Decay of the Cordilleran Ice Sheet at the end of the Pleistocene (Invited)

At its maximum, the last (Late Wisconsinan) Cordilleran Ice Sheet and its ensemble of coalescent alpine glaciers covered an area of about 2 million km2 and had an ice volume of 3-4 million km3 (ca. 6-8 m of eustatic drawdown of Earth’s oceans). The ice sheet covered all of British Columbia, southern and central Yukon Territory, southern Alaska, western Alberta, northern Washington State, northern Idaho, and northwest Montana, and the continental shelves of British Columbia and Pacific Alaska. This ice sheet grew discontinuously over a period of about 18 kyr from about 35 ka ago to 17 ka ago. In contrast, it disappeared in half that time; ice cover in northwest North America by 10.5 ka was no more extensive than it is today. Deglaciation occurred by frontal retreat along the margins of the ice sheet and downwasting and complex frontal retreat in its interior. The pattern of deglaciation in interior British Columbia and Yukon was particularly complex, with lowering of the ice surface dividing the ice sheet into a series of tongues in valleys, many of which eventually stagnated. The average loss of ice during the deglacial period was several hundred cubic kilometres per year, which is substantially larger than the average rate of ice loss from the Greenland Ice Sheet over the past decade. The relatively rapid decay of the Cordilleran Ice Sheet probably was caused by a combination of factors, including atmospheric warming, eustatic sea-level rise at the western margin of the ice sheet, and establishment of warmer near-surface waters in the North Pacific due to a change in ocean circulation. Climate, however, flickered during the early stages of deglaciation — intervals of warmth alternated with much cooler periods, including those corresponding to the Older and Younger Dryas chronozones. Deglaciation was so advanced by the beginning of the Younger Dryas that much of the ice sheet was unable to respond to the cooler climate. Some alpine glaciers and high-elevation ice caps, however, grew in size during the Younger Dryas interval. Climate flickering ended shortly around 11 ka, with prolonged warmer and probably drier conditions of the early Holocene.