THE NORTHERN UUMMANNAQ ICE STREAM SYSTEM, WEST GREENLAND: DEGLACIAL HISTORY AND CONTROLS UPON DEGLACIATION

Ice streams drain the majority of ice from contemporary and palaeo-ice sheets. Through time these leave a geomorphological imprint upon the landscape as a result of fluctuations in their thickness and extent. The Uummannaq Ice Stream System (UISS) is a Greenlandic palaeo-ice stream which flowed to the shelf-edge at the Last Glacial Maximum (LGM). Geomorphological mapping and surface exposure dating (15 ages) of the northern UISS constrained LGM ice stream activity to 1400 m a.s.l., with intervening plateaux remaining ice free or fostering cold-based icefields. Regional topography forced individual fjords to become coalescent into the Uummannaq Trough, forming the trunk zone of the UISS.

Deglaciation of the UISS began by 14.9 kyr, forced by increasing air temperature and rising sea level, further enhanced by the bathymetric overdeepening of the Uummannaq Trough. In Rink-Karrat Fjord ice reached the mid-fjord region by 11.6 kyr, where the ice margin stabilised. This stabilisation lasted for up to 5 kyr, forming a series of inset lateral moraines. These moraines appear similar in both form and position to other moraines from West Greenland which have been ascribed to the 9.3 and 8.2 ka events. However, the moraines from this study cannot be correlated to such climate events. Stabilisation occurred due to fjord constriction and bathymetric shallowing, despite continued air temperature increase during the Holocene. Following this, ice retreated, reaching the present ice margin after 5 kyr. The presence of a major ice stream within a mid-fjord setting during the Holocene Thermal Maximum (~11 – 5 kyr) is in direct contrast to other ice streams throughout West Greenland, which suggest ice had retreated beyond its present margin by 9-7 kyr. This demonstrates the potential importance of topographic control on calving margin stability, and its ability to override climatic forcing.