ICE STREAMS OF THE LAURENTIDE ICE SHEET: CHARACTERISTICS, CONTROLS AND TIMING

We present an analysis of ice streams in the Laurentide Ice Sheet (LIS) based on new mapping (onshore and offshore) and a review of previously published work. A total of 117 ice streams are identified based on a variety of evidence including their bedform imprint, large-scale topography, till properties, and ice rafted debris in ocean sediment records. During the Last Glacial Maximum (LGM), ice streams formed a drainage pattern that bears close resemblance to the present day velocity patterns in modern ice sheets. Large ice streams had extensive onset zones and were fed by multiple tributaries and, where ice drained through regions of high relief, the spacing of ice streams shows a degree of spatial self-organisation. Topography exerted a primary control on the location of ice streams, but there are large areas along the western and northern margin that were underlain by sedimentary bedrock, and where networks of ice streams switched direction repeatedly. As the LIS retreated onto its low relief interior, several ice streams show no correspondence with topography or underlying geology, perhaps facilitated by localised build-up of pressurised subglacial meltwater. These ice streams have no modern analogue. We determine the timing of operation of individual ice streams using the most recently published ice margin chronologies. A number of ice streams with topographic control operated continuously for thousands of years from the LGM and throughout the early stages of deglaciation, predominantly in the northwestern, northern (Innuitian Ice Sheet), and eastern sectors. Another group of ice streams operated on much shorter time scales of perhaps as little as a few hundreds of years during deglaciation. This group comprises large ice streams along the western and southern margin and smaller ephemeral ice streams elsewhere. While around 30% of the ice sheet circumference was drained by ice streams at the LGM (similar to that for the present day Antarctic ice sheets), this number decreases to 15% and 12% at 12 cal ka BP and 10 cal ka BP, respectively. The extent to which these changes in the ice stream drainage network represent a simple readjustment to a changing mass balance driven by climate, or internal ice dynamical feedbacks unrelated to climate (or both) is largely unknown and represents a key area for future work to address.