Evidence for the asynchronous retreat of large outlet glaciers in southeast Greenland at the end of the last glaciation

Abstract

Be exposure dating Fjord deglaciation a b s t r a c t Recent rapid changes in the marine-terminating sectors of the Greenland Ice Sheet (GrIS) have prompted concerns about the future stability of the ice sheet. Long-term records of ice sheet behaviour provide valuable context to assess the magnitude of current change and may help resolve the mechanisms driving deglaciation. We report 23 new 10 Be exposure ages which constrain the deglacial history of two large fjord systems in southeast (SE) Greenland. We compare our chronologies with existing data from the centre of the sector to examine the timing and style of deglaciation at a regional-scale. Glacial erratic 10 Be exposure ages demonstrate that Kangerdlugssuaq Fjord deglaciated at~11.8 ka at the end of the Younger Dryas (YD e 12.8e11.6 ka). Retreat at Kangerdlugssuaq Fjord coincided with known incursion of the warm Irminger Current (IC) onto the continental shelf; this is inferred to have initiated retreat. Comparison with recently published results from Sermilik Fjord and new 10 Be ages from Bernstorffs Fjord indicates deglaciation occurred~1 ka later in the south of the SE region. Sermilik Fjord (~10.9 ka) and Bernstorffs Fjord (~10.4 ka) deglaciated later; retreat likely occurred in response to dramatic climatic amelioration at the termination of the YD stadial. We suggest that the disparate timing of deglaciation across the SE region may be primarily explained by the varying influence of the warm IC; glaciers in southern SE Greenland were isolated from warm Atlantic waters during the YD by complex shelf bathymetry. In all fjord settings ice retreat was rapid and persistent, consistent with the absence of geomorphological evidence for stillstand or readvance events. Ice retreat was accompanied by rapid thinning and likely continued to well within present-day ice sheet margins. Glacial erratic 10 Be age determinations and geomorphological observations show no evidence for Holocene readvance events prior to the Little Ice Age (LIA).

New geophysical evidence for a revised maximum position of part of the NE sector of the Greenland ice sheet during the last glacial maximum

“…Evans et al 2008;Winkelmann et al 2010). Sheet (Dyke et al 2014) and the Norwegian Channel Ice Stream draining much of the SW Fennoscandian Ice Sheet (Svendsen et al 2015). …”

The Greenland Ice Sheet during the last glacial cycle: Current ice loss and contribution to sea-level rise from a palaeoclimatic perspective

“…Even in areas where moraine ridges are completely lacking it is possible to build a deglacial chronology by combining ages obtained from exposure dating of bedrock surfaces and radiocarbon dates of terrestrial deposits (e.g. Hughes et al, 2012;Carlson et al, 2014;Dyke et al, 2014;Winsor, 2014).…”

“…It was the period when the ice margin first retreated onto land after the LGM along most of the Greenland coast, and in many areas it retreated all the way to its present position and beyond (Hughes et al, 2012;Larsen et al, 2014;Carlson et al, 2014;Dyke et al, 2014;Winsor, 2014;Winsor et al, 2015).…”

“…Weidick et al, 2012), has been used to determine when the GrIS margin retreated close to, or beyond, its Late Holocene maximum extent in areas all around Greenland. A compilation of this evidence (Young and Briner, 2015) shows that the ice margin retreated back to its Late Holocene maximum position or beyond during the Early Holocene in northwest Greenland between 72°N and 78°N (Corbett et al, 2015;Young and Briner, 2015), in west Greenland south of~65°N (Kaplan et al, 2002;Weidick et al, 2012;Carlson et al, 2014;Larsen et al, 2014;Winsor et al, 2015), and in east Greenland south of Kangerdlussuaq Fjord (~68°N) Hughes et al, 2012;Dyke et al, 2014). The remaining part of the ice margin probably stayed outside its Late Holocene maximum until the Middle Holocene, as summarised in Section 9.2, and empirical data therefore points towards an ice sheet of fairly similar size as the present at the transition to the Middle Holocene.…”

“…Early Holocene retreat rates have been studied for Jakobshavn Isfjord (Young et al, 2011a) and the Godthåbsfjorden area in west-and southwest Greenland Winsor et al, 2015), and for several fjord systems in southeast Greenland (Hughes et al, 2012;Dyke et al, 2014). Sermilik Fjord became deglaciated over a period of about 1 ka, centred around 11.4 ka BP (original age from Hughes et al (2012) adjusted for new Arctic 10 Be production rate (Young et al, 2013b) in Dyke et al (2014)), and this corresponds to a mean retreat rate of~80 m a − 1 , which is the highest Early Holocene retreat rate reconstructed for tidewater glaciers in southeast Greenland . In Jakobshavn Isfjord, on the other hand, the glacier margin remained at a position close to the fjord mouth (cf.…”