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The ~31-km-wide Hiawatha structure, located beneath Hiawatha Glacier in northwestern Greenland, has been proposed as an impact structure that may have formed after the Pleistocene inception of the Greenland Ice Sheet. To date the structure, we conducted 40 Ar/ 39 Ar analyses on glaciofluvial sand and U-Pb analyses on zircon separated from glaciofluvial pebbles of impact melt rock, all sampled immediately downstream of Hiawatha Glacier. Unshocked zircon in the impact melt rocks dates to ~1915 million years (Ma), consistent with felsic intrusions found in local bedrock. The 40 Ar/ 39 Ar data indicate Late Paleocene resetting and shocked zircon dates to 57.99 ± 0.54 Ma, which we interpret as the impact age. Consequently, the Hiawatha impact structure far predates Pleistocene glaciation and is unrelated to either the Paleocene-Eocene Thermal Maximum or flood basalt volcanism in east Greenland. However, it was contemporaneous with the Paleocene Carbon Isotope Maximum, although the impact’s exact paleoenvironmental and climatic significance awaits further investigation.

Determining the sensitivity of the Greenland Ice Sheet (GrIS) to Holocene climate changes is a key prerequisite for understanding the future response of the ice sheet to global warming. In this study, we present new information on the Holocene glacial history of the GrIS in Inglefield Land, north Greenland. We use 10Be and in situ 14C exposure dating to constrain the timing of deglaciation in the area and radiocarbon dating of reworked molluscs and wood fragments to constrain when the ice sheet retreated behind its present-day extent. The 10Be ages are scattered ranging from ca. 92.7 to 6.8 ka, whereas the in situ 14C ages range from ca. 14.2 to 6.7 ka. Almost half of the apparent 10Be ages predate the Last Glacial Maximum and up to 89 % are to some degree affected by nuclide inheritance. Based on the few reliable 10Be ages, the in situ 14C ages and existing radiocarbon ages from Inglefield Land, we find that the deglaciation along the coast commenced at ca. 8.6–8.3 ka cal BP in the western part and ca. 7.9 ka in the central part, following the opening of Nares Strait and arrival of warm waters. The ice margin reached its present-day position at ca. 8.2 ka at the Humboldt Glacier and ca. 6.7 ka in the central part of Inglefield Land. Radiocarbon ages of reworked molluscs and wood fragments show that the ice margin was behind its present-day extent from ca. 5.8 to 0.5 ka cal BP. After 0.5 ka cal BP, the ice advanced towards its Little Ice Age position. Our results emphasize that the slowly eroding and possibly cold-based ice in north Greenland makes it difficult to constrain the deglaciation history based on 10Be ages alone unless they are paired with in situ 14C ages. Further, combining our findings with those of recently published studies reveals distinct differences between deglaciation patterns of northwest and north Greenland. Deglaciation of the land areas in northwest Greenland occurred earlier than in north Greenland, and periods of restricted ice extent were longer, spanning the Middle and Late Holocene. Overall, this highlights past ice sheet sensitivity to Holocene climate changes in an area where little information was available just a few years ago.

Present-day global warming is a threat to ice masses in the Arctic and estimated future temperatures in this region will be higher than the global average due to Arctic amplification. Mass loss from the Greenland Ice Sheet (GrIS) has increased significantly over the last decades as a consequence of rising atmospheric and ocean temperatures. In particular, the marine-based parts of the ice sheet have shown to be vulnerable to present-day warming. Ice marginal fluctuations of the ice sheet during the Holocene (last 11,700 years) is the most recent analogue for what might come in the future, and studying this period, can help extrapolate the current behaviour of the ice sheet into a long-term perspective. The northwest and north GrIS have shown a distinct sensitivity to current global warming, yielding widespread retreat and runoff. The northwest GrIS has been the largest contributor to sea level rise over the last decades, possibly due to its large marine-based area, and the north GrIS is expected to be one of the main contributors in the future as this region is currently undergoing dramatic changes. Despite this behaviour, research of and constraints on the Holocene glacial history in these regions were sparse just a few years ago. This PhD dissertation presents new extensive knowledge on ice marginal fluctuations and the sensitivity of the northwest and north GrIS to Holocene climate changes. Constraints were obtained using mainly 10Be exposure dating and radiocarbon dating. In addition, the first use of in-situ 14C exposure dating was applied in north Greenland, as the cold-based ice sheet complicated constraints in this region. Results on past ice marginal fluctuations were assessed with local and regional climate records. Our findings highlight an overall sensitivity of the GrIS to past climate changes. Especially, the large marine-based part of the ice sheet in northwest Greenland, shows distinct vulnerability to rising atmospheric temperatures at the onset of Holocene resulting in rapid ice retreat. We see a great control on the dynamics of the ice sheet from rising ocean temperatures during middle and late Holocene, which drove the ice sheet behind its present-day extent. A clear difference in ice retreat patterns between fjord and inter-fjord areas is seen in especially north Greenland, where ice retreat in the fjords commended earlier than on land. Land-based glaciers and ice caps responded to the Neoglacial cooling, but evidence from the marine-based outlet glaciers is sparse and possibly hidden beneath the ice sheet. Throughout the regions, a widespread re-advance was seen from both the GrIS, its outlets and local ice caps to late Holocene cooling culminating in the Little Ice Age. Since then, an overall retreat of ice in Greenland has taken place at an increasing rate. To conclude, we see an overall pattern of sensitivity in ice retreat confined to latitude in northwest and north Greenland. Earlier retreat and longer periods of restricted ice extent during the Holocene are evident for areas at lower latitudes. In summary, the results from this study provide a new much needed foundation for long-term predictions and future estimates of the response of the northwest and north GrIS to present and future global warming.

This article has been retracted: please see Elsevier Policy on Article Withdrawal (https://www.elsevier.com/about/our-business/policies/article-withdrawal). This article has been retracted: please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy). This article has been retracted at the request of the Authors. This abstract was submitted while a manuscript that contained partially redundant material was still under consideration at Science Advances.