Ice stream dynamics and pro-glacial lake evolution along the north-western margin of the Laurentide Ice Sheet.

Abstract

Ice streams drain ice sheets rapidly and are key regulators of their mass balance in both palaeo and contemporary settings. Present day ice streams can be identified, and their short-term activity monitored, by measuring the surface velocity of ice sheets. However, in order to understand their long-term behaviour, reconstructions of their activity in palaeo-ice sheets are necessary. Numerous palaeo-ice streams have been identified in the Laurentide Ice Sheet (LIS) and this has considerably refined our understanding of its dynamic behaviour and links to the ocean-climate system. In the north-west sector of the LIS, ice streaming has been hypothesised but detailed mapping of the area has not been carried out and so our understanding of palaeo-ice streaming is limited compared to other areas.
This thesis presents a new ice sheet reconstruction of the north-west sector of the LIS that incorporates ice stream activity and pro-glacial lake evolution. Mapping and analysis was carried out using a range of remote sensing imagery and Digital Elevation Models (DEMs), which enabled widespread, rapid and systematic coverage of the 800,000 km2 study area. More than 95,000 bedforms have been mapped, including glacial lineations, eskers, moraines and palaeo-channels. These data permit the identification and classification of 272 flow-sets which have been dated using an existing 14C database and relative cross-cutting relationships. Flow-sets are used to construct a robust and self-consistent ice sheet reconstruction, incorporating the activity of ice streams at a temporal resolution of up to 250-500 years.
The reconstruction reveals major changes in ice sheet configuration during Late Wisconsinan deglaciation and indicates that margin retreat was complex and dominated by the dynamic spatial and temporal evolution of seven ice stream systems. These ice streams were not synchronous but a peak in their activity occurred between 15 and 13 ka. Their location and behaviour was influenced by the availability of soft sediments, but their temporal switching was likely controlled by sub-glacial meltwater routing and ice piracy. Large proglacial lakes developed during deglaciaton but their evolution did not appear to control ice stream activity, as observed elsewhere in the ice sheet. However, major palaeo-channels are consistent with a previously hypothesised north-west drainage route for Glacial Lake Agassiz.