Controls upon the Last Glacial Maximum deglaciation of the northern Uummannaq Ice Stream System, West Greenland

The Uummannaq Ice Stream System (UISS) was a convergent cross-shelf ice stream system that operated in West Greenland during the Last Glacial Maximum (LGM). This paper presents new evidence constraining the geometry and evolution of the northern sector of the UISS and considers the factors controlling its dynamic behaviour. Geomorphological mapping, 21 new terrestrial cosmogenic nuclide (TCN) exposure ages, and radiocarbon dating constrain LGM warm-based ice stream activity in the north of the system up to 1400ma.s.l. Intervening plateaux areas either remained ice free, or were covered by cold-based icefields. Beyond the inner fjords, topography and bathymetry forced ice flow southwards into the Uummannaq Trough, where it coalesced with ice from the south, and formed the trunk zone of the UISS.Deglaciation of the UISS began at 14.9cal.kaBP. Rapid retreat from the LGM limit was forced by an increase in air temperatures and rising sea level, enhanced by the bathymetric over-deepening of the Uummannaq and Igdlorssuit Sund troughs. Ice reached the inner fjord confines in the northern Uummannaq area by 11.6ka and experienced an ice marginal stabilisation in Rink-Karrat Fjord for up to 5ka. This was a function of topographic constriction and bathymetric shallowing, and occurred despite continued climatic forcing. In the neighbouring Ingia Fjord this did not occur. Following this period of stability, ice within Rink-Karrat Fjord retreated, reaching the present ice margin or beyond after 5ka. The presence of a major ice stream within a mid-fjord setting, during the mid-Holocene and the Holocene Thermal Maximum (~11-5ka) is in direct contrast to records of other ice streams throughout West Greenland, which suggest ice had retreated beyond its present margin by 9-7ka. This demonstrates the potential importance of topographic control on calving margin stability, and its ability to override climatic forcing.