bluuebunny/crossref_metadata_2025 huggingface dataset:
Ice discharge of eastern Dome C drainage area, Antarctica, determined from airborne radar survey and satellite image analysis
Authors: Frezzotti Massimo , Tabacco Ignazio Ezio , Zirizzotti Achille
Published: November 2007

Eastern Dome C, southern Talos Dome and northern Taylor Dome are drained by the Priestley, Reeves, David, Mawson and Mackay outlet glaciers, which flow into the Scott Coast on the west side of the Ross Sea, Antarctica. Airborne radar surveys were conducted on these glaciers to determine ice thickness and bed morphology along transverse and longitudinal profiles of the grounded and floating segments. A new analysis of a Landsat Thematic Mapper satellite image using a tracking technique was used to measure ice velocity at grounding lines and along ice tongues. The integration of radar and satellite data helped to locate grounding lines and to calculate the ice discharge. Changes in ice fluxes of floating glaciers were used to determine basal melting and freezing rates. The ice discharge calculated is less than half that required for a zero net surface mass balance according to the inputs given by the accumulation estimates widely adopted at present. The basal melting rates of meteoric ice represent 50% of the net ablation rate.
A hot-water ice-coring drill
Authors: Engelhardt H. , Kamb B. , Bolsey R.
Published: November 2007

A new method of ice-core drilling uses an annulus of hot-water jets to melt out a cylindrical ice core. This lightweight device used in combination with a fast hot-water drill can quickly obtain ice cores from any depth.
Results from the EISMINT model intercomparison: the effects of thermomechanical coupling
Authors: Payne A. J. , Huybrechts P. , Abe-Ouchi A. , Calov R. , Fastook J. L. , Greve R. , Marshall S. J. , Marsiat I. , Ritz C. , Tarasov L. , Thomassen M. P. A.
Published: November 2007

This paper discusses results from the second phase of the European Ice Sheet Modelling Initiative (EISMINT). It reports the intercomparison of ten operational ice-sheet models and uses a series of experiments to examine the implications of thermomechanical coupling for model behaviour. A schematic, circular ice sheet is used in the work which investigates both steady states and the response to stepped changes in climate. The major finding is that the radial symmetry implied in the experimental design can, under certain circumstances, break down with the formation of distinct, regularly spaced spokes of cold ice which extended from the interior of the ice sheet outward to the surrounding zone of basal melt. These features also manifest themselves in the thickness and velocity distributions predicted by the models. They appear to be a common feature to all of the models which took part in the intercomparison, and may stem from interactions between ice temperature, flow and surface form. The exact nature of these features varies between models, and their existence appears to be controlled by the overall thermal regime of the ice sheet. A second result is that there is considerable agreement between the models in their predictions of global-scale response to imposed climate change.
Himalayan ice-core dating with snow algae
Authors: Yoshimura Yoshitaka , Kohshima Shiro , Takeuchi Nozomu , Seko Katsumoto , Fujita Koji
Published: November 2007

Snow algae in shallow ice cores (7 m long) from Yala Glacier in the Langtang region of Nepal were examined for potential use in ice-core dating. Ice-core samples taken at 5350 m a.s.l. in 1994 contained more than seven species of snow algae. In a vertical profile of the algal biomass, 11 distinct algal layers were observed. Seasonal observation in 1996 at the coring site indicated most algal growth occurred from late spring to late summer. Pit observation in 1991, 1992 and 1994 indicated that algal layer formation takes place annually. δ18O, chemical ions (Na+, Cl−, SO42− and NO3−) and microparticles failed to show any clear seasonal variation, particularly at depths exceeding 2 m, possibly due to heavy meltwater percolation. Snow algae in ice cores would thus appear to be accurate boundary markers of annual layers and should prove useful for ice-core dating in Himalayan-type glaciers.
Calculating basal thermal zones beneath the Antarctic ice sheet
Authors: Wilch Ellen , Hughes Terence J.
Published: November 2007

A procedure is presented for using a simple flowline model to calculate the fraction of the bed that is thawed beneath present-day ice sheets, and therefore for mapping thawed, frozen, melting and freezing basal thermal zones. The procedure is based on the proposition, easily demonstrated, that variations in surface slope along ice flowlines are due primarily to variations in bed topography and ice–bed coupling, where ice–bed coupling for sheet flow is represented by the basal thawed fraction. This procedure is then applied to the central flowlines of flow bands on the Antarctic ice sheet where accumulation rates, surface elevations and bed topography are mapped with sufficient accuracy, and where sheet flow rather than stream flow prevails.

In East Antarctica, the usual condition is a low thawed fraction in subglacial highlands, but a high thawed fraction in subglacial basins and where ice converges on ice streams. This is consistent with a greater depression of the basal melting temperature and a slower rate of conducting basal heat to the surface where ice is thick, and greater basal frictional heat production where ice flow is fast, as expected for steady-state flow. This correlation is reduced or even reversed where steady-state flow has been disrupted recently, notably where ice-stream surges produced the Dibble and Dalton Iceberg Tongues, both of which are now stagnating.

In West Antarctica, for ice draining into the Pine Island Bay polynya of the Amundsen Sea, the basal thawed fraction is consistent with a prolonged and ongoing surge of Pine Island Glacier and with a recently initiated surge of Thwaites Glacier. For ice draining into the Ross Ice Shelf, long ice streams extend nearly to the West Antarctic ice divide. Over the rugged bed topography near the ice divide, no correlation consistent with steady-state sheet flow exists between ice thickness and the basal thawed fraction. The bed is wholly thawed beneath ice streams, even where stream flow is slow. This is consistent with ongoing gravitational collapse of ice entering the Ross Sea embayment and with unstable flow in the ice streams.
Numerical modelling of ice-sheet dynamics across the Vostok subglacial lake, central East Antarctica
Authors: Mayer Christoph , Siegert Martin J.
Published: November 2007

A numerical model of the ice-sheet/ice-shelf transition was used to investigate ice-sheet dynamics across the large subglacial lake beneath Vostok station, central East Antarctica. European Remote-sensing Satellite (ERS-1) altimetry of the ice surface and 60 MHz radio-echo sounding (RES) of the ice-sheet base and internal ice-sheet layering were used to develop a conceptual flowline across the ice sheet, which the model used as input. The model calculates horizontal and vertical velocities and stresses, from which particle flow paths can be obtained, and the ice-sheet temperature distribution. An inverse approach to modelling was adopted, where particle flow paths were forced to match those identified from internal RES layering. Results show that ice dynamics across the inflow grounding line are similar to an ice-sheet/ice-shelf transition. Model particle flow paths match internal RES layering when ice is (a) taken away from the ice base across the first 2 km of the flowline over the lake and (b) added to the base across the remainder of the lake. We contend that the process causing this transfer of ice is likely to be melting of ice and freezing of water at the ice–water interface. Other explanations, such as enhanced rates of accumulation over the grounding line, or three-dimensional convergent/divergent flow of ice are inconsistent with available measurements. Such melting and refreezing would be responsible for circulation and mixing of at least the surface layers of the lake water. Our model suggests that several tens of metres of refrozen “basal ice” would accrete from lake water to the ice sheet before the ice regrounds.
The link between climate warming and break-up of ice shelves in the Antarctic Peninsula
Authors: Scambos Ted A. , Hulbe Christina , Fahnestock Mark , Bohlander Jennifer
Published: November 2007

A review of in situ and remote-sensing data covering the ice shelves of the Antarctic Peninsula provides a series of characteristics closely associated with rapid shelf retreat: deeply embayed ice fronts; calving of myriad small elongate bergs in punctuated events; increasing flow speed; and the presence of melt ponds on the ice-shelf surface in the vicinity of the break-ups. As climate has warmed in the Antarctic Peninsula region, melt-season duration and the extent of ponding have increased. Most break-up events have occurred during longer melt seasons, suggesting that meltwater itself, not just warming, is responsible. Regions that show melting without pond formation are relatively unchanged. Melt ponds thus appear to be a robust harbinger of ice-shelf retreat. We use these observations to guide a model of ice-shelf flow and the effects of meltwater. Crevasses present in a region of surface ponding will likely fill to the brim with water. We hypothesize (building on Weertman (1973), Hughes (1983) and Van der Veen (1998)) that crevasse propagation by meltwater is the main mechanism by which ice shelves weaken and retreat. A thermodynamic finite-element model is used to evaluate ice flow and the strain field, and simple extensions of this model are used to investigate crack propagation by meltwater. The model results support the hypothesis.
Controls on the distribution of surge-type glaciers in Svalbard
Authors: Jiskoot Hester , Murray Tavi , Boyle Paul
Published: November 2007

We analyzed the possible controls on the distribution of surge-type glaciers in Svalbard using multivariate logit models including 504 glaciers and a large number of glacial and geological attributes. Specifically we examined the potential effect of geological boundaries, mass-balance conditions and thermal regime on surging. It was found that long glaciers with relatively steep slopes overlying young fine-grained sedimentary lithologies with orientations in a broad arc clockwise from northwest to southeast are most likely to be of surge type. No relation between lithological boundaries and surge potential could be established. Possible explanations for length being conducive to surging are transport-distance-related substrate properties, distance-related attenuation of longitudinal stresses and the possible relation between thermal regime and glacier size. Analysis of glaciers with recorded radioecho sounding reveals that a polythermal regime, accumulation-area ratios close to balance and a large elevation span increase the surge potential. The logit models also enabled us to detect 19 new surge-type glaciers, to reclassify six glaciers as normal and to identify unusual surge-type glaciers. Our model results suggest that a polythermal regime and fine-grained potentially deformable beds are conducive to the surge potential of Svalbard glaciers.
1.4 GHz radar penetration and evidence of drainage structures in temperate ice: Black Rapids Glacier, Alaska, U.S.A.
Authors: Arcone Steven A. , Yankielun Norbert E.
Published: November 2007

We have tested the ability of a 1.12–1.76 GHz bandwidth airborne Frequency Modulation–Continuous Wave (FM-CW) radar with an effective pulse duration of 3 ns to penetrate temperate ice of the ablation zone of Black Rapids Glacier, central Alaska, U.S.A. We used high-gain horn antennas to suppress clutter, and tested over cold and nearly ideal surface conditions. Englacial horizons dipping to at least 60 m depth were found along three sections of one axial profile. More narrow-band (1.21–1.29 GHz), low-resolution (24 ns pulse duration) profiles from a fourth section detected events at about 100–150 m depth. Comparative profiles recorded with a 100 MHz short-pulse-type radar reproduce the horizons of two of the sections, and verify the penetration in all cases. All horizons are composed of diffractions. We interpret voids from the phase of the 100 MHz diffractions within one of the horizons. The diffraction nature of the horizons, the void interpretation and the proximity to a nearby and up-glacier pothole field lead us to conclude that the horizons within two of the sections are meandering drainage channels. A more complex, branching structure with near-surface horizons profiled within the third section much farther down-glacier may also be a complex drainage system fed by near-surface melting. The FM-CW signal-to-clutter-noise ratios of some of the targets predict that they could be detected at 200 m depth in the 1–2 GHz range. Significant performance improvements at maximum vertical resolution could be achieved with higher-gain antennas.
A new technique for ice-fabric analysis
Authors: Wilen L. A.
Published: November 2007

A new method for ice-fabric analysis which is easily automated is described. The technique relies on a set of digital images of an ice thin section viewed through crossed polarizers and rotated to various angles. From the acquired set of images, the c-axis orientation of all the grains in the image can be determined. The theoretical basis for the technique is described in detail, including corrections for refraction, as well as other corrections for off-axis grains and polarization/reflection effects. An experiment was performed comparing results of the new technique with those of the standard technique. Excellent agreement was obtained. It is expected that an automated system using this technique (currently almost complete) will have a large impact on the amount of information and physical knowledge which can be extracted from ice fabrics in the future.
Glaciofluvial crevasse and conduit fills as indicators of supraglacial dewatering during a surge, Skeiðarárjökull, Iceland
Authors: Bennett Matthew R. , Huddart David , Waller Richard I.
Published: November 2007

This paper documents the glaciological structures associated with the surge of Skeiðarárjökull, Iceland, in 1991. These structures are interpreted as units of stratified ice, low-angle fractures, vertical and sub-vertical fractures (crevasse traces) and thrusts. The inferred thrusts are debris-rich and, unusually, have both down-glacier and up-glacier dips close to the ice margin. Sediment infills consist of either massive sand or horizontally stratified sand units. The most significant debris-rich structures on the glacier surface, however, are supraglacial crevasse and conduit fills, which contain either massive or horizontally stratified silts, sands and granule-gravels. These sediments infill both vertical fractures (relict crevasses) and englacial conduits. At the stratigraphic base of these sediment fills there is evidence of syn-sedimentary deformation, suggesting that sedimentation occurred during crevasse closure and continued thereafter. We argue that these structures relate to an episode of supraglacial meltwater flow during the 1991 surge, caused by the build-up of subglacial water pressure in a linked-cavity system or some similar distributed drainage system beneath the glacier. The development of this high-level drainage route may have helped regulate basal water pressures and therefore the active phase of the surge. The idea that the supraglacial leakage of subglacial water may have played a role in terminating the surge is explored.
The orientation dependence of the strength of ice single crystals
Authors: Trickett Y. L. , Baker I. , Pradhan P. M. S.
Published: November 2007

Single crystals with a wide variety of orientations were cut from large pucks of laboratory-grown ice. Constant-strain-rate compression tests were performed on the crystals either at an axial strain rate of 1 × 10−5 s−1at –20°C or at axial strain rates from 1 × 10−6 s−1 to 1 × 10−4 s−1 at –10°C. In agreement with previous studies of ice flow, the compression tests showed a linearly rising stress with increasing strain, followed by a sharply declining stress after reaching a peak. With further strain, the sharp decline in stress slowed and the flow stress approached a plateau that was only weakly dependent on strain. For all crystallographic orientations, it was found that Schmid’s (critical resolved shear stress) law was obeyed by the peak stress. Slip lines clearly showed that basal slip was the deformation mode.
Numerical modelling of the ice sheet in western Dronning Maud Land, East Antarctica: impacts of present, past and future climates
Authors: Näslund J. O. , Fastook J. L. , Holmlund P.
Published: November 2007

Time-dependent ice-sheet modelling of a 176 000 km2 area in western Dronning Maud Land, East Antarctica, provided information on the ice sheet’s response to six climate-change scenarios. Another experiment was done to study changes in ice thickness, flow and basal temperature conditions between the present ice configuration and a simulated maximum palaeo-ice sheet. The input to the model included new datasets of bed and surface topography compiled for this study. The results of the six climate-change experiments, including a 0.5°C per century global-warming scenario, show that the ice sheet has a robust behaviour with respect to the different climate changes. The maximum change in ice volume was <5% of the initial volume in all climate runs. This is for only relatively short-term climate changes without major changes in global sea level, and also a simulated ice sheet without an ice shelf. The modelled long-term response time of the ice sheet, 20 kyr or more, indicates that the ice sheet may still be adjusting to the climate change that ended the Last Glacial Maximum. In the maximum palaeo-ice-sheet simulation, with a 5°C climate cooling and the grounding line located at the continental-shelf margin, ice thickness increased drastically downstream from the Heimefrontfjella mountain range but remained basically unaffected on the upstream polar plateau. Compared to present conditions, complex changes in basal temperatures were observed. The extent of areas with basal melting increased, for example in the deep trough of the Veststraumen ice stream. Areas at intermediate elevations in the landscape also experienced increased basal temperatures, with significant areas reaching the melting point. In contrast, high-altitude areas that today are clearly cold-based, such as around Heimefrontfjella and Vestfjella and the Högisen dome, experienced a 5–10°C decrease in basal temperatures in the palaeo-ice-sheet reconstruction. The results suggest that the alpine landscape within these mountain regions was formed by wet-based local glaciers and ice sheets prior to the late Cenozoic.
The accumulation pattern across Siple Dome, West Antarctica, inferred from radar-detected internal layers
Authors: Nereson N. A. , Raymond C. F. , Jacobel R.W. , Waddington E. D.
Published: November 2007

The spatial distribution of accumulation across Siple Dome, West Antarctica, is determined from analysis of the shapes of internal layers detected by radio-echo sounding (RES) measurements. A range of assumed accumulation patterns is used in an ice-flow model to calculate a set of internal layer patterns. Inverse techniques are used to determine which assumed accumulation pattern produces a calculated internal layer pattern that best matches the shape of internal layers from RES measurements. All of the observed internal layer shapes at Siple Dome can be matched using a spatially asymmetric accumulation pattern which has been steady over time. Relative to the divide, the best-fitting accumulation pattern predicts 40% less accumulation 30 km from the divide on the south flank of Siple Dome and 15–40% more accumulation 30 km from the divide on the north flank. The data also allow the possibility for a small time variation of the pattern north of the divide. The mismatch between the calculated and the observed layer shapes is slightly reduced when the accumulation rate north of the divide is higher in the past (> 5kyr BP) than at present. Sensitivity tests show that the predicted change in the spatial accumulation pattern required to cause the slight Siple Dome divide migration (inferred from previous studies) would be detectable in the internal layer pattern if it persisted for > 2 kyr. Our analysis reveals no evidence that such a change has occurred, and the possible change in accumulation distribution allowed by the data is in the opposite sense. Therefore, it is unlikely that the Siple Dome divide migration has been caused by a temporal change in the spatial pattern of accumulation. This conclusion suggests the migration may be caused by elevation changes in Ice Streams C and D at the boundaries of Siple Dome.
A simple snowpack/cloud reflectance and transmittance model from microwave to ultraviolet: the ice-lamella pack
Authors: Mätzler Christian
Published: November 2007

Modeling the reflectance and transmittance of strong volume scatterers is a delicate task. Slightly different approaches can lead to different results, making comparisons difficult. Here a simple, analytic multiple-scattering model is presented as a possible reference for comparisons and also for better understanding of the physics involved. The model quantifies the transmittance and reflectance of homogeneously distributed scatterers within slabs of any thickness. The simplicity of the model is given by the one-dimensional geometry, a system consisting of freely arranged ice lamellae in air. Although direct application of the model will be limited, it gives a spectral description of ice clouds and snowpacks over a very broad spectral range from microwave to ultraviolet. As well as the transmittance and reflectance, the model gives the emittance through Kirchhoff’s law. Comparison with other models shows, on the one hand, agreement with current snow models in the spectral description, and on the other, some quantitative inconsistencies between all of them. It appears that the lamella pack produces the same optical spectra as an average snow model, with spherical ice grains whose radius corresponds to about the lamella thickness, whereas microwave spectra appear to be slightly different.
Transmission of stress between an ice stream and interstream ridge
Authors: Whillans I. M. , van der Veen C. J.
Published: November 2007

Measurements of strain rate are used to describe the stress distribution at the lateral margin of an active West Antarctic ice stream. The goal is to describe the way in which drag at the sides operates to control the motion of this ice stream. The problem is inherently complex, but with some approximations a mainly analytic solution is obtained. Allowing for spatially varying ice strength shows that the shear strain rate decreases over a larger distance from the ice stream than for ice of uniform strength. The simplest explanation using the results of field measurements is that the upper 40% of the ice stream is able to support lateral stress. For the slow-moving interstream ridge, rather more of the thickness is strong. The lateral shear stress from the ice stream is transferred to a basal zone about 3 km wide where basal drag reaches 60 kPa. This basal zone controls the speed of the ice stream.
The elevation history of ice streams and the spatial accumulation pattern along the Siple Coast of West Antarctica inferred from ground-based radar data from three inter-ice-stream ridges
Authors: Nereson N. A. , Raymond C. F.
Published: November 2007

Measurements of the surface and internal layer geometry from ice-penetrating radar and global positioning system surveys on three inter-ice-stream ridges in West Antarctica (Siple Dome, ridge DE and ridge BC) are examined with ice-flow models to infer (1) the history of the divide position at each site and (2) the spatial pattern of accumulation across the ridges. We find that the divide position is most steady at Siple Dome, somewhat steady at ridge DE and highly variable at ridge BC. Data from Siple Dome and ridge DE show evidence for steady northward motion of the ice divide for the past few thousand years. The layers beneath ridge BC suggest a 5 km northward shift of the divide position within the past several hundred years. Assuming the divide shifts are all due to changing elevation of the bounding ice streams, we infer the relative elevation history for segments of Ice Streams B–E. The northward displacement of the divide for all ridges implies a progressive relative thinning of the ice streams from E to B, with most dramatic recent thinning (100 m in <103 years) of Ice Stream B relative to Ice Stream C. Analysis of the internal layer pattern across the ridges indicates a south–north accumulation gradient with higher accumulation rates on the northern flanks of the ridges in all three cases. The inferred accumulation distribution is nearly uniform on the northern flanks, decreases sharply within a few ice thicknesses across the divides, and then decreases gradually farther to the south. The north/south decrease is strongest for ridge DE and weakest for ridge BC. This spatial pattern and the reduction in gradient strength with distance from the Amundsen Sea is consistent with the hypothesis that storms from the Amundsen Sea carry moisture first south then west over West Antarctica and deposit more snow on the windward side of ridges due to orographic lifting. This pattern has been stable for at least the past several thousand years.
Evidence for rapid retreat and mass loss of Thwaites Glacier, West Antarctica
Authors: Rignot Eric
Published: November 2007

Thwaites Glacier, the second largest ice stream in West Antarctica, drains an area of 166 500 ± 2000 km2 which accumulates 55 ± 5 Gt a−1 (or 60 ± 6 km3 ice a−1) into the Amundsen Sea, unrestrained by an ice shelf. Using interferometric synthetic-aperture radar (InSAR) data collected by the European Remote-sensing Satellites (ERS-1 and -2) in 1996, an output flux of 71 ±7 Gt a−1 (or 77 ± 8 km3 ice a−1) is estimated at the grounding line, where ice thickness is deduced from hydrostatic equilibrium. A similar flux, 70 ± 7 Gt a−1 (or 76 ± 8 km3 ice a−1), is obtained at a gate located 20 km upstream, where ice thickness was measured in 1978 by ice-sounding radar. Total accumulation in between the two gates is 1.6 Gt a−1, or 1.8 km3 ice a−1. Ice discharge therefore exceeds mass accumulation by 30 ± 15%, and Thwaites Glacier must be thinning and retreating at present. The InSAR data show that the glacier floating ice tongue exerts no back pressure on the inland ice, calves into tabular icebergs along a significant fraction of its grounding line, and has a grounding-line thickness which exceeds a prior-calculated limit for stability. Glacier thinning is confirmed at the coast by the detection of a 1.4 ± 0.2 km retreat of its grounding line between 1992 and 1996 with InSAR, which implies 3.2 ± 0.6 m ice a−1 thinning at the glacier center and less near the sides. These results complement the decimeter-scale annual surface lowering observed with satellite radar altimetry several hundred km inland of the grounding line. The magnitude of ice thinning estimated at the coast, however, rules out temporal changes in accumulation as the explanation for surface lowering. Ice thinning must be due to changes in ice flow.
Spatial patterns of glacier motion during a high-velocity event: Haut Glacier d’Arolla, Switzerland
Authors: Mair Douglas , Nienow Peter , Willis Ian , Sharp Martin
Published: November 2007

The surface motion of Haut Glacier d’Arolla, Switzerland, was monitored at a high spatial and temporal resolution. Data are analyzed to calculate surface velocities, surface strain rates and the components of the glacier force budget before, during and after an early melt season speed-up or “spring event”. We investigate the extent to which variations in glacier motion can be attributed to hydrologically induced local forcing or to non-local forcing transmitted via horizontal stress gradients. Enhanced glacier motion is dependent on a change in the spatial distribution of areas of high drag across the glacier.
Numerical modeling of blowing and drifting snow in Alpine terrain
Authors: Gauer Peter
Published: November 2007

In mountainous regions, snow transport due to wind significantly influences snow distribution and, as a result, avalanche danger. A physically based numerical two-layer model is developed to simulate blowing and drifting snow in Alpine terrain. One layer describes the driving-wind field and the transport in suspension. The description is based on the atmospheric boundary-layer equations, using ane−∊model for the turbulent closure. The second layer describes the transport due to saltation, including erosion and deposition of snow. Here, conservation equations for mass and momentum are formulated for the mixture of snow and air. Particle trajectory calculations are used to parameterize quantities characterizing the saltation layer. Both layers are mutually coupled by boundary conditions. A two-way coupling between particles and airflow is taken into account. Comparisons between simulation results and field measurements around an Alpine crest show encouraging results.
Crevasse patterns at the onset to Ice Stream B, West Antarctica
Authors: Price S. F. , Whillans I. M.
Published: November 2007

Sequential satellite imagery and modeling are used to investigate crevasse patterns at the head of Ice Stream B tributary B1b. The crevasses, informally called the “chromosomes”, form at the upstream limit to B1b’s northern shear margin and chaotic crevasse zone. We find that the onset to crevasse formation, and by inference the onset to streaming flow, has migrated upstream over time at a mean rate of 230(16) m a−1. A possible cause for that migration is changes in net basal friction due to changes in basal water production rate and storage.
Glacier–volcano interactions deduced by SAR interferometry
Authors: Björnsson Helgi , Rott Helmut , Gudmundsson Sverrir , Fischer Andrea , Siegel Andreas , Gudmundsson Magnús T.
Published: November 2007

Glacier-surface displacements produced by geothermal and volcanic activity beneath Vatnajökull ice cap in Iceland are described by field surveys of the surface topography combined with interferograms acquired from repeat-pass synthetic aperture radar images. A simple ice-flow model serves well to confirm the basic interpretation of the observations. The observations cover the period October 1996–January 1999 and comprise: (a) the ice-flow field during the infilling of the depressions created by the subglacial Gjálp eruption of October 1996, (b) the extent and displacement of the floating ice cover of the subglacier lakes of Grímsvötn and the Skaftá cauldrons, (c) surface displacements above the subglacier pathways of the jökulhlaups from the Gjálp eruption site and the Grímsvötn lake, (d) detection of areas of increased basal sliding due to lubrication by water, and (e) detection of spots of temporal displacement that may be related to altering subglacial volcanic activity. At the depression created by the Gjálp eruption, the maximum surface displacement rate away from the radar decreased from 27 cm d−1to 2 cm d−1over the period January 1997–January 1999. The observed vertical displacement of the ice cover of Grímsvötn changed from an uplift rate of 50 cm d−1to sinking of 48 cm d−1, and for Skaftá cauldrons from 2 cm d to 25 cm d−1.
Meteorological conditions over Antarctic blue-ice areas and their influence on the local surface mass balance
Authors: Bintanja Richard , Reijmer Carleen H.
Published: November 2007

This paper addresses the causes of the prevailing meteorological conditions observed over an Antarctic blue-ice area and their effect on the surface mass balance. Over blue-ice areas, net accumulation is zero and ablation occurs mainly through sublimation. Sublimation rates are much higher than over adjacent snowfields. The meteorological conditions favourable for high sublimation rates (warm, dry and gusty) are due to the specific orographic setting of this blue-ice area, with usually a steep upwind mountainous slope causing strong adiabatic heating. Diabatic warming due to radiation, and entrainment of warm air from aloft into the boundary layer augment the warming. The prevailing warm, dry conditions explain roughly 50% of the difference in sublimation, and the different characteristics of blue ice (mainly its lower albedo) the other 50%. Most of the annual sublimation (∼70%) takes place during the short summer (mainly in daytime), with winter ablation being restricted to occasional warm, dry föhn-like events. The additional moisture is effectively removed by entrainment and horizontal advection, which are maximum over the blue-ice area. Low-frequency turbulent motions induced by the upwind mountains enhance the vertical turbulent transports. Strong gusts and high peak wind speeds over blue-ice areas cause high potential snowdrift transports, which can easily remove the total precipitation, thereby maintaining zero accumulation.
The isotopic record at an alpine glacier and its implications for local climatic changes and isotopic homogenization processes
Authors: Yuanqing He , Theakstone Wilfred H. , Tandong Yao , Yafeng Shi
Published: November 2007

Stratigraphic variations of oxygen isotopes in the snow which accumulates during the winter at the Norwegian glacier Austre Okstindbreen are not entirely eliminated after 1–2 months of ablation in the following summer. The relationship between regional temperature changes and δ18O values in the snowpack is affected by many natural factors, but 1989/90 winter air temperatures were reflected in the snow which remained on Austre Okstindbreen at 1350 m a.s.l. in July 1990. There were many variations of δ18O values in the 4.1m of snow above the 1989 summer surface, but variations in the underlying firn were relatively small. Meltwater percolation modifies the initial variations of δ18O values in the snowpack. At a site below the mean equilibrium-line altitude on Austre Okstindbreen, increased isotopic homogenization within a 10 day period in July accompanied an increase of the mean δ18O value. Although the isotopic record at a temperate glacier is likely to be influenced by more factors than is that at polar glaciers, it can provide an estimate of the approximate trend of local temperature variations.
The localization of strain and c-axis evolution in anisotropic ice
Authors: Wilson Christopher J. L. , Sim Hadi M.
Published: November 2007

Using a series of combined compression–simple-shear experiments, it has been possible to investigate strain distributions and crystal-orientation fabrics related to varying layer orientation in ice. A variety of flattening strains accompanied by layer buckling, simple shear and the development of a lenticular layering are produced in anisotropic ice masses. In samples where the creep curve has only just reached a minimum strain rate, the c-axis preferred orientation is similar to that in the starting material, with specific c-axis concentrations affected by the extent of preserved host grains. At shear strains where γ ≤ 1, it was found that the c-axis preferred orientations were highly variable depending on the magnitude of strain, strain distribution and upon the modification and degree of rotation of initial c-axis preferred orientation. However, once recrystallization dominates in high-strain zones (γ ≥ 1), there is a rapid development of an asymmetric two-maxima fabric with little evidence of any contributions from inherited fabric elements. The final c-axis pattern is asymmetric with respect to the direction of shortening, with a strong maximum at ∼80° to the shear zone, with a sense of asymmetry in the direction of the shear, and a secondary maximum lying at ∼50° to the plane of shearing.
History of lower Pine Island Glacier, West Antarctica, from Landsat imagery
Authors: Bindschadler Robert A.
Published: November 2007

A 28 year record of lower Pine Island Glacier, West Antarctica, constructed from 15 Landsat images, shows changes at the terminus, grounding zone and both margins. The north margin has expanded 5 km into the adjacent ice shelf in a sustained event that was underway in 1973 and may have begun in 1957. Between 1991 and 1997, this expansion ceased and a new region of rifting was created associated with an ice rise on the glacier’s floating tongue. Changes in the topography of a nearby ice rise are used to deduce an upper-bound estimate of a 134 m thinning of the adjacent ice shelf. On the south margin, widening was limited to 1 km over the observation period and is seen propagating downstream in an intermediate-dated image. New areas of crevassing are also evident in the grounding zone of the glacier. Ice loss by the calving of large tabular bergs vastly exceeds mass loss by calving of many small bergs. These observations are consistent with reported changes of the 1990s and indicate that changes in the flow of Pine Island Glacier have occurred over the full period of satellite observations.
The retreat of a tidewater glacier: observations and model calculations on Hansbreen, Spitsbergen
Authors: Vieli Andreas , Jania Jacek , Kolondra Lezek
Published: November 2007

Based on observations and model calculations, the retreat over the last two decades of Hansbreen, a tidewater glacier in southern Spitsbergen, Svalbard, is investigated. The observations of the calving-front position between 1982 and 1998 show an abrupt retreat in 1990, which is suggested to be related to a depression in the glacier bed. The observed seasonal variations of the front position are mainly due to variations of the calving rate. The observations of Hansbreen further indicate that during periods of slow front-position changes, melting at the water-line may play an important role in triggering the process of calving. The evolution of Hansbreen between 1982 and 1998 is simulated with a numerical model for the dynamics of tidewater glaciers. Using a flotation criterion for calving in which for each time-step the part of the glacier terminus which is below a critical height above buoyancy is removed, we are able to reproduce the observed rapid retreat of Hansbreen through the depression in the glacier bed. From the observations and model calculations, we conclude that the rapid retreat is mainly an effect of basal topography in the terminus region and not a direct response to a change in mass balance.
Temperature gradient and initial snow density as controlling factors in the formation and structure of hard depth hoar
Authors: Pfeffer W. Tad , Mrugala Randy
Published: November 2007

We investigate how temperature gradient and initial density influence depth-hoar growth in snow and seek to better define the range of conditions under which cohesive, or hard, depth hoar forms. Samples of 400 kg m−3sieved snow were exposed to temperature gradients of 20–80°C m−1, and samples of four different densities were exposed to a 40°C m temperature gradient. Following exposure to temperature gradients, penetrometer tests were made on samples to determine the presence of solid and/or hard depth hoar. Grain bond orientation was analyzed in section planes by two-dimensional stereological techniques where hard depth hoar developed. Results indicate that hard cohesive depth hoar forms from rounded-grain snow having a density of 400 kg m−3or greater, following exposure to a temperature gradient of 20°C m−1or greater. Hard depth hoar appears to consist of solid-type depth-hoar grains connected by necks, with vertically preferred directions of grain elongation and organization of grain-to-grain chains. This work corroborates Atikaya’s (1974) results, but extends his observation of formation of hard depth hoar to weaker temperature gradients for high-density snow. Our results also indicate that hard depth hoar is composed of faceted solid-type (anhedral) grains.
The effect of temporal variations in the surface mass balance and temperature-inversion strength on the interpretation of ice-core signals
Authors: Van Lipzig Nicole P. M. , Meijgaard Erik Van , Oerlemans Johannes
Published: November 2007

The proxy for temperature (δ signal) in ice cores is stored in the snow/ice during precipitation events and hence reflects the temperature at which precipitation is formed (here approximated by the inversion temperature Ti) weighted with the accumulation. Results from a 14 year integration (1980–93) with a regional atmospheric model (RACMO, ΔX = 55 km) show that the annual mean accumulation-weighted inversion temperature (Ti,w) and the annual mean Ti are not covariant in time at four out of the five deep-drilling sites considered, mainly due to year-to-year variations in the seasonality of precipitation. As a consequence, the surface temperature (Ts,core) derived from RACMO output, using a method analogous to the retrieval of the surface temperature from ice-core δ signals, deviates from the directly modelled surface temperature Ts on interannual time-scales. Results from a 5 year sensitivity integration, forced with a 2 K temperature increase, show an 18% overestimation of the increase in Ts,core relative to the increase in Ts due to a change in the relationship between the inversion strength and the surface temperature in a different climate regime. Similar errors may occur in deriving the temperature difference between Last Glacial Maximum and present-day climate from δ signals in ice cores.
Idealized study of a 2-D coupled sea-ice/atmosphere model during warm-air advection
Authors: Cheng Bin , Vihma Timo
Published: November 2007

We present a two-dimensional, coupled, mesoscale atmosphere–sea-ice model, and apply it to simulate the air–ice interaction during warm-air advection. The model was run into a steady state under various conditions with respect to the season, cloud cover and wind speed. The spatial and temporal evolution of the thermodynamics of the ice, snow and the atmospheric boundary layer (ABL) were investigated. The development of the stably stratified ABL downwind of the ice edge depended above all on the wind speed and cloud cover. If the turbulent heat flux from air to snow was large enough to compensate the radiative cooling of the surface, a downward conductive heat flux was generated in the upper ice and snow layers. The stronger was the surface heating (strong wind, overcast skies) and the shorter its duration (on a scale down to a few hours), the wider was the region where this downward flux occurred. From the point of view of ABL modelling, the interactive coupling between air and ice was most important when the wind was strong, while from the point of view of ice thermodynamic modelling the coupling was most important when the wind was weak.
Ice-shelf changes in Pine Island Bay, Antarctica, 1947-2000
Authors: Rignot Eric
Published: November 2007

Aerial photographs from 1947 and 1966, satellite optical imagery from 1973 and 1980, and interferometric synthetic aperture radar (InSAR) data from 1992, 1996 and 2000 are employed to detect ice-shelf changes in Pine Island Bay, Antarctica. The front position of the fast-flowing central ice shelf did not migrate discernibly over the past 50 years. New cracks and rifts appeared in the 1990s, however, that reveal a major weakening of the ice shelf. At the grounding-line center, the ice shelf thinned 21 m in 8 years. The northern, slow-moving ice shelf also shows signs of decay: (1) its calving front is retreating at an accelerating rate; and (2) the ice shelf is slowly unpinning from its bedrock anchors. These changes are taking place in a region well beyond the temperature-dependent limit of viability of ice shelves, and hence differ from those observed along the Antarctica Peninsula. They are likely due to a change in oceanic forcing, not to a change in air temperature. One possibility is that the documented intrusion of warm circumpolar deep water on the continental shelf has increased basal melting compared to that required to maintain the ice shelf in a state of mass balance, and that this has triggered a general retreat of ice in this sector.
Isotopic study on Dokriani Bamak glacier, central Himalaya: implications for climatic changes and ice dynamics
Authors: Nijampurkar Vasant , Rao Kameswara , Sarin Manmohan , Gergan Joseph
Published: November 2007

Measurements of natural and artificial radioisotopes (32Si, 210Pb and 137Cs) and oxygen isotopes (δ18O) have been carried out on surface snow and ice, shallow snow pits and an ice core collected from Dokriani Bamak glacier, central Himalaya, to study the dynamics of glacier ice and short-term climatic changes. Based on the 32Si and 210Pb activities in the meltwaters, the age of the snout ice is 400 years and the flow rate of ice along the glacier length is ∼14 m a−1. The specific activity of 137Cs, corresponding to 1963 fallout, in the surface ice at the equilibrium line yields a flow rate of 32 m a−1, a factor of two higher than that derived for the snout ice. The depth variation of 137Cs concentration in a shallow ice core yields a mean accumulation rate of 0.43 m a−1 for the glacier ice over the past decade. The δ18O of snout ice (−13.4‰) is significantly depleted compared to the average value of −9.2‰ in the shallow ice core, indicating that cooler climatic conditions prevailed around AD 1600. Based on the oxygen isotopic ratios in the shallow pits, an “altitude effect” of 0.9‰ per 100 m in δ18O variation is documented for this glacier.
Velocities, strain rates, stresses, crevassing and faulting on Glacier de Saint-Sorlin, French Alps, 1957–76
Authors: Lliboutry Louis
Published: November 2007

Stake surveys on Glacier de Saint-Sorlin, French Alps, during the period 1957–76 show that annual surface velocities fit a linear vectorial model, with a term depending on the site, another on the year, and an important random component. Strain rates, viscosities and stresses at shallow depth are computed using strain triangles of hectometric (102 m) size. Between 1961/62 and 1972/73 the isotropic point, where streamlines cease to converge, moved downstream about 200 m. This trend may explain increasing velocities. Crevasses appear where annual strain is > 1.2%. Faults limit the effective shear stress at the surface to about 0.38 bar. Ten modes of flow are distinguished, instead of only two for the two-dimensional model (compressive and extensive). The gradients of shallow stresses, which ensure extra driving forces, are computed with another mosaic of triangles of similar size (stress triangles). There are also important extra driving forces at the bottom, which force the flow to deviate from the direction of the steepest surface slope. Two criteria allow elimination of stress triangles where these unknown basal extra driving forces are important. Even so, no sliding law in terms of mean annual values can be obtained. This study shows that the classical perturbation theory, which explains advances and retreats by the arrival of kinematic waves, is unsuitable for glaciers of kilometric size.