Repeat aerial photography is used to obtain closely spaced measurements of velocity and elevation over a complete transect of Ice Stream tributary B2, including the shear margins, the fast ice of the ice stream and several unusual features, as well as the UpB camp. Persistent features, mainly crevasses, are tracked to provide 1541 values of velocity and 1933 values of elevation. These are used to describe ice flow in the ice stream. Within the ice stream, the dominant velocity gradient is lateral shear. Crevasse patterns are studied in relation to measured velocity gradients. Crevasses intersect one another at acute angles, indicating that their origin is deeper than the depth to which crevasses penetrate. One feature within the ice stream seems to be a raft of stiff ice. Others are crevasse trains. Also, there are spreading ridges, perhaps due to upwelling ice. There is no evidence of large sticky spots within the studied transect, i.e. no steep surface slopes with associated surface stretching just up-glacier and surface compression down-glacier.

In this paper we develop a theoretical model describing water motion in a coupled borehole-subglacial flow system. The theory applies to basal drainage systems having multiple and extensive interconnected flow paths. Within this domain it encompasses a broad range of flow regimes, from laminar Darcian flow in a thick permeable unit to turbulent sheet flow in a very thin layer. Important terms in the model are highlighted by recasting the problem in dimensionless form. The non-dimensional formulation indicates that there are four free parameters in the coupled system. These parameters characterize skin friction in the borehole, and diffusion, transmissivity and turbulent transport in the subglacial flow layer. Dimensionless results show that, under most circumstances, the effects of skin friction in the borehole are negligible. Diffusion, transmissivity and especially turbulent transport in the basal layer are found to influence subglacial water flow strongly. We use our model to predict fluctuations of borehole-water levels that result from different types of disturbances. We show how this framework can be used to estimate subglacial hydraulic properties by comparing model results with data collected during field experiments on Trapridge Glacier, Yukon Territory, Canada in 1989 and 1990.

Image-analysis procedures have been developed for a simple PC-based system to evaluate the textural features of ice thin sections automatically. The method yields parameters that describe both ice crystals and pores in an objective, reproducible manner. Specimens are recorded in linearly and circularly polarized light (ice grains) as well as in transmitted and incident plain light (pores) under standardized conditions. After preparative filtering, images are segmented through union of two Sobel-filtered images (ice crystals/grain boundaries) or through thresholding (ice crystals/pores). The quantification of texture is based on the evaluation of image contrast as well as on linear analysis and non-linear transforms to obtain information on grain-sizes and their distribution. In the study of ice pores, stereological effects of finite section thickness have to be corrected. The discussion focuses on errors involved in automated textural analysis and the glaciological yield of measured and derived parameters.

Tracers were injected into South Cascade Glacier to determine the flow condition and geometry of the subglacial water system. Results indicate that two distinct drainage basins on the glacier feed the two main streams flowing from the glacier. In the largest basin, two parallel drainage networks exist, one englacial and the other subglacial. The englacial system is an arboresecent network of conduits, whereas the subglacial system is a distributed flow system. Both systems connect to a single subglacial conduit which appears as a stream at the glacier’s terminus. The comparison between the travel time of the tracers and stream discharge indicated that the single conduit was pressurized in July and partly filled in August and September.

To estimate the flow geometry (e.g. path length, flow depth and velocity), the advection-diffusion equation was formulated to express the water velocity as a function of water depth. Longitudinal dispersion of the tracer was calculated from the shear in longitudinal water velocity. Results indicate that the flow is very wide compared to its depth and that the path is sinuous. The estimated flow speed in the conduits is an order of magnitude larger than the measured speed through the glacier, indicating that other flow processes, probably englacial, route the water much more slowly.

The other, smaller, basin drains the water from the surface to the subglacial distributed flow system. Based on the travel time of the individual concentration peaks, the water could be flowing through a linked-cavity system or interconnected bands of highly permeable debris separated by zones of less permeability.

Solute acquisition by Alpine glacial meltwaters is the result of the coupling of different pairs of reactions, one of which usually involves dissolved gases. Hence, the availability of atmospheric gases to solution is an important control on the composition of glacial meltwaters. The chemical compositions of the two main components of the bulk meltwater, quick flow and delayed flow, are dominated by different geochemical processes. Delayed flow waters are solute-rich and exhibit high p(CO2) characteristics. The slow transit of these waters through a distributed drainage system and the predominance of relatively rapid reactions, such as sulphide oxidation and carbonate dissolution, in this environment maximize solute acquisition. Quick-flow waters are dilute, both because of their rapid transit through ice-walled conduits and open channels, and because the weathering reactions are fuelled by relatively slow gaseous diffusion of (CO2) into solution, despite solute acquisition being dominated by rapid surface exchange reactions. As a consequence, quick flow usually bears a low or open-system p(CO2) signature. Bulk meltwaters are more likely to exhibit low p(CO2) values when suspended-sediment concentrations are high, which promotes post-mixing reactions. This conceptual model suggests that the composition of both quick flow and delayed flow is likely to be temporally variable, since kinetic, rather than equilibrium, factors determine the composition.

Basal ice stratigraphy in coastal ice cliffs at the Law Dome margin has revealed the basal accretion of clean and debris-bearing ice, marine congelation ice and granular marine ice inland of the margin. Co-isotopic analysis of δ18O and δD isotopes together with solute chemistry were applied to determine the modes of accretion and debris entrainment. The marine congelation ice and the granular marine ice were formed from the basal freezing of desalinated sea water and the episodic mixture of basal meltwater and sea water, respectively. Two different debris-entrainment mechanisms were identified. Debris-band ice with debris concentrations of 6.3–33% (by volume) was formed from proglacial raised beach and shallow marine sediment incorporated by an over-riding advance of the margin. Two other debris-bearing ice types, dispersed debris-poor ice with debris concentrations <0.3% (by volume) and laminated debris ice with debris concentrations 0.9–1.9% (by volume) were accreted further inland from the margin by basal regelation processes associated with the Robin (1976) heat-pump effect.

For our work in the Filchner-Ronne Ice Shelf Programme (FRISP), we have developed a new technique for measuring the bottom-melting rate with high reliability. The method is based on time-domain reflectometry (TDR) measurements of transmission lines inserted into melt holes. The TDR-data are digitally recorded on magnetic tape. System resolution has been estimated at 0.2 m. Hence, re-measuring after 1 year gives an accuracy of 10% for melting rates of 2m a-1. Two transmission lines for TDR measurements were installed during the German FRIS Expedition 1989–90. This paper describes the design of the system. Examples of recorded wave forms are given.

The pattern of elevation change along the length of glaciers caused by retreat from Neoglacial maximum to present is investigated and described in terms of a profile-shape factor f defined as the ratio of the average thickness change to the local thickness change at the present terminus. The factor f is relevant to estimation of the change in volume of a glacier in relation to its change in length and the response time to a change in climate. Thickness-change profiles on a selection of 15 mountain valley glaciers give f in the range 0.1–0.4 with an average of 0.28. The elevation changes are in all cases largest near the terminus and decrease headward more rapidly than a linear variation with distance that would correspond to f of 0.50, a value assumed to be characteristic in the past. The smaller values of f indicate reduced estimates of changes in volume versus change in lengths and smaller response times for glaciers. Total volume change for glaciers and ice caps (excluding ice sheets in Greenland and Antarctica) as estimated roughly by these means, is consistent with previous estimates from mass-balance measurements and supports the view that melting of these ice masses has made a significant contribution to sea-level change over the last century. The observed values of f do not explain an apparent discrepancy between alternative theoretical methods for estimating the response time of glaciers based on length and terminus velocity or based on the thickness and terminus-ablation rate. The discrepancy may be caused in part by the relationship between terminus velocity and ablation rate associated with strongly non-steady-state conditions during advance or retreat. The patterns of elevation change measured for short time intervals of a few decades or less correspond to a wide spread of profile-shape factor f from less than 0.1 to more than 1.2 indicative of transients in the short time-scale response.

An exact, analytic description of the steady, downhill flow of a viscous ice stream across a no-slip to free-slip transition in basal boundary condition suggests ways in which such transitions might be detected by observations of ice-stream surface properties. We find that the best expression of this transition is in the free-surface profile, which dips over the point of transition and becomes horizontal far downstream. The streamwise velocity at the surface shows a gradual change across the transition, and this is in disagreement with previous studies which suggest a marked change. Basal shear stress and basal pressure exhibit singularities at the point of transition. As concluded in previous study, the former singularity is a likely point of strong basal erosion. The latter singularity is problematical, however, because it violates the assumptions which make the exact solution possible. It is not clear how this problem can be overcome without appealing to thermodynamics, non-Newtonian material properties, cavitation or wake separation.

This paper is concerned with the motion of an unconfined finite mass of granular material down an inclined plane when released from a rest position in the shape of a circular or elliptical paraboloid. The granular mass is treated as a frictional Coulomb-like continuum with a constant angle of internal friction. The basal friction force is assumed to be composed of a Coulomb-type component with a bed-friction angle that is position-dependent and a viscous Voellmy-type resistive stress that is proportional to the velocity squared. The model equations are those of Hutter and others (in press b) and form a spatially two-dimensional set for the evolution of the avalanche height and the depth averaged in-plane velocity components; they hold for a motion of a granular mass along a plane surface.

Similarity solutions, i.e. solutions which preserve the shape and the structure of the velocity field, are constructed by decomposing the motion into that of the centre of mass and the deformation relative to it. This decomposition is possible provided the effect of the Voellmy drag on the deformation is ignored. With it, the depth and velocities relative to those of the centre of mass of the moving pile can be determined analytically. It is shown that the pile has a parabolic cap shape and contour lines are elliptical. The semi-axes and the position and velocity of the centre of mass are calculated numerically. We explicitly show that

• (i) For two-dimensional spreading, a rigid-body motion does not exist, no matter what be the values of the bed-friction angle and the coefficient of viscous drag.

• (ii) A steady final velocity of the centre of the mass cannot be assumed, but the motion of the centre of mass depends strongly on the value of the Voellmy coefficient.

• (iii) The geometry of the moving pile depends on the variation of the bed-friction angle with position, as well as on the value of the coefficient of viscous drag.

In order to estimate the accuracy of altimetric height measurements over ice sheets, an altimeter wave-form simulator has been developed, and different tracking methods have been tested. A large range of surface features, including large-scale and medium-scale features and micro-roughness have been taken into account for modeling of either surface-or volume-scattering. A large set of parameters affects the trailing edge of the radar wave form, so that re-tracking algorithms based on the detection of its leading edge provide better retrievals of the surface height than those based on the analysis of the whole wave form. A volume component is clearly present in the radar wave forms; its effect on the leading edge depends mostly on the snow grain-size (and therefore on the snow temperature) and on the pointing angle. However, on average, the induced error on the snow-surface height estimation should only be around 25 cm.

Ice observations and sediment samples were collected in the Beaufort Gyre in 1988. Fine sediment occurred in very small patches of turbid ice, as thin spotty surface layers, in mud pellets or in old snowdrifts. The latter were widespread south of 74°N, containing an estimated 22 tonnes of silt and clay km−2. Average particle concentration in sea ice (40mg l−1) was much higher than in sea water (0.8 mg l−1) or in new snow, but the sediment load was significantly smaller and of finer texture compared to that observed in a shelfal source area after a major entrainment event. About 30% of the sediment consisted of small pellets. Mud in pellets has similar texture, clay minerals and organic/inorganic carbon content as dispersed mud. Particle sizes <16μm dominate, sand is less than 1%, compared to as much as 8% in four samples obtained in 1971 and 1972. Organic carbon content is about 2%, illite dominates clay minerals (~50%), and diatoms suggest a shelf sediment source. From the prevalence of wind-reworked surficial deposits, the spotty occurrence of only small patches of turbid ice in old clean ice, and the virtual lack of sand-size material, we assume the sediment had drifted at least 2 years since entrainment and was distant from its source. Assuming one-third of the load is released each year, the estimated deposition rate would equal the measured Holocene rate (~2cm 1000year−1). Therefore, modern sea-ice rafting represents a substantial fraction of the total Arctic Ocean sediment budget.

The importance of the atmospheric boundary layer for the coupling between the climate and an ice sheet is investigated using a slab model of the atmospheric boundary layer. The model is shown to give reasonable agreement with observations over Antarctica and it is used to look at the effect of different ice-sheet shapes on the boundary layer. The importance of entrainment in bringing heat to the surface is highlighted and is shown to be particularly significant when the ice profile becomes steeper. The model could be used as part of an energy-balance model of snow in order to incorporate the interplay of the boundary layer and ice-sheet shape in the ablation process. The slab model could also be used in a GCM as a parameterization of these sub-grid scale processes which are at present ignored in models on a global scale.

The results of measurements of radio-wave velocities (RWV) by wide-angle reflection (WAR) methods in the temperate Abramov Glacier in the Alai Mountain Ridge and the “two-layered” sub-polar Fridtjovbreen and Hansbreen on Svalbard using a low-frequency (2–13 MHz) radar are considered and discussed. The experimental data obtained and the data from the literature show that the values of RWV could be a good indicator of the hydrothermal state of glaciers. As such, these data enable the identification of cold, temperate and transitional (two-layered) glaciers, and can be used for estimation of the water content in glaciers and changes in the hydrothermal state.

The Seasat A satellite scatterometer radar, initially designed to measure ocean-wind intensity and direction, also provided observations on the Antarctic ice sheet. The signal of the back-scatter coefficient decreases strongly from 10 to −20 dB when the incidence angle of the observations increases from 0° to 65°. An additional 5 dB signal is found, which is correlated with the direction and intensity of katabatic winds, independent of the incidence angle and polarization of the signal. By using simplified models of the volume-scattering within the snowpack (which is mostly sensitive to snow grain-size) and surface-scattering from the air-snow interface (which depends on roughness), it is evident that the signal of the scatterometer could result from the effects of snow dunes at low incidence angle, and of micro-roughness and volume back-scatter at incidence angles greater than 25°. The instrument therefore provides a means of measuring the direction and intensity of katabatic winds.

A new extraction system has been constructed and tested which allows the extraction of gases from air bubbles in ice without melting it. An ice sample of up to 20 g is crushed in a sealed container by a milling cutter and the gas escaping from the opened bubbles is flushed with helium to a Porapak column where it is stored until its injection into the gas Chromatograph. To avoid any contamination with CH4 produced by friction in the gear section, a helium-flushed rotary feed-through is used. CH4 analyses on ice samples of about 10 g from the last 1000 years give precise and reproducible results. In the future, it is planned to measure also the CO2 and N2O concentrations on the same sample.

We present the results of tests done to determine the tensile behavior of first-year columnar sea ice over a range of temperatures from −20° to −3°C and strain rates of 10−5 and 10−3s−1. The temperature of a test specimen was dictated by its in-situ location within the sea-ice sheet; samples located near the top of the sea-ice sheet were tested at the lower temperatures. A tensile load was applied along the cylindrical axes of the test specimens, which were perpendicular to the growth direction of the ice. Results showed that the maximum stress reached during a test was most strongly influenced by temperature, while the failure strain and the modulus were principally affected by the loading rate. A model relating the tensile strength of the ice to its porosity based on temperature-dependent variations in the brine-pocket geometry is evaluated.

Grand Pacific and Melbern Glaciers, two of the largest valley glaciers in British Columbia, have decreased over 50% in volume in the last few hundred years (total ice loss = 250–300 km3). Melbern Glacier has thinned 300–600 m and retreated 15 km during this period; about 7 km of this retreat occurred between the mid-1970s and 1987, accompanied by the formation of one of the largest, presently existing, ice-dammed lakes on Earth. Grand Pacific Glacier, which terminates in Tarr Inlet at the British Columbia-Alaska boundary, retreated 24 km between 1879 and 1912. This rapid deglaciation has destabilized adjacent mountain slopes and produced spectacular ice-marginal land forms. The sediments and land forms produced by historic deglaciation in Melbern–Grand Pacific valley are comparable, both in style and scale, to those associated with the decay of the Cordilleran ice sheet at the end of the Pleistocene (c. 14–10 ka BP). Rates of historic and terminal Pleistocene deglaciation also may be comparable.

Numerical models to assess the principal response of large ice caps to climatic changes are used as a guide to the interpretation of field evidence of changes in the glaciological regime in the coastal areas of Victoria Land and north Victoria Land, Antarctica. Based on the theoretical work, the following scenario is proposed: areas within about 300 km from the coast of Victoria Land experienced (i) significantly shallower ice slopes and a lesser degree of glaciation during most of the late Tertiary, (ii) steep slopes and thicker ice than today during glacial stages, and (iii) moderate thinning of the ice in intervening interglacial stages.

The model predicts, for central regions of Antarctica, a slightly thinner ice cap (lower snow-accumulation rate) during glacial stages, but an estimated 200 m thicker ice cap in warmer Tertiary climates than today. The calculated “Tertiary ice caps” indicate a probable tendency of periodic surges due to basal melting at the outer fringes.

Only modest changes of the ice thickness in reasonably good agreement with the model predictions for the current interglacial stage have been observed on four blue-ice fields, all located within 250 km of the coastline. Investigated ice fields include two meteorite traps — the Allan Hills Icefield and the Frontier Mountain meteorite fields. Antarctic meteorite traps are sustained by very specific glaciological conditions — in particular, only moderate changes in ice thickness over time. The sub-ice topography of these fields was determined by radar measurements and reveals a former, very different glaciological regime, which is interpreted as being associated with glacial processes, operative during the late Tertiary.

Field evidence for a hypsithermal event during the Holocene is presented.