Reply on RC2

Abstract

Nioghalvfjerdsbrae, or 79° N Glacier, is the largest marine-terminating glacier draining Northeast Greenland Ice Stream (NEGIS). In recent years, it’s ~70 km-long fringing ice shelf (hereafter referred to as 79° N ice shelf) has thinned, and a number of small calving events highlight its sensitivity to climate warming. With the continued retreat of 79° N ice shelf and the potential for accelerated discharge from NEGIS, which drains 16 % of the Greenland Ice Sheet (GrIS), it has become increasingly important to understand the long-term history of the ice shelf in order to put the recent changes into perspective and to judge their long-term significance. Here we reconstruct the Holocene dynamics of 79° N ice shelf by combining radiocarbon dating of marine mollusc from isostatically uplifted glacimarine sediments with a multi-proxy investigation of two sediment cores recovered from Blåsø, a large epishelf lake 2–13 km from the current grounding line of 79° N Glacier. Our reconstructions suggest that the ice shelf retreated between 8.5 and 4.4 cal. ka. BP, which is consistent with previous work charting grounding line and ice shelf retreat to the coast, and open marine conditions in Nioghalvfjerdsbrae. Ice shelf retreat followed a period of enhanced atmospheric and ocean warming in the Early Holocene. Based on our detailed sedimentological, microfaunal and biomarker evidence the ice shelf reformed at Blåsø after 4.4 cal. ka BP, reaching a thickness similar to present by 4.0 cal. ka BP. Reformation of the ice shelf coincides with decreasing atmospheric temperatures, increased dominance of Polar Water, a reduction in Atlantic Water and (near) perennial sea-ice cover on the adjacent continental shelf. Together with available climate archives our data indicate that 79° N ice shelf is susceptible to collapse when mean atmospheric and ocean temperatures are ~2 °C warmer than present, which could be achieved by the middle of this century under some climate model scenarios. Finally, the presence of ‘marine’ markers in the uppermost part of the Blåsø sediment cores could record modern ice shelf thinning, although the significance and precise timing of these changes requires further work.