This study clarifies changes in the chemical forms of microparticles during Termination I, the period of drastic climate change between the Last Glacial Maximum (LGM) and the Holocene. We determine the chemical forms of individual water-soluble microparticles through micro-Raman spectroscopy and compare the relative frequencies of different types with the ion concentrations in melted ice. Micro-Raman spectroscopy shows that Na2SO4·10H2O and MgSO4·11H2O are abundant in Holocene ice, while CaSO4·2H2O and other salts are abundant in LGM ice. Further, the number of CaSO4·2H2O particles is strongly correlated with the concentration of Ca2+ during Termination I. Taken together, the evidence strongly suggests that most of the Ca2+ exists as CaSO4·2H2O. The different compositions of microparticles from the Holocene and LGM can be explained by ion balance arguments.

Repeated measurements of density profiles and surface elevation along a 515 km section of the Greenland ice sheet have been used to determine elevation change rates and the error in determining mass balance from these rates which arises from short-term fluctuations in mass input, compaction and surface density. Over the 28 months from spring 2004 to summer 2006 the average error over 100 km sections of the traverse ranged from −0.006 to 0.100 ma−1. The lowest values, comparable with the system accuracy of the CryoSat radar altimeter (0.033 m a−1), were found below 3000 m. The surface density required to translate the elevation change into mass change decreased from 0.40 g cm−3 at an elevation of 2348 m to 0.33 g cm−3 at an elevation of 3264 m. From the density profiles the equivalent values for a time period of 10 years were found to be 0.48 and 0.38 g cm−3, respectively.

Observed meteorological data and a high-resolution (5 km) model were used to simulate Greenland ice sheet surface melt extent and trends before the satellite era (1960–79) and during the satellite era through 2010°. The model output was compared with passive microwave satellite observations of melt extent. For 1960–2010 the average simulated melt extent was 15 ± 5%. For the period 1960–72, simulated melt extent decreased by an average of 6%, whereas 1973–2010 had an average increase of 13%, with record melt extent in 2010. The trend in simulated melt extent since 1972 indicated that the melt extent in 2010 averaged twice that in the early 1970s. The maximum and mean melt extents for 2010 were 52% (∼9.5 × 105 km2) and 28% (∼5.2 × 105 km2), respectively, due to higher-than-average winter and summer temperatures and lower-than-average winter precipitation. For 2010, the southwest Greenland melt duration was 41–60 days longer than the 1960–2010 average, while the northeast Greenland melt duration was up to 20 days shorter. From 1960 to 1972 the melting period (with a >10% melt extent) decreased by an average of 3 days a−1. After 1972, the period increased by an average of 2 days a−1, indicating an extended melting period for the ice sheet of about 70 days: 40 and 30 days in spring and autumn, respectively.

A tomography-based methodology for the mass transport characterization of snow is presented. Five samples, characteristic for a wide range of seasonal snow, are considered. Their three-dimensional (3-D) geometrical representations are obtained by micro-computed tomography and used in direct pore-level simulations to numerically solve the governing mass and momentum conservation equations, allowing for the determination of their effective permeability and Dupuit–Forchheimer coefficient. The extension to the Dupuit–coefficient is useful near the snow surface, where Reynolds numbers higher than unity can appear. Simplified semi-empirical models of porous media are also examined. The methodology presented allows for the determination of snow’s effective mass transport properties, which are strongly dependent on the snow microstructure and morphology. These effective properties can, in turn, readily be used in snowpack volume-averaged (continuum) models such as strongly layered samples with macroscopically anisotropic properties.

Jökulhlaups in 2007 and 2008 from an ice-dammed lake at the northern margin of Russell Glacier, West Greenland, marked the onset of a renewed jökulhlaup cycle after 20 years of stability. We present a record of successive ice-dammed lake drainage events and associated ice-margin dynamics spanning ∼25 years. Robust calculations of lake volumes and peak discharges are made, based on intensive field surveys and utilizing high-spatial-resolution orthophotographs of the lake basin and ice margin. These data enable identification of controls on the behaviour of the ice-dammed lake and provide the first field-based examination of controls on jökulhlaup magnitude and frequency for this system. We find that Russell Glacier jökulhlaups have a much higher peak discharge than predicted by the Clague–Mathews relationship, which we attribute to an unusually short englacial/subglacial routeway and the presence of a thin ice dam that permits incomplete sealing of jökulhlaup conduits between lake drainage events. Additionally, we demonstrate that the passage of jökulhlaups through an interlinked system of proglacial bedrock basins produces significant attenuation of peak discharge downstream. We highlight that improved understanding of jökulhlaup dynamics requires accurate information about ice-dammed lake volume and ice-proximal jökulhlaup discharge.

We report measurements using a portable real aperture radar (Gamma Portable Radar Interferometer (GPRI)) for interferometric imaging of the surface ice motion on Gornergletscher, Switzerland, during the drainage of the adjacent ice-marginal lake Gornersee. The GPRI tracked the surface ice motion in line of sight over an area of ∼3 km2 down-glacier of Gornersee almost continuously during the drainage event. The displacement maps derived from the acquired interferograms capture the spatial distribution of the surface ice motion. Due to fast acquisition times of the microwave images, the GPRI was able to record sub-daily variations of the ice displacements, most likely caused by the impact of the Gornersee drainage on the ice motion of Gornergletscher. In situ point measurements of the ice displacement agree reasonably well with the results obtained by the GPRI and highlight the use of the GPRI for high-resolution measurements of glacier surface ice motion.

For more than a century Alpine glaciers have been retreating dramatically, and they are expected to shrink even more quickly over the coming decades. This study addresses the future evolution of Grosser Aletschgletscher, Switzerland, the largest glacier in the European Alps. A three-dimensional combined surface mass-balance and glacier dynamics model was applied. The ice flow was described with the full Stokes equations. The glacier surface evolution was obtained by solving a transport equation for the volume of fluid. Daily surface melt and accumulation were calculated on the basis of climate data. The combined model was validated against several types of measurements made throughout the 20th century. For future climate change, scenarios based on regional climate models in the ENSEMBLES project were used. According to the median climatic evolution, Aletschgletscher was expected to lose 90% of its ice volume by the end of 2100. Even when the model was driven using current climate conditions (the past two decades) the glacier tongue experienced a considerable retreat of 6 km, indicating its strong disequilibrium with the present climate. By including a model for the evolution of supraglacial debris and its effect in reducing glacier melt, we show that this factor can significantly slow future glacier retreat.

A mass-balance model using upper-air meteorological data for input was calibrated with surface mass balance measured mainly during 1977–78 at 67 sites on Columbia Glacier, Alaska, between 135 and 2645 m a.s.l. Root-mean-square error, model vs measured, is 1.0 m w.e. a−1, with r2 = 0.88. A remarkable result of the analysis was that both precipitation and the factor in the positive degree-day model used to estimate surface ablation were constant with altitude. The model was applied to reconstruct glacier-wide components of surface mass balance over 1948–2007. Surface ablation, 4 km3 ice eq. a−1 (ice equivalent), has changed little throughout the period. From 1948 until about 1981, when drastic retreat began, the surface mass balance was positive but changes in glacier geometry were small, so the positive balance was offset by calving, ∼0.9 km3 ice eq. a−1 . During retreat, volume loss of the glacier accounted for 92% of the iceberg production. Calving increased to ∼4.3 km3 ice eq. a−1 from 1982 to 1995, and after that until 2007 to ∼8.0 km3 ice eq. a−1, which was about twice the loss by surface ablation, whereas prior to retreat it was only about a quarter as much. Calving is calculated as the difference between glacier-wide surface mass balance and geodetically determined volume change.

The Altels hanging glacier in Switzerland broke off on 11 September 1895. The ice volume of this catastrophic rupture was estimated as 4 × 106 m3, the largest icefall event ever observed in the Alps. However, the causes of this collapse are not entirely clear. Based on previous studies, we reanalyzed this break-off event, with the help of a new numerical model, initially developed by Faillettaz and others (2010) for gravity-driven instabilities. The simulations indicate that a break-off event is only possible when the basal friction at the bedrock is reduced in a restricted area, possibly induced by the storage of infiltrated water within the glacier. Further, our simulations reveal a two-step behavior: (1) a first quiescent phase, without visible changes, with a duration depending on the rate of change in basal friction; (2) an active phase with a rapid increase of basal motion over a few days. The general lesson obtained from the comparison between the simulations and available observations is that detectable precursors (crevasse formation and velocity increase) of the destabilization process of a hanging glacier, resulting from a progressive warming of the ice/bed interface towards a temperate regime, will appear only a few days prior to the break-off.

A surface cooling of ∼1.7°C occurred over the ∼two millennia prior to ∼1700 CE at the West Antarctic ice sheet (WAIS) Divide site, based on trends in observed bubble number-density of samples from the WDC06A ice core, and on an independently constructed accumulation-rate history using annual-layer dating corrected for density variations and thinning from ice flow. Density increase and grain growth in polar firn are both controlled by temperature and accumulation rate, and the integrated effects are recorded in the number-density of bubbles as the firn changes to ice. Number-density is conserved in bubbly ice following pore close-off, allowing reconstruction of either paleotemperature or paleo-accumulation rate if the other is known. A quantitative late-Holocene paleoclimate reconstruction is presented for West Antarctica using data obtained from the WAIS Divide WDC06A ice core and a steady-state bubble number-density model. The resultant temperature history agrees closely with independent reconstructions based on stable-isotopic ratios of ice. The ∼1.7°C cooling trend observed is consistent with a decrease in Antarctic summer duration from changing orbital obliquity, although it remains possible that elevation change at the site contributed part of the signal. Accumulation rate and temperature dropped together, broadly consistent with control by saturation vapor pressure.

Buried surface hoar and near-surface faceted crystals are known to lead to deadly avalanches. Over the course of three winter seasons a field investigation detailing the environmental conditions leading to the formation of these crystals was performed. Weather stations on north- and south-facing aspects were established. The weather data were accompanied by detailed daily observations and grain-scale photographs of the snow surface. During the three seasons, 35 surface hoar and 47 near-surface facets events were recorded. The mean weather conditions for the entire dataset (all three seasons and both stations) were compared to the nights when surface hoar formed. The comparison yielded five parameters that were statistically linked to the formation of surface hoar: incoming longwave radiation, snow surface temperature, wind velocity, relative humidity and the air/snow temperature difference. A similar comparison between the daytime mean values for all days with near-surface facet events revealed three parameters with statistically significant differences. Thus, these parameters (short- and longwave radiation and relative humidity) could be statistically linked to facet formation. This research also suggests that environmental conditions in the daytime hours before and after surface hoar formation are statistically similar to the conditions causing near-surface facet formation.

An in-depth analysis of seven samples from the Siple Dome (Antarctica) ice core, using optical microscopy and electron backscatter diffraction, illustrates rotational recrystallization or polygonization in the fine-grained region of the core between 700 and 800 m. Between 640 and 700 m, the microstructure is characterized by a bimodal grain-size distribution and a broken girdle fabric with evidence of polygonization. From 727 to 770 m, mean grain size decreases and a single-maximum fabric is found, and, by 790 m, mean grain size has again increased and a multiple-maxima fabric manifests itself. We compare grain-size distribution, c- and a-axis orientation, and misorientation between adjacent grains. We found that misorientations between adjacent grains in the 727–770 m region were predominantly low-angle and typically around a common a-axis, suggesting polygonization. This conclusion is supported by radar evidence of a physical disturbance at 757 m, which may be correlated with higher than usual strain in the 700–800 m range. Below 770 m, larger less regular misorientations and textural evidence show that migration recrystallization is the primary recrystallization mechanism.

A large part of the ice flux within ice caps occurs through spatially limited fast-flowing units. Some of them permanently maintain fast flow, whereas others operate in an oscillatory mode, characterized by short-lived active phases followed by long quiescent phases. This surge-type behaviour results from intrinsic rather than external factors, thus complicating estimates of glacier response to climate change. Here we present numerical model results from Austfonna, an ice cap on Svalbard that comprises several surge-type basins. Previous studies have suggested a thermally controlled soft-bed surge mechanism for Svalbard. We systematically change the parameters that govern the nature of basal motion and thereby control the transition between permanent and oscillatory fast flow. Surge-type behaviour is realized by a relatively abrupt onset of basal sliding when basal temperatures approach the pressure-melting point and enhanced sliding of marine grounded ice. Irrespective of the dynamic regime, the absence of considerable volumes of temperate ice, both in the observed and simulated ice cap, indicates that fast flow is accomplished by basal motion over a temperate bed. Given an idealized present-day climate, the equilibrium ice-cap size varies significantly, depending on the chosen parameters.

While glacier fabric reflects the accumulated strain, detailed azimuthal information is required to link the microstructure to the flow, and this is not easily gathered at depth. Borehole logging provides a way to obtain a log of azimuthal orientation of tilted stratigraphic features that can be used to orient the core with respect to glacier flow. We demonstrate this using acoustic borehole logs and the ice core from a 162 m borehole in Upper Fremont Glacier, Wind River Range, Wyoming, USA. We measured the dip of tilted dust and bubble layers in the actual ice core, identified them on the borehole log, then used their strike to orient the core sections containing them. We examined the crystal orientation fabric of our samples, using electron backscatter diffraction in a scanning electron microscope. When we compared the orientation of the tilted layers in some samples with the fabric, we found that the normal to the foliation and the c-axes maxima both pointed in the direction of maximum shear stress. This illustrates the usefulness of borehole logs for orienting ice cores after removal from the borehole, and for developing a better understanding of fabric development.

Abrupt changes in crystal-orientation fabric (COF), and therefore viscosity, are observed near the base of the ice sheet throughout West Antarctica. We report on active-source seismic observations from WAIS Divide, mid-stream and downstream on Thwaites Glacier, and the onset region of Bindschadler Ice Stream. These data reveal a prevalence of englacial seismic reflectivity in the bottom quarter of the ice sheet. The observed seismic reflectivity is complex but largely bed-conformable, with long-spatial-wavelength features observed in the flow direction and short-wavelength features observed across flow. A correspondence of englacial structures with bed features is also observed. We determine the origin of the reflectivity to be abrupt changes in the COF of ice, based on the following: (1) observations of englacial reflectivity are consistent with current knowledge of COF within ice sheets, (2) englacial reflectivity caused by COF contrasts requires the simplest genesis, especially at ice divides, and (3) amplitude analysis shows that the observed englacial reflectivity can be explained by contrasts in seismic velocity due to COF changes. We note that the downstream increase in the quantity and complexity of observations indicates that direct observations of COF at ice divides likely underestimate the role that fabric plays in ice-sheet dynamics.

We have measured the in situ average electric field attenuation length, 〈Lα〉, for radio-frequency signals broadcast vertically through the Ross Ice Shelf, Antarctica. We chose a location, Moore Embayment, south of Minna Bluff, known for its high reflectivity at the ice–sea interface. We confirmed specular reflection and used the return pulses to measure the average attenuation length from 75–1250 MHz over the round-trip distance of 1155 m. We find 〈Lα〉 to vary from ∼500 m at 75 MHz to ∼300 m at 1250 MHz, with an experimental uncertainty of 55–15 m.

Three positive feedback mechanisms that accelerate ice-sheet melting are assessed in a warming climate, using a numerical ice model driven by atmospheric climate models. The Greenland ice sheet (GrIS) is the modeling test-bed under accelerated melting conditions. The first feedback is the interaction of sea water with ice. It is positive because fresh water melts ice faster than salty water, owing primarily to the reduction in water heat capacity by solutes. It is shown to be limited for the GrIS, which has only a small ocean interface, and the grounding line of some fast glaciers becomes land-terminating during the 21st century. The second positive feedback, strain heating, is positive because it produces further ice heating inside the ice sheet. The third positive feedback, granular basal sliding, applies to all ice sheets and becomes the dominant feedback during the 21st century. A numerical simulation of Jakobshavn Isbræ over the 21st century reveals that all three feedback processes are active for this glacier. Compared with the year 2000 level, annual ice discharge into the ocean could increase by ∼1.4 km3 a−1 (∼5% of the present annual rate) by 2100. Granular basal sliding contributes ∼40% of this increase.

We present a novel automated processing method for obtaining layer dip from radio-echo sounding (RES) data. The method is robust, easily applicable and can be used to process large (several terabytes) ground and airborne RES datasets using modest computing resources. We give test results from the application of the method to two Antarctic datasets: the Fletcher Promontory ground-based radar dataset and the Wilkes Subglacial Basin airborne radar dataset. The automated RES processing (ARESP) method comprises the basic steps: (1) RES noise reduction; (2) radar layer identification; (3) isolation of individual ‘layer objects’; (4) measurement of orientation and other object properties; (5) elimination of noise in the orientation data; and (6) collation of the valid dip information. The apparent dip datasets produced by the method will aid glaciologists seeking to understand ice-flow dynamics in Greenland and Antarctica: ARESP could enable a shift from selective regional case studies to ice-sheet-scale studies.

Tidewater glaciers deposit sediment at their terminus, thereby reducing the relative water depth. Reduced water depth can lead to increased glacier stability through decreased rates of iceberg calving, glacier thinning and submarine melting. Here we investigate sedimentation processes at the termini of Kronebreen and Kongsvegen, Svalbard. We mapped the fjord floor bathymetry in August 2009 and calculate sedimentation rates based on our bathymetry and that from a similar study in 2005. A grounding-line fan is developing near the current position of the subglacial stream. An older, abandoned grounding-line fan that likely formed between ∼1987 and 2001 is degrading near the middle of the ice front. Our findings indicate that sediment gravity flows reduce the height of the sediment mound forming at the glacier terminus. The future impact of glacimarine sedimentation processes on glacier stability will depend on the net balance between the observed gravity flows and sediment deposition.

We assess basal roughness beneath Pine Island Glacier (PIG), West Antarctica, based on a recent airborne radio-echo sounding dataset. We identify a clear relationship between faster ice flow and decreased basal roughness in significant parts of PIG. The central portion and two of its tributaries are particularly smooth, but the majority of the tributaries feeding the main trunk are rougher. We interpret the presence of a smooth bed as being a consequence of the deposition of marine sediments following disappearance of the West Antarctic ice sheet in the Pliocene or Pleistocene, and, conversely, a lack of marine sedimentation where the bed is rough. Importantly, we also identify a patchy distribution of marine sediments, and thus a bed over which the controls on flow vary. While there is a notable correspondence between ice velocity and bed roughness, we do not assume a direct causal relationship, but find that an indirect one is likely. Where low basal roughness results in low basal resistance to flow, a lower driving stress is required to produce the flux required to achieve mass balance. This, in turn, means that the surface in that area will be lower than surrounding areas with a rougher bed, and this will tend to draw flow into the area with low bed roughness. Since our studies shows that bed roughness beneath the tributaries of the trunk varies substantially, there is a strong likelihood that these tributaries will differ in the rate at which they transmit current velocity changes on the main trunk into the interior of the glacier basin.