Dendroglaciological analysis of supraglacial trees represents an example of applied dendro- geomorphological methods in reconstructing glacier variations. Supraglacial trees react to glacier ice and debris movement, assuming typical shapes with modified radial growth. In this paper, based on treering analysis of Larix decidua Mill., we investigate the relationship between the distribution and growth of trees located on the most famous and representative debris-covered glacier in the Italian Alps (Miage glacier, Valle d’Aosta) and the superficial movements of ice and debris in the lower part of the tongue. Different growth anomalies (e.g. pointer years, compression wood, abrupt growth changes) were identified and dated. Three reference tree-ring chronologies based on undisturbed larches growing outside the glacier were constructed for comparison with tree-ring data from supraglacial trees. The oldest sampled trees colonized the glacier surface just before 1960. The simultaneous presence of different disturbance indicators occurred mainly between 1984 and 1990 on the southern lobe and during the period 1989–93 on the northern glacier lobe. These results fit with glaciological data documenting volume and surface-level variations in the same period.

The presence of cave ice is documented in many karst regions but very little is known about the age range of this potential paleoclimate archive. This case study from the Monlesi ice cave, Swiss Jura Mountains, demonstrates that dating of cave ice is possible using a multi-parameter approach. Ice petrography, debris content and oxygen isotope composition have the potential for identification of annual growth layers, but require a continuous core from the ice deposits, limiting application of this approach. Furthermore, complete melting of ice accumulations from individual years may occur, causing amalgamation of several annual bands. Use of 3H content of the ice and 14C dating of organic debris present in the ice proved to be of limited utility, providing rather broad bounds for the actual age. Initial estimates based on 210Pb analyses from clear ice samples gave results comparable to those from other methods. The most reliable techniques applied were the determination of ice turnover rates, and the dating of anthropogenic inclusions (a roof tile) in the ice. These suggest, respectively, that the base of the cave ice was a minimum of 120 and a maximum of 158 years old. Therefore, our data support the idea that mid-latitude and low-altitude subsurface ice accumulations result from modern deposition processes rather than from presence of Pleistocene relict ice.

The interdependence of rapid ice flow, surface topography and the spatial distribution of subglacial water are examined by linking existing theories. The motivation is to investigate whether the acceleration of an ice-stream tributary contains a positive feedback that encourages the retention of subglacial water that leads to faster flow. Periodically varying surface and bed topographies are related through a linear ice-flow perturbation theory for various values of mean surface slope, perturbation amplitude and basal sliding speeds. The topographic variations lead to a periodic variation in hydraulic potential that is used to infer the tendency for subglacial water to be retained in local hydraulic potential minima. If water retention leads to enhanced basal sliding, a positive feedback loop is closed that could explain the transition from slower tributary flow to faster-streaming flow and the sustained downstream acceleration along the tributary–ice-stream system. A sensitivity study illustrates that the same range of topographic wavelengths most effectively transmitted from the bed to the surface also strongly influences the behavior of subglacial water. A lubrication index is defined to qualitatively measure the heterogeneity of the subglacial hydrologic system. Application of this index to field data shows that the transition from tributary to ice stream closely agrees with the location where subglacial water may be first stored.

Previous observations have shown that rift propagation on the Amery Ice Shelf (AIS), East Antarctica, is episodic, occurring in bursts of several hours with typical recurrence times of several weeks. Propagation events were deduced from seismic swarms (detected with seismometers) concurrent with rapid rift widening (detected with GPS receivers). In this study, we extend these results by deploying seismometers and GPS receivers in a dense network around the tip of a propagating rift on the AIS over three field seasons (2002/03, 2004/05 and 2005/06). The pattern of seismic event locations shows that icequakes cluster along the rift axis, extending several kilometers back from where the rift tip was visible in the field. Patterns of icequake event locations also appear aligned with the ice-shelf flow direction, along transverse-to-rift crevasses. However, we found some key differences in the seismicity between field seasons. Both the number of swarms and the number of events within each swarm decreased during the final field season. The timing of the slowdown closely corresponds to the rift tip entering a suture zone, formed where two ice streams merge upstream. Beneath the suture zone lies a thick band of marine ice. We propose two hypotheses for the observed slowdown: (1) defects within the ice in the suture zone cause a reduction in stress concentration ahead of the rift tip; (2) increased marine ice thickness in the rift path slows propagation. We show that the size–frequency distribution of icequakes approximately follows a power law, similar to the well-known Gutenberg–Richter law for earthquakes. However, large icequakes are not preceded by foreshocks nor are they followed by aftershocks. Thus rift-related seismicity differs from the classic foreshock and aftershock distribution that is characteristic of large earth quakes.

During the outburst flood of a glacier-dammed lake on Gornergletscher, Switzerland, in July 2004, the drained lake water triggered anomalous glacier motion. At the onset of the outburst, the ice-flow direction in the vicinity of the lake became closer to the central flowline. When the lake discharge magnitude decreased, the flow direction altered such that the ice moved back to the azimuth of the initial motion. At one of the survey points, where the ice flows parallel to the central flowline, the ice accelerated along the pre-event flow direction followed by a 180° backward motion that lasted over 2 days. These observations indicate the impact of the lake outburst on the subglacial and englacial stress conditions; however, the reversal in the flow direction is difficult to explain by drawing on our current understanding of glacier mechanics. The timing and the timescale of the flow-direction changes suggest that the elastic glacier motion and its rebound played a role under the rapidly changing stress conditions, but the Young’s modulus of ice is too large to cause the observed ice motion. Other processes, including basal separation and subglacial sediment deformation, are discussed as possible mechanisms for the reversal of the ice motion.

The 3623 m long, 5G core collected at Vostok station, Antarctica, contains alternating layers of meteoric ice with two distinctly different microstructures. In this paper, we present the microstructure and impurity content of a number of specimens ranging in depth from 97 to 3416 m, describe in detail the characteristics of the different layers and propose a mechanism for their microstructural development. Digital image analysis, ion chromatography, scanning electron microscopy and energy dispersive X-ray spectroscopy were used to measure texture and the location and type of impurities; electron backscatter diffraction was used to determine crystal orientation. The ice associated with interglacial periods is characterized by relatively coarse grains and a strong preferred orientation of the c axes in a plane encompassing the coring direction, producing a vertical-girdle fabric. In contrast, ice from glacial periods is characterized by a much smaller grain size and a strong singlemaximum fabric, where the c axes are clustered around the vertical. Calcium is uniquely present in the grain boundaries of the fine-grained glacial layers, and its effect on grain-boundary mobility and the misorientation dependence of mobility can explain the development of the discontinuous microstructure seen in glacial ice at Vostok station.

We studied the diffusion rate of stable isotopes of water ice in a controlled laboratory experiment, where isotopically different layers of fabricated firn were stacked. This experiment was done in order to study the diffusion rate between the isotopically different layers and to compare the measurements with forward-calculated diffusion rates of the fabricated firn stack, using three different analytical and numerical methods. The layers varied in thickness within the stack, and the studied period includes variations in firn temperature. We sampled the firn stack on five different dates along the 144 day long experiment. We obtained a quantitatively good match between the calculated and measured diffusion rates, with a root-mean-square similarity of 67–80% in the depth-averaged diffusion rates. Despite the good average match, we found some qualitative discrepancy between our calculations and our measurements. The calculated diffusion rates predict >5% faster isotopic decay of thinner layers and 13% slower decay of thicker layers than our measurements show. We speculate that grain size and grain fabric may influence the tortuosity of the firn pack, and that the current generalization of tortuosity, being a function of firn density as the only free parameter, is an oversimplification.

We describe exact solutions to the thermomechanically coupled shallow-ice approximation in three spatial dimensions. Although artificially constructed, these solutions are very useful for testing numerical methods. In fact, they allow us to verify a finite-difference scheme, that is, to show that the results of our numerical scheme converge to the correct continuum values as the grid is refined in three dimensions. Comparison of numerical results with exact solutions has helped us to precisely quantify and understand some of the numerical errors we are making. Our verified numerical scheme shows the basal temperature spokes which arose in the EISMINT (European Ice Sheet Modelling INiTiative) II intercomparison (Payne and others, 2000). A careful analysis describes these warm spokes as numerical errors which occur when the derivative of the strain-heating term with respect to the temperature is large. On the other hand, the appearance of basal temperature spokes in a verified numerical scheme strongly suggests that they are a feature of the EISMINT II experiment F continuum problem. In fact, they are clear evidence of an unstable equilibrium point of the continuum problem. This paper is a sequel to Bueler and others (2005) which addresses exact solutions and verification in the isothermal case.

Four tributaries of Panmah Glacier have surged in less than a decade, three in quick succession between 2001 and 2005. Since 1985, 13 surges have been recorded in the Karakoram Himalaya, more than in any comparable period since the 1850s. Ten were tributary surges. In these ten a full run-out of surge ice is prevented, but extended post-surge episodes affect the tributary and main glacier. The sudden concentration of events at Panmah Glacier is without precedent and at odds with known surge intervals for the glaciers. Interpretations must consider the response of thermally complex glaciers, at exceptionally high altitudes and of high relief, to changes in a distinctive regional climate. It is suggested that high-altitude warming affecting snow and glacier thermal regimes, or bringing intense, short-term melting episodes, may be more significant than mass-balance change.

Analysis of aerial photographs, GPS mapping and comparison of digital elevation models have been used to quantify the losses in extent and volume observed (1981–2005) in the glaciers of the Maladeta massif (Spanish Pyrenees). The data are examined in relation to different climatic (temperature, precipitation) and topographic factors that control glacial retreat both at the general and local scales. The evolution observed in the massif is characterized by the remarkable surface and volumetric shrinkage registered in all the glaciers: (1) glacial ice decreased in extent 35.7%, reducing from 240.9 ha to 155.0 ha; (2) losses in total ice volume reached 0.0137 km3 (75.6 m w.e.); (3) mean altitude of the studied glaciers increased 43.5 m. These changes seem to have been forced by climatic change (reduction in the snowfall contributions and increase in the maximum temperatures) during the past few decades in this Pyrenean region. In addition, local variables such as the orientation of each glacier, their altitude and their initial size seem to have induced significant spatial differences in the magnitude of the losses.

Analyses of confined and unconfined compression combined with shear, and of biaxial stress laterally confined or unconfined, are presented for a general deviatoric viscous relation describing the response of an incompressible material. At present, numerical models for ice-sheet flow commonly adopt a very simple viscous law throughout the ice sheet, in which the deviatoric stress is coaxial with the strain rate, and the single response function depends on only one invariant, and is determined by single stress component tests which cannot verify the validity of the simplification. The analysis presented here is concerned with two-stress-component experimental configurations which could determine the general quadratic form of a viscous relation, with two response functions depending on two invariants. It is shown that the two combined compression and shear tests can also check the consistency of a viscous fluid assumption, but not so the biaxial stress tests. Each test allows a direct assessment of the significance of the quadratic term. It is then shown that a significant quadratic term changes the relative stress magnitudes in the commonly adopted reduced model for ice-sheet flow, and that the crucial simplifications are not achieved.

Powder-snow avalanches are natural hazards which affect the way populations live in mountainous areas. Field measurements from avalanches remain one of the most significant and useful sources of information about their dynamics and behaviour. In this paper, we consider all the video data from the Swiss Vallée de la Sionne test site from the years 2003–05. General scaling laws are sought for the avalanche front velocity based on plume theories. Avalanche Froude numbers are found, comparing three different length scales: the cube root of the fracture volume; the avalanche height; and the depth of entrained snow cover. We discuss the difficulties in defining the volume of a powder-snow avalanche: should we include just the head or also the turbulent wake that extends back to the starting zone? This relates to whether we use a compact model for the avalanche, such as the KSB model (Ancey, 2006; Turnbull and others, 2006) or a plume model (Turner, 1973). Observations are made regarding the lateral spreading behaviour of the avalanches. We show that the slow lateral spreading can be explained by large internal velocities and anisotropic turbulence generated by the large-scale motion in the avalanche head.

Colinear-polarized 5 MHz radar profiling data were obtained on Mýrdalsjökull, a temperate glacier in Iceland. Radar transects, and therefore polarization planes, were aligned approximately parallel, transverse and oblique to the ice flow direction. Echoes from the shallower half to two-thirds of the ice were 10–20dB stronger on the oblique and longitudinal transects than those on the transverse transects. Anisotropy as a function of depth is clearly seen at the sites where the transects cross. Strong scattering on longitudinal transects apparently caused extinction of a radar-reflecting layer that was continuously profiled on the transverse transects. A radio-wave scattering model shows that scattering from a longitudinal water-filled conduit parallel to the glacier surface can explain the observed azimuthal variations of the echo. We conclude that low-frequency (~MHz) radio waves can help to characterize englacial water regimes.

Gornersee, Switzerland, is an ice-marginal lake, which drains almost every year, subglacially, within a few days. We present an analysis of the lake outburst events between 1950 and 2005, as well as results of detailed field investigations related to the lake drainage in 2004 and 2005. The latter include measurements of lake geometry, water pressure in nearby boreholes and glacier surface motion. A distributed temperature-index melt model coupled to a linear-reservoir runoff model is used to calculate hourly discharge from the catchment of Gornergletscher in order to distinguish between the melt/precipitation component and the outburst component of the discharge hydrograph. In this way, drainage volume and timing are determined. From 1950 there is a clear trend for the outburst flood to occur earlier in the melt season, but there is no trend in lake discharge volumes. Peak discharges from the lake lie significantly below the values obtained using the empirical relation proposed by Clague and Mathews (1973). The shapes of the 2004 and 2005 lake outflow hydrographs differ substantially, suggesting different drainage mechanisms. From water balance considerations we infer a leakage of the glacier-dammed lake in 2005, starting 1 week prior to the lake outburst. During the drainage events, up to half of the lake water is temporarily stored in the glacial system, causing substantial uplift of the glacier surface.

Meltwater and runoff from Keqicar Baqi glacier, a large glacier in the southwestern Tien Shan, northwestern China, are simulated using a modified degree-day model including potential clear- sky direct solar radiation, coupled with a linear reservoir model for the period 1 July to 12 September 2003. There is good agreement between modelled and measured meltwater at ablation stakes and between simulated and observed runoff at the glacier terminus. A reconstruction of glacier meltwater and runoff from an assumed debris-free surface during the study period shows that the effect of the debris layer on glacier meltwater generation crucially affects glacier runoff. The model is also used to calculate glacier runoff given the climate scenario resulting from a doubling of CO2 as projected by the ReCM2 regional climate model. The projected changes in temperature and precipitation vary from 0 to 2.7°C and from 0 to 25%, respectively. Results indicate that glacier runoff increases linearly with temperature over these ranges whether or not the debris layer is taken into consideration. The effect of change in temperature is much more noticeable than that for change in precipitation. Due to the debris layer predominantly covering (and insulating) the ablation area, the response of glacier runoff is less sensitive to temperature increase with a debris-covered surface than with a debris-free surface. Glacier runoff is also markedly reduced when a reduced glacier area is prescribed in the +2.7°C scenario.

Global positioning system (GPS) campaigns were conducted during the 2003 and 2004 austral summer seasons to obtain insight into the velocity and strain-rate distribution on Schirmacher Glacier, central Dronning Maud Land, East Antarctica. GPS data were collected at 21 sites and analyzed to estimate the site coordinates, baselines and velocities. The short-term precision of the base station, MAIT, is estimated from the daily coordinate repeatability solutions during the two years. All GPS points on the glacier were constrained with respect to MAIT and nearby International GPS Service stations. Horizontal velocities of the glacier sites lie between 1.89 ± 0.01 and 10.88 ± 0.01 ma−1 to the north-northeast, with an average velocity of 6.21 ± 0.01 m a−1. The principal strain rates provide a quantitative measurement of extension rates, which range from (0.11 ± 0.01) × 10−3 to (1.48 ± 0.85) × 10−3a−1, and shortening rates, which range from (0.04 ± 0.02) × 10−3 to (0.96 ± 0.16) × 10−3a−1. The velocity and strain-rate distributions across the GPS network in Schirmacher Glacier are spatially correlated with topography, subsurface undulations, fracture zones/crevasses and the partial blockage of the flow by nunataks and the Schirmacher Oasis.

Interpretation of ice-core records requires accurate knowledge of the past and present surface topography and stress–strain fields. The European Project for Ice Coring in Antarctica (EPICA) drilling site (75.0025° S, 0.0684° E; 2891.7 m) in Dronning Maud Land, Antarctica, is located in the immediate vicinity of a transient and forking ice divide. A digital elevation model is determined from the combination of kinematic GPS measurements with the GLAS12 datasets from the ICESat. Based on a network of stakes, surveyed with static GPS, the velocity field around the drilling site is calculated. The annual mean velocity magnitude of 12 survey points amounts to 0.74 m a–1. Flow directions mainly vary according to their distance from the ice divide. Surface strain rates are determined from a pentagonshaped stake network with one center point close to the drilling site. The strain field is characterized by along-flow compression, lateral dilatation and vertical layer thinning.

The deformation, resulting from a surge in 1985, of Glaciar Horcones Inferior is analyzed using structural geological models. During the surge, previously continuous debris cover was deformed by the formation of regularly separated and rotated ice blocks, suggesting a system of linked rotational extensional faults. Block tilting was measured from photographs taken shortly after the surge, showing rotation of the debris-covered surface. Fault inclination was assumed to be coincident with the debris-free side of the block. Glacier advance during the surge was obtained by comparing pre-surge aerial photographs with the position of maximum advance after the surge. Glacier thinning was estimated from the debris surface average lowering (relief generated at lateral scarps coincident with shear zones) and ice thickness measurements after surge termination. Three independent sets of information, geometry of the deformation (i.e. depth of detachment, fault traces, fault spacing, block rotation), glacier thinning and net advance, limit possible interpretations. Surface geometry suggests a domino-style or a linked planar rotational extensional fault system. In the observed configuration, however, these models can only explain a 12–13% extension. Glacier thinning suggests 30% local extension, and total glacier advance implies 16% minimum extension, which does not account for some frontal compression, as observed. A linked curved rotational extensional fault model fits the data well, implying a significant degree of internal deformation within each block. This model satisfactorily explains the observed deformation produced by the surge. It may also explain some modes of fast glacier flow, since the observed style of block tilting is present in other glaciers with high relief.

Basal water pressure and water flow patterns are significant factors in controlling the behavior of an ice sheet, because they influence ice-sheet thickness, stability and extent. Water produced by basal melting may infiltrate the subsurface, or occur as sheet or channelized flow at the ice/bed interface. We examine subglacial groundwater conditions along a flowline of the Scandinavian ice sheet through Nordfjord, in the western fjords region of southern Norway, using a steady-state, twodimensional groundwater-flow model. Meltwater input to the groundwater model is calculated by a two-dimensional, time-dependent, thermomechanically coupled ice-flow model oriented along the same flowline. Model results show that the subglacial sediments could not have transmitted all the meltwater out of the fjord during times of ice advance and when the ice sheet was at its maximum position at the edge of the continental shelf. In order for pore-water pressures to remain below the overburden pressure of the overlying ice, other paths of subglacial drainage are necessary to remove excess water. During times of retreat, the subglacial aquifer is incapable of transmitting all the meltwater that was probably generated. Pulses of meltwater reaching the bed could explain nonclimatically driven margin readvances during the overall retreat phase.

An understanding of the reflection and transmission of light by snow is important for snow thermodynamics, hydrology, ecology and remote sensing. Snow has an intricate microstructure replete with ice/air interfaces that scatter light. Spectral observations of light reflection and transmission, from 400 to 1000 nm, were made in temperate snowpacks, under cold and under melting conditions. The optical observations were made using a dual-detector spectroradiometer. One detector was placed above the snow surface to monitor the incident and reflected solar irradiance, and the second detector was placed at the base of snow cover to measure downwelling irradiance. The optical measurements were supplemented by a physical characterization of the snow, including depth, density and an estimate of grain size. In general, transmitted light levels were low and showed a strong spectral dependence, with maximum values between 450 and 550 nm. For example, a 10 cm thick snow layer reduced visible transmission (500 nm) to about 5% of the incident irradiance, and infrared transmission (800 nm) to less than 1%. Extinction coefficients were in the range 3–30 m−1, and tended to decrease slightly as the snow aged and increase as snow density increased.