Rapid early Holocene ice retreat in West Greenland

The possible demise of the Greenland ice sheet and its effect on global sea level rank among the most serious climate threats to society. To improve our knowledge about the future behaviour of the ice margin, we studied the ice sheet's response to early Holocene warming in West Greenland using 47 cosmogenic 10Be exposure ages, 26 optically-stimulated luminescence ages as well as 15 new and 28 previously published radiocarbon ages. Paired bedrock and boulder ages show that the entire area was covered by warm-based ice during the Last Glacial Maximum (LGM), although glacial erosion was insufficient to completely remove the upper rock surface containing 10Be inherited from a previous period of exposure in bedrock samples above an elevation of 800 m. Our compilation of 10Be and 14C ages demonstrates that the ice sheet retreated from the outer-coast to the present ice margin between c. 11.4 and 10.4 cal. ka BP in the Godthåbsfjord system and between 10.7 ± 0.6 and 10.1 ± 0.4 ka ago in Buksefjord, whereas the coast at Sermilik became ice free at c. 10.5 cal. ka BP. We find no significant changes in the retreat rates between the deep Godthåbsfjord system and the Buksefjord-Sermilik region, which is characterized by only a few narrow and shallow fjords. However, deglaciation was initiated c. 700–900 years earlier in the Godthåbsfjord system indicating that the deep fjords probably triggered land-based deglaciation by dynamic ice loss leading to an overall rapid early Holocene ice retreat and drawdown of the ice sheet in West Greenland. These results demonstrate that even if there was a topographic control on the onset of deglaciation, fast ice retreat is not restricted to deep fjord systems but may occur independently of the topographic setting.