RESOLVING THE ROLE OF COLD-BASED ICE ALONG THE NE LAURENTIDE MARGIN: NEW INSIGHTS FROM A DECADE OF COSMOGENIC EXPOSURE DATING

Ice-sheet reconstructions have long relied on classical ice-erosional and ice-constructional features to define ice-sheet boundaries. However, for ice to perform work on the landscape requires differential movement between the ice and the underlying terrain. Along most of the southern margins of the great Pleistocene ice sheets, these conditions are common. However, at high latitudes, where ice-age temperatures were well below freezing in summer at sea level, ice sheets may have been frozen to their beds wherever the ice was relatively thin and/or slow moving. Whether time or changes in basal thermal conditions provide the best explanations for diagnostic changes in landform characteristics in the Arctic has been debated for decades, but with little secure evidence for most viewpoints. We have been tackling the question of cold ice – landscape interactions for the past decade along eastern Baffin Island, the NE margin of the former Laurentide Ice Sheet, utilizing isotopes of Be, Al, and C in surficial quartz created by the flux of cosmic rays striking Earth's surface. Basal thermal conditions during the last glacial maximum (LGM) were dependent on ice thickness and evolved during the glacial cycle. Prior to 15 ka, Laurentide ice covered most of the extensive, low-lying forelands, but accomplished virtually no work, and its passage can only be demonstrated by the ages of large erratic blocks left on the surface of undisturbed pre-LGM landscapes. Even meltwater channels apparently relate to a pre-LGM glacial cycle. After 14 ka, the landscape is characterized by classic ice-constructional and ice-erosional features. Fast-flowing ice in the major fiord systems lowered bedrock surfaces at sea level by an amount sufficient to remove any previously acquired cosmogenic isotopes, but the efficiency of erosion weakens rapidly with elevation. These dramatic contrasts in landscape impacts are also apparent in the modern regime. Small, cold-based ice caps mantling the rolling interior of Baffin Island are now rapidly retreating from their Little Ice Age maxima, reflecting a snow-line rise of at least 600 m. Ice-cap retreat reveals undisturbed ancient landscapes of soft sediment. Cosmogenic radiocarbon dating offers the ability to place 20th century warming in a longer-term perspective.