The Middle Wisconsinan History of the Laurentide Ice Sheet

Abstract

Evidence for Middle Wisconsinan ice limits and climates comes from sites scattered around the periphery of the Laurentide Ice domain and from the Hudson Bay Lowlands. Interpretations are based on dated wood, peat, shell and sediment; biological climate indicators (chiefly cool-climate indicators); and stratigraphie sequences of both glacial and nonglacial units. The best evidence comes from the prairie provinces and the Great Lakes areas, both of which indicate substantial ice retreat from earlier southern glacial limits, and cool, boreal-tundra climates. The western arctic may have experienced an early warm period but both the western arctic — northwestern plains and eastern maritime areas may later have become ice accumulation areas. Three maps portray various possible ice limits. The first shows substantial ice cover in the arctic, but reduced ice cover in the prairies and Great Lakes, and expanded maritime ice caps (rather than Laurentide Ice) in the southeast and on Baffin Island. This ice mass distribution may reflect Middle Wisconsinan shifts in air masses and ocean currents. Ice volumes generated by this model are in accord with the marine oxygen isotope record and perceived global sea level changes. A modification to this model, which resolves some of the controversy in the Hudson Bay Lowlands, shows a calving bay penetrating into the heart of the ice sheet, induced by dynamic instability of the marine-based ice mass in Hudson Bay during relatively high glacial isostatic and eustatic seas. A third reconstruction portrays extensive climatically induced déglaciation and retains Laurentide ice only in parts of the northwest and Labrador-Ungava, with local ice in the Appalachian-Atlantic region. This model is based on alternative genetic interpretations of lithologic units and reassessment of age assignments.

Pre-LGM Northern Hemisphere ice sheet topography

“…We have here chosen to base our synthesis primarily on previous spatial reconstructions (Dredge and Thorleifson, 1987;Clague, 1989;Clark et al, 1993;Kleman et al, 1997Kleman et al, , 2010Lundqvist, 2004;Svendsen et al, 2004;Mangerud, 2004;Winsborrow et al, 2004;Lambeck et al, 2006) that cover this time interval. A full consideration of primary morphological and stratigraphic data is offered in several of the key source publications.…”

“…For the extent of North American ice sheets before the LGM, we have based our 2-D outlines primarily on the recent review by Kleman et al (2010) and references therein, particularly Dredge and Thorleifson (1987), Clague (1989), Boulton and Clark (1990), Clark et al (1993), and Stea et al (2004). For the Keewatin sector of the Laurentide Ice Sheet there is good evidence for one or two, probably closely related, stages of a pre-LGM ice sheet with a dome center in the northern Keewatin or south-central Arctic region (Kleman et al, 2002(Kleman et al, , 2010McMartin and Henderson, 2004).…”

Late Wisconsinan and Holocene History of the Laurentide Ice Sheet*

“…On the southern Interior Plains ice advanced to its Late Wisconsinan limit from a severely retracted Middle Wisconsinan position (DREDGE and THORLEIFSON, 1987). This advance occurred after 27 ka at Watino, Alberta, after 24 ka at Medicine Hat, Alberta and at Zelena, Manitoba, and after 21 ka in the area covered by the Des Moines Lobe (FULTON etal., 1984;CLAYTON and MORAN, 1982).…”

Deglaciation of the Laurentide Ice Sheet from the Last Glacial Maximum

“…Following an MIS 4 maximum that may have been almost as large as the MIS 2 (LGM) volume according to some models (Marshall et al, 2000;Stokes et al, 2012), the ice sheet retreated to a mid-Wisconsinan (early MIS 3) minimum at some point between 60 and 40 ka (Dredge and Thorleifson, 1987;Clark et al, 1993;Kleman et al, 2010;Stokes et al, 2012). Indeed, the extent of the ice sheet during MIS 3 is very poorly constrained (e.g.…”

“…Indeed, the extent of the ice sheet during MIS 3 is very poorly constrained (e.g. see review in Dredge and Thorleifson, 1987), with numerical modelling indicating a relatively large ice sheet that stored up to 30 m of sea level equivalent at 55 ka (Marshall et al, 2000;Stokes et al, 2012), but with a suite of new dates raising the possibility that the Hudson Bay Lowlands, close to the geographic centre of the ice sheet, were completely ice free between ~50 and ~40 ka (see Dalton et al, 2016). Following the MIS 3 minimum, the ice sheet underwent gradual expansion that was punctuated by episodes of successively less recession (e.g.…”