Search

Radio Echo Sounding (RES) surveys conducted in May 2010 and April 2011 revealed a 2 km(2) flat area with increased bed reflectivity at the base of Isunnguata Sermia at the western margin of the Greenland Ice Sheet. This flat reflector was located within a localized subglacial hydraulic potential (hydropotential) minimum, as part of a complex and elongated trough system. By analogy with comparable features in Antarctica, the initial interpretation of such a feature was a potential subglacial lake. In September 2013 a co-located seismic survey revealed a 1,750 m by 540 and 37 m thick stratified lens-shaped bedform at the base of a subglacial trough system. Amplitude Versus Angle (AVA) analysis yields a derived reflection coefficient R = 0.09 +/- 0.14 indicative of consolidated sediments possibly overlain by dilatant till. The bed and flank on the northern side of the trough consist of unconsolidated, possibly water-bearing sediments with R = -0.10 +/- 0.08, whereas on the southern side it consists of more consolidated material. We interpret the trough as a key component of the wider subglacial drainage network, for which the sediments on its northern side act as a localized water-storage reservoir. Given the observation of seasonally forming and rapidly draining supraglacial meltwater lakes in this area, we interpret the lens-shaped bedform as deposited by episodically ponding meltwater within the subglacial trough system. Our results highlight the importance of transient subglacial hydrological and sedimentological processes such as drainage events for the interaction of ice sheets and their substrates, to understand ice dynamics in a warming climate. Plain Language Summary A ground based radar survey in West Greenland showed an unusually flat, highly reflective zone in an otherwise rough bed suggesting a possible subglacial lake beneath the ice. The highly reflective zone was part of a drainage system transporting meltwater under the ice sheet. We performed a detail

Radio Echo Sounding (RES) surveys conducted in May 2010 and April 2011 revealed a 2 km2 flat area with increased bed reflectivity at the base of Isunnguata Sermia at the western margin of the Greenland Ice Sheet. This flat reflector was located within a localized subglacial hydraulic potential (hydropotential) minimum, as part of a complex and elongated trough system. By analogy with comparable features in Antarctica, the initial interpretation of such a feature was a potential subglacial lake. In September 2013 a co-located seismic survey revealed a 1,750 m by 540 and 37 m thick stratified lens-shaped bedform at the base of a subglacial trough system. Amplitude Versus Angle (AVA) analysis yields a derived reflection coefficient R = 0.09 ± 0.14 indicative of consolidated sediments possibly overlain by dilatant till. The bed and flank on the northern side of the trough consist of unconsolidated, possibly water-bearing sediments with R = −0.10 ± 0.08, whereas on the southern side it consists of more consolidated material. We interpret the trough as a key component of the wider subglacial drainage network, for which the sediments on its northern side act as a localized water-storage reservoir. Given the observation of seasonally forming and rapidly draining supraglacial meltwater lakes in this area, we interpret the lens-shaped bedform as deposited by episodically ponding meltwater within the subglacial trough system. Our results highlight the importance of transient subglacial hydrological and sedimentological processes such as drainage events for the interaction of ice sheets and their substrates, to understand ice dynamics in a warming climate.

Microstructures from deep ice cores reflect the dynamic conditions of the drill location as well as the thermodynamic history of the drill site and catchment area in great detail. Ice core parameters (crystal lattice-preferred orientation (LPO), grain size, grain shape), mesostructures (visual stratigraphy) as well as borehole deformation were measured in a deep ice core drilled at Kohnen Station, Dronning Maud Land (DML), Antarctica. These observations are used to characterize the local dynamic setting and its rheological as well as microstructural effects at the EDML ice core drilling site (European Project for Ice Coring in Antarctica in DML). The results suggest a division of the core into five distinct sections, interpreted as the effects of changing deformation boundary conditions from triaxial deformation with horizontal extension to bedrock-parallel shear. Region 1 (uppermost approx. 450m depth) with still small macroscopic strain is dominated by compression of bubbles and strong strain and recrystallization localization. Region 2 (approx. 450-1700m depth) shows a girdle-type LPO with the girdle plane being perpendicular to grain elongations, which indicates triaxial deformation with dominating horizontal extension. In this region (approx. 1000m depth), the first subtle traces of shear deformation are observed in the shape-preferred orientation (SPO) by inclination of the grain elongation. Region 3 (approx. 1700-2030m depth) represents a transitional regime between triaxial deformation and dominance of shear, which becomes apparent in the progression of the girdle to a single maximum LPO and increasing obliqueness of grain elongations. The fully developed single maximum LPO in region 4 (approx. 2030-2385m depth) is an indicator of shear dominance. Region 5 (below approx. 2385m depth) is marked by signs of strong shear, such as strong SPO values of grain elongation and strong kink folding of visual layers. The details of structural observations are compared w

The concept of a positive feedback between ice flow and enhanced melt rates in a warmer climate fuelled the debate regarding the temporal and spatial controls on seasonal ice acceleration. Here we combine melt, basal water pressure and ice velocity data. Using 20 years of data covering the whole ablation area, we show that there is not a strong positive correlation between annual ice velocities and melt rates. Annual velocities even slightly decreased with increasing melt. Results also indicate that melt variations are most important for velocity variations in the upper ablation zone up to the equilibrium line altitude. During the extreme melt in 2012, a large velocity response near the equilibrium line was observed, highlighting the possibility of meltwater to have an impact even high on the ice sheet. This may lead to an increase of the annual ice velocity in the region above S9 and requires further monitoring.

The concept of a positive feedback between ice flow and enhanced melt rates in a warmer climate fuelled the debate regarding the temporal and spatial controls on seasonal ice acceleration. Here we combine melt, basal water pressure, and ice velocity data. We show using twenty years of data covering the whole ablation area that there is no strong feedback between annual ice velocities and melt rates. Annual velocities even slightly decreased with increasing melt. Results also indicate that melt variations are most important for velocity variations in the upper ablation zone up to the equilibrium line altitude. During the extreme melt in 2012 a large velocity response near the equilibrium line was observed, highlighting the possibility of rapidly changing bed conditions in this part of the ice sheet that may lead to a doubling of the annual ice velocity.

Efforts to extract a Greenland ice core with a complete record of the Eemian interglacial (130,000 to 115,000 years ago) have until now been unsuccessful. The response of the Greenland ice sheet to the warmer-than-present climate of the Eemian has thus remained unclear. Here we present the new North Greenland Eemian Ice Drilling ('NEEM') ice core and show only a modest ice-sheet response to the strong warming in the early Eemian. We reconstructed the Eemian record from folded ice using globally homogeneous parameters known from dated Greenland and Antarctic ice-core records. On the basis of water stable isotopes, NEEM surface temperatures after the onset of the Eemian (126,000 years ago) peaked at 8 ± 4 degrees Celsius above the mean of the past millennium, followed by a gradual cooling that was probably driven by the decreasing summer insolation. Between 128,000 and 122,000 years ago, the thickness of the northwest Greenland ice sheet decreased by 400 ± 250 metres, reaching surface elevations 122,000 years ago of 130 ± 300 metres lower than the present. Extensive surface melt occurred at the NEEM site during the Eemian, a phenomenon witnessed when melt layers formed again at NEEM during the exceptional heat of July 2012. With additional warming, surface melt might become more common in the future.

We present the design and first results from two experiments using a wireless subglacial sensor system (WiSe) that is able to transmit data through 2500 m thick ice. Energy consumption of the probes is minimized, enabling the transmission of data for at least 10 years. In July 2010 the first prototype of the system was used to measure subglacial pressure at the base and a temperature profile consisting of 23 probes in two 600 m deep holes at Russell Glacier, a land-terminating part of the West Greenland ice sheet near Kangerlussuaq. The time series of subglacial pressure show very good agreement between data from the WiSe system and the wired reference system. The wireless-measured temperature data were validated by comparison with the theoretical decrease of melting point with water pressure inside the water-filled hole directly after installation. To test the depth range of the WiSe system a second experiment using three different probe types and two different surface antennas was performed inside the 2537 m deep hole at NEEM. It is demonstrated that, with the proper combination of transmission power and surface antenna type, the WiSe system transmits data through 2500 m thick ice.

To better understand the spatio-temporal variability of the glaciological environment in Dronning Maud Land (DML), East Antarctica, a 2800-km-long Japanese-Swedish traverse was carried out. The route includes ice divides between two ice-coring sites at Dome Fuji and EPICA DML. We determined the surface mass balance (SMB) averaged over various time scales in the late Holocene based on studies of snow pits and firn cores, in addition to radar data. We find that the large-scale distribution of the SMB depends on the surface elevation and continentality, and that the SMB differs between the windward and leeward sides of ice divides for strong-wind events. We suggest that the SMB is highly influenced by interactions between the large-scale surface topography of ice divides and the wind field of strong-wind events that are often associated with high-precipitation events. Local variations in the SMB are governed by the local surface topography, which is influenced by the bedrock topography. In the eastern part of DML, the accumulation rate in the second half of the 20th century is found to be higher by similar to 15% than averages over longer periods of 722 a or 7.9 ka before AD 2008. A similar increasing trend has been reported for many inland plateau sites in Antarctica with the exception of several sites on the leeward side of the ice divides., QC 20120413

For the deep ice-core drilling community, the 2005/06 Antarctic season was an exciting and fruitful one. In three different Antarctic locations, Dome Fuji, EPICA DML and Vostok, deep drillings approached bedrock (the ice-water interface in the case of Vostok), emulating what had previously been achieved at NorthGRIP, Greenland, (summer 2003 and 2004) and at EPICA Dome C2, Antarctica (season 2004/05). For the first time in ice-core drilling history, three different types of drill (KEMS, JARE and EPICA) simultaneously reached the depth of 'warm ice' under high pressure. After excellent progress at each site, the drilling rate dropped and the drilling teams had to deal with refrozen ice on cutters and drill heads. Drills have different limits and perform differently. In this comparative study, we examine depth, pressure, temperature, pump flow and cutting speed. Finally, we compare a few parameters of ten different deep drills. Udgivelsesdato: Dec

In the mid-1990s, excellent results from the GRIP and GISP2 deep drilling projects in Greenland opened up funding for continued ice-coring efforts in Antarctica (EPICA) and Greenland (NorthGRIP). The Glaciology Group of the Niels Bohr Institute, University of Copenhagen, was assigned the task of providing drilling capability for these projects, as it had done for the GRIP project. The group decided to further simplify existing deep drill designs for better reliability and ease of handling. The drill design decided upon was successfully tested on Hans Tausen Ice Cap, Peary Land, Greenland, in 1995. The 5.0 m long Hans Tausen (HT) drill was a prototype for the ~11 m long EPICA and NorthGRIP versions of the drill which were mechanically identical to the HT drill except for a much longer core barrel and chips chamber. These drills could deliver up to 4 m long ice cores after some design improvements had been introduced. The Berkner Island (Antarctica) drill is also an extended HT drill capable of drilling 2 m long cores. The success of the mechanical design of the HT drill is manifested by over 12 km of good-quality ice cores drilled by the HT drill and its derivatives since 1995. Udgivelsesdato: december

Precise knowledge of the phase relationship between climate changes in the two hemispheres is a key for understanding the Earth's climate dynamics. For the last glacial period, ice core studies(1,2) have revealed strong coupling of the largest millennial-scale warm events in Antarctica with the longest Dansgaard - Oeschger events in Greenland(3-5) through the Atlantic meridional overturning circulation(6-8). It has been unclear, however, whether the shorter Dansgaard - Oeschger events have counterparts in the shorter and less prominent Antarctic temperature variations, and whether these events are linked by the same mechanism. Here we present a glacial climate record derived from an ice core from Dronning Maud Land, Antarctica, which represents South Atlantic climate at a resolution comparable with the Greenland ice core records. After methane synchronization with an ice core from North Greenland(9), the oxygen isotope record from the Dronning Maud Land ice core shows a one-to-one coupling between all Antarctic warm events and Greenland Dansgaard - Oeschger events by the bipolar seesaw(6). The amplitude of the Antarctic warm events is found to be linearly dependent on the duration of the concurrent stadial in the North, suggesting that they all result from a similar reduction in the meridional overturning circulation.

We present a new combination of existing methods to identifythe origin of internal layers in an ice sheet by unprecedented accuracy.Most continuous internal reflection horizons observed by radio-echo soundingare known to form isochrones and can be followed over large distances.With an ice core at either end of the profile, the reflection horizons presenttime markers that are used to synchronize the ice-core records. Using electricalproperties along an ice core as input to a numerical model which simulates thepropagation of electromagnetic waves in the ice we reproduce the reflectioncharacteristics of a radar profile near the ice core. The depth of origin ofreflections are identified by removing individual peaks in conductivity in theinput record, thus also removing the corresponding reflections in the syntheticradargram. A pilot study at the EPICA drilling site in Dronning Maud Land,Antarctica, shows that it is possible to locate the origin of internal reflectionswith an accuracy of 0.5 m in a depth of 2000 m and more. Our approach imposeslittle constrains on the input records, making it applicable to a number ofdrilling sites, and has several advantages over usual methods where merelyreflector traveltimes (respective depths) and ice-core profiles are compared.Both, dielectric profiling and electrical conductivity measurements can be usedas electrical model input. As we use pronounced series of reflections to calibratethe traveltime-depth relation, only a coarse density record is required. Inaddition, as we do not require explicite electromagnetic wave speeds, systematicphysical errors in the ice-core or radio-echo sounding data have no effect onthe final result. Application of this method to deep-drilling locations inAntarctica connected by radio-echo sounding will improve their relativesynchronization and will help to answer the question of phase relations ofclimate changes observed in the ice-core records at different locations.

Two deep ice cores from central Greenland, drilled in the 1990s, have played a key role in climate reconstructions of the Northern Hemisphere, but the oldest sections of the cores were disturbed in chronology owing to ice folding near the bedrock. Here we present an undisturbed climate record from a North Greenland ice core, which extends back to 123,000 years before the present, within the last interglacial period. The oxygen isotopes in the ice imply that climate was stable during the last interglacial period, with temperatures 5-8°C warmer than today. We find unexpectedly large temperature differences between our new record from northern Greenland and the undisturbed sections of the cores from central Greenland, suggesting that the extent of ice in the Northern Hemisphere modulated the latitudinal temperature gradients in Greenland. This record shows a slow decline in temperatures that marked the initiation of the last glacial period. Our record reveals a hitherto unrecognized warm period initiated by an abrupt climate warming about 115,000 years ago, before glacial conditions were fully developed. This event does not appear to have an immediate Antarctic counterpart, suggesting that the climate see-saw between the hemispheres (which dominated the last glacial period) was not operating at this time.

The Antarctic Vostok ice core provided compelling evidence of the nature of climate, and of climate feedbacks, over the past 420,000 years. Marine records suggest that the amplitude of climate variability was smaller before that time, but such records are often poorly resolved. Moreover, it is not possible to infer the abundance of greenhouse gases in the atmosphere from marine records. Here we report the recovery of a deep ice core from Dome C, Antarctica, that provides a climate record for the past 740,000 years. For the four most recent glacial cycles, the data agree well with the record from Vostok. The earlier period, between 740,000 and 430,000 years ago, was characterized by less pronounced warmth in interglacial periods in Antarctica, but a higher proportion of each cycle was spent in the warm mode. The transition from glacial to interglacial conditions about 430,000 years ago ( Termination V) resembles the transition into the present interglacial period in terms of the magnitude of change in temperatures and greenhouse gases, but there are significant differences in the patterns of change. The interglacial stage following Termination V was exceptionally long - 28,000 years compared to, for example, the 12,000 years recorded so far in the present interglacial period. Given the similarities between this earlier warm period and today, our results may imply that without human intervention, a climate similar to the present one would extend well into the future.

For the first time, annually resolved accumulation rates have been determined in central Antarctica by means of counting seasonal signals of ammonium, calcium, and sodium. All records, obtained from three intermediate depth ice cores from Dronning Maud Land, East Antarctica, show rather constant accumulation rates throughout the last 9 centuries with mean values of 63, 61, and 44 mm H2O yr−1 and a typical year-to-year variation of about 30%. For the last few decades, no trend was detected accounting for the high natural variability of all records. A significant weak intersite correlation is apparent only between two cores when the high-frequency part with periods less than 30 years is removed. By analyzing the records in the frequency domain, no persistent periods were found. This suggests that the snow accumulation in this area is mainly influenced by local deposition patterns and may be additionally masked by redistribution of snow due to wind. By comparing accumulation rates over the last 2 millennia a distinct change in the layer thickness in one of the three cores was found, which might be attributed either to an area upstream of the drilling site with lower accumulation rates, or to deposition processes influenced by surface undulations. The missing of a clear correlation between the accumulation rate histories at the three locations is also important for the interpretation of small, short time variations of past precipitation records obtained from deep ice cores.