Showing papers in "Journal of Glaciology in 2000"

The Link Between Climate Warming and Break-Up of Ice Shelves in the Antarctic Peninsula

Abstract: 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.

Sensitivity of mass balance of five Swiss glaciers to temperature changes assessed by tuning a degree-day model

Abstract: A degree-day model is used to assess the sensitivity of the mass balance of five Swiss glaciers to temperature changes. The model uses temperature data extrapolated from nearby climate stations, and is tuned by varying precipitation to make the model fit the observed distribution of mass balance with altitude. Once the model is tuned, the effect of temperature change is simulated by recalculating the mass balance with the same parameters as before, but with a temperature increase of 1°C throughout the year. The largest mass-balance changes, involving increased ablation of > 1 m w.e. a−1 °C−1, occur at the snout, with a progressively smaller increase with altitude. The area-averaged sensitivities for the five glaciers are −0.7 to −0.9 m w.e. a−1 °C−1. If annual precipitation also increased by 20% it would partly offset the effect of the 1°C higher temperatures but could not compensate for it.

Effect of summer accumulation on glacier mass balance on the Tibetan Plateau revealed by mass-balance model

Abstract: The characteristics and sensitivities of a cold-based glacier on the Tibetan Plateau, where the summer monsoon provides most of the mass input to glaciers, are discussed using an energy-balance model incorporating the process of water refreezing. The model accurately represents the observational results related to the mass balance of Xiao Dongkemadi glacier on the central plateau during 1992/93. Our data revealed that the mass balance of cold glaciers cannot simply be described by the surface mass/heat balances, because about 20% of infiltrated water is refrozen and thus does not run off from the glacier. Model calculations demonstrate that glaciers in an arid environment can maintain their mass since the monsoon provides precipitation during the melting season. Snowfall in summer keeps surface albedo high and largely restrains ablation. Nevertheless, the calculations also make clear that glaciers on the plateau are more vulnerable than those of other regions because of summer accumulation. In the monsoon climate, warming would cause not only a decrease in accumulation, but also a drastic increase in ablation in combination with surface-albedo lowering. Therefore, although glaciers on and around the plateau can be sustained by summer accumulation, they are more vulnerable to warming than winter-accumulation-type glaciers.

Measurement and parameterization of albedo variations at Haut Glacier d'Arolla, Switzerland

Abstract: Spatial and temporal variations of surface albedo on Haut Glacier d’Arolla, Switzerland, during the 1993 and 1994 ablation seasons are described. Correlation and regression analyses are used to explain the albedo variations in terms of independent meteorological and surface property variables. Parameterizations are developed which allow estimation of albedo variation in surface energy-balance models. Snow albedo is best estimated from accumulated daily maximum temperatures since snowfall. On “deep” snow (≥0.5 cm w.e. depth) a logarithmic function is used, while on “shallow” snow (<0.5 cm w.e. depth) an exponential function is used to enable the albedo to decay to the underlying ice or debris albedo. The transition from “deep” to “shallow” snow is calculated as a function of decreasing snow depth (combined r 2 = 0.65). This new parameterization performs better than earlier schemes because accumulated daily maximum temperatures since snowfall correlate strongly with snow grain-size and impurity concentration, the main physical controls on snow albedo. Ice albedo may be parameterized by its relationship to elevation (r 2 = 0.28), but this approach results in only a small improvement over the assumption of a constant mean ice albedo.

Results from the EISMINT model intercomparison : the effects of thermomechanical coupling

Abstract: This paper discusses results from the second phase of the European Ice sheet Modelling Initiative (EISMINT). It reports the intercompartison 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 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 regimne 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.

Controls on the distribution of surge-type glaciers in Svalbard

Abstract: 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.

Glacier motion dominated by processes deep in underlying till

Abstract: Black Rapids Glacier is a 40 km long surge-type glacier in the central Alaska Range. In spring 1997 a wireline drill rig was set up at a location where the measured surface velocities are high and seasonal and annual velocity variations are large. The drilling revealed a layer of subglacial till, up to 7 m thick, that is believed to be water-saturated. At one location a string of instruments, containing three dual-axis tiltmeters and one piezometer, was successfully introduced into the till. The tiltmeters monitored the inclination of the borehole at the ice-till interface and at 1 and 2 m into the till, for 410 days. They showed that no significant deformation occurred in the upper 2 m of the till layer, and no significant amount of the basal motion was due to sliding of the ice over the till. The measured surface velocity at the drill site is about 60 m a -1 , of which 20-30 m a can be accounted for by ice deformation. Almost the entire amount of basal motion, 30-40 m a -1 , was taken up at a depth of > 2 m in the till, possibly in discrete shear layers, or as sliding of till over the underlying bedrock. We propose that the large-scale mobilization of such till layers is a key factor in initiating glacier surges.

Relating glacier mass balance to meteorological data using a Seasonal Sensitivity Characteristic

Abstract: We propose to quantify the climate sensitivity of the mean specific balance B of a glacier by a seasonal sensitivity characteristic (SSC). The SSC gives the dependence of B on monthly anomalies in temperature and precipitation. It is calculated from a mass-balance model. We show and discuss examples for Franz-Josef Glacier (New Zealand), Nigardsbreen (Norway), Hintereisferner (Austria), Peyto Glacier (Canadian Rockies), Abramov Glacier (Kirghizstan) and White Glacier (Canadian Arctic). With regard to the climate sensitivity of B, the SSCs clearly show that summer temperature is the most important factor for glaciers in a dry climate. For glaciers in a wetter climate, spring and fall temperatures also make a significant contribution to the overall sensitivity. The SSC is a 2 × 12 matrix. Multiplying it with monthly perturbations of temperature and precipitation for a particular year yields an estimate of the balance for that year. We show that, with this technique, mass-balance series can be (re)constructed from long meteorological records or from output of atmospheric models.

A millennium of variable ice flow recorded by the Ross Ice Shelf, Antarctica

Abstract: An enhanced composite Advanced Very High Resolution Radiometer (AVHRR) image is used to map flow stripes and rifts across the Ross Ice Shelf, Antarctica. The patterns of these flow-related features reveal a history of discharge variations from the ice streams feeding the eastern part of the shelf. The most profound variations are visible in the track of rifts downstream of Crary Ice Rise, flow-stripe bends to the west of this ice rise and adjacent to Steershead ice rise, and changes in the northern margin of Ice Stream B. The track of rifts downstream of Crary Ice Rise indicates that the ice rise has existed for at least 700 years. The character of this track changes about 350 km downstream, indicating a rearrangement of flow patterns about 550 years ago. The large bulge in the flow stripes to the west of Crary Ice Rise is shown in detail, with bent flow stripes extending for several hundred kilometers along flow; this feature formed from the south, possibly due to a change in the discharge of Ice Stream A. The AVHRR image documents a complex history associated with the shutdown of Ice Stream C, with changes in the margins of Ice Stream C and the northern margin of Ice Stream B, and the grounding of Steershead ice rise with an associated bending and truncation of flow stripes. Landsat imagery shows a region that appears to be actively extending just downstream of the ice rise, as the shelf continues to respond to recent changes in ice-stream discharge. We present a four-stage flow history which accounts for the features preserved in the ice shelf.

Bed properties of Siple Dome and adjacent ice streams, West Antarctica, inferred from radio-echo sounding measurements

Abstract: We have used ground-based radio-echo sounding (RES) profiles to reveal the spatial distribution of basal and internal ice properties across Siple Dome, West Antarctica, and under the dormant ice streams on its flanks. The RES-detected bed-reflection power, corrected for the effects of instrumentation and ice-thickness variation, is nearly constant across Siple Dome at a value suggesting spatially homogeneous basal properties of ice frozen to bedrock. Till, if present under the dome, must be thin ( 1 m) thawed or frozen till layer. The evidence that the dormant SIS is not frozen directly to underlying bedrock (but is separated by a water or till layer) is a further indication that it was once an active ice stream, and suggests that streaming motion may have ceased before the basal layer was frozen. The absence of a thick till layer beneath Siple Dome is consistent with its apparent stability as an inter-ice-stream ridge in the past and may suggest that it will remain as a stable limitation of ice-stream width in the future.

A numerical investigation of ice-lobe-permafrost interaction around the southern Laurentide ice sheet

Abstract: Permafrost existed around and under marginal parts of the southern Laurentide ice sheet during the Last Glacial Maximum. The presence of permafrost was important in determining the extent, form and dynamics of ice lobes and the landforms they produced because of influences on resistance to basal motion and subglacial hydrology. We develop a two-dimensional time-dependent model of permafrost and glacier-ice dynamics along a flowline to examine: (i) the extent to which permafrost survives under an advancing ice lobe and how it influences landform development and hydrology, and (ii) the influence of permafrost on ice motion and surface profile. The model is applied to the Green Bay lobe, which terminated near Madison, Wisconsin, during the Last Glacial Maximum. Simulations of ice advance over permafrost indicate that the bed upstream of the ice-sheet margin was frozen for 60-200 km at the glacial maximum. Permafrost remained for centuries to a few thousand years under advancing ice, and penetrated sufficiently deep (tens of meters) into the underlying aquifer that drainage of basal meltwater became inefficient, likely resulting in water storage beneath the glacier. Our results highlight the influence of permafrost on subglacial conditions, even though uncertainties in boundary conditions such as climate exist.

Englacial water distribution in a temperate glacier from surface and borehole radar velocity analysis

Abstract: We have obtained common offset, common midpoint (CMP) and borehole vertical (VRP) ground-penetrating radar profiles close to the margin of Falljokull, a small, steep temperate valley glacier situated in southeast Iceland. Velocity analysis of CMP and VRP surveys provided a four-layered velocity model. This model was verified by comparison between the depths of englacial reflectors and water channels seen in borehole video, and from the depths of boreholes drilled to the bed. In the absence of sediment within the glacier ice, radar velocity is inversely proportional to water content. Using mixture models developed by Paren and Looyenga, the variation of water content with depth was determined from the radar velocity profile. At the glacier surface the calculated water content is 0.23−0.34% (velocity 0.166 m ns−1), which rises sharply to 3.0−4.1% (velocity 0.149 m ns−1) at 28 m depth, interpreted to be the level of the piezometric surface. Below the piezometric surface the water content drops slowly to 2.4−3.3% (velocity 0.152 m ns−1) until ∼102 m depth where it falls to 0.09−0.14% (velocity 0.167 m ns−1). The water content of the ice then remains low to the glacier bed at about 112 m. These results suggest storage of a substantial volume of water within the glacier ice, which has significant implications for glacier hydrology, ice rheology and interpretations of both radar and seismic surveys

The use of bulk and profile methods for determining surface heat fluxes in the presence of glacier winds

Abstract: A one-dimensional second-order closure model and in situ observations on a melting glacier surface are used to investigate the suitability of bulk and profile methods for determining turbulent fluxes in the presence of the katabatic wind-speed maximum associated with glacier winds. The results show that profile methods severely underestimate turbulent fluxes when a wind-speed maximum is present. The bulk method, on the other hand, only slightly overestimates the turbulent heat flux in the entire region below the wind-speed maximum and is thus much more appropriate for use on sloping glacier surfaces where katabatic winds dominate and wind-speed maxima are just a few meters above the surface.

Monsoon and dust signals recorded in Dasuopu glacier, Tibetan Plateau

Abstract: During summer 1997, a 15 m firn core was recovered from Dasuopu glacier (28°23' N, 85°44' E; 7000 m a.s.l.) on the northwest margin of Xixabangma Feng in the central Himalaya. Oxygen isotope values and concentrations of Ca 2+ , Mg 2+ , NH 4 + , SO 4 2 and NO 3 were measured over the 10 years of snow accumulation captured in the firn core. The seasonal variations of δ 18 O values and major-ion concentrations in the Dasuopu core indicate that summer monsoon and dust signals are clearly recorded in Dasuopu glacier. Annual variations in δ 18 O values are controlled by the amount effect, with more negative (i.e. lighter) δ 18 O values representing summer monsoon precipitation characteristic of tropical regions. Higher concentrations of Ca 2+ , Mg 2+ and SO 4 2- reflect the influx of mineral aerosols from the vast arid and semi-arid desert regions to the north and west during the spring dust-storm period. High spring concentrations of NH 4 + and NO 3 appear to reflect changes in regional biogenic-source strength.

Energy balance of ice streams

Abstract: Analysis of the cross-flow transmission of force from the central parts of a well-lubricated ice stream to its margins shows that there is a corresponding shift in the lateral location of motion-induced heat generation. The rate of basal heat generation in the center can be substantially smaller than the local rate of potential energy loss given by driving stress times the speed of downslope motion. The basal heating is a maximum for an intermediate level of lubrication for which speed is about 40% of the speed over a friction-less bed and base stress is about 25% of the driving stress. Stable and unstable balances between meltwater production and drainage on the bed are identified. A stable steady state with a speed less (more) than that giving maximum heat generation is termed drainage-(production-) limited, since an increase in speed would lead to increased (decreased) basal melting and must (need not) be balanced by increased drainage. It is shown that gradual evolution of the basal water drainage system and the factors affecting basal melting can cause discontinuous jumps between fast- and slow-moving states. A simplified analysis applied to six cross-sections of West Antarctic Ice Streams B, D, E and Rutford Ice Stream shows them to be diverse in the level of support from the sides and corresponding shift of mechanical heating sideward from their central parts. The cross-sections of Ice Stream B near “Upstream B” may be production-limited, because of especially high lubrication and related support from the sides. Cross-sections in the upper part of Ice Stream D, Ice Stream E and Rutford Ice Stream are in a drainage-limited condition. Substantial reduction of basal heat generation by side drag (in most cases) and expected high heat flow into the basal ice associated with low thickness (in some cases) tends to favor basal freezing. Nevertheless, all of the examined cross-sections except one are expected to experience basal melting with a modest geothermal heat-flux density of 60 m W m−1 or less in some cases. The lower part of Ice Stream B is an exception, where the analysis indicates that geothermal flux density must exceed 80–100 m W−1 m to maintain melting. If this high geothermal flux is not present, then the base of the lower part of Ice Stream B may be freezing, which would suggest continued deceleration of this part of Ice Stream B.

Analysis of a 3 year meteorological record from the ablation zone of Morteratschgletscher, Switzerland : energy and mass balance

Abstract: Since 1 October 1995, an automatic weather station has been operated on the tongue of Morteratschgletscher, Switzerland. The station stands freely on the ice, and sinks with the melting glacier surface. It is located at 2100 m a.s.l., and measures air temperature, wind speed and direction, incoming and reflected solar radiation, pressure and snow temperature. A sonic ranger, mounted to stakes drilled into the ice, measures surface height from which melt rates and snow accumulation can be derived. In this paper the data for the period 1 October 1995 to 30 September 1998 are used to evaluate the surface energy balance. The turbulent energy fluxes are calculated with the bulk method. The turbulent exchange coefficient Ch is used as a control parameter. With Ch = 0.00127 the calculated melt equals the observed melt, which is 17.70 m w.e. over the 3 years. When averaged over the time when melting occurs (i.e. 35% of the time), the mean surface heat flux equals 191 W m [exp. -2]. Net shortwave radiation contributes 177 W m [exp. -2], net longwave radiation -25 W m [exp. -2], the sensible-heat flux 31 W m [exp. -2]. and the latent-heat flux 8 W m [exp. -2].

Glaciochemical dating of an ice core from upper Grenzgletscher (4200 m a.s.l.)

Abstract: One crucial condition for the interpretation of ice-core records is the establishment of an accurate time-scale. This task is especially difficult for glacier sites in a complex topography such as the Alps, due to the often irregular deposition of fresh precipitation. In this work, dating techniques were applied to an Alpine ice core from upper Grenzgletscher, Monte Rosa massif (4200 m a.s.l.), representing about two-thirds of the total glacier thickness. They are based on (i) the radioactive decay of the isotope 210 Pb, (ii) seasonally varying signals such as the concentrations of NH 4 + and the isotopic ratio δ 18 O, and (iii) stratigraphic markers from Saharan dust falls, atmospheric nuclear weapon tests and the reactor accident in Chernobyl. From the combined application of these dating methods, a time period of 1937-94 covered by the ice core was derived. Dating uncertainty is <1 year for the period 1970-94 and ± 2 years for the period 1937-69. The observed thinning of the annual layers as a function of depth could be well described by a simple kinematic glacier flow model.

Late Pleistocene climate conditions in the north Chilean Andes drawn from a climate-glacier model

Abstract: A climate-glacier model was used to reconstruct Late-glacial climate conditions from two case-study glaciers at 18° and 22° S in the arid (sub) tropical western Andes of northern Chile. The model uses (i) the geometry of the Late-glacial maximum glaciation, (ii) modern diurnal and annual cycles, amplitudes and lapse rates of the climate, (iii) empirical-statistical sublimation, melt and accumulation models developed for this area, and (iv) dynamic ice flow through two known cross-sections for steady-state conditions. The model is validated with modern conditions and compares favorably with the glaciological features of today. The mass-balance model calculates the modern equilibrium-line altitude at 18° S as high as 5850 m (field data 5800 m), whereas no glaciers exist in the fully arid southern area at 22° S despite altitudes above 6000 m and continuous permafrost. For Late-glacial times, the model results suggest a substantial increase in tropical summer precipitation (AP = +840 (-50/+ 10) mm a -1 for the northern test area; +1000 (- 10/+ 30) mm a -1 for the southern test area) and a moderate temperature depression (ΔT = -4.4 (-0.1/+0.2) °C at 18° S; -3.2 (±0.1) °C at 22° S). Extratropical frontal winter precipitation (June-September) was <15% of the total annual precipitation. A scenario with higher winter precipitation from the westerlies circulation belt does not yield a numerical solution which matches the observed geometry of the glaciers. Therefore, we conclude that an equatorward displacement of the westerlies must be discarded as a possible explanation for the late Pleistocene glaciation in the Andes of northern Chile.

Texture and strength changes of buried surface-hoar layers with implications for dry snow-slab avalanche release

Abstract: Buried layers of surface hoar are the failure plane for many slab avalanches, including fatal human-triggered avalanches in various mountain regions. These layers may persist as weak layers in the snow cover for weeks or months. It is therefore essential for operational avalanche forecasters to monitor the evolution of persistent weak layers, such as buried surface hoar. Traditional grain-shape observations of isolated grains with a magnifier and crystal screen do not show bonding that is decisive for strength. In this study we used in situ microphotography and observations of texture to complement strength measurements from shear frame tests. Buried layers of surface hoar consist of crystals most of which extend from the layer below to the layer above, and may exhibit a columnar or truss-like structure. Observations and measurements show that texture and crystal size change little over periods of up to several months during which the snowpack remains dry. Under these conditions, layer thickness decreases while density and strength increase. Based on field measurements, we argue that the increase in strength is primarily due to penetration of the surface-hoar crystals into the adjacent layers, especially at the bottom of the buried surface-hoar layer, where bonding is critical. The weak bonding at the bottom implies that shear failure occurs at the lower interface rather than within the weak layer. On slopes, we find that surface-hoar crystals that were initially surface-normal are tilted downslope faster than predicted by published shear strain rates for settled snow, indicating that shear strain is concentrated in these layers. The characteristic texture of buried surface hoar (columnar or truss-like) permits collapsing at the time of fracture. The gravitational energy released by the displacement of the slab may contribute to the extensive fracture propagation associated with buried surface-hoar layers.

Reconstructing past atmospheric CO2 concentration based on ice-core analyses: open questions due to in situ production of CO2 in the ice

Abstract: Analysis of air extracted from bubbles of polar ice showed the anthropogenic increase of atmospheric CO2 concentration during the past few hundred years (preindustrial concentration 280 ppmv) and, unexpectedly, that the concentration also increased due to natural causes from 200 to 280 ppmv during the transition from the last glacial epoch to the Holocene. However, more detailed reconstructions based on ice-core analyses suggested that some of the additional observed variations were actually due to the modification of CO2 concentration in the bubbles by chemical reactions between impurities in the ice. Detailed analyses of acidity, the carbonate concentration, the concentration of oxidation agents like hydrogen peroxide (H2O2), and organic compounds (e.g. formaldehyde (HCHO)) along short intervals of ice cores, representing only a few annual layers, allowed us to investigate these chemical reactions. The records are not conclusive but they indicate that the oxidation of organic compounds is at least as important as acid–carbonate reactions. Ice containing a low mean carbonate and a low H2O2 concentration, as well as a small scattering of CO2 results obtained on several adjacent samples representing a few annual layers at most, is at present the best guarantee for reliable results of the atmospheric CO2 concentration.

A model of ablation-dominated medial moraines and the generation of debris-mantled glacier snouts

Abstract: Medial moraines form striking dark stripes that widen non-linearly, steepen laterally and increase in relief down-glacier from the equilibrium line. Coalescence of these low-ablation-rate features can feed back strongly on the mass balance of a glacier snout. Ablation-dominated medial moraines originate from debris delivered to glacier margins, producing a debris-rich septum between tributary streams of ice below their confluence. Emergence of this ice below the equilibrium line delivers debris to the glacier surface, which then moves down local slopes of evolving morainal topography. A quantitative description of moraine evolution requires specification of the debris concentration field within the glacier, treatment of the melt-rate dependence on debris thickness, and characterization of processes that transport debris once it emerges onto the ice surface. Debris concentration at glacier tributary junctions scales with the erosion rates and the lengths of the tributary-valley walls, and inversely with the tributary ice speeds. Melt rate is damped exponentially by debris, with a ∼10 cm decay scale. Debris flux across the glacier surface scales with the product of debris thickness and local slope. Analytical and numerical results show that medial moraines should develop cross-glacier profiles with parabolic crests and linear slopes, and should widen with age and hence distance down-glacier. Debris should be both thin and uniform over the moraine. Observed faster-than-linear growth of moraine widths with distance reflects the increasing ablation rate down-glacier. Increase in medial moraine cover reduces the local average ablation rate, allowing the glacier to extend further down-valley than meteorology alone would suggest. This feedback is especially effective when moraines merge.

The accumulation pattern across Siple Dome, West Antarctica, inferred from radar-detected internal layers

Abstract: 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.

Glacier calving : a numerical model of forces in the calving-speed/water-depth relation

Abstract: Empirical data suggest that the rate of calving of grounded glaciers terminating in water is directly proportional to the water depth. Important controls on calving may be the extent to which a calving face tends to become oversteepened by differential flow within the ice and the extent to which bending moments promote extrusion and bottom crevassing at the base of a calving face. Numerical modelling suggests that the tendency to become oversteepened increases roughly linearly with water depth. In addition, extending longitudinal deviatoric stresses at the base of a calving face increase with water depth. These processes provide a possible physical explanation for the observed calving-rate/water-depth relation.

Changes in the margin of Ice Stream C, Antarctica

Abstract: We present results from satellite imagery, ice-motion surveys and ice-penetrating radar studies of part of the north margin of Ice Stream C, one of the ice streams draining the West Antarctic ice sheet to the “Ross Embayment”. Our studies suggest that the shutdown of Ice Stream C about 150 years ago was not a single event, but a sequence involving stagnation of ice and migration of the ice-stream boundary. Ground-based studies confirm the inference from imagery that a series of former shear zones exist, decreasing in age towards the ice-stream center. A region of ice-stream trunk, including a former margin, lies sheared and folded between the (recent) inner and (older) outer margins of the area. ice-motion and topographic surveys give some constraint on the time of shutdown of the outer margin. The results provide a forum for discussing shutdown mechanisms. Possible causes for the stepwise migration of the north margin of Ice Stream C include a gradual decrease in ice flux, a reduction in the available water or hydrostatic pressure in the basal till, or a freezing of the till layers on the northern side.

A comparison of radio-echo sounding data and electrical conductivity of the GRIP ice core

Abstract: The depth of reflecting layers in Arctic ice sheets has been determined by electromagnetic echo sounding, using a varying distance between transmitter and receiver to determine the radar wave velocity. The depth of the radar reflecting layers is compared with a profile of electrical conductivity measurements (ECMs) from the Greenland Ice Core Project (GRIP) ice core, in order to determine the velocity of the radar waves in the ice cap. By using several reflecting layers, it is possible to isolate the firn correction of the wave velocity and to estimate the accuracy of the calculated electromagnetic wave velocity. The measured firn correction is compared with the correction calculated from the density profile, and a comparison between the depth profiles of ECM and radar based on the corrected electromagnetic wave velocity is presented. This profile shows that acid layers, which originate from major volcanic eruptions, show up as reflecting radar horizons.

Mass balance of the Lambert Glacier-Amery Ice Shelf system, East Antarctica : a comparison of computed balance fluxes and measured fluxes

Abstract: We combine European Remote-sensing Satellite (ERS-1) radar altimeter surface elevations (Fricker and others, 2000) with six different accumulation distributions to compute balance fluxes for the Lambert Glacier-Amery Ice Shelf drainage system. These interpolated balance fluxes are compared with fluxes derived from in situ measurements of ice thickness and velocity at 73 stations of the Lambert Glacier basin traverse and at 11 stations further downstream, to assess the system's state of balance. For the upstream line we obtain a range of imbalance estimates, from -23.8% to +19.9% of the observed flux, reflecting the sensitivity to the accumulation distributions. For some of the accumulation distributions the imbalance estimates vary significantly between different parts of the line. Imbalance estimates for the downstream line range from -17.7 % to +70.2%, with four of the estimates exceeding +30%, again reflecting the sensitivity of the result to input accumulation, and strongly suggesting that the mass balance of the region between the two lines is positive. Our results confirm the importance of accurate estimates of accumulation in ice-sheet mass-balance studies. Furthermore, they suggest that it is not possible to accurately determine the state of balance of large Antarctic drainage basins on the basis of currently available accumulation distributions.

Dynamic behaviour analysis of glacier de Saint Sorlin, France, from 40 years of observations, 1957-97

Abstract: Mass-balance and dynamic measurements carried out on glacier de Saint Sorlin since 1957 provide a good opportunity to study the dynamics of this glacier. Ice-flow analysis shows that dynamic changes have been important over the last 40 years and that these changes are not consistent with the concepts usually used in glacier modelling. Present velocities are larger than the 1960 velocities, although the thickness decreased everywhere (10-30 m in the ablation zone). A simple numerical ice-flow model which does not include longitudinal stress gradients has been used to investigate these phenomena. This model allows us to infer the sliding velocity from observed surface and calculated deformation velocities. We conclude that: (1) the sliding velocity cannot be described by Weertman analysis or empirical relations which link the sliding to the thickness and surface slope; (2) the inferred sliding velocity is uniform over at least half of the glacier; and (3) there is no clear link between the sliding process and the quantity of water coming from surface ablation. Furthermore, it may not be reasonable to calibrate model flow parameters from geometry changes because the surface geometry is relatively insensitive to velocity changes over some decades.

Rheology of ice at the bed of Engabreen, Norway

Abstract: A three-dimensional finite-element model is used to analyze field data collected as dirty basal ice flowed past an instrumented obstacle at the bed of Engabreen, a temperate glacier in northern Norway. The ice is modeled as an incompressible power-law fluid, with viscosity η = (B/2) Π (1-n)/2n D , where Π D is the second invariant of the stretching tensor, and B and n are two parameters. Using measurements obtained in 1996 and 1997, two values of B are obtained, one using the measured normal stress difference across the obstacle, and the other using the measured bed-parallel force over the instrument. These two values are not equal, probably owing to small frictional forces at the bed unaccounted for in the numerical model. Hence, B ranges between 1.9 × 10 7 and 3.2 × 10 7 Pa s 1/3 in 1996, and between 2.2 × 10 7 and 4.1 × 10 7 Pa s 1/3 in 1997. These values are smaller than measured elsewhere for clean glacier or laboratory ice. Field measurements of water content, fabric and texture of the basal ice suggest that unbound water between thin sediment layers and lamellae of clean ice may act as a lubricant and significantly weaken the ice. Near-isotropic fabrics indicate that preferred fabric orientation does not enhance the deformation.

An analysis of balance velocities over the Greenland ice sheet and comparison with synthetic aperture radar interferometry

Abstract: Balance velocities for the Greenland ice sheet have been calculated from a new digital elevation model (DEM), accumulation rates and an existing ice-thickness grid, using a fully two-dimensional finite-difference scheme. The pattern of velocities is compared with velocities derived from synthetic-aperture radar (SAR) interferometry for three different regions of the ice sheet. Differences between the two estimates of velocity highlight the respective strengths and weaknesses of the datasets and techniques used. A comparison with ten global positioning system-derived velocities indicates that the balance-velocity scheme and input datasets used here provide a remarkably good representation of the velocity distribution inland from the margins. These balance-velocity data, therefore, could help constrain numerical ice-sheet models. The balance velocities were found to be unreliable close to the ice-sheet margins due to larger errors in ice thickness, surface slope and ablation rate in this region. Comparison of the balance velocities with SAR interferometry in the region of the “Northeast Greenland Ice Stream” indicates the importance of the smoothing distance that must be applied to the DEM before calculating balance velocities. A smoothing distance of 20 times the ice thickness gave good agreement between the two measures of velocity.