EVOLUTION OF LAURENTIDE ICE CATCHMENT AREAS THROUGH ICE LOBE ACTIVITY AND TILL PROVENANCE

Imagine a Laurentide Ice Sheet similar to the modern West Antarctic Ice Sheet, with the balance dominated by discharge through fast moving ice streams that feed outlet glaciers or ice shelves. Ice flux to terrestrial outlet glaciers or ice lobes would have varied as the ice catchment area evolved. Catchment evolution would have included the headward migration of ice stream onset areas, changes in the number and configuration of tributary ice streams, changes in width of ice streams, and shutting down of streaming flow on century time scales. These mechanical controls on ice flux would have strongly overprinted climatic controls on ice advance. However, climate would have affected how quickly an ice lobe ablated after advancing to lower latitudes and commonly stagnating.

Sediment that is conveyed by ice streams to ice lobes can be studied to increase our understanding of the deglacial evolution of the Laurentide Ice Sheet. The sediment record of southern Laurentide ice lobes is nearly continuous and accessible at the surface as is the sediment record of their source areas. It is clear that sediment is incorporated far up-ice in the ice catchment areas and mainly conveyed by the lobes, that ice lobes were fed by evolving ice streams, that multiple ice streams were active at any given time but were not synchronous, and that outlet glaciers competed for space in the glacial foreground. Ice streams also changed dominance over time and many had a profound reorganization after the last Glacial Maximum.

The southern Laurendtide ice lobes were active during the interval between the 19-kyr meltwater pulse (mwp) and mwp 1A. They were the primary means by which ice was redistributed to lower latitudes during this period. Meltwater was released gradually as the ice lobes stagnated except in instances where it was temporarily stored in proglacial lakes and released through a spillway, catastrophically in some cases.