Understanding the microstructure of ice underpins the interpretation of ice-core measurements and many ice-sheet properties. A detailed study of polar snow and ice using scanning electron microscope (SEM) and X-ray analysis revealed the micro-structural distribution of soluble impurities. Sublimation under vacuum (etching) concentrated impurity from both the bulk and grain boundaries on to the specimen surfaces in detectable quantities. Sublimation in the cold room before examination (pre-etching) collected previously unobservable quantities of impurity at triple junctions. A heterogeneous distribution of impurities was observed. Chloride was frequently found to originate from the lattice, but not usually at triple junctions. Other impurities, particularly sodium chloride, were detected at grain boundaries and bubble surfaces. Sulphate was often found at triple junctions in specimens containing a high bulk concentration of the acid, frequently in conjunction with cations. This suggests the possibility that veins were only filled with significant amounts of impurity when the surrounding grain boundaries were saturated. The model of impurity arrangement inferred from the results reconciles differences between previous SEM studies; additionally it is consistent with and explains recent electrical conduction observations. The disconnected arrangement of impurity-filled grain boundaries and veins limits opportunities for significant post-depositional solute movement.

The North Greenland Icecore Project (NorthGRIP) palaeoclimatic information back to about 120 kyr BP. The size distributions of ice crystals in the upper 880 m of the NorthGRIP ice core, which cover a time-span of approximately 5300 years, have been obtained previously. The distributions evolve towards a universal curve, indicating a common underlying physical process in the formation of crystals. We identify this process as an interplay between fragmentation of the crystals and diffusion of their grain boundaries. The process is described by a two-parameter differential equation to which we obtain the exact solution. The solution is in excellent agreement with the measured distributions.

Interferometric radar images collected by ERS-1, ERS-2 and RADAR- SAT-1 are used to observe the rupture tip of rifts that propagate along Hemmen Ice Rise on the Ronne Ice Shelf, Antarctica. Interferograms generated in 1992 and 1997 allow for the observation of ice deformation accumulated over 9 and 24 days respectively. These interferograms are combined, in order to separate the continuous process of creep deformation from the more cyclic motion caused by variations in ocean tide. An examination of local gradients in creep deformation reveals the pattern of ice deformation around and near the rupture tips and rifts with great precision (up to 10 cm a-1). We compare the observations with a deformation model for ice and obtain the following results: (1) The tidal oscillation of the Ronne Ice Shelf only yields small deformations along the rifts and near the rupture tips. (2) Along the ice front, the rifts and at the rupture tips, vertical bending is observed which is well explained by a model of viscous deformation of ice. Furthermore, the model indicates that the deformation pattern observed at the rupture tips is a sensitive indicator of the propagation state of the rifts (i.e. active vs inactive). (3) The viscous adjustment of ice is the dominant mode of deformation, masking the deformation pattern predicted by linear elastic fracture mechanics (LEFM). (4) Yet, at a spatial scale equivalent to the length of a rift, the propagation rate is well predicted by LEFM.

An essential problem in snow science is to predict the changing form of ice grains within a snow layer. Present theories are based on the idea that form changes are driven by mass diffusion induced by temperature gradients within the snow cover. This leads to the well-established theory of isothermal- and temperature-gradient metamorphism. Although diffusion theory treats mass transfer, it does not treat the influence of this mass transfer on the form — the curvature radius of the grains and bonds — directly. Empirical relations, based on observations, are additionally required to predict flat or rounded surfaces. In the following, we postulate that metamorphism, the change of ice surface curvature and size, is a process of thermodynamic optimization in which entropy production is minimized. That is, there exists an optimal surface curvature of the ice grains for a given thermodynamic state at which entropy production is stationary. This state is defined by differences in ice and air temperature and vapor pressure across the interfacial boundary layer. The optimal form corresponds to the state of least wasted work, the state of minimum entropy production. We show that temperature gradients produce a thermal non-equilibrium between the ice and air such that, depending on the temperature, flat surfaces are required to mimimize entropy production. When the temperatures of the ice and air are equal, larger curvature radii are found at low temperatures than at high temperatures. Thus, what is known as isothermal metamorphism corresponds to minimum entropy production at equilibrium temperatures, and so-called temperature-gradient metamorphism corresponds to minimum entropy production at none-quilibrium temperatures. The theory is in good agreement with general observations of crystal form development in dry seasonal alpine snow.

Short-term variations in horizontal and vertical surface motion were studied with high temporal resolution during the ablation season in Lauteraargletscher, Bernese Alps, Switzerland. Horizontal surface flow speed oscillated diurnally, showing a correlation with the water level in a borehole. Flow speed increased as a function of the water level, with an asymptote at the ice overburden level. This observation implied that the flow variations were principally controlled by the local water pressure which enhanced basal motions. Detailed examination of the diurnal variations, however, showed that the speed was larger when the pressure was increasing than when it was decreasing. Greater speed with increasing pressure was interpreted by subglacial water-cavity opening and/or longitudinal stress coupling with the upper reaches of the glacier. Upward surface movements were observed when the glacier flow speed increased. Simultaneous measurement of internal vertical strain in a borehole showed that the uplift had two different sources: vertical straining of ice and volume increase of subglacial water cavities. The vertical surface movement was largely affected by the vertical strain, and the uplift events could not be simply attributed to cavity opening.

The net mass balance on Gulkana Glacier, Alaska, U.S.A., has been measured since 1966 by the glaciological method, in which seasonal balances are measured at three index sites and extrapolated over large areas of the glacier. Systematic errors can accumulate linearly with time in this method. Therefore, the geodetic balance, in which errors are less time-dependent, was calculated for comparison with the glaciological method. Digital elevation models of the glacier in 1974, 1993 and 1999 were prepared using aerial photographs, and geodetic balances were computed, giving – 6.0 ± 0.7 m w.e. from 1974 to 1993 and -11.8 ± 0.7 m w.e. from 1974 to 1999. These balances are compared with the glaciological balances over the same intervals, which were – 5.8 ± 0.9 and -11.2 ± 1.0 m w.e. respectively; both balances show that the thinning rate tripled in the 1990s. These cumulative balances differ by <6%. For this close agreement, the glaciologically measured mass balance of Gulkana Glacier must be largely free of systematic errors and be based on a time-variable area-altitude distribution, and the photography used in the geodetic method must have enough contrast to enable accurate photogrammetry.

We have investigated the formation of 10-50 mm long ice spikes that sometimes appear on the free surface of water when it solidifies. By freezing water under different conditions, we measured the probability of ice-spike formation as a function of: (1) the air temperature in the freezing chamber, (2) air motion in the freezing chamber (which promotes evaporative cooling), (3) the quantity of dissolved salts in the water, and (4) the size, shape and composing material of the freezing vessel. We found that the probability of ice-spike formation is greatest when the air temperature is near -7°C, the water is pure and the air in the freezing chamber is moving. Even small quantities of dissolved solids greatly reduce the probability of ice-spike formation. Under optimal conditions, approximately half the ice cubes in an ordinary ice-cube tray will form ice spikes. Guided by these observations, we have examined the Bally-Dorsey model for the formation of ice spikes. In this model, the density change during solidification forces super cooled water up through a hollow ice tube, where it freezes around the rim to lengthen the tube. We propose that any dissolved solids in the water will tend to concentrate at the tip of a growing ice spike and inhibit its growth. This can qualitatively explain the observation that ice spikes do not readily form using water containing even small quantities of dissolved solids.

Ice-spike observations in nature have sparked much interest in the scientific and non-scientific communities alike, yet most research performed thus far has been largely qualitative. We have conducted a quantitative, systematic laboratory investigation in order to assess theories explaining ice-spike growth and to determine the conditions conducive to it. We observed ice-spike growth using time-lapse digital photography, using two water types in two different containers. We observed that ice spikes occurred much more frequently in distilled water than in tap water. Digital images were analyzed to determine the growth rate of the ice spikes. Water temperature was recorded throughout the freezing process, and the cooling rate was used to estimate a bulk heat transfer coefficient. Finally, a simple model, based on mass conservation, was derived and was found to give useful predictions of ice-spike growth rate.

To better understand how internal radar echoes depend on ice-flow conditions and radar polarization, we surveyed two basins in East Antarctica using 179 MHz airborne radar. We compared radar echoes from three ice-flow conditions: parallel sheet flow in the main stream of a basin, convergent flow towards an ice stream, and longitudinal compression by nunataks. We detected a distinct zone of high radar scattering several hundred meters thick at middle depths in the latter two regions. This high-scattering zone was detected only when the radar polarization plane was parallel to the compression axis in ice. Such a high-scattering zone was not found in the parallel-flow region, regardless of the polarization. Using a recently developed theory of radar scattering in ice, we interpret the high-scattering zone as being caused by crystal-orientation-fabric alternations among adjacent ice layers due to difference in horizontal strain components. We argue that the spatial variation of the high-scattering zone is crucial for understanding past and present flow features.

Spatial and temporal variations of the flow of Hansbreen, a tidewater glacier in southern Spitsbergen, Svalbard, are investigated. During summer 1999, surface flow velocities were measured in the ablation zone of Hansbreen with a temporal resolution of 1—2 hours. Short events with strongly increased surface velocities and a typical duration of 1—2 days were observed. These “speed-up events” are related to periods of strongly increased water input to the glacier, due to rainfall or enhanced surface melt. A close relation is found between the surface velocities and water pressure recorded in a moulin. However, there are indications from a short time lag between velocity and water-pressure peak as well as from observed vertical surface uplifts that basal motion is related to basal water storage rather than directly to basal water pressure. The observed short-term velocity variations and associated processes on Hansbreen are very similar to those observed on land-based valley glaciers and suggest that the relevant mechanisms and physical processes that control the flow and its temporal variations are similar. In contrast to the flow of land-based glaciers, sliding velocities on Hansbreen are observed to be high all year round and velocities increase towards the calving front.

The upper 20—30 m of ice-rich permafrost at three sites overridden by the northwest margin of the Laurentide ice sheet in the Tuktoyaktuk Coastlands, western Arctic Canada, comprise massive ice beneath ice-rich diamicton or sandy silt. The diamicton and silt contain (1) truncated ice blocks up to 15 m long, (2) sand lenses and layers, (3) ice veins dipping at 20—30°, (4) ice lenses adjacent and parallel to sedimentary contacts, and (5) ice wedges. The massive ice is interpreted as intrasedimental or buried basal glacier ice, and the diamicton and silt as glacitectonite that has never thawed. Deformation of frozen ground was mainly ductile in character. Deformation was accompanied by sub-marginal erosion of permafrost, which formed an angular unconformity along the top of the massive ice and supplied ice clasts and sand bodies to the overlying glacitectonite. After deformation and erosion ceased, postglacial segregated ice and ice- wedge ice developed within the deformed permafrost.

It has been an underlying assumption in many studies that near-surface layers imaged by ground-penetrating radar (GPR) can be interpreted as depositional markers or isochrones. It has been shown that GPR layers can be approximately reproduced from the measured electrical properties of ice, but these material layers are generally narrower and more closely spaced than can be resolved by typical GPR systems operating in the range 50−400 MHz. Thus GPR layers should be interpreted as interference patterns produced from closely spaced and potentially discontinuous material layers, and should not be assumed to be interpretable as precise markers of isochrones. We present 100 MHz GPR data from Lyddan Ice Rise, Antarctica, in which near-surface (<50 m deep) layers are clearly imaged. The growth of the undulations in these layers with depth is approximately linear, implying that, rather than resulting from a pattern of vertical strain rate, they do correspond to some pattern of snowfall variation. Furthermore, comparison of the GPR layers with snow-stake measurements suggests that around 80% of the rms variability in mean annual accumulation is present in the GPR layers. The observations suggest that, at least in this case, the GPR layers do approximate isochrones, and that patterns of snow accumulation over Lyddan Ice Rise are dominated by extremely persistent spatial variations with only a small residual spatial variability. If this condition is shown to be widely applicable it may reduce the period required for measurements of surface elevation change to be taken as significant indications of mass imbalance.

The melt cycle of snow is investigated by combining ground-based microwave radiometric measurements with conventional and meteorological data and by using a hydrological snow model. Measurements at 2000 m a.s.l in the basin of the Cor-devole river in the eastern Italian Alps confirm the high sensitivity of microwave emission at 19 and 37 GHz to the snow melt−freeze cycle, while the brightness at 6.8 GHz is mostly related to underlying soil. Simulations of snowpack changes performed by means of hydrological and electromagnetic models, driven with meteorological and snow data, provide additional insight into these processes and contribute to the interpretation of the experimental data.

Mertz Glacier, East Antarctica, is characterized by a 140 km long, 25 km wide floating ice tongue. In this paper, we combine a large number of remotely sensed datasets, including in situ global positioning system measurements, satellite radar altimetry, airborne radio-echo sounding and satellite synthetic aperture radar imagery and interferometry. These various datasets allow us to study the interaction of the ice tongue with the tides and currents. However, the inverse barometer effect needs to be applied to sea-level variations affecting the tongue. We find that the tide-induced currents exert a small lateral pressure on the tongue which, when integrated over the large surface of the tongue, induce a flexure of up to 2 m amplitude per day. Simple elastic modelling of the flexure confirms the observations and helps validate the boundary conditions necessary to explain different eastward and westward tongue deflections. In addition, the along-flow velocity of the tongue does vary daily from 1.9 to 6.8 md-1 depending on the tidal current. When the current pushes the tongue toward the eastern boundary of the fjord, the tongue is retarded by the drag and the velocity decreases. The accumulated stress is released, allowing the tongue to flow very rapidly when the current pushes the tongue westward. These forcing and boundary conditions on the floating ice flow are important and must be taken into account when studying glacier discharge and calving.

During two cruises in 1998 and 1999, we examined drift and ridging characteristics of sea ice in the Ross Sea, Antarctica. Mean ice thickness in the western Ross Sea in autumn was 0.5 m, while higher level-ice thicknesses, greater areal coverages of ridges and higher sails were found in the central and eastern Ross Sea in summer. Near the continent, ice drifted westward near the coast and turned eastward further north. We use a regional sea-ice−mixed-layer−pycnocline model to initiate backward trajectories at the time and location of field observations and examine the dynamic and thermodynamic processes that determine ice thickness along these trajectories. Model results agree with previously published field data to indicate that thermodynamic and dynamic thickening and snow-ice formation each contribute significantly to the ice mass of the summer ice field in the central and eastern Ross Sea. For first-year ice in the western Ross Sea, model results and field data both indicate that thermodynamic thickening is the dominant process that determines ice thickness, with dynamic thickening also contributing 20% to the net ice-thickening rate. However, model results fail to reproduce the prevalence of snow- ice formation that was seen in field data.

We measured vertical strain in the firn at Siple Dome, Antarctica, using two systems, both of which measure relative displacements over time of metal markers placed in an air-filled borehole. One system uses a metal-detecting tuned coil, and the other uses a video camera to locate the markers. We compare the merits of the two systems. We combine steady-state calculations and a measured density profile to estimate the true vertical-velocity profile. This allows us to calculate a depth-age scale for the firn at Siple Dome. Our steady-state depth-age scale has ages ≈10-15% younger at any given depth when compared to depth-age scales derived by layer counting in a core 40 m away. The age of a visible ash layer at 97 m in the core is 665 ± 30 years, in agreement with a similar analysis conducted at Taylor Dome, Antarctica, where the same ash is also seen, providing an additional dated tie point between the two cores.

The Holocene portion of the Siple Dome (Antarctica) ice core was dated by interpreting the electrical, visual and chemical properties of the core. The data were interpreted manually and with a computer algorithm. The algorithm interpretation was adjusted to be consistent with atmospheric methane stratigraphic ties to the GISP2 (Greenland Ice Sheet Project 2) ice core, 10Be stratigraphic ties to the dendrochronology 14 C record and the dated volcanic stratigraphy. The algorithm interpretation is more consistent and better quantified than the tedious and subjective manual interpretation.