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 emissions 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.
This work was funded by NERC Standard Grant NE/N011228/1 and some radiocarbon analysis funded by NEIF under grant NE/S011587/1 (allocation number 2169.1118). We thank the Alfred Wegner Institute, and particularly Angelika Humbert and Hicham Rafiq, for their significant logistic support through the iGRIFF project. Additional support was provided from Station Nord (Jorgen Skafte), Nordland Air, Air Greenland and the Joint Arctic Command. Naalakkersuisut, Government of Greenland, provided Scientific Survey (VU-00121) and Export (046/2017) licences for this work. Finally, we would like to thank our Field Ranger Isak (Nanu-Travel) and dog Ooni for keeping us safe in the field and taking great pleasure in beating JAS at cards.