to the Eagle moraine; asynchronous ice lobe behavior between the Rainy Lobe, and adjacent Koochiching and Superior Lobes; moraine building events that last between 50 and 200 years; and multiple glacial lake drainage events, both into and out of the lake basins from which the varves derive. Our expectation is that this varve chronology will continue to be developed and refined, as it promises to be the longest and most geographically extensive glacial varve dataset in North America.