Search

A rarely considered element of Arctic system change in the Pleistocene is the possibility and consequences of any large high-latitude impact. We report the discovery of a large impact crater hidden beneath Hiawatha Glacier in northwest Greenland. From airborne radar surveys, we identify a 31-kilometer-wide, circular bedrock depression beneath up to a kilometer of ice. This depression has an elevated rim that cross-cuts tributary subglacial channels and a subdued central uplift that appears to be actively eroding. From ground investigations of the deglaciated foreland, we identify overprinted structures within Precambrian bedrock along the ice margin that strike tangent to the subglacial rim. Glaciofluvial sediment from the largest river draining the crater contains shocked quartz and other impact-related grains. Geochemical analysis of this sediment indicates that the impactor was a fractionated iron asteroid, which must have been more than a kilometer wide to produce the identified crater. Radiostratigraphy of the ice in the crater shows that the Holocene ice is continuous and conformable, but all deeper and older ice appears to be debris-rich or heavily disturbed. The age of this impact crater is presently unknown, but from our geological and geophysical evidence, we conclude that it is unlikely to predate the Pleistocene inception of the Greenland Ice Sheet.

High Arctic environments are particularly sensitive to climate changes, but retrieval of paleoecological data is challenging due to low productivity and biomass. At the same time, Arctic soils and sediments have proven exceptional for long-term DNA preservation due to their constantly low temperatures. Lake sediments contain DNA paleorecords of the surrounding ecosystems and can be used to retrieve a variety of organismal groups from a single sample. In this study, we analyzed vascular plant, bryophyte, algal (in particular diatom) and copepod DNA retrieved from a sediment core spanning the Holocene, taken from Bliss Lake on the northernmost coast of Greenland. A previous multi-proxy study including microscopic diatom analyses showed that this lake experienced changes between marine and lacustrine conditions. We inferred the same environmental changes from algal DNA preserved in the sediment core. Our DNA record was stratigraphically coherent, with no indication of leaching between layers, and our cross-taxon comparisons were in accordance with previously inferred local ecosystem changes. Authentic ancient plant DNA was retrieved from nearly all layers, both from the marine and the limnic phases, and distinct temporal changes in plant presence were recovered. The plant DNA was mostly in agreement with expected vegetation history, but very early occurrences of vascular plants, including the woody Empetrum nigrum, document terrestrial vegetation very shortly after glacial retreat. Our study shows that multi-taxon metabarcoding of sedimentary ancient DNA from lake cores is a valuable tool both for terrestrial and aquatic paleoecology, even in low-productivity ecosystems such as the High Arctic.

Climate analogues, also denoted Space-For-Time, may be used to identify regions where the present climatic conditions resemble conditions of a past or future state of another location or region based on robust climate variable statistics in combination with projections of how these statistics change over time. The study focuses on assessing climate analogues for Denmark based on current climate data set (E-OBS) observations as well as the ENSEMBLES database of future climates with the aim of projecting future precipitation extremes. The local present precipitation extremes are assessed by means of intensity-duration-frequency curves for urban drainage design for the relevant locations being France, the Netherlands, Belgium, Germany, the United Kingdom, and Denmark. Based on this approach projected increases of extreme precipitation by 2100 of 9 and 21% are expected for 2 and 10 year return periods, respectively. The results should be interpreted with caution as the best region to represent future conditions for Denmark is the coastal areas of Northern France, for which only little information is available with respect to present precipitation extremes.

The Greenland Ice Sheet loses mass through surface meltwater runoff and discharge from marine terminating outlet glaciers. The spatial variability and magnitude of these processes have been studied and described in detail for the past decades. Here, we combine the mass loss between the LIA to 2010 with a SMB model extending back to ~1900 in order to investigate the spatial distribution of mass loss processes. We use high quality aerial stereo photogrammetric imagery recorded between 1978 and 1987 to map morphological features such as trim lines and end moraines marking the maximum ice extent of the LIA, which enables us to obtain vertical point-based differences associated with former ice extent. These point measurements are combined with contemporary ice surface differences derived using NASA's Airborne Topographic Mapper (ATM) from 2003-2010, NASA's Ice, Cloud, and land Elevation Satellite (ICESat) from 2003-2009, NASA's Land, Vegetation, and Ice Sensor (LVIS) from 2010, and ASTER (Silcast AST14DMO) co-registered to ICESat, to estimate mass loss throughout the 20th and early 21st Century. The mass balance estimates of the GrIS since retreat from maximum LIA is combined with a SMB model for the period for three intervals, LIAmax (~1900) - 1978/87, 1978/87 - 2003, and 2003 - 2010. Across the GrIS the total mass loss if found to be spatially- and temporally variable. However, when assessing the mass loss due to SMB and mass loss due to dynamic ice loss, we find that that the ratios between these components are variable between the different sectors of the GrIS, e.g. in the southeast sector of the GrIS we find substantial mass loss, possibly driven by high precipitation rates but also the presence of a large number of marine terminating glaciers. Furthermore many areas currently undergoing changes correspond to those that experienced considerable thinning throughout the 20th century. Consequently, comparing the 20th century thinning pattern to that of the las

In this study, we use a combination of 10Be exposure ages and threshold lakes to constrain the ice sheet history in Godthåbs- and Buksefjorden, west Greenland (63-64°N) during the Holocene. The 10Be cosmogenic exposure ages have been used to quantify both the ice retreat and thinning of the west Greenland Ice Sheet in 3 transects from the coast to the present ice margin. Preliminary results (n=47) indicate initial deglaciation of coastal areas around 11 ka in concert with existing radiocarbon chronology, followed by a rapid retreat from the outer coast to the present ice margin around 10 ka. Boulder samples from the highest peaks demonstrate that the ice was warm-based whereas bedrock samples often contain an inherited signal. These results may have implications for other studies in Greenland, which have inferred thin LGM ice based on 10Be ages of bedrock samples. The threshold lakes are used to record episodes when the ice sheet was close-to-the present extent, represented by beds of clastic sediments whereas gyttja-deposition reflects periods when the glacier was behind the threshold, and no glacial meltwater input. Our results demonstrates that following the initial deglaciation the ice margin remained close to the present ice margin until around 7 cal. ka BP before it retreated behind the present extent during the Holocene Thermal Maximum (HTM). Here it remained until the Little Ice Age (LIA) where it readvanced to the present position around 0.5 cal. ka BP. Our results correspond to other recent findings from south and west Greenland that show that the ice sheet first began to retreat behind the present ice margin around 7 cal. ka BP. However, it differs from the other areas because the ice sheet first reappears at the present ice position during the LIA (i.e. no Neoglacial advance) and this suggest that the ice sheet in this area may have been more retracted and probably more sensitive to climate change than other areas in south and west Greenland.

As the planet cooled from peak warmth in the early Cenozoic, extensive Northern Hemisphere ice sheets developed by 2.6 Ma ago, leading to changes in the circulation of both the atmosphere and oceans. From not, vert, similar2.6 to not, vert, similar1.0 Ma ago, ice sheets came and went about every 41 ka, in pace with cycles in the tilt of Earth’s axis, but for the past 700 ka, glacial cycles have been longer, lasting not, vert, similar100 ka, separated by brief, warm interglaciations, when sea level and ice volumes were close to present. The cause of the shift from 41 ka to 100 ka glacial cycles is still debated. During the penultimate interglaciation, not, vert, similar130 to not, vert, similar120 ka ago, solar energy in summer in the Arctic was greater than at any time subsequently. As a consequence, Arctic summers were not, vert, similar5 °C warmer than at present, and almost all glaciers melted completely except for the Greenland Ice Sheet, and even it was reduced in size substantially from its present extent. With the loss of land ice, sea level was about 5 m higher than present, with the extra melt coming from both Greenland and Antarctica as well as small glaciers. The Last Glacial Maximum (LGM) peaked not, vert, similar21 ka ago, when mean annual temperatures over parts of the Arctic were as much as 20 °C lower than at present. Ice recession was well underway 16 ka ago, and most of the Northern Hemisphere ice sheets had melted by 6 ka ago. Solar energy reached a summer maximum (9% higher than at present) not, vert, similar11 ka ago and has been decreasing since then, primarily in response to the precession of the equinoxes. The extra energy elevated early Holocene summer temperatures throughout the Arctic 1–3 °C above 20th century averages, enough to completely melt many small glaciers throughout the Arctic, although the Greenland Ice Sheet was only slightly smaller than at present. Early Holocene summer sea ice limits were substantially smaller than their 20t

The warmest millennia of at least the past 250,000 years occurred during the Last Interglaciation, when global ice volumes were similar to or smaller than today and systematic variations in Earth's orbital parameters aligned to produce a strong positive summer insolation anomaly throughout the Northern Hemisphere. The average insolation during the key summer months (M, J, J) was ca 11% above present across the Northern Hemisphere between 130,000 and 127,000 years ago, with a slightly greater anomaly, 13%, over the Arctic. Greater summer insolation, early penultimate deglaciation, and intensification of the North Atlantic Drift, combined to reduce Arctic Ocean sea ice, allow expansion of boreal forest to the Arctic Ocean shore across vast regions, reduce permafrost, and melt almost all glaciers in the Northern Hemisphere. Insolation, amplified by key boundary condition feedbacks, collectively produced Last Interglacial summer temperature anomalies 4–5 °C above present over most Arctic lands, significantly above the average Northern Hemisphere anomaly. The Last Interglaciation demonstrates the strength of positive feedbacks on Arctic warming and provides a potentially conservative analogue for anticipated future greenhouse warming.

The warmest millennia of at least the past 250,000 years occurred during the Last Interglaciation, when global ice volumes were similar to or smaller than today and systematic variations in Earths orbital parameters aligned to produce a strong positive summer insolation anomaly throughout the Northern Hemisphere. The average insolation during the key summer months (M, J, J) was ca 11% above present across the Northern Hemisphere between 130,000 and 127,000 years ago, with a slightly greater anomaly, 13%, over the Arctic. Greater summer insolation, early penultimate deglaciation, and intensification of the North Atlantic Drift, combined to reduce ArcticOcean sea ice, allow expansion of boreal forest to the Arctic Ocean shore across vast regions, reduce permafrost, and melt almost all glaciers in the Northern Hemisphere. Insolation, amplified by key boundary condition feedbacks, collectively produced Last Interglacial summer temperature anomalies 45 1C above present over most Arctic lands, significantly above the average Northern Hemisphere anomaly. The Last Interglaciation demonstrates the strength of positive feedbacks on Arctic warming and provides a potentially conservative analogue for anticipated future greenhouse warming

This issue is the final synthesis report by QUEEN (Quaternary Environments of the Eurasian North), an à la carte programme of the ESF (European Science Foundation) Standing Committee for Life and Environmental Sciences (LESC), supported through funds from Denmark, Finland, Germany, Norway, Sweden, Switzerland and the United Kingdom. In addition to several national projects, the European Union project Ice Sheets and Climate in the Eurasian Arctic (Eurasian Ice Sheets) formed part of the program during the period 1998 to 2000. Support from many Russian institutions and individual researchers was essential for organizing the numerous expeditions into remote areas of the northernmost Eurasia and the adjacent Arctic Ocean. QUEEN was initially suggested in 1995 when several research groups of western European and Russian scientists studying the Quaternary in remote and poorly known regions of the Eurasian North decided that they could improve the scope and depth of their investigations through international coordination and cooperation. By that time several of the European groups had already acquired substantial experience in collaborative studies of Quaternary processes and sedimentation history along the polar North Atlantic margins through a similar project, PONAM (Elverhøi et al., 1998), and by working on a bilateral basis with their Russian colleagues.QUEEN was launched in 1996 for a 5-yr period and ended in 2002 after an extension for another 2 yr. The ESF-based funding for QUEEN allowed to conduct annual workshops for discussing the spectacular new observations from areas which had been largely inaccessible in earlier, political more troublesome times, and to jointly plan and coordinate field work for each successive year. A particular emphasis was on bringing numerous young researchers from Russia together with their western partners for scientific exchange within an international framework. Three of the workshops have yielded collections of papers summarizing the