ICE STREAM INITIATION AND SHUT-DOWN: EVIDENCE FROM THE DUBAWNT LAKE PALAEO-ICE STREAM, NW CANADIAN SHIELD

Ascertaining the triggers for ice streaming and the controls on their location is essential in order to understand the behaviour of former ice sheets and their interactions with the ocean-climate system. The prevailing paradigm for the Laurentide Ice Sheet is that ice streams predominantly occurred in topographic troughs that caused localised flow acceleration, or over areas of soft fine-grained sediment that facilitated rapid ice flow through subglacial till deformation or basal sliding. The location of the Dubawnt Lake ice stream in Keewatin on the low relief, relatively hard bedrock of the north-western Canadian Shield conflicts with this paradigm and raises a number of questions regarding its activity: why did the ice stream initiate in this location, and what caused it to switch on (and off)? We use a combination of digital satellite imagery (Landsat ETM+ and ASTER) and Digital Elevation Models (DEMs) to explore the idea that the development of a glacial lake was an important trigger for this ice stream. Using reconstructed ice flow patterns and DEMs the location and depth of ice-dammed lakes are predicted at a range of margin positions. Calving into a deep (>100 m) lake in the Thelon Basin is likely to have been influential in triggering the ice stream by inducing high calving rates, drawing down ice and propagating fast ice flow. After ice stream initiation the margin is inferred to have advanced into the lake and through the lake basin. Identification of lake shorelines superimposed on the ice stream flow pattern is clear evidence that a lake formed in a similar location post-ice stream activity. Transverse ridges (ribbed moraines) are also superimposed on the ice stream bedforms (drumlins and mega scale glacial lineations) predominantly in the upstream (onset) zone. We argue that their occurrence is a manifestation of ice stream shut-down and speculate that sticking and slipping as basal coupling increased and/or localised basal freeze-on, may have formed them. It is concluded that glacial lakes are likely to have been an important control on ice sheet dynamics and the deglaciation of the Laurentide Ice Sheet. This work also highlights the potential of detailed examination of palaeo-ice stream beds to test theories of ice stream shut-down (freeze-on, water piracy, etc.).