We apply the force-budget technique using the isothermal block-flow model, on Storglaciaren, Sweden, to investigate the ratio between basal drag and driving stress in relation to a bedrock ridge in the bed topography during a peak melt season. The input data consist of glacier surface velocities collected using differential global positioning system surveying of a stake net and geometry from previous radar soundings and digitized ice surface maps. The study focuses on the effects of transverse bedrock ridges upon basal stress conditions. The pattern of the calculated ratio of basal drag and driving stress shows a rhythmical position of relatively high and low basal drags on the stoss and lee sides, respectively, of the bedrock thresholds. One of the zones of low basal drag corresponds to the location where the highest basal sliding rate has been measured previously by borehode deformation studies. This zone also aligns with the area where the drainage system is suggested to change from englacial to subglacial.

Evacuation of the ground-water intruding into a coal mine beneath Høganesbreen, Svalbard, is difficult and expensive. To solve this problem, it was proposed that the mine be connected to the ice–bedrock interface. Pumping hot water from the mine should establish a flow path along the glacier bed where the ground-water would drain gravitationally. In this paper, we assess the requirements for maintaining such a drainage system in open-channel conditions. To obtain the bedrock topography, we determined the ice thickness by ground-penetrating radar and subtracted it from the surface elevation measured by global positioning system. A measured temperature profile at the site where the mine should connect to the glacier bed (140m depth) revealed that the basal ice is below the pressure-melting point. The locations of major subglacial conduits were estimated using a hydraulic-potential approach. We adopted a model oftime-dependent discharge through a Röthlisberger channel to calculate a set of scenarios using different flow-law parameters. Results of the simulations suggest that for the given conditions, water flow would be pressurized, thereby inhibiting the gravitational drainage of the mine.

Tracer injections into a subglacial channel at Unteraargletscher, Switzerland, were repeated at intervals of about 2 hours over two diurnal discharge cycles in August and September 2000. Records of dye concentration reveal a pronounced hysteresis in the velocity–dispersion relationship, thereby indicating alterations in the drainage system. Theoretical considerations for Röthlisberger channels suggest an evolution of the conduit cross-section in response to a diurnally varying discharge. We studied the relation between conduit cross-section and tracer dispersion with numerical tracer experiments. The velocity field for steady flow through a given conduit geometry is calculated using a commercial flow solver. Tracer transport is represented by a scalar volume which is advected by the velocity field. Experiments were conducted for several scenarios by varying flow velocity, conduit geometry and conduit roughness. Results show only a weak dependence of dispersion on conduit size. In contrast, changes in roughness of the conduit walls reveal a strong effect on tracer dispersion. Therefore, to explain the observed hysteresis in the velocity–dispersion relationship, we suggest that the evolution of a subglacial flow path might involve changes in roughness.

Channels incised into bedrock, or Nye channels, often form an important component of subglacial drainage at temperate glaciers, and their structure exerts control over patterns and rates of (a) channel erosion, (b) water flow-velocity and (c) water pressure. The latter, in turn, exerts a strong control over basal traction and, thus, ice dynamics. In order to investigate these controls, it is necessary to quantify detailed flow processes in subglacial Nye channels. However, it is effectively impossible to acquire such measurements from fully pressurized, subglacial channels. To solve this problem, we here apply a three-dimensional, finite-volume solution of the Reynolds averaged Navier– Stokes (RANS) equations with a one-equation mixing-length turbulence closure to simulate flow in a 3 m long section of an active Nye channel located in the immediate foreground of Glacier de Tsanfleuron, Switzerland. Numerical model output permits high-resolution visualization of water flow through the channel reach, and enables evaluation of the experimental manipulation of the pressure field adopted across the overlying ice lid. This yields an increased theoretical understanding of the hydraulic behaviour of Nye channels, and, in the future, of their effect on glacier drainage, geomorphology and ice dynamics.

Central Asian glaciers, compared to other glaciers in the world, should exhibit a different response to changing climate owing to (i) the extreme continentality that gives rise to aridity and large seasonal temperature variations, and (ii) the coincidence of accumulation and ablation seasons in summer. A detailed 4year field survey has been carried out on Sofiyskiy glacier in the south Chuya range(Altai mountains), Russia. Field observations reveal that this glacier retreated steadily during the 20th century at a rate of 18.3 m a–1, considerably faster than Maliy Aktru glacier, 30km to the north. Based on radio-echo sounding (RES), the subglacial topography of Sofiyskiy glacier has been reconstructed, and stake and snow-pit measurements enable the determination of surface velocity and mass balance. Basal sliding plays an important role in Sofiyskiyglacier’s behaviour, as is shown by a force-balance analysis. An analysis of RES measurements yields the bed reflection power (BRP). Using the three-layer reflectivity model of Born and Wolf (1986), the BRP is compared with the theoretical bed reflectivity for two contrasting layers of varying porosity and thickness. Results indicate that Sofiyskiy glacier possibly exhibits a polythermal regime.

The upper 10 m of the firn of a Svalbard glacier is imaged along the centre line using a 500 MHz ground-penetrating radar, and a 10 m firn core taken along the profile. Complex reflection coefficients are calculated from the high-resolution capacitance and conductance measurements made on the snow core. The reflection coefficient depth series is converted to the time domain and convolved with model radar monopulses to synthesize traces that compare well with radar traces recorded near the ice core. Differences are probably due to cm-scale physical and chemical inhomogeneities that are smoothed when imaged by the radar beam, which integrates information over areas that are of the same order of magnitude as the depth to the layer.

Detailed ground-penetrating radar (GPR) surveys on the firn–ice transition zones of Kongsvegen, Nordenskjoldbreen, Hansbreen and Werenskioldbreen, Svalbard, are presented. We discuss features seen in GPR radargrams in all polythermal glaciers down-glacier of the firn line. The firn line of a polythermal glacier in GPR data is easily detected with both 50 and 200 MHz radar. By comparison with satellite images, characteristic patterns of reflection are shown to be caused by superimposed ice, but the reflections from the interface between the superimposed ice and the underlying glacier ice are of unknown origin. Layered reflections within the superimposed ice and firn area are likely from layers of ice lenses and relatively low-density firn in the firn area, and from bubble-rich and bubble-poor ice in the superimposed-ice zone. Dipping foliations in the glacier ice down-glacier of the firn line show that the layered reflections from the firn line are preserved, though in much weaker form, even when the firn has been transformed to solid ice.

We studied the scattering of radio waves off strata within the ice sheet at Mizuho station, Antarctica, to determine the most plausible scattering mechanisms at this location. We measured the effects of birefringence and anisotropic scattering boundaries on the return signal using the following set of experimental conditions: (1) co-polarization and cross-polarization antenna arrangements, (2) all orientations of the antenna system, (3) 60 and 179 MHz frequencies, and (4) pulse lengths of 150–1000 ns. Analyses of the propagated radio waves suggested that the signal is dominated by anisotropic scatter-ingboundaries at 179 MHz, but effects from birefringence also occurred. At depths of 250– 750 m, the scattering was stronger when the polarization plane was along the flowline. In contrast, at depths of about 900–1500 m, scattering was stronger when the polarization plane was perpendicular to the flowline. We suggest that the scattering below about 250 m is related to a layered ice stratum of crystal-orientation fabrics with different types or different cluster strengths. Although our study was at a single site, similar remote measurements over wider regions should provide valuable information about the physical structure of this vast ice sheet.

Radio-wave velocity measurements in temperate and polythermal glaciers, combined with dielectric mixture formulae by Looyenga or Paren, have been used during the last decade to estimate the water content in temperate ice. We have used a similar mixture formula by Riznichenko, but based on elastic properties of the material, to estimate the water content from seismic velocity data. To compare the suitability of the two methods, we have used seismic and radar data from a temperate glacier on an Antarctic island. The estimated water contents are within 0.4–2.3% (average 1.2 ±0.6%) when radio-wave velocities are used, and within 0.9–3.2% (average 2.2±0.9%) when seismic velocities are used. These results are similar to those directly measured from ice cores and to those estimated from radar data on other temperate glaciers. The water-content estimates from seismic data are higher than those from radar data, which we attribute to the different behaviour of seismic and radar velocities as functions of density. Near-surface conditions (ice–firn conditions, presence of crevasses, etc.) have a strong influence on the propagation of elastic and electromagnetic waves, and thus on the accuracy of the velocity determinations and water-content estimates, and so should not be disregarded.

Spatially extensive internal layers have been traced in airborne radio-echo sounding (RES) data collected over Greenland during the late 1990s. By linking internal layers within individual flight-lines at crossover points, it is possible to identify spatially continuous layers that are interpreted as isochronous surfaces. Several of the survey lines pass over the GRIP core site, and this allows us to use the published GRIP age–depth relationship to accurately date these surfaces. Two layers, with ages of 3891 and 6956 years BP, have been traced over a large part of North Greenland. Accurately dated and spatially continuous isochrones are valuable for both assimilation within, and verification of, numerical models. For example, comparison of isochronous surfaces from a numerical simulation with those layers observed in RES data can be used to inform the choice of parameters (e.g. rheology) and climate history used to force a numerical model. To demonstrate the potential of the RES data, two layers for North Greenland were used to determine palaeo-accumulation rates. The inversion from layer depth to accumulation rate requires a three-dimensional velocity field. This velocity field is constructed by combining a two-dimensional balance-velocity field with an assumed vertical structure for the horizontal velocity. The isochronous-layer derived accumulation rates were compared with the Bales and others (2001) rates. A larger east–west gradient was found across the central ice divide for the derived accumulation rate, suggesting a trend in the Holocene accumulation rates for this region. The layers were also compared with isochronous surfaces derived from simulations of a three-dimensional thermodynamic ice-sheet model. Using the isochronous-layer derived accumulation rates to force the model improved the match between modelled and observed layers.

Results from 15 years of work on glacier-bed mapping by radio-echo soundings on Finsteraar-, Lauteraar- and Unteraargletscher, Switzerland, are summarized, and a new and greatly improved map of the ice-thickness distribution presented. In contrast to the tongue of Unteraargletscher, its two main tributaries, Lauteraar and Finsteraar, are both deep and narrow, and ice-thickness determination depends on the detection of more than just the primary reflection. Migrating the data led to considerably improved bed determination. Wherever possible, additional information on ice thicknesses gained from numerous hot-water drillings to the glacier bed is used as an independent verification of the results of the radar measurements, and a fair agreement is found.

Comparisons between computed balance velocities, obtained from two different computing schemes, and global positioning system (GPS)-derived velocities were made in the Lambert Glacier basin region, East Antarctica. The two computing schemes used for the balance-velocity computations (a flowline (FL) scheme (Remy and Minster, 1993) and a finite-difference (BW) scheme (Budd and Warner, 1996; Fricker and others, 2000)) were first evaluated and compared. One of the key issues studied was the spatial resolution of the digital elevation model (DEM), representing the surface topography of the ice sheet, and the sensitivity of the balance velocities to the length of smoothing applied to the DEM. Comparison with the GPS velocities validated the two schemes to within 5–25% but showed the high sensitivity of the flowline method to the length scale of the smoothing. The finite-difference scheme was found to be robust to the chosen smoothing scale, but the balance-velocity values increased when a finer-resolution DEM was used. Both FL and BW computing schemes tended to overestimate the balance velocities in comparison with the GPS values; some of this discrepancy can be attributed to ice-sheet sliding.

Isochronic continuous horizons between 20 and 90 m depth in a ground-penetrating radar (GPR) profile, recorded in Dronning Maud Land, Antarctica, are identified by comparison of synthetic and measured single radar traces. The measured radar-gram is derived from a stacked GPR profile; the synthetic radargram is computed by convolution of the complex reflection coefficient profile, based on dielectric profiling (DEP) data of a 150 m ice core, with a depth-invariant wavelet. It reproduces prominent reflections of the measured radargram to a considerable degree. Analyzing matching peaks in both radargrams enables us to identify isochronic reflections and transfer individual volcanic-event datings to the GPR profile. Reflections are primarily caused by changes in permittivity; changes in conductivity are of minor importance. However, several peaks in permittivity andconductivity show a good correlation and indicate that some reflections are related to acidic layers. The results demonstrate the possibility of reproducing radargrams from ice-core property profiles, a necessary step for the interpretation of remotely sensed radar data and the general significance of connecting ice-core and radar data for correct interpretations. Problems related to forward modeling, data gaps, origin of permittivity peaks, and GPR profiles used for comparison, are discussed.

A new digital elevation model of the surface of the Greenland ice sheet and surrounding rock outcrops has been produced from a comprehensive suite of satellite and airborne remote-sensing and cartographic datasets. The surface model has been regridded to a resolution of 5 km, and combined with a new ice-thickness grid derived from ice-penetrating radar data collected in the 1970s and 1990s. A further dataset, the International Bathymetric Chart of the Arctic Ocean, was used to extend the bed elevations to include the continental shelf. The new bed topography was compared with a previous version used for ice-sheet modelling. Near the margins of the ice sheet and, in particular, in the vicinity of small-scale features associated with outlet glaciers and rapid ice motion, significant differences were noted. This was highlighted by a detailed comparison of the bed topography around the northeast Greenland ice stream.

The Mars polar caps are highly interesting features of Mars and have received much recent attention, with new and exciting data on morphology, basal units and layered outcroppings. We have examined the climatological, glaciological and geological issues associated with a subsurface exploration of the Mars North Polar Cap and have determined that a fine-scale optical examination of ice in a borehole, to study the stratigraphy, geochemistry and geochronology of the ice, is feasible. This information will enable reconstruction of the development of the cap as well as understanding the properties of its ice. We present visible imagery taken of dust inclusions in archived Greenland ice cores as well as in situ images of accreted lithologic inclusions in West Antarctica, and we argue for use of this kind of data for Mars climate reconstruction as has been successful with Greenland and Antarctic ice-core analysis.

Assuming that the permanent north polar cap of Mars consists of flowing water ice, the relationship between topography and flow is examined along a profile extending from the pole in the 160° E direction This profile is intersected by scarps and troughs that are characteristic of the north polar cap The flow is calculated by a finite-element ice-flow model which includes divergence of the flow, longitudinal stresses and temperature effects. Ice-flow velocities are generally on the order of 0.1–1 mm a–1 but are enhanced at scarps and troughs to cm a–1. Ice flow smooths out the troughs. Troughs affect the flow to the bottom of the cap. Beneath a trough, ice is dragged upward. Longitudinal stresses are able to drag the lowest part of the ice past smaller troughs. At the pole-facing side of major troughs, ice is stagnant or flows slowly poleward. Implications for formation mechanisms of scarps and troughs are discussed.

If an ice sheet is frozen to its bed, deep ice directly under a divide experiences low deviatoric stress and is relatively hard, because the rheology of polar ice is described by a power-law constitutive relation. In steady state, stratigraphic layers tend to form an arch (“Raymond bump”) in this region. However, when the basal ice can slide, the stresses are redistributed, and longitudinal extension due to sliding is associated with increased deviatoric stress in the deep ice under the divide. This increased deviatoric stress weakens the tendency to form a Raymond bump. To find a realistic spatial distribution of sliding under an ice divide, we incorporate a thin layer of viscous till in a finite-element plane-strain flow model. The resulting basal “sliding” velocity varies approximately linearly with distance from the ice divide. By varying the till viscosity, we can adjust the amount of basal motion. We find that the Raymond bump decays exponentially with the fraction of total ice flux carried by sliding: the arch is 50% smaller when the sliding flux is only 7% of the total ice flux. This implies that the possibility of a wet bed must be considered when inferring past ice-divide locations from radar internal layering.

It has been shown recently that ice streams are fed by fast-flowing tributaries occupying well-defined subglacial troughs and with shared source areas. Here, ice-penetrating radio-echo sounding (RES) data are analyzed in conjunction with ice surface velocities derived from interferometric synthetic aperture radar (InSAR), to determine the englacial properties of tributaries feeding Ice Stream D, West Antarctica. All of Ice Stream D’s tributaries are coincident with “buckled” internal ice-sheet layers, most probably deformed by the processes responsible for enhanced ice flow. Between the tributaries well-preserved internal layers occur. The data reveal that no lateral migration of the ice-stream tributaries has occurred recently. This is consistent with thermomechanical ice-flow modelling, which indicates that the flow of Ice Stream D is controlled by a subglacial trough and is unaffected by changes to the flow of neighbouring Ice Stream C.

In order to reconstruct the palaeoglaciation in Fennoscandia and northern Asia during the late-Weichselian ice-age phase, simulations with the dynamic and thermodynamic ice-sheet model SICOPOLIS are carried out. Our focus is on the Last Glacial Maximum (LGM) around 20 kyr BP. Climate forcing is based on mean annualsur-face temperature and precipitation derived from present data and Palaeoclimatic Modelling Intercomparison Project (PMIP) UKMO21 results for the LGM. These distributions are interpolated via a glacial index defined by the Greenland Icecore Project (GRIP) δ18O record. The extent of the Scandinavian and the Barents ice sheets is reproduced in good agreement with the Quaternary Environments of the Eurasian North (QUEEN) reconstruction, but the Kara Sea and Taymyr Peninsula areas are excessively glaciated. The fast-flow regions derived from the simulations, which are generally connected to regions with a temperate base and temperate ice above, are compared to hypothesized palaeo-ice-stream locations, especially in the Norwegian Channel and the Baltic area. In the Norwegian Channel, temperate basal conditions with temperate ice above prevail and favour fast flow. In the Baltic area, ice-sheet advance is generally accompanied by slow ice velocities (<200 m a–1). Some temporary fast-flow features occur due to transitional temperate-base conditions, and higher velocities arise in retreat phases.

Two aspects of thermal coupling with bedrock are considered: the coupled time-dependent problem of co-evolving temperatures in lithosphere and ice; and the influence of basal topography on steady temperature distribution within the ice. The nature of the time-dependent coupling is found to depend on the horizontal velocity. As has been suggested, there is a cooling of steady temperatures on bedrock highs, but this is phase-shifted downstream when horizontal velocities increase. This observation may have consequences for geomorphological processes such as plucking and protection. The effect of bedrock channelling on steady temperature is considered. The positive anomaly of basal temperature due to channelling increases as the transverse wavelength decreases, but not monotonically, reaching a plateau when both the wavelengths of the basal topography are around 100 km.