Jakobshavn Isbræ is a major ice stream that drains the west-central Greenland ice sheet and becomes afloat in Jakobshavn Isfjord (69° N, 49° W), where it has maintained the world’s fastest-known sustained velocity and calving rate (7 kma−1) for at least four decades. The floating portion is approximately 12 km long and 6 km wide. Surface elevations and motion vectors were determined photogrammetrically for about 500 crevasses on the floating ice, and adjacent grounded ice, using aerial photographs obtained 2 weeks apart in July l985. Surface strain rates were computed from a mesh of 399 quadrilateral elements having velocity measurements at each corner. It is shown that heavy crevassing of floating ice invalidates the assumptions of linear strain theory that (i) surface strain in the floating ice is homogeneous in both space and time, (ii) the squares and products of strain components are nil, and (iii) first- and second-order rotation components are small compared to strain components. Therefore, strain rates and rotation rates were also computed using non-linear strain theory. The percentage difference between computed linear and non-linear second invariants of strain rate per element were greatest (mostly in the range 40–70%) where crevassing is greatest. Isopleths of strain rate parallel and transverse to flow and elevation isopleths relate crevassing to known and inferred pinning points.

Records of surface motion, englacial tilt and repeat inclinometry are used to determine patterns of surface, internal and basal motion across the tongue of Haut Glacier d’Arolla, Switzerland, over temporal scales ranging from days to months. Findings are interpreted with reference to contemporaneous measurements of subglacial water pressures, and prior knowledge of the glacier’s subglacial drainage-system structure. Long-term inclinometry results show pronounced extrusion flow over a subglacial drainage axis, with basal velocities up to twice those measured at the glacier surface. Deformation profiles are more conventional away from the drainage axis, with basal velocities ∼60–70% of surface velocities. Comparison of long-term tilt rates from repeat inclinometry and englacial tiltmeters shows close correspondence. Englacial tiltmeter data are used to reconstruct internal velocity profiles and to split surface velocities into internal deformation and basal motion contributions over spring, summer and autumn/winter periods. Although, spatial patterns of surface movement are similar between periods, patterns of internal and basal motion are not. Results are interpreted in terms of the location of sticky and slippery spots, with temporally changing patterns of basal drag reflecting changing distributions of water pressure.

Liligo Glacier is a small glacier located in a transverse valley, which flows on the south side of Baltoro Glacier, Karakoram, Pakistan. Terminus variations of Liligo Glacier since 1892 were reconstructed using various methods and sources (historical documents, cartography, photographs, satellite images and field surveys). The glacier is characterized by two phases of strong advance (beginning and end of the 20th century), separated by at least half a century of retreat. The advance rates, together with some ice-surface features such as the heavily crevassed surface and terminus morphology, are considered to be indicative of a surge-type glacier.

Calving speeds and calving mechanisms in fresh water contrast with those in tidewater. We obtained calving speeds for six lake-calving glaciers in New Zealand’s Southern Alps, and surveyed the depths and temperatures of their ice-contact lakes. The glaciers are temperate, grounded in shallow (≤20 m) water, and exhibit compressive flow at their termini. These data increase the global dataset of fresh-water calving statistics by 40%, bringing the total to 21 glaciers. For this dataset, calving rates (uc) correlate positively with water depths (hw) (r2 = 0.83), the relationship being expressed by: uc = 17.4 + 2.3 hw. This is an order of magnitude lower than values of uc at temperate tidewater glaciers. For a subset of 10 glaciers for which ice-proximal water temperature (tw) data are available, uc also correlates positively with tw, supporting a physical relation between calving and melting at and below the water-line. Fluctuations of New Zealand lake-calving glaciers in the period 1958–97 show that although the transition from non-calving to calving dramatically increases frontal retreat rates, the onset of calving does not isolate terminus change from climatic forcing. In terms of climatic sensitivity, lake-calving glaciers occupy an intermediate position between tidewater glaciers (least sensitive) and non-calving glaciers (most sensitive).

Numerous field and theoretical studies have pointed to an important role for subaqueous and waterline melting in the dynamics of calving glaciers. These processes remain unquantified because of the dangerous nature of the environment, a data gap which hampers both theoretical and numerical modelling studies. Here we report laboratory experiments designed to quantify waterline and subaqueous ice-melt rates in saline and fresh water. Experiments were conducted at temperatures of 1–10°C, a range typical of ice-contact environments, and at salinities of 0,17.5 and 35 ppt. Results indicate that melt rates are slightly faster under fresh-water conditions, and that different thermohaline combinations can produce contrasting ice-front geometries. Thermo-notches develop quickly at higher temperatures ( >4°C), and form most rapidly and to a greater extent in saline water. Contrasts in melt rates and ice-front geometry are controlled by temperature-driven density contrasts and circulation patterns. Rates of up to 0.8 md–1 suggest that mass loss by subaqueous melting is a significant process at calving termini.

Sequential optical images of high spatial resolution were used for the first time to derive surface ice velocities of Glaciar Upsala, a fast-moving fresh-water calving glacier in southern Patagonia. Cross-correlation methods applied to four Landsat ETM+ images acquired in 2000–01 yielded average velocities of around 1600 m a−1, similar to values measured in the field in November 1993. The derived velocities show almost no seasonal variation for the analyzed calving termini. During the period of satellite coverage, clear readvances were detected in the autumn–winter period, followed by recessions during summers. Between 24 April 1999 and 14 October 2001, the glacier front has been fluctuating seasonally within about 400 m, in contrast to the previous dramatic recession. During the last 2.5 years, Glaciar Upsala west terminus had a net advance of around 300 m. In addition, the available satellite images allowed us to determine recent calving speeds and confirm the improved calving-rate/water-depth relationship, recently proposed by incorporating new data from Patagonian glaciers.

The disruption of internal layering visible in radio-echo sounding (RES) data from East Antarctica has, to date, been attributed to ice flow around bedrock topography. However, observations of internal layer disruption in the Siple Coast ice streams of West Antarctica have led to the suggestion that increased strain at the margins of ice streams may also be responsible for the disruption of internal layers. Here we present a re-analysis of the extensive RES dataset collected between 1967 and 1979 over a large part of Wilkes Land, East Antarctica, and relate the location of continuous and disrupted internal layers to modelled balance velocities. We show that the mean balance velocity associated with all areas of disrupted layers is 2.5 times higher than that associated with areas of continuous layers. We also demonstrate that disrupted layers are associated not only with ice streams, but also with areas of enhanced ice flow, which penetrate inland from the grounding line up to several hundred kilometres into the interior of East Antarctica. Continuous layers always overlie disrupted layers, suggesting either a depth dependency in the process responsible for layer disruption, or subsequent deposition of continuous layers. In some cases, disrupted layers occur outside fast-flow features, and continuous layers occur within fast-flow features. Such regions are explained by short-term flow patterns, but might also be attributed to inaccuracies in the balance-velocity calculations.

The distribution of basal traction on a transect along Pine Island Glacier, West Antarctica, is estimated by inverting observed surface velocities using a control method and a simple numerical stream-flow model. This model calculates the horizontal flow along a transect, based on the assumptions that the horizontal flow is independent of ice depth and that the driving stresses are balanced by resistive forces at the glacier bed and margin and by gradients in longitudinal stress. Basal traction is assumed to be linearly related to the basal velocity. For the lateral shear traction a parameterization based on an inversion of Glen’s flow law is used. The application of the control method allows us to calculate the set of model parameters (e.g. the basal friction coefficient) that gives the best fit between modelled andobserved surface velocities. The model is used to investigate the stress regime of Pine Island Glacier, in particular to estimate the importance of basal, lateral and longitudinal stresses relative to each other. In the flat region just behind the grounding line, basal drag, lateral drag and the longitudinal stress gradient are the same order of magnitude. In the steep region up-glacier from the grounding line, the driving stresses are highest and balanced predominantly by basal resistive stresses. Further upstream, in the trunk of the glacier, lateral and basal drag predominate.

The surface velocity of Pine Island Glacier, West Antarctica, during the period 1992–2000 is measured with synthetic aperture radar feature-tracking techniques. Over the observation period, we find a monotonic acceleration with a spatially uniform amplitude of about 12% of the surface velocity. The acceleration extends > 80 km inland of the grounding line into a zone of prominent arcuate crevasses.The upper limit of these crevasses has migrated up-glacier by 0.2 km a−1 correlated with a velocity increase of similar size in the crevassed zone. On the other hand, there is no clear correlation between the velocity variations and observations of grounding-line migration. These findings suggest ongoing dynamic thinning of Pine Island Glacier, providing independent confirmation of recent interferometric results obtained by Rignot and others (2002).

The surface velocity of Thwaites Glacier (TG), West Antarctica, during the period 1992–2000 is measured with synthetic aperture radar feature-tracking techniques. We find no indication of interannual velocity variations of the grounded ice further than about 20 km inland of the grounding line. The velocity of the floating TG Tongue shows cyclicalvariations with an amplitude of 10%; a minimum around 1997 is bracketed by similarly sized maxima in 1995 and 2000. The observed velocity variations can be explained by time-dependent rotation and deformation superimposed on the steady flow of TG Tongue. The orientation of the rotation is clockwise during the entire observation period; the mean center of the rotation is close to a small ice rise, situated at the east side of the tongue about 20 km past the grounding line. The recent calving of TG Tongue in February 2002 is consistent with continued clockwise rotation that eventually led to cracking from west to east across the tongue. The rotation and deformation of TG Tongue is caused by forces unrelated to glacier dynamics. Analysis of European Centre for Medium-Range Weather Forecasts wind data suggests a synoptic-scale origin for the external forcing that causes the rotation.

The acoustic impedance of the subglacial material beneath 7.2 km profiles on four ice streams in Antarctica has been measured using a seismic technique. The ice streams span a wide range of dynamic conditions with flow rates of 35–464 m a–1. The acoustic impedance indicates that poorly lithified or dilated sedimentary material is ubiquitous beneath these ice streams. Meanacoustic impedance across each profile correlates well with basal shear stress and the slipperiness of the bed, indicating that acoustic impedance is a good diagnostic not only for the porosity of the subglacial material, but also for its dynamic state (deforming or non-deforming). Beneath two of the ice streams, lodged (non-deforming) and dilated (deforming) sediment coexist but their distribution is not obviously controlled by basal topography or ice thickness. Their distribution may be controlled by complex material properties or the deformation history. Beneath Rutford Ice Stream, lodged and dilated sediment coexist and are distributed in broad bands several kilometres wide, whileon Talutis Inlet there is considerable variability over much shorter distances; this may reflect differences in the mechanism of drainage beneath the ice streams. The material beneath the slow-moving Carlson Inlet is probably lodged but unlithified sediment; this is consistent with the hypothesis that Carlson Inlet was once a fast-flowing ice stream but is now in a stagnant phase, which could possibly be revivedby raised basal water content. The entire bed beneath fast-flowing Evans Ice Stream is dilated sediment.

We have constructed a numerical model that simulates the response of subglacial sediments to basal freeze-on. The model is set up to emulate the basal zone of drilling sites in the Ross Sea sector of the West Antarctic ice sheet. We treat basal freeze-on at an ice–sediment interface as a thermodynamic process that couples the flow of water, heat and solutes in unfrozen subglacial sediments underlying a freezing ice base. The coupling of these flows occurs through the Clapeyron equation, which specifies the dependence of the basal freezing/melting temperature on ice pressure, water pressure, solute concentration and surface tension effects. Thermally driven water flow is induced when an ice base becomes supercooled below the pressure-melting point because ice–water surface tension inhibits ice growth in small pore spaces of fine-grained subglacial sediments. Our model results show that basal freeze-on is capable of inducing considerable changes in the basal zone of both ice streams and interstream ridges. These changes are associated with specific signatures that compare with borehole observations and geophysical surveys. Water-pressure levels are reduced, and thick layers of debris-laden basal ice develop. These basal ice layers and underlying sediments contain a distinct isotopic signal. The predicted stable-isotope ratios reflect Rayleigh-type isotopic fractionation whose significance increases with increasing freezing rates. Supercooling of the ice base induces also measurable changes in the ice-temperature profile of the glacier. Till porosity represents another quantity whose evolution is influenced strongly by basal freeze-on. In particular, measurements of vertical porosity distribution beneath stopped ice streams could be used to back-calculatethe timing of the onset of basal freezing. Our model results show that the basal zone of ice streams and interstream ridges responds sensitively to changes in basal melting/freezing rates. This sensitivity may allow reconstruction of past conditions beneath ice streams and interstream ridges from measurements made on basal ice samples and subglacial sediment samples. Our model results also indicate that meltwater from fast-flowing ice streams may be driven towards the freezing ice base of interstream ridges.

Narrow lateral shear margins are the most distinctive visual feature of the West Antarctic ice streams. Large shear stresses within these layers support the majority of the gravitational driving stress within a fast-flowing ice stream. The present contribution looks upstream, to the tributaries that feed ice-stream onsets, and considers the effects of both horizontal and vertical shear on their flow. Numerical and direct simulations of vertical and horizontal shear are used. Vertical shear, simulated using an anisotropic flow law, is of particular interest. We conclude that by isolating overlying ice from large-amplitude variations in bed elevation,“vertical shear margins” play an important role in sustaining relatively rapid tributary flow.

Changes in the discharge of West Antarctic ice streams are of potential concern with respect to global sea level. The six relatively thin, fast-flowing Ross ice streams are of interest as low-slope end-members among Antarctic ice streams. Extensive research has demonstrated that these “rivers of ice” have a history of relatively high-frequency , asynchronous discharge variations with evolving lateral boundaries. Amidst this variability, a ∼1300 km grounding-line retreat has occurred since the Last Glacial Maximum. Numerical studies of Ice Stream D (Parizek and others, 2002) indicate that a proposed thermal-regulation mechanism (Clarke and Marshall, 1998; Hulbe and MacAyeal, 1999; Tulaczyk and others, 2000a, b), which could buffer the West Antarctic ice sheet against complete collapse, may be over-ridden by latent-heat transport within melt-water from beneath inland ice. Extending these studies to Ice Stream A, Whillans Ice Stream and Ice Stream C suggests that further grounding-line retreat contributing to sea-level rise is possible thermodynamically However, the efficiency of basal water distribution may be a constraint on the system. Because local thermal deficits promote basal freeze-on (especially on topographic highs), observed short-term variability is likely to persist.

We have used a recently derived map of the velocity of Whillans Ice Stream and Ice Streams A and C, West Antarctica, to help estimate basal melt. Ice temperature was modeled with a simple vertical advection–diffusion equation,“tuned” to match temperature profiles. We find that most of the melt occurs beneath the tributaries, where larger basal shear stresses and thicker ice favor greater melt (e.g. 10–20mm a−1). The occurrence of basal freezing is predicted beneath much of the ice plains of Ice Stream C andWhillans Ice Stream. Modeled melt rates for when Ice Stream C was active suggest there was enough meltwater generated in its tributaries to balance basal freezing on its ice plain. Net basal melt for Whillans Ice Stream is greater due to less steep basal temperature gradients. Modeled temperatures on Whillans Ice Stream, however, were constrained by a single temperature profile at UpB. Basal temperature gradients for Whillans branch 1 and Ice Stream A may have conditions more similar to those beneath Ice Streams C and D, in which case, there may not be sufficient melt to sustain motion. This would be consistent with the steady deceleration of Whillans Ice Stream over the last few decades.

We show that the ice plain in the mouth of Whillans Ice Stream (formerly Ice Stream B), Antarctica, moves by stick–slip motion. During a spring-tide period, rapid motions regularly occur near high tide and during falling tide. This correlation is weaker during a neap-tide period when the tidal magnitudes are less. Precise timing of these motion events suggests that they propagate through the region with a mean velocity of 88 m s−1.We hypothesize that this speed is associated with the propagation of shear waves through a wet subglacial till. Motion events are also seen on more smoothly flowing floating ice. Event delays are very short between grounded and floating stations, suggesting the events propagate through the ice shelf as an elastic wave. We further hypothesize the events are caused by the interaction of a sticky bed, the accumulation of stored elastic strain through the compression of ice by upstream inflow, and tidal forcing. Motion events seem to be triggered either by reduction of vertical normal stresses at high tide or by the increase of shear stresses from sub-shelf ocean currents during falling tide. Event magnitudes are not related to the length of the preceding quiescent period, suggesting significant viscous dissipation within the till.

Ice-stream tributaries connect the relatively slow-moving interior of the West Antarctic ice sheet (WAIS) with the fast-flowing Siple Coast ice streams. Basal water underneath these ice streams reduces basal resistance and enables the fast motion of the ice. Basal melting being the only source for this water, it is important to include the distribution of basal melting and freezing into numerical models assessing the stability of the WAIS. However, it is very difficult to constrain its distribution from existing field observations. Past borehole observations confirmed the presence of a wet bed at Byrd Station in the WAIS interior and at different locations within Siple Coast ice streams. However, the recent discovery of a 12–25m thick sediment-laden bubble-free basal ice layer at the UpC boreholes indicates that basal freezing is either currently occurring or had occurred upstream during the last glacial–interglacialcycle.We use a flowline model of ice thermodynamics to assess and quantify the spatial and temporal distribution of basal melting and freezing beneath Ice Stream C tributaries, taking into account the geothermal flux, shear heating and heat conduction away from the bed. Under the assumption that the ice was moving over a weak bed (τb =1–10 kPa) our model is able to reproduce a layer of frozen-on ice similar in thickness to the UpC “sticky spot” basal ice layer. Increased basal melting in the early Holocene possibly could have initiated the Holocene decay of the WAIS, whereas increased freezing rates over the past few thousand years could have decreased the amount of basal water in the system, resulting in a strengthening of the bed. This is consistent with current force-budget calculations for ice-stream tributaries and with observed stoppages and slow-downs of ice streams.

A powerful seismic technique that exploits the phase of the ice-bottom reflections shows that soft till is widespread beneath a West Antarctic ice stream very close to the onset of streaming flow. The amplitude vs offset (AVO) method measures the change in amplitude of the reflection as a function of increasing angle of incidence. For a decrease in acoustic impedance with depth, the reflection phase is negative at low angles of impedance but positive at intermediate angles. The change in phase by 180° is an obvious and robust measure of the relative acoustic impedance contrasts. This technique is only usable when there is a change in phase vs offset, conditions which obtain for “UpB-type” tills (high water pressures and porosity, low compressional- and shear-wave velocities, similar to those observed at Upstream B camp). I have applied this technique to the far upstream regions of Ice Stream C and find that a dilatant ( and presumably deforming), relatively thick (meters) till layer has formed beneath the ice stream within tens of km of the region identified as the transition from inland flow to ice-stream flow. These results suggest that the onset of rapid basal motion is linked to the formation of this deforming subglacial layer.

Radio-echo sounding (RES) techniques are used to examine spatial changes in bed reflectivity across relict ice streams inWest Antarctica. Measurements from adjacent interstream ridges are used to correct the measured power returned from the bed for attenuation and losses due to geometric spreading, scattering and absorption. RES measurements near boreholes drilled on Ice Stream C (ISC) indicate high coefficients of bed reflectivity (R > 0.1) in locations where the bed was thawed and boreholes connected to the basal water system, and low reflectivity coefficients (R < 0.02) at locations that were frozen and not connected. Intermediate values of bed reflectivity were measured at locations where the connection to the basal water system was weak. Measurements across four relict margins show that bed reflectivity usually jumps from low to high values several kilometers inside the outermost buried crevasses. We interpret this to be a transition from frozen to thawed basal conditions and discuss implications of these observations.