GLACIOLOGICAL AND CHRONOLOGICAL CONSTRAINTS OF LAURENTIDE ICE SHEET OCCUPATION OF THE SUPERIOR BASIN DURING THE YOUNGER DRYAS

The glacial history of the Laurentide Ice Sheet (LIS) in the Lake Superior Basin during the Younger Dryas cold period (11 – 10 ¹⁴C ka) is not well constrained.  Reliable chronologic control is limited to wood radiocarbon dates in till and in-situ trees buried by outwash dating from 9.73 to 10.42 ¹⁴C ka (average ~10 ¹⁴C ka) along the southern margin of modern Lake Superior.  Several bulk radiocarbon dates are located within the basin (e.g. 10.8 and 10.65 ¹⁴C ka near Sault Ste. Marie, Ontario ), but are subject to contamination and may be too old.  All other wood radiocarbon dates within the Superior Basin are <10 ¹⁴C ka.  To further address deglacial chronology in the Lake Superior Basin, we i) obtained basal radiocarbon dates from Livingstone sediment cores inside and outside of the Hartmann, Steep Rock, Brule Creek and Marks Moraines in northwestern Ontario, and ii) logged cutbank exposures and collected macrofossils for radiocarbon dating along the Whitefish and Kamininistiquia Rivers west of Thunder Bay, Ontario.  Radiocarbon dates of 10,190 ± 40 ¹⁴C yrs BP (Beta-195959) and 10,250 ± 75 ¹⁴C yrs BP (ETH-31430) from sediment cores north of the Brule Creek Moraine indicate deglaciation occurred ~10.3 ka ¹⁴C yrs BP.  Further north, inside the Hartmann Moraine, a date of 9705 ± 145 ¹⁴C yrs BP (ETH-31001) indicates ice retreated from the Hartmann Moraine <~10 ka ¹⁴C yrs BP.  These dates suggest a gradual retreat of the LIS (Rainy Lobe) in northwestern Ontario during the latter part of the Younger Dryas.  However, calving loss would promote rapid retreat of the Superior Lobe once ice retreated from the south shore of Lake Superior , especially in the eastern basin where water depths reach 400 meters.  A >200 kilometer readvance from the north shore to the south shore of Lake Superior during the Younger Dryas seems unlikely given known maximum advance rates of modern and Holocene glaciers and the chronological constraints on events in the basin.  Therefore, it would seem plausible that ice remained in the Superior Basin during the Younger Dryas and that the Marquette readvance was only a minor advance.