Ocean forcing of Ice Sheet retreat in central west Greenland from LGM to the early Holocene

Abstract

*Manuscript Click here to view linked References 2 2 Three radiocarbon dated sediment cores from trough mouth fans on the central west Greenland continental slope were studied to determine the timing and processes of Greenland Ice Sheet (GIS) retreat from the shelf edge during the last deglaciation and to test the role of ocean forcing (i.e. warm ocean water) thereon. Analyses of lithofacies, quantitative x-ray diffraction mineralogy, benthic foraminiferal assemblages, the sea-ice biomarker IP 25 , and  18 O of the planktonic foraminifera Neogloboquadrina pachyderma sinistral from sediments in the interval from 17.5-10.8 cal ka BP provide consistent evidence for ocean and ice sheet interactions during central west Greenland (CWG) deglaciation. The Disko and Uummannaq ice streams both retreated from the shelf edge after the last glacial maximum (LGM) under the influence of subsurface, warm Atlantic Water. The warm subsurface water was limited to depths below the ice stream grounding lines during the LGM, when the GIS terminated as a floating ice shelf in a sea-ice covered Baffin Bay. The deeper Uummannaq ice stream retreated first, (ca. 17.1 cal ka BP), while the shallower Disko ice stream retreated at ca. 16.2 cal ka BP. The grounding lines were protected from accelerating mass loss (calving) by a buttressing ice shelf and by landward shallowing bathymetry on the outer shelf. Calving retreat was delayed until ca. 15.3 cal ka BP in the Uummannaq Trough and until 15.1 cal ka BP in the Disko Trough, during another interval of ocean warming. Instabilities in the Laurentide, Innuitian and Greenland ice sheets with outlets draining into northern Baffin Bay periodically released cold, fresh water that enhanced sea ice formation and slowed GIS melt. During the Younger Dryas, the CWG records document strong cooling, lack of GIS meltwater, and an increase in iceberg rafted material from northern Baffin Bay. The ice sheet remained in the cross-shelf troughs until the early Holocene, when it retreated 3 3 rapidly by calving and strong melting under the influence of atmosphere and ocean warming and a steep reverse slope toward the deep fjords. We conclude that ocean warming played an important role in the palaeo-retreat dynamics of the GIS during the last deglaciation.

Ice stream reorganization and glacial retreat on the northwest Greenland shelf

“…The landforms also show no evidence of significant reworking by iceberg scours or current winnowing, and it is therefore assumed that they are from the last glacial cycle. Although an older glaciation cannot be completely ruled out, this assumption is in line with numerical modeling studies of LGM ice sheet extent [ Lecavalier et al ., ] and dated landform records to the south [ Jennings et al ., ; Sheldon et al ., ].…”

“…The precise timing of the mid‐shelf GZW formation is uncertain. Logically, one might expect a period of slow retreat or even readvance during the Younger Dryas (12.9–11.7 ka) (Figure ), as has been observed in the Disko and Uummannaq Troughs [ Dowdeswell et al ., ; Hogan et al ., ; Jennings et al ., , ]. In particular, the landform assemblage in the Uummannaq Trough, with evidence for a shelf edge maximum and a mid‐shelf stillstand during retreat, is similar to that observed here.…”

“…If the mid‐shelf GZW was deposited during the Younger Dryas and ice margin retreat from the shelf edge began in unison with ice streams to the south at 17–16 ka [ Jennings et al ., ], then retreat between the shelf edge and mid‐shelf GZWs (~80 km) would have occurred at ~20–25 km kyr −1 . Likewise, if grounding‐line retreat from the mid‐shelf GZW began after the Younger Dryas, it would have retreated ~100 km to the coastline by ~10 ka, a retreat rate of ~60 km kyr −1 .…”

“…This was associated with cold freshwater release in northern Baffin Bay that occurred intermittently after 14.3 cal ka B.P. and helped promote formation of the large mid‐shelf GZW in the Uummannaq Trough [ Jennings et al ., ]. Reduced northward advection of heat across Uummannaq implies cooler conditions in the MBT and similar landform assemblages between the two troughs might suggest that oceanographic changes resulted in a comparable evolution of the ice sheet margin.…”

Sea ice, ice‐rafting, and ocean climate across Denmark Strait during rapid deglaciation (∼16–12 cal ka BP) of the Iceland and East Greenland shelves

“…14 C dates from core tops on the Kangerlussuaq Trough Mouth Fan are ∼15–16 14 C ka BP (Andrews et al ., ; Dunhill, ) (Fig. B) and evidence from NE and West Greenland indicates that the Greenland Ice Sheet started to retreat shortly after the LGM (O'Cofaigh et al ., ; Jennings et al ., ). Many dates on either side of Denmark Strait range between 12 and 15 14 C ka BP (Fig.…”