Jakobshavn Isbrae Research Articles

Late Holocene environmental change in Disko Bugt, west Greenland: interaction between climate, ocean circulation and Jakobshavn Isbrae

Late Holocene environmental change in Disko Bugt, west Greenland: interaction between climate, ocean circulation and Jakobshavn Isbrae

Present-day vertical crustal deformations in West Greenland from repeated GPS observations

SUMMARY A GPS network, consisting of 10 sites, was established in the ice-free area of West Greenland and was observed for the first time in 1995. In 2002 a complete re-observation was carried out. These repeated GPS observations served as a basis for the determination of vertical crustal deformations. The data analysis was performed using the Bernese Software version 5.0. For the central site Kangerlussuaq a negative uplift rate (subsidence) of (−3.1 ± 1.1) mm yr −1 was obtained, related to the reference frame IGb00. The regional pattern is characterized by an east‐west gradient of up to 4 mm yr −1 between the outer coast and the subsiding area along the present ice margin, which can be explained to a great extent as a result of the late Holocene re-advance of the Greenland ice sheet. Relative sea-level changes could be calculated taking the present eustatic sea-level rise into account. The present-day sea level rises at the outer coast between Maniitsoq and Paamiut, and in the large fjords with increasing rates of more than 4m m yr −1 in their innermost parts. These findings are in agreement with the general picture obtained from geomorphological and archaeological research. For Sisimiut and the Disko Bay area the present sea-level change is almost zero, whereas the crustal uplift rate for Ilulissat was observed to be 1.6 mm yr −1 .W econclude that this present vertical uplift rate in Ilulissat is affected by the retreat of the ice margin during the last 150 yr and the present negative mass balance of the Jakobshavn Isbrae and its drainage basin.

Early Holocene history of the west Greenland Ice Sheet and the GH-8.2 event

The margin of the Greenland Ice Sheet retreated rapidly during the first few thousand years of the Holocene. During this period of relative warmth, known as the Holocene thermal maximum, ice core records identify a significant short-lived cooling event at approximately 8.4–8.0 ka cal. yr BP (the ‘GH-8.2 event’) associated with a 5–7 °C fall in mean annual air temperature over the centre of the ice sheet. In this paper we constrain the history of the ice sheet margin in Disko Bugt (west Greenland) and that of a major ice stream, Jakobshavns Isbrae, during the early Holocene, which incorporates the interval of the GH-8.2 event. Our work is based on a new relative sea-level curve and minimum age estimates for the timing of deglaciation from two field sites, combined with a review of previously published research from the study area. We identify important differences in the chronology of ice margin recession during the early Holocene, most noticeably, the margin of Jakobshavns Isbrae retreated well inland of the adjacent ice sheet at this time. We conclude that the early Holocene ‘Fjord Stade’ moraines in Disko Bugt do not record a uniform ice sheet margin response to the GH-8.2 event. Rather, these moraines are diachronous and formed between c. 10–8 ka cal. yr BP, their age varying as a function of the interplay between topography and ice sheet/ice stream dynamics. We hypothesise that one cause for the lack of an identifiable response to the GH-8.2 event is because topographic controls dominated ice sheet behaviour at this time. In lowland areas, any increase in ice sheet mass balance was probably associated with an increase in calving rather than any major advance of a grounded ice sheet margin.

Early Holocene palaeoceanography and deglacial chronology of Disko Bugt, West Greenland

Two new cores from Disko Bugt, a large marine embayment in West Greenland, are used to assess the deglacial chronology and palaeoceanography of the area during the early Holocene. Deglaciation of the main part of Disko Bugt is directly dated for the first time to a minimum of ca 10.2 ka cal. BP. By this time the Jakobshavn Isbrae ice stream had retreated to the eastern margins of Disko Bugt. Initial deglaciation took place before the relatively warm West Greenland Current (WGC) was initiated. A strengthening in the WGC is identified from ca 9.2 ka cal. BP accompanied by retreat of the ice stream to a shallow bank immediately west of the present day Jakobshavn Isfjord. While in this position the calving and melting ice stream produced large volumes of sediment and meltwater. The large volumes of meltwater prevented the relatively warm waters of the WGC reaching the eastern margins of Disko Bugt. The ice stream remained grounded on this bank for over a thousand years until ca 7.9 ka cal. BP when reduced sedimentation rates and an increase in boreal fauna point to its retreat inside the present day fjord system.

Streamlined bedrock terrain and fast ice flow, Jakobshavns Isbrae, West Greenland: implications for ice stream and ice sheet dynamics

Streamlined bedrock terrain and fast ice flow, Jakobshavns Isbrae, West Greenland: implications for ice stream and ice sheet dynamics

Large fluctuations in speed on Greenland's Jakobshavn Isbræ glacier

It is important to understand recent changes in the velocity of Greenland glaciers because the mass balance of the Greenland Ice Sheet is partly determined by the flow rates of these outlets. Jakobshavn Isbrae is Greenland's largest outlet glacier, draining about 6.5 per cent of the ice-sheet area, and it has been surveyed repeatedly since 1991 (ref. 2). Here we use remote sensing data to measure the velocity of Jakobshavn Isbrae between 1992 and 2003. We detect large variability of the velocity over time, including a slowing down from 6,700 m yr(-1) in 1985 to 5,700 m yr(-1) in 1992, and a subsequent speeding up to 9,400 m yr(-1) by 2000 and 12,600 m yr(-1) in 2003. These changes are consistent with earlier evidence for thickening of the glacier in the early 1990s and rapid thinning thereafter. Our observations indicate that fast-flowing glaciers can significantly alter ice discharge at sub-decadal timescales, with at least a potential to respond rapidly to a changing climate.

Jakobshavn Isbræ, West Greenland: the 2002–2003 collapse and nomination for the UNESCO World Heritage List

Jakobshavn Isbræ (also known as Sermeq Kujalleq or Ilulissat Isbræ) is situated at about 69°10′N and 50°00′W in West Greenland. This major outlet from the Inland Ice has an extremely high rate of movement (nearly 1 m/hour) and thus a high production of icebergs, which via the icefjord float westwards through Disko Bugt to Davis Strait (Fig. 1). Estimates of the iceberg production are in the range of 35 ± 10 km3 ice per year, more than 10% of the entire calf-ice production of the Inland Ice (e.g. Bauer l968; Bindschadler 1984). The icefjord into which Sermeq Kujalleq calves is Kangia, best known in glaciological literature as Jakobshavn Isfjord. Spectacular changes of the glacier were observed during 2002 and 2003 at the same time as it was nominated for inclusion in the UNESCO World Heritage List under the name ‘Ilulissat Icefjord’.

Late Weichselian deglacial history of Disko Bugt, West Greenland, and the dynamics of the Jakobshavns Isbrae ice stream

New relative sea‐level (RSL) data from Disko Bugt, a large marine embayment in West Greenland, are used to examine the deglacial history of the Jakobshavns Isbrae ice stream. RSL data show rapid deglaciation after 10.3 ka cal. yr BP. Once deglaciation began, a bedrock high in the west of the bay exerted no discernible influence on the deglacial chronology. Following initial rapid retreat, ice stream recession slowed as it approached the eastern shores of the bay. Seabed elevations increase here and the ice stream terminus lingered for several thousand years before retreating into the narrow bedrock‐confined Jakobshavns Isfjord. The seabed topography of Disko Bugt includes several deep channels which probably record the former course of the ice stream. Using a simple water depth/calving velocity relationship it is estimated that the maximum calving velocity on deglaciation was c. 4.8 km a‐1. This is less than the present rate (6–7 km a‐1), although ice discharge was two to four times that observed today. Initiation of rapid ice stream retreat was probably caused by ice stream thinning and increased surface melting. A critical point in time was the retreat of the ice stream from shallow continental shelf waters (c. 400 m) into the deep bedrock trough (>800 m) which marks the entrance to Disko Bugt.

Late Weichselian deglacial history of Disko Bugt, West Greenland, and the dynamics of the Jakobshavns Isbrae ice stream

New relative sea-level (RSL) data from Disko Bugt, a large marine embayment in West Greenland, are used to examine the deglacial history of the Jakobshavns Isbrae ice stream. RSL data show rapid deglaciation after 10.3 ka cal. yr BP. Once deglaciation began, a bedrock high in the west of the bay exerted no discernible influence on the deglacial chronology. Following initial rapid retreat, ice stream recession slowed as it approached the eastern shores of the bay. Seabed elevations increase here and the ice stream terminus lingered for several thousand years before retreating into the narrow bedrock-confined Jakobshavns Isfjord. The seabed topography of Disko Bugt includes several deep channels which probably record the former course of the ice stream. Using a simple water depth/calving velocity relationship it is estimated that the maximum calving velocity on deglaciation was c. 4.8 km a-1. This is less than the present rate (6–7 km a-1), although ice discharge was two to four times that observed today. Initiation of rapid ice stream retreat was probably caused by ice stream thinning and increased surface melting. A critical point in time was the retreat of the ice stream from shallow continental shelf waters (c. 400 m) into the deep bedrock trough (>800 m) which marks the entrance to Disko Bugt.

Surface roughness characterizations of sea ice and ice sheets: case studies with MISR data

This work is an examination of potential uses of multiangular remote sensing imagery for mapping and characterizing sea ice and ice sheet surfaces based on surface roughness properties. We use data from the Multi-angle Imaging SpectroRadiometer (MISR) to demonstrate that ice sheet and sea ice surfaces have characteristic angular signatures and that these angular signatures may be used in much the same way as spectral signatures are used in multispectral classification. Three case studies are examined: sea ice in the Beaufort Sea off the north coast of Alaska, the Jakobshavn Glacier on the western edge of the Greenland ice sheet, and a region in Antarctica south of McMurdo station containing glaciers and blue-ice areas. The MISR sea ice image appears to delineate different first-year ice types and, to some extent, the transition from first-year to multiyear ice. The MISR image shows good agreement with sea ice types that are evident in concurrent synthetic aperture radar (SAR) imagery and ice analysis charts from the National Ice Center. Over the Jakobshavn Glacier, surface roughness data from airborne laser altimeter transects correlate well with MISR-derived estimates of surface roughness. In Antarctica, ablation-related blue-ice areas, which are difficult to distinguish from bare ice exposed by crevasses, are easily detected using multiangular data.

Calving bays

A mechanism is presented for subaerial calving of ice slabs and subaqueous calving of ice ledges from ice walls and ice shelves in lacustrine and marine glacial environments. The calving rate is equated with the maximum forward velocity of slab-bending at the time of calving. The rate of slab calving above water is shown to control the rate of block calving from ice ledges below water. Slabs theoretically calve along surface crevasses that are at a distance behind the calving front of about one-tenth the subaerial height of the ice wall or ice shelf. Blocks theoretically calve along bottom crevasses when subaqueous ice ledges produced by subaerial slab calving approach one-half of the ice thickness below water. These calculations are supported by studies of a calving ice wall on Deception Island, Antarctica, and by observations of calving from ice walls along the Antarctic Peninsula, from the Ross and Getz Ice Shelves in Antarctica, from large tabular icebergs in the Ross and Amundsen Seas of Antarctica, from Jakobshavns Isbrae in Greenland, and from tidewater glaciers in Alaska, Greenland, and Svalbard. Calving bays accelerate the retreat of Quaternary ice sheets along lacustrine and marine ice margins at the termination of Quaternary glaciation cycles. Rapid calving retreat causes sudden outbursts of icebergs from marine ice streams and meltwater from ice-dammed lakes, events that may trigger abrupt climatic changes. In the case of the Laurentide Ice Sheet, retreat of a calving bay along Hudson Strait would not only be accompanied by an iceberg outburst into the North Atlantic, the calving bay could also continue into Hudson Bay and carve out the heart of the Laurentide Ice Sheet itself, thereby terminating the last cycle of Quaternary glaciations.

Penetration depth of interferometric synthetic‐aperture radar signals in snow and ice

Digital elevation models of glaciated terrain produced by the NASA/Jet Propulsion Laboratory (JPL) airborne interferometric synthetic‐aperture radar (InSAR) instrument in Greenland and Alaska at the C‐ (5.6 cm wavelength) and L‐band (24‐cm) frequencies were compared with surface elevation measured from airborne laser altimetry to estimate the phase center of the interferometric depth, or penetration depth, δp. On cold polar firn at Greenland summit, δp = 9±2m at C‐ and 14±4m at L‐band. On the exposed ice surface of Jakobshavn Isbrae, west Greenland, δp = 1±2 m at C‐ and 3±3 m at L‐band except on smooth, marginal ice where δp=15±5 m. On colder marginal ice of northeast Greenland, δp reaches 60 to 120 m at L‐band. On the temperate ice of Brady Glacier, Alaska, δp is 4±2 m at C‐and 12±6 m at L‐band, with little dependence on snow/ice conditions. The implications of the results on the scientific use of InSAR data over snow/ice terrain is discussed.

Calculation of Ice Velocities in the Jakobshavn Isbrae Area Using Airborne Laser Altimetry

The ability to measure velocities of fast-moving glaciers is essential to the understanding and interpretation of their dynamics and related processes. This is especially important in the Jakobshavn drainage basin of the Greenland ice sheet, where a significant portion of the ice sheet mass loss occurs and the iceberg calving rate is high. In this article, we present a method for using airborne laser altimetry data to determine surface velocities in portions of the Jakobshavn drainage basin that have distinct elevation features. By examining correlations between repeat elevation surveys, the motion of these features, the most distinct of which are crevasses, is tracked. Results show that for areas with a standard deviation in elevation in excess of 1 m, displacement of features can be tracked to a precision of 1–2 m, and the corresponding velocities can be retrieved. The maximum velocity of the Jakobshavn Isbrae, at its calving front, is found to be nearly 7 km yr −1. Ice stream boundaries and flow patterns can also be observed in the derived velocity data.

Mapping ice sheet margins from ERS-1 SAR and SPOT imagery

We demonstrate a new technique to automatically map the margin of ice sheets using ERS-1 SAR and SPOT imagery. The technique relies on a series of image processing techniques including edge enhancement, dynamic thresholding, region growing, edge detection, and edge following. The mapping technique is used to measure temporal displacement of the ice sheet margin north and south of the Jakobshavn Glacier in Greenland. During the 4 year interval between 1988 and 1992 we find ice sheet margin advance/retreat rates of about +/- 12m/year, consistent with independent observation. We combine the 1988 and 1992 data to assign an average ice margin location for the observation interval assuming a constant displacement rate. The resulting snapshot of the ice sheet margin can be used as a benchmark for future change detection studies. In both cases the uncertainties are better than 200m.

Radar interferogram filtering for geophysical applications

The use of SAR interferometry is often impeded by decorrelation from thermal noise, temporal change, and baseline geometry. Power spectra of interferograms are typically the sum of a narrow‐band component combined with broad‐band noise. We describe a new adaptive filtering algorithm that dramatically lowers phase noise, improving both measurement accuracy and phase unwrapping, while demonstrating graceful degradation in regions of pure noise. The performance of the filter is demonstrated with SAR data from the ERS satellites over the Jakobshavns glacier of Greenland.

Jakobshavn Glacier, west Greenland: 30 years of spaceborne observations

Early 1960's reconnaissance satellite images are compared to more recent image and map data in an interannual and seasonal study of West Central Greenland margin fluctuations. From 1962 to 1992, ice sheet margins to the north and south of Jakobshavn Glacier retreated despite a decline in average summer temperatures. The retreat may be reversing along the southern flank of the ice stream where regional mass balance estimates are positive. From 1950 to 1996, the terminus of Jakobshavn Glacier seasonally fluctuated ∼2.5 km around its annual mean position. The total calving flux during the summer is more than six times that during winter. We identified that summer melting and the break‐up of sea ice and icebergs in the fjord are important in controlling the rate of iceberg production. If correct, calving rates may be expected to increase should climate become warmer in the near future.

Tidal flexure at ice shelf margins

Hinge zones accommodate the differential tidal movement between floating ice shelves, which move with the sea tide, and grounded ice sheets, which do not. Observations on Rutford Ice Stream and Ronne Ice Shelf, Antarctica, using the kinematic method of Global Positioning System, have yielded the first continuous tidal displacement profiles from an ice sheet hinge zone. The form of these tidal displacement profiles indicates that the flexure can be fit to an elastic beam model using a parameter space optimization technique. The model matches the observed tidal deflection profile to within 5 cm which is similar to the observational uncertainty. The same model and parameter fitting technique is then successfully applied to various other published and unpublished tidal displacement data from Doake Ice Rumples, Bach Ice Shelf, and Ekström Ice Shelf, Antarctica, and Jakobshavns Glacier, Greenland. I conclude that the elastic beam model with a single value of the elastic modulus (0.88±0.35 GPa) adequately describes almost all the data and so can usefully be applied elsewhere. Earlier studies of tidal dissipation in ice shelf hinge zones, based on a viscous or transient creep rheology, showed that perhaps 30% of global tidal energy dissipation could be occurring in ice shelf hinge zones. This study suggests, however, that an elastic rheology may be equally appropriate, in which case this estimation of the tidal energy dissipation is excessively high.

Fast glacier flow: Ice streams, surging, and tidewater glaciers

Fast flowing glaciers are unusual but important. Jakobshavns Glacier in western Greenland illustrates this point (Figure 1). Considered to be the world's fastest glacier, its maximum observed flow rate (measured in midsummer near the floating terminus) is 8360 m yr−1 [Lingle et al., 1981]. Except for a tidal influence near the terminus, the flow is believed to be fairly steady [Bindschadler, 1984].

Deluge II and the continent of doom: rising sea level and collapsing Antarctic ice

Hughes, Terence J. 1987 06 01: Deluge II and the continent of doom: rising sea level and collapsing Antarctic ice. Boreas, Vol. 16, pp. 89–100. Oslo. ISSN 0300–9483.Many cultures in both the Old and New Worlds have preserved legends of a Great Flood. In the Biblical deluge, ‘the springs of the great deep broke through and the sluices of heaven opened’ (Genesis 7: 11). The rise in sea level, as opposed to prolonged rainfall, is a conceivable cause of global flooding because the last stages in collapse of late Wisconsin/Weichselian ice sheets occurred in the late prehistorical period, from 8,000 to 6,000 B.C. A possible mechanism that might collapse large parts of ice sheets in a short time is found in Jakobshavns Isbrae, which drains the west‐central part of the Greenland Ice Sheet. This mechanism, called the Jakobshavns Effect, is described and its possible role in Holocene collapse of former Northern Hemisphere ice sheets (Deluge I) and future collapse of parts of the Antarctic Ice Sheet (Deluge II) is examined. Rapid global flooding by this mechanism is extremely unlikely; however, we lack the information needed to eliminate the possibility.

Measurement of Short Period Motion On A Fast Moving Glacier and Analysis of Data (Abstract)

The investigation of glacier motion over short time periods for relatively long duration and over large longitudinal extent can yield valuable insight into the dynamics of glacier surging, basal sliding, ice stream development, and calving mechanisms. In this paper, we discuss techniques for monitoring short term horizontal and vertical motion employed on the often highly-fractured surface of Variegated Glacier, Alaska, prior to its recent surge (1980—84) and on the fast-moving outlet glacier — ice stream system of Jakobshavns Glacier, Greenland. The short period measurements described here were made continuously over one to several months, and, in many cases, encompass seasonal and longer term fluctuations as well. The positions of a relatively large number of surface markers (15-35) were followed as functions of time. Application of standard terrestrial surveying techniques and modern microwave and UHF positioning methods to these short period studies are discussed. We then describe methods of reduction and analysis on the resulting large data sets, which may be treated as quasi-stationary time series. Examples of correlation with other glacier variables, such as basal water pressure, seismicity, and stream discharge are given and the propagation of movement events discussed.