At the Last Glacial Maximum (LGM), about 18-21 ka (21,000-25,000cal.yr), ice over Canada and the northern USA formed a nearly continuous cover which merged with the Greenland Ice Sheet (e.g. Dyke & Prest, 1987a; Dyke et al., 2003, Dyke, 2004). Beyond this area, glaciers expanded over the higher mountains in the western USA, Alaska and the Yukon Territory. Contiguous ice sheets included the Cordilleran (mostly over British Columbia, and in southern Yukon and Alaska at its maximum about 16.5ka (19,700cal.yr)), Innuitian (in Arctic Canada), Laurentide (mostly over the remainder of Canada and adjacent northern USA) and Greenland (Fig. 28.1). The largest and most variable of these was the Laurentide Ice Sheet (LIS), which has been estimated to have accounted for 50% of the increased ice volume on Earth during the LGM (Boulton et al., 1985). By about 5ka (5700cal.yr), ice remained only over Greenland, the highlands of Baffin and Queen Elizabeth islands of Arctic Canada, and the highest elevations of the western Cordillera.

The baseline surface runoff from precipitation and meltwater from the southern margin of the LIS had profound effects on North America and on the North Atlantic Ocean and climate (Teller, 1990a,b; Licciardi et al., 1999). Supplementing this runoff were periodic outbursts of water stored in ice-marginal lakes and, possibly, in subglacial reservoirs (Shaw, 1989, 2002; Teller et al., 2002). Iceberg production also added freshwater to the oceans, with the most intense periods known as Heinrich events, based on ice-rafted detritus found in Atlantic Ocean sediments (Ruddi-man, 1977; Heinrich, 1988; Bond et al., 1999; Clarke et al., 1999). In this chapter, we review the history of surface runoff from the southern margin of the LIS during the last period of deglaciation, with particular reference to major abrupt changes in runoff magnitude and routing and their possible relationship to changes in thermohaline circulation (THC) and the North Atlantic Deep Water (NADW) formation. Recent advances in modelling ice-ocean-atmosphere and lake-ice-atmosphere feed-back processes are also discussed in the context of their importance in forcing ice-marginal fluctuations and diversions of runoff routing. Finally, the sensitivity of North Atlantic THC to freshwater runoff during the last deglaciation is explored with particular reference to events of abrupt climate change.

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