Ice sheet extent and early deglacial history of the southwestern sector of the Greenland Ice Sheet

Abstract

Corresponding author. Tel: 0044-191-344-1935; Fax: 0044-191-334-1801. D.H.Roberts@durham.ac.uk (D.H.Roberts). AbstractThe offshore and coastal geomorphology of southwest Greenland records evidence for the advance and decay of the Greenland Ice Sheet during the Last Glacial Maximum. Regional ice flow patterns in the vicinity of Sisimiut show an enlarged ice sheet that extended southwestwards on to the shelf, with an ice stream centred over Holsteinsborg dyb. High level periglacial terrain composed of blockfield and tors is dated to between 101 -142 ka using 26 Al and 10Be cosmogenic exposure ages. These limit the maximum surface elevation of the Last Glacial Maximum ice sheet in this part of southwest Greenland to ca 750 -810 m asl, and demonstrate that terrain above this level has been ice free since MIS 6. Last Glacial Maximum ice thickness on the coast of ca 700 m implies that the ice sheet reached the mid to outer continental shelf edge to form the Outer Hellefisk moraines. Exposure dates record ice surface thinning from 21.0 to 9.8 ka, with downwasting rates varying from 0.06 to 0.12m yr -1 . This reflects strong surface ablation associated with increased air temperatures running up to the Bølling Interstadial (GIS1e) at ca 14 ka, and later marine calving under high sea levels. The relatively late retreat of the Itilleq ice stream inland of the present coastline is similar to the pattern observed at Jakobshavn Isbrae, located 250 km north in Disko Bugt, which also retreated from the continental shelf after ca 10 ka. We hypothesise that the ice streams of West Greenland persisted on the inner shelf until the early Holocene because of their considerable ice thickness and greater ice discharge compared with the adjacent ice sheet. IntroductionMass balance observations of the Greenland Ice Sheet (GIS) show that it is undergoing rapid change (Howat et al., 2005;Chen et al., 2006;Joughin, 2006;Rignot and Kanagaratnam, 2006;Velicogna and Wahr, 2006;Holland et al., 2008). These short term changes may be a response to climate warming, or natural instability in ice sheet behaviour. There is a need, therefore, to develop longer-term perspectives on ice sheet 2 behaviour in order to improve our knowledge of the internal and external environmental controls that drive ice sheet fluctuation.Furthering our understanding the long term behaviour of the western sector of the GIS is important for several reasons. Firstly, early geomorphological research established the Sisimiut region as a benchmark study area for the deglacial chronology of West Greenland (Weidick, 1972;Ten Brink and Weidick, 1975;Kelly, 1985;Funder, 1989). However, the deglacial chronology relied largely on radiocarbon dated marine shells and direct dating of deglacial terrain was not possible. Moreover, recent work has begun to challenge aspects of the resulting regional deglacial chronology. In the Disko Bugt region, for example, ice margin retreat occurred later than originally thought, whilst the early Holocene "Fjord stade" moraines, previously da...

Coupled ice sheet–climate modeling under glacial and pre-industrial boundary conditions

“…Further continuously active glaciers follow on the southern east coast. On the west coast, ice streams surge repeatedly in the region of Jakobshaven Isbrae (E) and the trough north of it, matching proxy observations in Roberts et al (2009). Further north, in the region of Kong Oscar Glacier (F), two tributaries show surge-type activity.…”

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

“…New constraints upon ice stream extent, thickness, and deglacial behaviour from the LGM to present have been established since the early 2000's both onshore (Bennike and Bjorck, 2002, Håkansson et al, 2007, Håkansson et al, 2009, Roberts et al, 2008, Roberts et al, 2009, Roberts et al, 2010, Roberts et al, 2013 and offshore (Ó Cofaigh et al, 2004, Evans et al, 2009, Dowdeswell et al, 2010. These have provided evidence for the existence of large ice streams across the West, Northeast and East Greenland continental shelves during the last glacial cycle (Lykke-Andersen, 1998, Ó Cofaigh et al, 2004, Evans et al, 2009, Dowdeswell et al, 2010.…”

Permafrost degradation risk zone assessment using simulation models

“…Sediments are mainly found in local bedrock depressions. All of Greenland was ice covered during the last glaciation (Weichsel-Wisconsin), and the ice retreated from the west coast around 10 000 years ago (Bennike and Björck, 2002;Roberts et al, 2009). At that time, the local sea level is expected to have been 70 to 150 m above the present level (Rasch, 2000), and a series of marine silt and clay rich sediments were deposited on the sea bottom.…”