Deformation rates of single hexagonal crystals, deforming by glide on the basal plane, are described as a function of stress state and crystal orientation. These results are used to infer the deformation rate of crystal aggregates assuming that the stress distribution within the crystal aggregate is homogeneous. Analytical equations for the deformation rate of anisotropic ice aggregates are derived for vertically symmetric girdle fabric. This type of fabric is approximated by a uniform distribution of c-axis orientations between a cone angle and a smaller girdle angle relative to the symmetry axis. For simple shear stress acting on a single-maximum fabric there is a slight de-enhancement for cone angles of 60–90°. In uniaxial compression a maximum enhancement of ∼1.7 occurs at a cone angle of 57°. A pure shear stress state has similar features, with the additional complication that it causes a non-zero transverse strain rate, except for perfect vertical alignment of crystals and isotropic fabric. In combined states of stress the contribution of each stress component to the strain rate depends on fabric. A single enhancement factor is not adequate to describe the effects of anisotropy for complex stress states.

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.

A simple flux model using twice-daily measurements of wind, humidity and temperature from standard upper-air levels in a distant radiosonde estimated winter balance of South Cascade Glacier, Washington, U.S.A., over 1959–98 with error 0.24 m w.e. Correlation between net and winter balance is strong; the model estimates net balance with error 0.53 m w.e. Over the past 40 years, average net balance of South Cascade Glacier has been strongly negative (−0.46 m w.e.), and it has been shrinking steadily. In comparison, 200 km west-southwest at Blue Glacier, the average balance has been less negative (−0.13 m w.e); that glacier has undergone little change over the 40 years. Balance histories of the two glaciers are positively correlated (r = +0.54), and South Cascade has been more out of balance than Blue, presumably because it is still adjusting to climate change since the Little Ice Age. Recent warming and drying has made the net balance of both glaciers strongly negative since 1976 (−0.84 m w.e. at South Cascade, −0.56 m w.e. at Blue). If South Cascade Glacier were in balance with the 1986–98 climate, it would be about one-quarter of its present area.

We present newly acquired airborne radar data showing ice thickness and surface elevation for Pine Island Glacier, Antarctica. These data, when combined with earlier measurements, suggest the presence of a lightly grounded area immediately above the grounding line of Pine Island Glacier. We identify this region as an “ice plain”. It lies close to the centre line of the glacier, has an elevation above buoyancy of <50 m and extends inland for >28 km. The upstream edge of the ice plain is defined by a “coupling line”. The configuration of the ice plain implies that nearby thinning of the ice stream would result in substantial grounding-line retreat. We suggest that the grounding-line retreat of Pine Island Glacier, observed between 1992 and 1996, probably commenced sometime after 1981.

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−1 to 2 cm d−1 over 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−1 to sinking of 48 cm d−1, and for Skaftá cauldrons from 2 cm d to 25 cm d−1.

Methanesulfonate was investigated as a potential contributor to the sulfur budget, based on analysis of a firn core from Collins Ice Cap, King George Island, Antarctica (62°10′ S, 58°50′ W). The anion was found to be present at a mean concentration of 0.17 μeq L−1, with a maximum of 0.73 μeq L−1. Dating based on the δ18O profile suggests that the principal peaks of methanesulfonate are associated with snow deposited in summer and autumn. A careful examination of MSA, SO42− and nssSO42− profiles indicates that two of the three peaks in the MSA profile may result mainly from migration and relocation of MSA. The mechanism responsible for this might be similar to that for deep cores from other Antarctic glaciers, supporting the migration hypothesis proposed by prior researchers and extending it to near-temperate ice. Due to the post-depositional modification, the main part of the MSA profile of the firn is no longer indicative of the seasonal pattern of MSA in the atmosphere, and the basis for calculation of the MSA/nssSO42− ratio should be changed. The MSA/nssS042 ratio obtained by a new computation is 0.22, 10% higher than that ignoring the effect of MSA migration.

The numerous debris bands in the terminus region of Matanuska Glacier, Alaska, U.S.A., were formed by injection of turbid meltwaters into basal crevasses. The debris bands are millimeter(s)-thick layers of silt-rich ice cross-cutting older, debris-poor englacial ice. The sediment grain-size distribution of the debris bands closely resembles the suspended load of basal waters, and of basal and proglacial ice grown from basal waters, but does not resemble supraglacial debris, till or the bedload of subglacial streams. Most debris bands contain anthropogenic tritium (3H) in concentrations similar to those of basal meltwater and ice formed from that meltwater, but cross-cut englacial ice lacking tritium. Stable-isotopic ratios (δ18O and δD) of debris-band ice are consistent with freezing from basal waters, but are distinct from those in englacial ice. Ice petrofabric data along one debris band lack evidence of active shearing. High basal water pressures and locally extensional ice flow associated with overdeepened subglacial basins favor basal crevasse formation.

Debris-laden basal ice is exposed along an ice cliff near Hamna Glacier, Sôya Coast, East Antarctica. The basal ice is about 6.8 m thick and shows conspicuous stratigraphic features. The upper 5.5 m consists of alternating layers of bubble-free and bubbly ice. δ values of the bubble-free ice layers are enriched by 2.4 ±1.0‰ (standard deviation) for δ18O compared to values of neighboring bubbly ice layers above, and slopes of δ18O vs δD are close to 8. Such layers are suggested to have been formed by refreezing of meltwater in an open system. In contrast, part of the bubbly ice layers shows neutral profiles for stable isotopes, suggesting that these ice masses are undisturbed ice-sheet ice which was not affected by melting and freezing. The massive alternating layers are thus considered to have been formed by folding of refrozen and non-melted layers. The lower 1.3 m consists predominantly of bubble-free massive ice. The profile of co-isotopic values shows a change of about 3.0‰ for δ18O at the interface between bubble-free and bubbly ice. Since the isotopic change occurred over a wider thickness than the upper 5.5 m, the basal ice is suggested to have been formed by refreezing of meltwater on a larger scale than the upper 5.5 m.

Spatial patterns in residual bed reflection power (BRPr), derived from ground-based radio-echo sounding, were mapped and interpreted in terms of the thermal and hydrological conditions at the base of a high-Arctic polythermal glacier (John Evans Glacier, Ellesmere Island, Canada). BRPr is the residual from a statistical relationship between measured bed reflection power and ice thickness that describes the rate of dielectric loss with depth in the glacier. We identified three types of thermal structure: (a) Positive BRPr and an internal reflecting horizon occur over the glacier terminus. The reflecting horizon is interpreted as the boundary between warm and cold ice, and suggests the presence of a warm basal layer. (b) Positive BRPr occurs without an internal reflector in the upper part of the ablation zone. This suggests that ice is at the pressure-melting point only at the bed. (c) Negative BRPr without an internal reflector occurs in all other regions, suggesting cold ice at the bed. Where BRPr is positive, its pattern is similar to the pattern of subglacial water flow predicted from the form of the subglacial hydraulic equipotential surface. This suggests that hydrological conditions at the glacier bed are a major control on BRPr, probably because the dielectric contrast between ice and water is higher than that between ice and other subglacial materials.

The dynamics of grounded tidewater glaciers is investigated with a time-dependent numerical flow model, which solves the full equations for the stress and velocity fields and includes a water-pressure-dependent sliding law. The calving criterion implemented in the model shifts the calving front at each time-step to the position where the frontal ice thickness exceeds flotation height by a prescribed value. With this model, the linear relation between calving rate and water depth proposed on empirical grounds is qualitatively reproduced for the situation of a slowly retreating or advancing terminus, but not for situations of rapid changes. Length changes of tidewater glaciers, i.e. especially rapid changes, are dominantly controlled by the bed topography and are to a minor degree a direct reaction to a mass-balance change. Thus, accurate information on the near-terminus bed topography is required for reliable prediction of the terminus changes due to climate changes. The results also confirm the suggested cycles of slow advance and rapid retreat through a basal depression. Rapid changes in terminus positions preferably occur in places where the bed slopes upwards in the ice-flow direction.

This paper presents detailed observations of the regularly rippled surface on an Antarctic blue-ice area near Svea, at five sites. The wavelength of the ripples was found to be 20–24 cm, while the wave height (crest–trough) was 1–2 cm. The ripple crests are generally oriented perpendicular to the direction of the strongest winds. Repeated measurements show that wave heights increase throughout the summer, with most ablation occurring in the wave troughs. This implies that traditional methods of measuring ablation (such as stakes when a rod on the ice surface is used to define a mean surface height) tend to underestimate total ablation because they sample only crests. One site exhibited significant migration of the surface ripples of about 2 cm month−1 in the downwind direction, whereas three other sites showed no significant wave movement. The formation and the specific characteristics of the surface ripples are most likely caused by self-amplifying interaction mechanisms between the free ice surface and the overlying turbulent atmosphere, which necessarily involve spatial variations in sublimation. A simple model was used to quantify the interaction between the surface ripples, the airflow aloft and the sublimation rate. The model is able to predict wavelengths and migration rates that are in reasonable agreement with the observations.

Interferometric synthetic aperture radar (InSAR) observations of ice-shelf tidal deformation reveal the wide transition between grounded and floating ice as well as local areas where the ice shelf is only grounded at low tide, a condition that we call ephemeral grounding. Ephemeral grounding creates a subtle, local disturbance on the vertical motion field of the ice-shelf surface in response to changes in oceanic tide which is detected with millimetric precision using InSAR. These ice-shelf features are, however, not expected to produce a noticeable disturbance on the ice-shelf velocity field. To illustrate the influence of ephemeral grounding on ice-shelf creep flow, we use a finite-element model in which ephemeral grounding is incorporated through a variable basal friction coefficient. The results show that while ice rises (permanently grounded areas) have a pronounced influence on the ice-shelf velocity field, areas of ephemeral grounding have a vanishingly small influence. What is thus of most interest is the capacity for observations of ephemeral grounding to reveal subtle changes in ice-shelf thickness over time. We discuss an example in the Thwaites Glacier area, West Antarctica, where multi-year data show how ice rises become ephemeral grounding and subsequently disappear. This result is consistent with the grounding-line retreat and ice thinning of Thwaites Glacier.

Microfabrics formed by flat particles and voids in till samples from the upper Ürümqi river valley, Tien Shan, northwestern China, were analyzed using three mutually perpendicular thin sections. Chi-squared tests were used to determine significance levels of the orientations of both particles and voids. The basal tills in an end moraine, on the stoss sides of two roche moutonnées, on the stoss sides of a drumlin with a rock core, and on the lee side of the same drumlin far from the rock core have strong particle and void microfabrics. Field evidence suggests that these tills were deformed in a subglacial setting. Thus, consistent with recent laboratory studies of till fabric development, deformation is believed to be responsible for the strong particle and void microfabrics. In the same end moraine, particle microfabrics in an overlying till were strong, but void microfabrics were weak. This till is believed to have been formed by dumping and rolling of debris from hill slopes and of ablation moraine from the glacier surface. The weak void microfabric is interpreted to indicate that the till was not consolidated and hence not sheared under the weight of the glacier, despite the strong particle microfabric. Basal tills on the lee sides of the roche moutonnees, and on the lee side of the drumlin but near its core, have weak microfabrics.

Surface strain rates around the southeastern dome of Hans Tausen Iskappe in Peary Land, North Greenland (82.5° N, 27.5° W), are determined from global positioning system surveys of a strain net. Average longitudinal surface strain rate increases towards the dome, from (1.4 ± 0.2) × 10−4 a−1 at 5–10 ice thicknesses from the divide to (2.4 ± 1.0) × 10−4 a−1 within 1 ice thickness from the divide. Analysis of the data shows that the ice cap is presently building up within the strain net with an average rate of 〈∂H/∂t〉 = + 0.04 ± 0.02 m a−1. Assuming a uniform thickening, the shape factor of the horizontal velocity (the ratio between the vertically averaged horizontal velocity and the horizontal surface velocity) decreases towards the dome, from 0.9 at a distance of 10 ice thicknesses from the dome to 0.5 at the dome based on application of the continuity equation. Our results indicate that a region with anomalous flow is formed around the dome, supporting recent indications reported by Vaughan and others (1999). It is not possible from our data to constrain parameters of the flow law, because there is no independent estimate of the significant present thickening of the central part of the ice cap and its pattern around the dome.

A new model, the variable width/depth ratio (VWDR) model, is used to analyze longitudinal variations in cross-section morphology along glacial valleys. In the VWDR, the cross-sectional shape of a valley is expressed as a function of the width/depth ratio at various heights above the valley floor. Two parameters, m, a measure of the breadth of the valley floor, and n, a measure of the steepness of the valley sides, appear in the model. We have used the VWDR model to study morphological variations of cross-sections along glacial valleys in the middle Tien Shan mountains, China, and find that: (1) in valleys without tributaries, m increases (the valley floor becomes wider) and n becomes more negative (valley sides become steeper) from the head of the valley to the equilibrium line, and then m decreases and n becomes less negative to the end of the valley; (2) in valleys with tributaries, a similar pattern is observed, with an oscillating maximum in m and minimum in n in those sections where a tributary enters the main valley. These characteristics are believed to reflect a maximization of glacial erosion potential in the vicinity of the equilibrium line and in locations of confluence.

In order to interpret measurements of ice-sheet surface elevation changes in terms of climatic or dynamic trends, it is necessary to establish the range of stochastic variability of elevation changes resulting from interannual fluctuations of accumulation rate and firn density. The analyses presented here are intended to facilitate such interpretations by defining benchmarks that characterize elevation-change variability in central Greenland, in the current climate and over the past millennium. We use a time- dependent firn-densification model coupled to an ice- and heat-flow model, forced by annual accumulation rate and temperature reconstructions from the Greenland Ice Sheet Project II (GISP2) ice core, to examine the elevation changes resulting from this climatic forcing. From these results, effective firn densities are calculated. These are factors that convert water-equivalent accumulation-rate variability to surface elevation variability. A current-climate benchmark is defined by applying this conversion to Van der Veen and Bolzan’s water-equivalent statistics, and to a 50 year accumulation variability estimate from the GISP2 core. Elevation-change statistics are compiled for the past millennium to define longer-term benchmarks, which show that multi-century variability has been substantially larger than current variability estimated by Van der Veen and Bolzan. It is estimated here that the standard deviation of net elevation change over 5 and 10 year intervals has been 0.27 and 0.38 m, respectively. An approximate method for applying these quantitative results to other dry-snow sites in Greenland is suggested.

The thermal regime of high-altitude accumulation areas in the Swiss Alps was systematically investigated on the Jungfraufirn, Bernese Alps, on the Breithornplateau, Valais Alps, and on Grenzgletscher, Valais Alps. In 1991, 1992 and 1994, temperatures were measured in a deep hole (120 m deep) and in several shallow holes (14–30 m deep). Whereas the wide névé of the Jungfraufirn at 3400–3600 m a.s.l. and the 3800 m high Breithornplateau seems to be predominantly temperate, cold firn and ice temperatures were measured throughout on Grenzgletscher (3900–4450 m a.s.l.). Mean firn temperatures on Grenzgletscher vary strongly and range between −3° and −14°C. A comparison between the measured temperature profiles and a one-dimensional heat-conduction calculation shows that the release of latent heat by penetrating and refreezing meltwater decisively influences the thermal pattern of the firn pack. A multiple linear regression model, based on measured firn temperatures from the European Alps and the parameters altitude and aspect, yields aspect-dependent lower boundaries for the occurrence of cold firn ranging between 3400 (northerly aspects) and 4150 m a.s.l. (southerly aspects). A total of 120 glaciers with cold-firn areas are found when applying the model to glacier inventory data from the European Alps.

Ice-core and snow data from the Amundsen, Bellingshausen and Weddell Seas, Antarctica, show that the formation of superimposed ice and the development of seawater-filled gap layers with high algal standing stocks is typical of the perennial sea ice in summer. The coarse-grained and dense snow had salinities mostly below 0.1‰. A layer of fresh superimposed ice had a mean thickness of 0.04–0.12 m. Gap layers 0.04–0.08 m thick extended downwards from 0.02 to 0.14 m below the water level. These gaps were populated by diatom standing stocks up to 439 μg L−1 chlorophyll a. We propose a comprehensive heuristic model of summer processes, where warming and the reversal of temperature gradients cause major transformations in snow and ice properties. The warming also causes the reopening of incompletely frozen slush layers caused by flood-freeze cycles during winter. Alternatively, superimposed ice forms at the cold interface between snow and slush in the case of flooding with negative freeboard. Combined, these explain the initial formation of gap layers by abiotic means alone. The upward growth of superimposed ice above the water level competes with a steady submergence of floes due to bottom and internal melting and accumulation of snow.

We examine the ability of synthetic aperture radar (SAR) to detect the equilibrium line on the glacier Austre Okstindbreen, Norway, using multipolarization SAR images in C- and L-band acquired with the Electromagnetic Institute of the Technical University of Denmark’s airborne EMISAR sensor during the European Multisensor Airborne Campaign EMAC ’95. The late-summer snowline, approximating the equilibrium line, cannot be seen on the SAR images. Instead, photographs from Austre Okstind-breen show that a distinct boundary visible on the C-band SAR images corresponds to the firn line created by old snow from many previous years. This is better seen on the cross-polarized SAR images (HV and VH), which in general reveal more detail than the HH- and VV-polarized images. We model the stratigraphy from net balance and glacier velocity data to calculate the firn-line altitude (FLA). Modelled FLA and the observed boundary are separated by 50 m in elevation, but considering errors during co-registration and modelling we conclude that the observed boundary on Austre Okstindbreen is the firn line. Monitoring FLA rather than equilibrium-line altitude (ELA) for mass-balance studies with remote sensing is therefore suggested.