bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparisons of Sea-Ice Velocity Fields from ERS-1 SAR and a dynamic model
Authors: Leppäranta Matti , Sun Yan , Haapala Jari
Published: September 2017

Numerical models for sea-ice thickness distribution and velocity are used for ice-dynamics research and ice forecasting. In the modeling work, ERS-1 SAR is an excellent tool, in particular by providing spatial ice-velocity fields as described in the present Baltic Sea study. Ice velocities were extracted from SAR data with 3 and 6 day time intervals using the optical-flow method. A considerable stiffening of the ice pack was observed due to the change in the character of ice déformation under compression from rafting to ridging as the minimum ice thickness increased from 10 to 3O cm. The coastal alignment was strong in the ice motion and the coastal boundary layer width was 20-30 km. An analysis of the SAR data with an ice-dynamics model showed that the observed overall ice-velocity field could be produred using the Hibler viscous-plastic ice rheology. The compressive strength of the ice (over 10 km sc ales) was 2.5x 104 N m−2 ±50% for ridging and negligible for rafting of very thin ice. The shear strength was significant and the normal yield ellipse aspect ratio of 2 was valid. The 3 day time interval is val id for updating an ice model but for detailed ice-dynamics investigations a data frequency of 1 d−1 or higher would be preferable.
Drumlinization and drumlin-forming instabilities: viscous till mechanisms
Authors: Hindmarsh Richard C. A.
Published: September 2017

Glacially induced flow naturally tends to thin and extended till cover through shock formation, even in the absence of longitudinal gradients in the applied stress. Thicker till cover has an increased effective pressure at its surface and base, a lower sliding velocity or deformation rate and above a critical thickness, a decrease in wave velocity with thickness, leading to reverse-facing shocks moving downstream. For sliding and for some theologies of internal deformation, a decrease in sediment flux with thickness occurs, implying backward-moving kinematic waves and reverse-facing, reverse-moving shock.

Downstream-facing shocks are also formed which move upstream if the till is sliding and downstream if the till is deforming internally. Eventually, shocks coalesce, leaving an upstream-lacing shock for sliding and a downstream-facing shock for internal deformation. It is observed that some drumlins have downstream blunt ends only.

Fairly realistic three-dimensional drumlin shapes can be produced from symmetric sediment bodies and barchan shapes can be produced from linear forms perpendicular to the ice-sheet flow.

The fact that viscous theories produce drumlinoid forms suggests that on this scale till behaves viscously and the the lower length scale for drumlins represents the plastic/viscous transition scale.
Imaging of firn and bubbly ice in coaxial reflected light: a new technique for the characterization of these porous media
Authors: Arnaud Laurent , Gay Michel , Barnola Jean-Marc , Duval Paul
Published: September 2017

A new technique for characterizing the structure of firn and bubbly ice is presented. This technique, based on observation of etched (sublimation) surfaces in coaxial reflected light, enables une to see simultaneously the pore network of the firn or bubbles in the ice and the crystal boundaries. At the same time, the main stages of image processing used to transform the initial photographs into clean binary images are described.
Detection of abrupt changes in glacier mass balance in the Tien Shan Mountains
Authors: Cao M. S.
Published: September 2017

Based on glacier mass-balance records collected, compiled and published by the World Glacier Monitoring Service (WGMS), an attempt is made here to determine whether there were abrupt decadal-scale changes in annual balance on glaciers in the Tien Shan Mountains. There appears to have been an abrupt change in the mid-1970s; two-thirds of the change was caused by an increase in annual ablation and the other one-third by a decrease in the annual accumulation. Thus, the change was caused mainly by a summer-temperature increase, supplemented by a decrease in annual snowfall. Climate records suggest that this event, in effect, is in response to a shift hi the background state of the coupled Ocean -atmosphere system over the Tropical Pacific during the mid-1970s.
Experiments on the damage process in ice under compressive states of stress
Authors: Stone B. M. , Jordaan I.J. , Xiao J. , Jones S.J.
Published: September 2017

During ice-structure interaction, ice will fail in a brittle manner dominated by two processes. The first corresponds to the formation of macrocracks and the consequent spalling-off of large ice pieces. The second includes an intense shear-damage process in zones, termed critical zones, where high pressures are transmitted to the structure. The shear-damage process results in microstructural changes including microcrack formation and recrystallization. A range of tests on laboratory-prepared granular ice have been conducted to determine the fundamental behaviour of ice under various stress states and stress history, particularly as it relates to changes in microstructure. The test series was designed to study three aspects: the intrinsic creep properties of intact, undamaged ice; the enhancement of creep and changes in microstructure due to damage; and the effects of different stress paths. Tests on intact ice with triaxial confining pressures and low deviatoric stresses, aimed at defining the intrinsic creep response in the absence of microcracking, showed that an accelerated creep rate occurred at relatively low deviatoric stresses. Hence, a minimum Creep rate occurred under these conditions. Recrystallization to a smaller grain-size and void formation were observed. Ice damaged uniaxially and triaxially prior to testing showed enhancement of creep under both uniaxial and triaxial loading conditions Creep rates in triaxially damaged ice were found to be non-linear with high deviatoric stresses, corresponding to a power-law dependence of creep rate. Uniaxially damaged specimens contained microcracks parallel to the stressed direction which tended to close under triaxial confinement. Damage under triaxial conditions at low confining pressures produced small recrystallized grains near zones of microcracking. At high confining pressures, a fine-grained recrystallized structure with no apparent cracking was observed uniformly across the specimen. The recrystallization process contributes significantly to the enhanced creep rates found in damaged specimens.
Vapor transport, grain growth and depth-hoar development in the subarctic snow
Authors: Sturm Matthew , Benson Carl S.
Published: September 2017

Measurements from the subarctic snowpack are used to explore the relationship between grain growth and vapor flow, the fundamental processes of dry-snow metamorphism. Due to extreme temperature gradients, the subarctic pack undergoes extensive depth-hoar metamorphism. By the end of the winter a five-layered structure with a pronounced weak layer near the base of the snow evolves. Grain-size increases by a factor of 2–3. while the number of grains per unit mass decreases by a factor of 10. Observed growth rates require significant net inter-particle vapor fluxes. Stable-isotope ratios show that there are also significant net layer-to-layer vapor fluxes. Soil moisture enters the base of the pack and mixes with the bottom 10 cm of snow, while isotopically light water vapor fractionates from the basal layer and is deposited up to 50 cm higher in the pack. End-of-winter density profiles for snow on the ground, compared with snow on tables, indicate the net layer-to-layer vapor flux averages 6 x 10−7 kg m−2 s−1, though detailed measurements show the net flux is episodic and varies with time and height in the pack, with peak net fluxes ten limes higher than average. A model, driven by observed temperature profiles, reproduces the layer-to-layer flux pattern and predicts the observed weak layer at the base of the snow. Calculated layer-to-layer vapor fluxes are ten times higher than inter-particle fluxes, which implies that depth-hoar grain growth is limited by factors other than the vapor supply. This finding suggests that gain and loss of water molecules due to sublimation from grains takes place at a rate many times higher than the rate at which grains grow, and it explains why grains can metamorphose into different forms so readily.
Model of wind pumping for layered snow
Authors: Colbeck S. C.
Published: September 2017

Layering affects the air flow through snow caused by surface pressure variations. The horizontal and total fluxes are high in hoar layers but the pressure perturbations and vertical components of the flow do not penetrate as deeply as in homogeneous snow. That is because the layers “pipe” the flow horizontally toward the area of low pressure. An ice layer at the surface reduces the total flow everywhere. The flow decreases as ice-layer thickness increases and, in general, flow changes with permeability. However, the magnitude of the effect is proportionately weaker when the ice layers are further from the surface. The residence time is reduced when hoar layers are present due to shorter flow paths, reduced penetration into the deeper snow and higher speeds.
Recession of the southern part of Barnes Ice Cap, Baffin Island, Canada, between 1961 and 1993, determined from digital mapping of Landsat TM
Authors: Jacobs John D. , Simms Élizabeth L. , Simms Alvin
Published: September 2017

Changes along the margin of the southern half of the 5900 km2 Barnes Ice Cap have been assessed using 1993 Landsat TM imagery in comparison with digitized 1:50 000 NTS maps based on 1961 photogrammetry. The average recession over the 183 km long southern perimeter was found to be at least 4 m a−1, with no significant difference between the southeast and southwest sectors. Viewed in conjunction with the sustained retreat previously reported for the northwest margin, these results indicate that a general reduction in the size of Barnes Ice Cap is occurring. The present retreat phase began under a regional climate warming in the late 19th to early 20th century period and continues, while the record of the ablation-season temperature since the mid-century has not shown any significant trend.
Meteorological controls on glacier mass balance: empirical relations suggested by measurements on glacier de Sarennes, France
Authors: Vincent C. , Vallon M.
Published: September 2017

Glacial mass-balance reconstruction for a long-term time-scale requires knowledge of the relation between climate change and mass-balance fluctuations. A large number of mass-balance reconstructions since the beginning of the century are based on statistical relations between monthly meteorological data and mass balance. The question examined in this paper is: are these relationships reliable enough for long-term time-scale extrapolation? From the glacier de Sarennes long mass-balance observations series, we were surprised to discover large discrepancies between relations resulting from different time periods. The importance of the albedo in relation to ablation and mass balance is highlighted, and it is shown that it is impossible to ignore glacier-surface conditions in establishing the empirical relation between mass-balance fluctuations and climatic variation; to omit this parameter leads to incorrect results for mass-balance reconstruction in the past based on meteorological data.
Rheology of till beneath Storglaciären, Sweden
Authors: Hooke Roger LeB. , Hanson Brian , Iverson Neal R. , Jansson Peter , Fischer Urs H.
Published: September 2017

In order to study, in situ, the rheology of a deforming subglacial till, various instruments were emplaced in till beneath Storglaciären, Sweden. Boreholes were used to gain access to the till beneath about 100 m of ice. Tiltmeters provided an estimate of the shear strain rate in the till. Two other instruments yielded measures of till strength. In addition, water pressures were recorded in boreholes and in the till, a computer-controlled distance meter provided an effectively continuous record of the surface velocity and data from frequent surveys of a stake network were used to estimate the mean basal drag, based on a force-balance calculation.

Tilt rates varied directly with effective pressure, so decreases in water pressure apparently increased the coupling between the glacier and the bed. Surface speed was either out of phase with tilt or varied independently of tilt. Thus, increases in speed were apparently a consequence either of longitudinal coupling or of reduced coupling between the glacier and the bed; they were not a result of till deformation! Till strength varied directly with effective pressure, which is consistent with it being a Mohr – Coulomb, or frictional material. The devices measuring till strength are presumed to have been pulled through the till at a speed that varied in phase with the surface speed but till strength did not vary systematically with surface speed. This implies that the residual strength of the till is insensitive to strain rate. Thus, the appropriate constitutive equation for till rheology may be of the form:

where k is a constant. This is consistent with experimental data reported in the geotechnical literature.
Determination of the Surface and Bed Topography in Central Greenland
Authors: Hodge Steven M. , Wright David L. , Bradley Jerry A. , Jacobel Robert W. , Skou Neils , Vaughn Bruce
Published: September 2017

The surface and bottom topography of the central Greenland ice sheet was determined from airborne ice-radar soundings over a 180 km by 180 km grid centered on the 1974 “Summit” site (lat. 72°18′N., long. 37°55′W.), using the Technical University of Denmark 60 MHz ice radar. Over 6100 km of high-quality radar data were obtained, covering over 99'% of the grid, along lines spaced 12.5 km apart in both north-south and east-west directions. Aircraft location was done with an inertial navigation system (INS) and a pressure altimeter, with control provided by periodically flying over a known point at the center of the grid. The ice radar was used to determine ice thickness; the surface topography was determined independently using height-above-terrain measurements from the aircraft’s radar altimeter. The calculated surface topography is accurate to about ±6 m, with this error arising mostly from radar-altimeter errors. The ice thickness and bottom topography are accurate to about ±50 m, with this error dominated by the horizontal navigation uncertainties due to INS drift; this error increases to about ±125 m in areas of rough bottom relief (about 12% of the grid).

The highest point on Greenland is at lat. 72°34′ N., long. 37°38′W., at an altitude of 3233 ± 6 m a.s.l. The ice surface at this point divides into three sectors, one facing north, one east-south-east, and one west-south-west, with each having a roughly uniform slope. The ice divide between the last two sectors is a well-defined ridge running almost due south. The ice is about 3025 m thick at the summit. Excluding the mountainous north-east corner of the grid, where the ice locally reaches a thickness of about 3470 m and the bed dips to about 370 m below sea-level, the maximum ice thickness, approximately 3375 m, occurs about 97 km south-south-west of the summit. The average bed altitude over the entire grid is 180 m and the average ice thickness is 2975 ± 235 m. The ice in most of the south-west quadrant of the grid is over 3200 m thick, and overlies a relatively smooth, flat basin with altitudes mostly below sea-level. There is no predominant direction to the basal topography over most of the grid; it appears to be undulating, rolling terrain with no obvious ridge/valley structure. The summit of the ice sheet is above the eastern end of a relatively large, smooth, flat plateau, about 10–15 km wide and extending about 50 km to the west. If the basal topography were the sole criterion, then a site somewhere on this plateau or in the south-west basin would be suitable for the drilling of a new deep ice core.
Relationship Between the Development of Depth Hoar and Avalanche Release in the Tian Shan Mountains, China
Authors: Weilin Ma , Ruji Hu
Published: September 2017

In spring and winter, there is a considerable amount of solid precipitation in the western part of the Tian Shan mountains. Avalanches frequently occur here, and often endanger national economic construction and human life. Observation shows that the avalanche occurrences are closely related to the physical characteristics of the snow, especially to the development of depth hoar in the snow cover. The conditions for the development of depth hoar are determined by thickness of the snow cover, air temperature, ground-temperature regime, and the duration of negative temperatures. The results of this study have revealed that the optimum depth of snow cover for the development of depth hoar is about 80 cm in the Tian Shan mountains, and the mean maximum depth of the snow cover in this region is 78 cm. Therefore, depth hoar develops extraordinarily well. The thickness of depth hoar can reach more than 80% of the total snow-cover depth. That is one of the main reasons why avalanches still occur frequently under the conditions of a limited snow-cover depth in the western part of the Tian Shan mountains.
Avalanche Weather Forecasting at the Northwest Avalanche Center, Seattle, Washington, U.S.A.
Authors: Ferguson S.A. , Moore M.B. , Marriott R.T. , Speers-Hayes P.
Published: September 2017

Since its inception in 1975, the Northwest Avalanche Center (NWAC) has developed and produced micro- and mesoscale weather forecasts to support avalanche forecast and control needs for the Olympic and Cascades Mountains of Washington and Oregon, U.S.A. This paper describes NWAC’s array of data, observational results, and analytical techniques that make “avalanche weather” forecasting possible. In addition, NWAC’s operational program and the general terrain and climate of the area are described. A sample forecast is also included.
Observations on the Drainage of an Ice-Dammed Lake in West Greenland
Authors: Russell Andrew J. , Aitken John F. , Jong Carmen De
Published: September 2017

A small ice-dammed lake near Søndre Strømfjord, West Greenland, drained suddenly during the night of 15–16 July 1988. This drainage was observed from within the lake basin near the tunnel mouth. The drainage had four main effects: the collapse of adjacent moraine ridges; the shearing of ice around the tunnel mouth; rapid incision of the lake bed; and the transport of sediment from the lake basin into the glacier. All of the above were confined to a period of 15–30 min following the exposure of a circular tunnel c. 6 m in diameter.
Remote-Sensing Studies of Kvitøyjøkulen, an Ice Cap on Kvitøya, North-East Svalbard
Authors: Bamber Jonathan L. , Dowdeswell Julian A.
Published: September 2017

Airborne 60 MHz radio echo-sounding data were collected over 250 km of flight track from an ice cap (Kvitøyjøkulen) on Kvitøya in north-eastern Svalbard. These were combined with Landsat imagery to provide ice-surface and bed topographic information and inferences about the thermal and climatic conditions on the ice cap. Quantitative analysis of radio-echo strength measurements and the existence of offshore plumes of sediment-laden melt water indicate significant basal melting and a warmer thermal regime than would be expected from the geographical location of the ice cap. Analysis of the dielectric absorption of radio waves by the ice and data from a recently recovered ice core from a neighbouring ice cap (Austfonna) suggest considerable anthropogenic contamination of the precipitation falling on Kvitøya, most probably originating from Eurasia.
In-Situ Tensile Tests of Snow-Pack Layers
Authors: Jamieson J.B. , Johnston C.D.
Published: September 2017

During the winter of 1987–88, an average of seven tensile tests was made for each of 66 snow layers in the Rocky Mountains of western Canada. The precision of the mean strength for seven tests, expressed in terms of the coefficient of variation, was 15% with 90% confidence. Snow with a faceted micro-structure was approximately half as strong as partly settled or rounded snow of the same density. Notch sensitivity in the strength data and critical strains of 1% or less indicate that the test fractures were essentially brittle.
Towards Identification of Optimum Radar Parameters for Sea-Ice Monitoring
Authors: Kim Y. S. , Moore R. K. , Onstott R. G. , Gogineni S.
Published: September 2017

Various field experiments have shown that microwave radars can be used to distinguish multi-year from first-year ice, although optimum radar parameters are not yet fully defined.

This paper presents the results from two theoretical models that, using selected physical parameters of sea ice, are able to predict the backscattering from multi-year and first-year ice under cold conditions. The possible ranges of the backscattering coefficient under various conditions (surface roughness, salinity, temperature, density, and air-bubble size) are calculated for multi-year and first-year ice by adjusting the parameters within the reported range of values.

Although the calculations show no specific resonance that would favor any particular frequency or incidence angles, the results confirm the experimental findings that Ku- and X-band frequencies, and incidence angles greater than 30°, are better for distinguishing sea-ice types than lower frequencies.
δD – δ18O Relationships in Ice Formed by Subglacial Freezing: Paleoclimatic Implications
Authors: Souchez R. A. , de Groote J. M.
Published: September 2017

A freezing slope, distinct from that of precipitation, is displayed on a δD–δ18O diagram by basal ice in different circumstances. However, if the subglacial reservoir allowed to freeze is mixed in the course of time with an input having a lighter isotopic composition, basal ice cannot be distinguished from glacier ice in terms of slope. Such a situation is encountered at the base of Grubengletscher and is indicated by a computer simulation using the open-system model of Souchez and Jouzel (1984). Suggested implications for the paleoclimatic interpretation of deep ice cores recovered from the bottom part of polar ice sheets are given.
On Supercooling and Ice Formation in Turbulent Sea-water
Authors: Omstedt Anders
Published: September 2017

Laboratory data on supercooling and frazil-ice formation in sea-water are analysed using a boundary-layer model. The model is based on a turbulent channel-flow boundary-layer theory, in which buoyancy effects become important because of vertical gradients in temperature, salinity, and suspended frazil-ice crystals. The frazil-ice crystals are treated as thin uniform plates. By assuming a mean face diameter, a mean thickness, and a mean Nusselt number of 10−3 m, 10−4 m, and 4, respectively, the general experimental findings are well reproduced by the model.
Transfer of Basal Sliding Variations to the Surface of a Linearly Viscous Glacier
Authors: Balise Michael J. , Raymond Charles F.
Published: September 2017

The transfer of basal velocity anomalies to the surface of a glacier is investigated using a model of a planar parallel-sided slab (thickness H) of linear viscous rheology. Surface velocity parallel (us) and normal (vs) to the surface is calculated for various spatial distributions of basal velocity anomalies with components parallel (ub) and normal (vb) to the surface. Four scales of differing behavior can be identified depending on the spatial length L of the basal anomalies. At very short scales (L ≤ 1H) there is essentially no response at the surface. At short scales (1H ≤ L ≤ 5H), a basal anomaly ub induces a response in both us and vs. The spatial pattern of us is such that velocity peaks in us can be shifted from peaks in ub, and may differ in number. The amplitude of us is up to about 0.3|ub|. The amplitude of the cross-component effect vs may be greater than the amplitude of us. A basal anomaly vb induces a response in both vs and us. The pattern of vs is the same as the pattern of vb, and the amplitude of vs is up to about 0.7 |vb|. The amplitude of the cross-component effect us is less than the amplitude of vs. At intermediate scales (5H ≤ L ≤ 10H), results differ from the short scale in two respects; velocity peaks in us correspond with peaks in ub; and surface amplitudes are increased, except for cross-component effects for which surface amplitudes are of the same order as at the short scale. These cross-component effects at the short and intermediate scales show in particular that substantial anomalous surface-normal motions can be induced by deformation, even though the basal velocity anomaly is parallel to the surface. At long scales (10H ≤ L), the velocity anomaly at the surface is essentially the same as the anomaly at the bed. For all scales, the longitudinal strain-rate averaged over depth is larger in magnitude than the longitudinal strain-rate at the surface and, at the short scale, it may differ in sign, so that vs cannot be easily estimated from surface strain-rate. Although the simplifications of the model do not allow its rigorous quantitative application to field measurements, the results indicate the need for caution in interpreting surface-velocity variations in terms of basal velocity anomalies. It is important to establish the spatial pattern of surface motions for any chance of a confident interpretation in terms of basal motions.
Ice Avalanches: Some Empirical Information about their Formation and Reach
Authors: Alean Jürg
Published: September 2017

A study intended to be mainly of practical use in the prediction of ice-avalanche hazards was carried out. About 100 ice avalanches, mostly from the Alps, were documented. Starting zones of these ice avalanches could be classified by using simple terrain characteristics. Ice avalanches from some starting zones at relatively low altitudes and with large, homogeneously inclined bedrock planes occur predominantly in summer and autumn. No such seasonal variation in frequency was found in connection with other types of starting zones occurring either at higher altitudes or involving abrupt changes of the bedrock’s gradient. One- and two-parameter models for the estimation of run-out distances of landslides and snow avalanches were tested for their use with ice avalanches. Introduction of the second (v2proportional) frictional parameter leads only to moderately improved accuracy in the prediction of run-out distances. For relatively short run-out distances (several hundred meters), an alternative method of estimation, based on terrain characteristics, is proposed.
Two-Dimensional, Time-dependent Modelling of an Arbitrarily Shaped Ice Mass with the Finite-Element Technique
Authors: Hodge Steven M.
Published: September 2017

The two-dimensional, time-dependent flow of an arbitrarily shaped ice mass can be successfully modeled with the finite-element technique on a small computer. Methods developed for automatically generating the mesh data greatly simplify the data preparation and optimize the numerical simulations. Using quadratic basis functions permits the flow to be approximated quite adequately by only two element rows (five nodes vertically). Mixed-order basis functions, however, must be used so that numerical oscillations do not set in, and the ends of the ice mass, where the thickness tends to zero, must be treated carefully. Time simulations to a steady-state condition are necessary to test such numerical models adequately.

South Cascade Glacier, Washington, is currently close to equilibrium. A bedrock sill dominates the bed topography in the lower half of the glacier, rising to a height of about 20% of the ice thickness. This sill produces a maximum increase in the overall thickness of about 6–7% compared to what the thickness would have been if the sill were not present. Finally, this glacier does not appear to be sliding much, if at all, despite its maritime alpine environment. This could help explain the difficulties encountered when trying to measure sliding and basal water pressures on the same glacier (Hodge, 1979), or it could imply that drag exerted by the valley walls has a significantly greater effect than conventional shape-factor concepts imply.
The 1987–88 surge of West Fork Glacier, Susitna Basin, Alaska, U.S.A.
Authors: Harrison W. D. , Echelmeyer K. A. , Chacho E. F. , Raymond C. F. , Benedict R.J.
Published: September 2017

A surge of West Fork Glacier, a temperate glacier in the Susitna Basin of the Alaska Range, began soon after the end of the 1987 melt season and terminated on 6 July 1988. Reconnaissance measurements of balance, elevation and speed had been made from 1981 to 1983. Daily measurements of surface speed at two points 9 km apart and of the characteristics of the stream draining the glacier were begun during the surge and continued through the following year. The maximum displacement of the ice during the surge was about 4 km; the maximum change in surface elevation was about 120 m. Between the time of the start of detailed observations on 12 February 1988 and the onset of a complex termination phase during the last month of the surge, the speed was almost constant, and the water discharge was totally free of turbidity, indicating that no basal water was escaping from the glacier. During the termination phase, sharp changes in speed occurred, almost simultaneously at the two observation sites; each deceleration event was accompanied by high sediment concentration and high water discharge. This behavior is similar to that observed on Variegated Glacier during its 1982-83 surge. The mechanism of triggering (related to surface water input and the disruption of the internal drainage system) and the cause of the fast motion were probably the same for both surges, even though there are substantial differences in size and mass-balance characteristics.
Borehole water-level variations and implications for the subglacial hydraulics of South Cascade Glacier, Washington State, U.S.A.
Authors: Fountain Andrew G.
Published: September 2017

Boreholes were drilled in South Cascade Glacier to investigate the hydraulics of subglacial water flow. Results indicate that subglacial water pressures are generally close to local ice-overburden pressures and that a subglacial debris layer probably exists. Calculations indicate that the range of hydraulic conductivity of this layer is 10−7-10−4m−1. The borehole water levels generally increased during the ablation season and may be caused by a seasonal evolution in the permeability of the debris layer. Water in the debris layer drains to a subglacial conduit, the existence of which is inferred by large diurnal variations in the water level of one borehole. These levels commonly reached the bottom of the glacier, indicating near-atmospheric pressures in the conduit.
Ice triaxial deformation and fracture
Authors: Rist M.A. , Murrell S.A.F
Published: September 2017

An experimental investigation into the mechanical behaviour of polycrystalline ice in triaxial compression has been conducted using conditions generally favourable to brittle fracture and microcracking. Under triaxial stresses at high strain rate, ice failure occurs by abrupt shear fracturing, generally inclined at about 45° to the maximum principal stress. At −20°C, such failure is suppressed by the imposition of a small confining pressure, allowing a transition to ductile-type flow accompanied by distributed microcracking, but at —40°C shear fracture persists under confinement of up to at least 50 MPa. For low confining pressures (< 10 MPa), brittle strength is strongly pressure-dependent; above this it is pressure-independent. Evidence is presented that suggests this may reflect a change from a fracture process influenced by friction to fracture initiated by localized yielding. Ductile yield strength is found to be little influenced by confining pressure despite the inhibition of cracking that leads to greatly contrasting observed crack densities. Flow conforms to the well-known power law for ice withQ= 69 J mol−1andn= 4.2 over the temperature range −20° to −4-5° C Under these conditions, microcracking in ice appears to remain remarkably stable and non-interacting.
Calibration of the δ18O isotopic paleothermometer for central Greenland, using borehole temperatures
Authors: Cuffey Kurt M. , Alley Richard B. , Grootes Pieter M. , Bolzan John M. , Anandakrishnan Sridhar
Published: September 2017

We calibrate the δ18O paleothermometer for central Greenland using borehole temperatures, a thermal model forced by a measured δ18O record and a formal inverse technique. The calibration is determined mostly by temperature fluctuations of the last several centuries, including the Little Ice Age. Results are generally insensitive to model variables, including initial condition, basal boundary condition, parameterization of snow thermal properties, ice thickness and likely errors in temperature and isotope measurements. Results of this borehole calibration also seem to be in agreement with modern spatial gradients of δ18O and temperature. We suggest that calibrations of isotopic paleothermometers using borehole temperatures are a useful paleoclimate tool, because they are independent of spatial gradients and include the effects of prehistoric temperatures over ice sheets.
Ice flow around large obstacles as indicated by basal ice exposed at the margin of the Greenland ice sheet
Authors: Knight Peter G. , Sugden David E. , Minty Christopher D.
Published: September 2017

Spatial variations in the debris-bearing basal ice layer exposed at the ice-sheet margin in West Greenland reflect the geography of basal melting and ice flow around large obstacles close to the margin. This paper demonstrates the character of the basal ice layer, which comprises fine material incorporated in an interior, subglacial environment and coarser material entrained in an ice-marginal environment. We develop an empirical model of ice flow close to a lobate margin of the ice sheet in which ice convergence and divergence, and limited subglacial melting affect the character and distribution of the basal ice at the margin. There is a tendency for the convergence and divergence to thicken the basal layer in lobate areas and to thin it in inter-lobate areas. Under certain circumstances, basal melting may remove much of the layer from beneath the snouts of larger lobes, thus causing the basal layer to be thickest in an intermediate location.
Observations of flexural waves on the Erebus Ice Tongue, McMurdo Sound, Antarctica, and nearby sea ice
Authors: Squire Vernon A. , Robinson William H. , Meylan Michael , Haskell Timothy G.
Published: September 2017

New strain data relating to flexural oscillations of the Erebus Glacier Tongue (EGT), McMurdo Sound, Antarctica, are presented and are analysed in the frequency domain. The data were collected during November 1989, just 3 months prior to the most recent calving of the ice tongue which occurred in March 1990. A broad-band oscillation centred on 50 s is found in both the strain measurements collected on the EGT and those collected on the sea ice nearby. The oscillation is shown, at least in part, to be propagating with a phase velocity of approximately 65 ms−1 in a direction away from the snout towards the grounding line, rather than being wholly due to a standing-wave pattern in the EGT. A coupling model between the sea ice and the EGT is proposed and is shown to compare reasonably well with the data.
Fracture toughness and crack growth of brackish ice using chevron-notched specimens
Authors: Stehn Lars
Published: September 2017

Field-test equipment called FIFT (a Field Instrument for Fracture toughness Tests on ice) was used in both field and laboratory fracture-toughness tests on brackish sea ice from the Gulf of Bothnia. An experimental calibration was performed and a compliance expression was then derived for the Short Rod Chevron Notched (SRCN) specimen. Using the SRCN configuration, for which the initial crack growth is shown to be stable, and measured load-point displacements, preliminary crack-growth velocities are found. The obtained estimated crack velocity is, on average,ȧe= 20 ms−1, albeit with a large standard deviation. The results indicate that critical crack (crack-jumping) growth occurs. The apparent fracture toughness,KQ, was found to have a pronounced dependency on porosity in the form of brine volume. The results obtained are derived from a linearly elastic fracture mechanics (LEFM) theory. Consequently, the tests were designed to satisfy small-scale yielding requirements in terms of notch sensitivity and brittleness. The linearity of the load vs crack-opening displacement curves together with a size-effect study, showing that the specimen is notch-sensitive for grain-sizes ranging from 1.6 to nearly 100 mm, indicate that LEFM could be applicable.
Solute Acquisition in Glacial Melt Waters. II Argentière (French Alps): Bulk Melt Waters with Open-System Characteristics
Authors: Thomas A.G. , Raiswell R.
Published: September 2017

Variations in dissolved cations, total alkalinity, sulphate, and field pH are recorded for subglacial melt and bulk melt waters (those emerging from the portal) at Argentière (France), in peak and recession flow conditions. Calcium and bicarbonate are the major ions and the bulk melt waters are demonstrated to have acquired solutes by weathering and dissolution in a system open to atmospheric carbon dioxide. Subglacial melt waters have closed-system characteristics, are close to saturation with calcite and quartz, and must be in near-equilibrium with weathered particulates. Recession-flow bulk melt waters are chemically similar to subglacial melt but have open-system characteristics, either due to re-equilibration with the atmosphere for ground-water mixing.
Chemical Models of Solute Acquisition in Glacial Melt Waters
Authors: Raiswell R.
Published: September 2017

Chemical models for the evolution of bulk melt waters can be constructed by assuming that rock mineral weathering and solute acquisition are controlled by carbonate equilibria. Different open- and closed- system models are defined by variations in the rates of weathering relative to the rate of hydrogen-ion supply by the dissolution and dissociation of CO2, and can be recognized by characteristic pH and ∑+ relationships. The observed and inferred compositions of englacial and subglacial melt suggest that mixing is unlikely to be conservative and that closed-system conditions result where post-mixing weathering reactions occur. The final composition of bulk melt waters is determined by the mixing ratio between englacial and subglacial melt, the extent of post-mixing evolution, and opportunity for mixing with ground waters or re-equilibration with the atmosphere.
Interpretation of the Chemical and Physical Time-Series Retrieved from Sentik Glacier, Ladakh Himalaya, India
Authors: Mayewski Paul A. , Lyons W. Berry , Ahmad N. , Smith Gordon , Pourchet M.
Published: September 2017

Spectral analysis of time series of ac. 17 ± 0.3 year core, calibrated for totalßactivity recovered from Sentik Glacier (4908m) Ladakh, Himalaya, yields several recognizable periodicities including subannual, annual, and multi-annual. The time-series, include both chemical data (chloride, sodium, reactive iron, reactive silicate, reactive phosphate, ammonium,δD,δ(18O) and pH) and physical data (density, debris and ice-band locations, and microparticles in size grades 0.50 to 12.70μm). Source areas for chemical species investigated and general air-mass circulation defined from chemical and physical time-series are discussed to demonstrate the potential of such studies in the development of paleometeorological data sets from remote high-alpine glacierized sites such as the Himalaya.
Field pH Determinations in Glacial Melt Waters
Authors: Metcalf Richard C.
Published: September 2017

Laboratory pH analyses of glacial melt waters are unrepresentative of in situ values, due primarily to CO2 gas exchange between the sample and the atmosphere, and solute enrichment from chemical reaction with sediment and colloidal particles. A method is presented which enables field pH measurements that are reproducible within ±0.04 pH units to be made in glacial melt waters, using commonly available digital pH meters with combination electrodes.

During initial spring snow melt in May 1981 at Gornergletscher, Switzerland, melt waters in the proglacial stream leaving the glacier terminus were oversaturated with respect to atmospheric p(CO2), and rapidly increased pH during CO2 outgassing at in situ temperature and pressure. Summer ice melt from glaciers which are temperate in the ablation zone are usually undersaturated by about ten times with respect to atmospheric p(CO2), and rapidly lower their pH values to achieve equilibrium upon encountering the atmosphere, as observed at Gornergletscher during July and August 1981. Gornergletscher summer proglacial stream waters, sometimes show pH increases from rock weathering, with the rate limited by the transfer rate of CO2 across the air-water interface to drive the weathering reactions. Throughout the year, any water parcel at equilibrium with atmospheric CO2 is generally at an equilibrium pH value, if filtration prohibits solute enrichment. For these reasons, laboratory pH measurements are unacceptable for quantitative studies of melt-water chemistry and should be discontinued.
“Sun Spirals” on Melting Snow
Authors: Jahn Alfred
Published: September 2017

The light oblong elements like blades of grass and wooden sticks resting on the surface of melting snow change their position and arrangement (direction). They turn under the direct influence of the varying angle of the Sun’s rays due to the movement of the Sun. Turning of the elements gives a circular form of arrangement that has been termed ablation “sun spirals”.
Possible Late Palaeozoic Glaciation in the Central Parts of the Yemen Arab Republic
Authors: El-Nakhal Hamed A.
Published: September 2017

Occasional outsize clasts, some of which are striated, and laminae bent against the enclosed clasts have been recorded throughout the shales and siltstones forming the lower part of the Kohlan Group in the central parts of the Yemen Arab Republic. Such features are interpreted as an indication of continental glaciation. On stratigraphical grounds, these shales and siltstones are assigned to the Permian.
Optical Properties of Blowing Snow
Authors: Pomeroy J.W. , Male D.H.
Published: September 2017

Calculation procedures are developed and results shown for the exact calculation of extinction and meteorological visual range using the blowing-snow mass in the atmosphere and particle radius. Results of the calculations show: (1) For monochromatic radiation, geometrical optics approximations of the extinction efficiency are found to provide results of only moderate accuracy in calculating the extinction of radiation by a single particle. (2) For broad-band radiation, the geometrical optics approximation is sufficiently accurate for many single-particle measurement instruments and applications, except in the infra-red band where Mie theory should be used. (3) For typical blowing-snow particle-size distributions, the shape parameter of the distribution of particle radii and the mean particle radius are very important in broad-band extinction and visual-range modelling. Estimates of blowing-snow quantities from broad-band extinction measurements or visual range from blowing-snow quantities should address the shape and mean value of the snow-particle radius distribution.
On the Albedo of Snow in Antarctica: A Contribution to I.A.G.O.
Authors: Wendler Gerd , Kelley John
Published: September 2017

As part of a larger experiment, detailed albedo measurements were carried out during the austral summer of 1985-86 in the dry-snow zone (1560 m) of Terre Adélie, eastern Antarctica. The following results were found:

(1)

Mean albedo values were high (around 82.6%). On clear days, the albedo showed some dependency on the solar elevation. The dependency was slight for solar elevations above 12° but became larger with low Sun angles.

(2)

The albedo was found to be a function of cloud amount and type, increasing with the amount and thickness of clouds. In white-out conditions, very high albedos were found (>90%).

(3)

The albedo showed a dependency on the type of snow. New snow displayed higher values than older snow, whose crystals had been destroyed by mechanical action.

(4)

A simple model was developed to assess the influence of sastrugi on the albedo. This model could explain the asymmetric diurnal variation about solar noon of the measured albedo above a sastrugi field.

The above four dependencies might explain the considerable discrepancies which can be found in the literature concerning the snow albedo of Antarctica.
Mass Balance Along Two Transects of the West Side of the Greenland Ice Sheet
Authors: Kostecka J.M. , Whillans I.M.
Published: September 2017

The mass balance is computed along the Ohio State University (OSU) transect near the Arctic Circle and along the Expédition Glaciologique Internationale au Groenland (EGIG) line. Measured surface velocities are compared with velocities calculated from up-glacial accumulation rate, flow-line spreading, ice thickness, and the depth variation in horizontal velocity. The depth variation in velocity is calculated using the constitutive relation for ice, calculated temperatures within the glacier, computed shear and longitudinal stresses, and allowance for impurity content and ice-crystal orientation. The resulting mass balance is +0.6 ± 0.14 m a−1 for the OSU transect and 0 ± 0.07 m a−1 along the EGIG line. The errors arise mainly from uncertainties in measured accumulation rate and flow-line spreading, and perhaps in flow-enhancement factors due to ice anisotropy or impurities. The results for the EGIG line differ from prior estimates mainly because earlier works placed greater emphasis on short-term accumulation rates.
A Pressure-sintering Model for the Densification of Polar Firn and Glacier Ice
Authors: Wilkinson D.S.
Published: September 2017

A comprehensive multi-mechanism theory of pressure sintering has been applied to the densification of two polar ice sheets. The comparison, which is made using pressure-sintering mechanism maps, indicates that power-law creep is the controlling mechanism between 50% and 98% theoretical density. Lattice diffusion becomes dominant at low porosities. The densification rates predicted by the theory are in good agreement with the data, and suggest that a reasonable estimation of the densification behaviour of a polar ice sheet can be made using the theory, based on information obtained from a relatively shallow core.