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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).

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The Greenland Ice Sheet has experienced significant mass loss in recent years. A substantial component of this is attributable to the retreat of marine-terminating outlet glaciers, which lose mass through increases in calving, submarine melting and terrestrial meltwater discharge. In terms of iceberg production, Jakobshavn Isbræ is the largest marine-terminating glacier in Greenland, yet relatively little is known about its history before the first glacier margin observations in 1851. Two marine sediment cores obtained 15 and 19 km northwest from the mouth of Jakobshavn Isfjord were analysed to reconstruct the past behaviour of Jakobshavn Isbræ and to investigate the response of the glacier system to ocean forcing. These records provide long-term (~2000) context for assessing the significance of the rapid changes in glacier stability over the last century. The X-ray imagery and high-resolution grain size analysis from both cores reveal distinct multi-centennial-scale changes in the flux of iceberg-rafted debris (IRD) from Jakobshavn Isbræ. Foraminiferal analysis shows that variability in the relatively warm West Greenland Current (WGC) may have been an important driver of calving activity at Jakobshavn Isbræ. We find that iceberg rafting and WGC inflow were relatively high from onset of the record, at 60 BC, until AD 1100. Subsequently, the inflow of the WGC into Disko Bugt decreased. This was accompanied by a dramatic reduction in IRD from AD 1500 to 1850, which is attributed to the establishment of a floating ice tongue. We also show that ocean warming in the 20th century is part of a longer-term warming trend in the WGC which started at around AD 1700. Finally, these new records underline the complexity of glaciomarine sediments; IRD variability was driven by the inflow of the WGC but was also modulated by a complex interplay of air temperature, sea-ice coverage and ice margin proximity.

Køge Bugt, in southeast Greenland, hosts three of the largest glaciers of the Greenland Ice Sheet; these have been major contributors to ice loss in the last two decades. Despite its importance, the Holocene history of this area has not been investigated. We present a 9100 year sediment core record of glaciological and oceanographic changes from analysis of foraminiferal assemblages, the abundance of ice-rafted debris, and sortable silt grain size data. Results show that ice-rafted debris accumulated constantly throughout the core; this demonstrates that glaciers in Køge Bugt remained in tidewater settings throughout the last 9100 years. This observation constrains maximum Holocene glacier retreat here to less than 6 km from present-day positions. Retreat was minimal despite oceanic and climatic conditions during the early-Holocene that were at least as warm as the present-day. The limited Holocene retreat of glaciers in Køge Bugt was controlled by the subglacial topography of the area; the steeply sloping bed allowed glaciers here to stabilise during retreat. These findings underscore the need to account for individual glacier geometry when predicting future behaviour. We anticipate that glaciers in Køge Bugt will remain in stable configurations in the near-future, despite the predicted continuation of atmospheric and oceanic warming.