bluuebunny/crossref_metadata_2025 huggingface dataset:
Buckling rate and overhang development at a calving face
Authors: Hanson Brian , Hooke Roger Le B.
Published: November 2007

Using the finite-element we have modeled the stress field near the calving face of an idealized tidewater glacier under a variety of assumptions about submarine calving-face height, subaerial calving-face height, and ice rheology. These simulations all suggest that a speed maximum should be present at the calving face near the waterline. In experiments without crevassing, the decrease in horizontal velocity above this maximum culminates in a zone of longitudinal compression at the surface somewhat up-glacier from the face. This zone of compression appears to be a consequence of the non-linear rheology of ice. It disappears when a linear rheology is assumed. Explorations of the near-surface stress field indicate that when pervasive crevassing of the surface ice is accounted for in the simulations (by rheological softening), the zone of compressive strain rates does not develop. Variations in the pattern of horizontal velocity with glacier thickness support the contention that calving rates should increase with water depth at the calving face. In addition, the height of the subaerial calving face may have an importance that is not visible in current field data owing to the lack of variation in height of such faces in nature. Glaciers with lower calving faces may not have sufficient tensile stress to calve actively, while tensile stresses in simulated higher faces are sufficiently high that such faces will be unlikely to build in nature.
Temporal and spatial variability of the surface mass balance in Dronning Maud Land, Antarctica, as derived from automatic weather stations
Authors: Reijmer Carleen H. , Broeke Michiel R. van den
Published: November 2007

Measurements of changes in surface height carried out with sonic altimeters mounted on automatic weather stations in Dronning Maud Land (DML) and on Berkner Island, Antarctica, are used to derive the surface mass balance. The surface mass balance is positive at all sites, i.e. accumulation outweighs ablation. The spatial and temporal variability in accumulation is high. Accumulation occurs in numerous small events and a few large events per year. The larger events contribute more to the annual accumulation than the small events; ∼50% of all accumulation is contributed by 10–25% of all events. The accumulation generally decreases with increasing distance from the coast and elevation. Annual averaged values range from ∼375 ± 59 mm w.e. a−1 near the coast to ∼33 ± 25 mm w.e. a−1 on theAntarctic plateau and are in good agreement with long-term averaged annual accumulation rates obtained from snow pits and firn cores. The records show seasonal dependency of the amount of accumulation, with a maximum in winter in the coastal and escarpment region of DML and in summer on Berkner Island and on the plateau. The seasonal cycles are significant on Berkner Island, and in the coastal area and part of the escarpment region.
The snow cover on lakes of the Arctic Coastal Plain of Alaska, U.S.A.
Authors: Sturm Matthew , Liston Glen E.
Published: November 2007

Shallow lakes cover >25% of Alaska’s Arctic Coastal Plain. These remain frozen and snow-covered from October to June. The lake snow is thinner, denser, harder and has less water equivalent than snow on the surrounding tundra. Itcontains less depth hoar than land snow, yet paradoxically is subject to stronger temperature gradients. It also has fewer layers and these have been more strongly affected by wind. Dunes and drifts are better developedon lakes; they have wavelengths of 5–20 m, compared to <5 m on land. Because of these differences, lake snow has roughly half the thermal insulating capacity of land snow. The winter mass balance on lakes is also different because (1) some snow falls into the water before the lakes freeze, (2) some snow accumulates in drifts surrounding the lakes, and (3) prevailing winds lead to increased erosion and thinner snow on the eastern lake sides. Physical models that extrapolate land snow over lakes without appropriate adjustments for depth, density, distribution and thermal properties will under-predict ice thickness and winter heat losses.
Isotopic diffusion in polycrystalline ice
Authors: Rempel Alan W. , Wettlaufer J. S.
Published: November 2007

Quantitative ice-core paleoclimatology must account for post-depositional processes, such as vapor-phase diffusion in the firn. After pore close-off, diffusion continues to smooth the stable-isotope records δ18O and δD that are eventually recovered from the ice, leading to the loss of high-frequency information. Johnsen and others (1997) found much higher rates of diffusive smoothing in the Greenland Icecore Project (GRIP) Holocene ice than would be predicted by diffusion through solid ice alone, and Nye (1998) argued that transport through liquid veins might explain this apparent excess diffusion. However, the analysis of Johnsen and others (2000) indicates that the required vein dimensions may be unrealistically large. Here, we model the diffusion of stable isotopes in polycrystalline ice and show that the predictions of Nye (1998) and those of Johnsen and others (2000) actually represent two end-members in a range of potential behavior. Our model determines which of these asymptotic regimes more closely resembles the prevailing conditions and quantifies the role of pre-melted liquid in the smoothing of isotopic signals. The procedure thereby ties together the two approaches and provides a rostrum for accurate analysis of isotope records and paleotemperature reconstructions.
Links between short-term velocity variations and the subglacial hydrology of a predominantly cold polythermal glacier
Authors: Copland Luke , Sharp Martin J. , Nienow Peter W.
Published: November 2007

The surface velocity of a predominantly cold polythermal glacier (John Evans Glacier, Ellesmere Island, Canada) varies significantly on both seasonal and shorter time-scales. Seasonal variations reflect the penetration of supraglacial water to the glacier bed through significant thicknesses of cold ice. Shorter-term events are associated with periods of rapidly increasing water inputs to the subglacial drainage system. Early-season short-term events immediately follow the establishment of a drainage connection between glacier surface and glacier bed, and coincide with the onset of subglacial outflow at the terminus. A mid-season short-term event occurred as surface melting resumed following cold weather, and may have been facilitated by partial closure of subglacial channels during this cold period. There is a close association between the timing and spatial distribution of horizontal and vertical velocity anomalies, the temporal pattern of surface water input to the glacier, and the formation, seasonal evolution and distribution of subglacial drainage pathways. These factors presumably control the occurrence of highwater-pressure events and water storage at the glacier bed. The observed coupling between surface water inputs and glacier velocity may allow predominantly cold polythermal glaciers to respond rapidly to climate-induced changes in surface melting.
Velocity of radio-wave propagation in ice at Vostok station, Antarctica
Authors: Popov Sergey V. , Sheremet’Yev Alexander N. , Masolov Valery N. , Lukin Valery V. , Mironov Anatoliy V. , Luchininov Vadim S.
Published: November 2007

During the austral summer field season of the Russian Antarctic Expedition in 1999/2000, wide-angle reflections experiments were performed in the vicinity of the Russian station Vostok. A 60 MHz ice radar system with 12-bit digital recording was used. The measurements were taken along two perpendicular lines directed south–north and east–west with a distance of 200 m between marks. We used a one-layer model (without snow–firn zone influence) for the calculations. We calculate that the average velocity of radio-wave propagation in the ice sheet is168.4 ± 0.5 m μs−1.The same velocity was derived from hyperbolic diffractions from internal discontinuities. The results allow more accurate depth interpretation of radio-echo soundings.
Statistical behaviour of the deformation for first loading of polycrystalline ice
Authors: Gold Lorne W.
Published: November 2007

A statistical analysis of the lengths of grain-boundary and transgranular cracks induced during the initial straining of columnar-grain ice by a compressive load applied perpendicular to the long direction of the columns is presented. The analysis shows that the crack lengths are randomly distributed and form distinct but correlated populations.The lognormal distribution function is shown to be a good descriptor of the populations for 5–90% of their range. Statistical models are presented for the lognormal behaviour of the crack-length distribution and for the strain dependence of the crack density. The models assume that a change in the value of the random variable of the respective population depends on the population value of the variable at the time of the change. It is shown that the model for the strain dependence of the crack density is suitable for the strain dependence of the acoustic emission, measured in both columnar-grain and granular ice subject to constant compressive loads. Evidence is also presented for a lognormal dependence of the dislocation density on strain. The analysis demonstrates that the cracks that form during the initial straining of polycrystalline ice are independent, random events and that the resulting crack populations are precursors to failure by fracture.
Ice crystals grown from vapor onto an orientated substrate: application to snow depth-hoar development and gas inclusions in lake ice
Authors: Adams Edward E. , Miller Daniel A.
Published: November 2007

A laboratory experiment was conducted in which new ice crystals were nucleated from the vapor phase onto large existing ice crystals obtained from Antarctic lake ice. Flat, smooth ice-crystal surfaces were prepared, with c axes oriented either vertically or horizontally. When these were subjected to a supersaturated vapor environment, multiple individual crystals nucleated onto the substrates adopting the same crystallographic orientation as the parent. A dominant grain-growth scenario for kinetic-growth metamorphism in snow, which in some ways is analogous to the oriented morphologies in lake ice, is hypothesized. In the lake-ice-growth scenario, optimally oriented crystals will grow at the expense of those less preferentially positioned.The proposed dominant grain-growth theory for snow is in agreement with the observed decrease in the number of grains and the proximal similarity of crystal habit in kinetic-growth metamorphism in snow. Similarly, kinetic crystal growth on the interior of gas inclusions in Antarctic lake ice will also acquire the crystallographic orientation of the substrate ice. These small-faceted interior crystals significantly influence light scattering and penetration in the lake-ice cover.
Taku Glacier (Alaska, U.S.A.) on the move again: active deformation of proglacial sediments
Authors: Motyka Roman J. , Echelmeyer Keith A.
Published: November 2007

Taku Glacier is one of the few glaciers in Alaska, U.S.A., that has advanced over the last century: 7 km since 1890. This advance slowed substantially during the past decade, but in summer 2001 the glacier terminus began to readvance at a rate of 30 cm d−1. The advance produced dramatic proglacial sediment deformation up to 200 m in front of the terminus. Two to three large bulges and several secondary bulges developed in the proglacial sediments as a result of glacial compression along a 1 km wide portion of the terminus. The bulge nearest the terminus was 10 m high and 65 m wide. The middle bulge (7 m high) advanced at 15 cm d−1 and the distal bulge (3 m high and 50 m wide) at 9 cm d−1. Crenulations and prominent fractures developed in the overlying vegetation layer. The frontal lobes of the bulges were steep and overlaid a shear zone, where sediments were being thrust up and over the ground surface. Ice-proximal push moraines, 1–10 m high, formed along much of the 9 km wide terminus, although deformation was minimal at some locations.
Depth- and time-dependent vertical strain rates at Siple Dome, Antarctica
Authors: Elsberg Daniel H. , Harrison William D. , Zumberge Mark A. , Morack John L. , Pettit Erin C. , Waddington Edward D. , Husmann Eric
Published: November 2007

As part of a project to investigate the flow of ice at low effective stress, two independent strain-gauge systems were used to measure vertical strain rate as a function of depth and time at Siple Dome, Antarctica. The measurements were made from January 1998 until January 2002 at the ice divide and a site 7km to the northeast on the flank. The strain-rate profiles place constraints on the rheology of ice at low stress, show the expected differences between divide and flank flow (with some structure due to firn compaction and probably ice stratigraphy), and suggest that the flow of the ice sheet has not changed much in the last 8.6 kyr. The strain rates show an unexpected time dependence on scales ranging from several months to hours, including discrete summer events at the divide. Time dependence in strain rate, water pressure, seismicity, velocity and possibly basal motion has been seen previously on the Siple Coast ice streams, but it is especially surprising on Siple Dome because the bed is cold.
Processes involved in the propagation of rifts near Hemmen Ice Rise, Ronne Ice Shelf, Antarctica
Authors: Larour Eric , Rignot Eric , Aubry Denis
Published: November 2007

Interferometric radar images collected by ERS-1, ERS-2 and RADAR- SAT-1 are used to observe the rupture tip of rifts that propagate along Hemmen Ice Rise on the Ronne Ice Shelf, Antarctica. Interferograms generated in 1992 and 1997 allow for the observation of ice deformation accumulated over 9 and 24 days respectively. These interferograms are combined, in order to separate the continuous process of creep deformation from the more cyclic motion caused by variations in ocean tide. An examination of local gradients in creep deformation reveals the pattern of ice deformation around and near the rupture tips and rifts with great precision (up to 10 cm a-1). We compare the observations with a deformation model for ice and obtain the following results: (1) The tidal oscillation of the Ronne Ice Shelf only yields small deformations along the rifts and near the rupture tips. (2) Along the ice front, the rifts and at the rupture tips, vertical bending is observed which is well explained by a model of viscous deformation of ice. Furthermore, the model indicates that the deformation pattern observed at the rupture tips is a sensitive indicator of the propagation state of the rifts (i.e. active vs inactive). (3) The viscous adjustment of ice is the dominant mode of deformation, masking the deformation pattern predicted by linear elastic fracture mechanics (LEFM). (4) Yet, at a spatial scale equivalent to the length of a rift, the propagation rate is well predicted by LEFM.
Stratigraphy and glaciotectonic structures of permafrost deformed beneath the northwest margin of the Laurentide ice sheet, Tuktoyaktuk Coastlands, Canada
Authors: Murton Julian B. , Waller Richard I. , Hart Jane K. , Whiteman Colin A. , Pollard Wayne H. , Clark Ian D.
Published: November 2007

The upper 20—30 m of ice-rich permafrost at three sites overridden by the northwest margin of the Laurentide ice sheet in the Tuktoyaktuk Coastlands, western Arctic Canada, comprise massive ice beneath ice-rich diamicton or sandy silt. The diamicton and silt contain (1) truncated ice blocks up to 15 m long, (2) sand lenses and layers, (3) ice veins dipping at 20—30°, (4) ice lenses adjacent and parallel to sedimentary contacts, and (5) ice wedges. The massive ice is interpreted as intrasedimental or buried basal glacier ice, and the diamicton and silt as glacitectonite that has never thawed. Deformation of frozen ground was mainly ductile in character. Deformation was accompanied by sub-marginal erosion of permafrost, which formed an angular unconformity along the top of the massive ice and supplied ice clasts and sand bodies to the overlying glacitectonite. After deformation and erosion ceased, postglacial segregated ice and ice- wedge ice developed within the deformed permafrost.
Study of the snow melt–freeze cycle using multi-sensor data and snow modeling
Authors: Cagnati Anselmo , Crepaz Andrea , Macelloni Giovanni , Pampaloni Paolo , Ranzi Roberto , Tedesco Marco , Tomirotti Massimo , Valt Mauro
Published: November 2007

The melt cycle of snow is investigated by combining ground-based microwave radiometric measurements with conventional and meteorological data and by using a hydrological snow model. Measurements at 2000 m a.s.l in the basin of the Cor-devole river in the eastern Italian Alps confirm the high sensitivity of microwave emission at 19 and 37 GHz to the snow melt−freeze cycle, while the brightness at 6.8 GHz is mostly related to underlying soil. Simulations of snowpack changes performed by means of hydrological and electromagnetic models, driven with meteorological and snow data, provide additional insight into these processes and contribute to the interpretation of the experimental data.
On the mechanics of ice-stream shear margins
Authors: Schoof Christian
Published: November 2007

We investigate the mechanics of ice-stream shear margins based on the assumption that the underlying bed behaves plastically. Sliding is assumed to occur if a prescribed, locally defined yield stress is attained, while no sliding is assumed possible if basal shear stress is lower than the yield stress. Mathematically, the ice-flow problem takes the form of a contact problem, in which the zones of sliding are part of the solution and cannot be prescribed arbitrarily. Simplistic assumptions about the location of till failure, or about mechanical conditions at the bed, predict stress singularities at the margins which lead to corresponding singularities in the basal melt rate. The ice-flow problem is solved using a complex variable method, and an associated quasi-static thermal problem is also solved using a Green’s function. High stress concentrations, which coincide with high rates of strain heating, are found on the ice-stream side of the margins, where basal melting is also greatest. Our results further indicate that a temperate zone may form over time above the bed in the margins. These findings differ from earlier studies based on different sliding laws, suggesting a high sensitivity of margin behaviour to basal conditions.
Antarctic ice-sheet balance velocities from merged point and vector data
Authors: Wu Xiaolan , Jezek Kenneth C.
Published: November 2007

We estimate Antarctic ice-flow balance velocities, which are the average speeds that ice must flow downslope through a volume assuming that there are equal amounts of ice entering and leaving the ice volume. We use the OSU (Ohio State University) digital elevation model of Antarctica, recent ice accumulation rate data and the BEDMAP ice-thickness data compilations for Antarctica to characterize the physical properties of the ice sheet that are included in the balance-velocity calculation. We adapt a flux algorithm from the hydrology literature that enables us to calculate the flux distribution from any cell in any order. Flux from one cell to its neighbors is partitioned as a simple function of surface slope direction. Digitized flow-stripe directions from satellite images minimize errors in flow direction where surface slopes are low or complex. We estimate errors in balance velocity arising from errors in the data and show semi-quantitatively how properties of the algorithm bias the balance-velocity result. We find a favorable comparison between our model and observed velocity data as well as the balance-velocity patterns reported by other researchers.
Ice motion and driving forces during a spring ice shove on the Alaskan Chukchi coast
Authors: Mahoney Andrew , Eicken Hajo , Shapiro Lewis , Grenfell Tom C.
Published: November 2007

An ice shove along the Alaskan Chukchi Sea coast occurred in June 2001, affecting the communities of Barrow and Wainwright, some 150 km apart. Aerial photography before and after the event allowed measurement of ice displacement vectors near Barrow where up to 395 m of ice motion was accommodated almost entirely in discrete ridges up to 5 m high. The forces required to build these ridges are estimated at 35−62 kN m-1, and driving forces of the whole event are investigated. Most ice deformation at or near the beach coincided with local onshore winds, but the event was preceded by the compaction of pack ice in the central Chukchi Sea and the closure of the coastal flaw lead, driven by the larger-scale wind field acting over several days beforehand. Whether this acted to impart pack-ice stress to the coast or simply to create a critical fetch of consolidated ice is uncertain. The near-melting near-isothermal state of the ice may have been a complicit factor and affected the behavior of the land-fast ice. Coastal morphology and bathymetry affected the location of deformation. This study highlights the range of scales at which processes act and culminate to have implications for Arctic communities.
The thickness and internal structure of Fireweed rock glacier, Alaska, U.S.A., as determined by geophysical methods
Authors: Bucki Adam K. , Echelmeyer Keith A. , MacInnes Scott
Published: November 2007

Geophysical investigations on rock glaciers are often difficult because rock glaciers are covered by an unconsolidated debris mantle a few meters thick, are typically <50 m thick and are composed of an ice—rock mixture of unknown composition. Transient electromagnetics (TEM) is a method that allows some of these difficulties to be minimized, and data collection is relatively efficient. TEM, with calibration from terminus exposure, was used to determine the thickness (~60 m) of Fireweed rock glacier, Alaska, U.S.A., under complex valley geometry. A conductive layer beneath the rock glacier was identified, and its distribution is consistent with a till-like layer. Seismic refraction, used to resolve the debris-mantle thickness (2–4 m), suggests the presence of a discontinuity at 18–28 m depth within the rock glacier. The discontinuity is also indicated in the radio-echo sounding and the TEM data, but to a lesser extent. This discontinuity is important because the motion of the rock glacier may occur across this as a “shear plane”.
Spatio-temporal variability in volcanic sulphate deposition over the past 2 kyr in snow pits and firn cores from Amundsenisen, Antarctica
Authors: Traufetter Fidan , Oerter Hans , Fischer Hubertus , Weller Rolf , Miller Heinz
Published: November 2007

In the framework of the European Project for Ice Coring in Antarctica (EPICA), a comprehensive glaciological pre-site survey has been carried out on Amundsenisen, Dronning Maud Land, East Antarctica, in the past decade. Within this survey, four intermediate-depth ice cores and 13 snow pits were analyzed for their ionic composition and interpreted with respect to the spatial and temporal variability of volcanic sulphate deposition. The comparison of the non-sea-salt (nss)-sulphate peaks that are related to the well-known eruptions of Pinatubo and Cerro Hudson in AD 1991 revealed sulphate depositions of comparable size (15.8±3.4 kg km–2) in 11 snow pits. There is a tendency to higher annual concentrations for smaller snow-accumulation rates. The combination of seasonal sodium and annually resolved nss-sulphate records allowed the establishment of a time-scale derived by annual-layer counting over the last 2000 years and thus a detailed chronology of annual volcanic sulphate deposition. Using a robust outlier detection algorithm, 49 volcanic eruptions were identified between AD 165 and 1997. The dating uncertainty is ±3 years between AD 1997 and 1601, around ±5 years between AD 1601 and 1257, and increasing to ±24 years at AD 165, improving the accuracy of the volcanic chronology during the penultimate millennium considerably.
Velocity map of the Thwaites Glacier catchment, West Antarctica
Authors: Lang Oliver , Rabus Bernhard T. , Dech Stefan W.
Published: November 2007

The two-dimensional surface velocity of Thwaites Glacier, West Antarctica, was mapped with 23 ascending- and 22 descending-orbit European Remote-sensing Satellite synthetic aperture radar (ERS SAR) interferograms (time range 1995–2000). The velocity map covers 175 500 km2 from the Amundsen Coast to the southern turning point of the satellite orbit and comprises >80% of the Thwaites catchment. Relative velocity errors are <10% except for rare regions (about 5% of the total area) of unfavorable look geometry. Six individual tributaries were identified; their center-line velocities increase from 0 at the catchment boundary to ~0.3 km a–1 when they join the main glacier trunk. On the main trunk, velocity increases to ~1.8 km a–1 at the grounding line and 3.6 km a–1 on the floating tongue. As at neighboring Pine Island Glacier, no strong longitudinal velocity gradients are found except near the grounding line. Within expected error bounds, the flow pattern appears temporally stationary, i.e. flowlines agree with the delineation of flow suggested by the pattern of velocity magnitude. A potential temporal shift of tributary boundaries must consequently be <4.4 m a–1.
Grain area as a statistical weight for polycrystal constituents
Authors: Gagliardini Olivier , Durand Gaël , Wang Yun
Published: November 2007

By using recently developed automatic instruments for fabric and texture measurements on ice, both the c-axis orientation and area of the individual crystals can be determined. Each grain can then be associated with its volume fraction, defined as a function of its measured cross-sectional area, to describe the microstructure of a poly-crystal. The relevance of this approach is studied using a three-dimensional microstructure obtained from the Potts model. In particular, the area weighting is compared to the classical implicit equal weighting used by glaciologists, which assumes that all the grains have the same volume fraction (discrete uniform distribution). Then, using the measurements of c-axis orientation and crystal size performed on the North Greenland Icecore Project (NorthGRIP) ice core, we compare area-weighted and equal-weighted fabrics. All these comparisons are made with respect to the orientation tensor. According to the ability of the Potts model to reproduce the ice microstructure, it is shown that using the grain cross-sectional area to infer its volume fraction improves the description of the actual polycrystal fabric.
The retreat of Jones Ice Shelf, Antarctic Peninsula
Authors: Fox Adrian J. , Vaughan David G.
Published: November 2007

In recent decades, several ice shelves along the Antarctic Peninsula have diminished in size as a result of climate warming. Using aerial photographic, satellite and survey data we document a similar retreat of Jones Ice Shelf, which was another small ice shelf on the west coast of the Antarctic Peninsula. This ice shelf was roughly stable between 1947 and 1969, but in the early 1970s it began to retreat and had completely disappeared by early 2003. Jones Ice Shelf has two ice fronts only a few kilometres apart and its retreat provides a unique opportunity to examine how different ice fronts retreat when subjected to similar climate forcing. We mapped the retreat of both the east and west ice fronts of Jones Ice Shelf and found that, although individual episodes of retreat may be related to particularly warm summers, the overall progress of retreat of the two ice fronts has been rather different. This suggests that in this case the course of retreat is controlled by the geometry of the embayment and location of pinning points as well as climatic events.
Fluctuation of the seasonal snowpack in a mountainous area of the heavy-snow district in the warm-temperate zone of Japan
Authors: Matsuura Sumio , Matsuyama Koji , Asano Shiho , Okamoto Takashi , Takeuchi Yoshitsugu
Published: November 2007

The seasonal snowpack and meteorological factors associated with the accumulation and ablation of the snowpack were monitored for 11 years in a mountainous area in the warm-temperate zone of Japan. No notable rise was observed in mean wintertime air temperature, but an increase was seen in the difference between the maximum and minimum air temperatures. Precipitation exhibited annual variability but no notable reduction over the measurement period. The length of the continuous snow-cover period increased slightly over the 11 years, but no trend in variability was observed. The maximum snow depth and maximum water equivalent of snow varied greatly from year to year, depending on the amount of snowfall. In a heavy-snow year, about 1600 mm of water, which is almost the mean annual precipitation for the whole of Japan, was found to be temporarily stored in the snowpack.
Glacier area changes in the Pumqu river basin, Tibetan Plateau, between the 1970s and 2001
Authors: Jin Rui , Li Xin , Che Tao , Wu Lizong , Mool Pradeep
Published: November 2007

Glacier area changes in the Pumqu river basin, Tibetan Plateau, between the 1970s and 2001 are analyzed, based on the Chinese Glacier Inventory and ASTER images. A new glacier inventory is obtained by visually interpreting the remote-sensing images and the digital elevation model. By comparing the two inventories, glacier area changes over the past 30 years are revealed. The results show that the area loss is about 9.0% and the shrinkage trend continues according to the meteorological data.
Application of micro-X-ray fluorescence to chemical mapping of polar ice
Authors: Fourcade M.C. Morel , Barnola J.M. , Susini J. , Baker R. , Durand G. , de Angelis M. , Duval P.
Published: November 2007

Synchrotron-based micro-X-ray fluorescence (μXRF) equipment has been used to analyze impurities in polar ice. A customized sample holder has been developed and the μXRF equipment has been adapted with a thermal control system to keep samples unaltered during analyses. Artificial ice samples prepared from ultra-pure water were analyzed to investigate possible contamination and/or experimental artefacts. Analyses of polar ice from Antarctica (Dome C and Vostok) confirm this μXRF technique is non-destructive and sensitive. Experiments can be reproduced to confirm or refine results by focusing on interesting spots such as crystal grain boundaries or specific inclusions. Integration times and resolution can be adjusted to optimize sensitivity. Investigation of unstable particles is possible due to the short analysis time. In addition to identification of elements in impurities, μXRF is able to determine their speciations. The accuracy and reliability of the results confirm the potential of this technique for research in glaciology.
The application of a non-linear back-propagation neural network to study the mass balance of Grosse Aletschgletscher, Switzerland
Authors: Steiner Daniel , Walter A. , Zumbühl H.J.
Published: November 2007

Glacier mass changes are considered to represent key variables related to climate variability. We have reconstructed a proxy for annual mass-balance changes in Grosse Aletschgletscher, Swiss Alps, back to AD 1500 using a non-linear back-propagation neural network (BPN). The model skill of the BPN performs better than reconstructions using conventional stepwise multiple linear regression. The BPN, driven by monthly instrumental series of local temperature and precipitation, provides a proxy for 20th-century mass balance. The long-term mass-balance reconstruction back to 1500 is based on a multi-proxy approach of seasonally resolved temperature and precipitation reconstructions (mean over a specific area) as input variables. The relation between the driving factors (temperature, precipitation) used and the reconstructed mass-balance series is discussed. Mass changes in Grosse Aletschgletscher are shown to be mainly influenced by summer (June–August) temperatures, but winter (December–February) precipitation also seems to contribute. Furthermore, we found a significant non-linear part within the climate–mass-balance relation of Grosse Aletschgletscher.
Hydrogeological analysis of slug tests in glacier boreholes
Authors: Kulessa Bernd , Hubbard Bryn , Williamson Mike , Brown Giles H.
Published: November 2007

Slug testing allows estimation of subglacial hydraulic properties by evaluating the response of a coupled borehole–subglacial flow system to an artificial displacement of its steady-state hydraulic head. However, existing models developed specifically for application to slug-test data collected in glacier boreholes are challenging to apply in practice. Here, we demonstrate that conventional linear methods, which are relatively readily applicable and widely used in groundwater studies, can also be used to estimate subglacial hydraulic properties. Overdamped, underdamped and critically damped slug-test data were recorded in fluctuating boreholes drilled to the bed of Haut Glacier d’Arolla, Valais, Switzerland. We find that non-linear effects in the data are negligible, supporting the application of conventional hydrogeological methods. Results suggest that the hydraulic conductivity of the unconsolidated sediments within the area influenced by a major subglacial drainage channel is enhanced (several 10–2 m s–1; typical of gravelly sand) compared to areas outside the zone of influence (~0.1 × 10–2ms–1; typical of silty sand). A distance to a flow boundary within the subglacial aquifer, inferred on the basis of slug-test analysis, was found to coincide with the actual location of the subglacial channel. Sensitivity analyses reveal that uncertainties in borehole and filter radii, as well as the storage coefficient of the subglacial aquifer, are more likely to affect the accuracy of the hydraulic and distance estimates than uncertainties regarding the repeatability of the slug-test responses and curve-fitting procedures involved in the conventional hydrogeological methods.
Influx of meltwater to subglacial Lake Concordia, East Antarctica
Authors: Tikku Anahita A. , Bell Robin E. , Studinger Michael , Clarke Garry K. C. , Tabacco Ignazio , Ferraccioli Fausto
Published: November 2007

We present evidence for melting at the base of the ice that overlies Lake Concordia, an 800 km2 subglacial lake near Dome Concordia, East Antarctica, via a combination of glaciohydraulic melting (associated with the tilted ice ceiling and its influence on lake circulation/melting temperature) and melting by extreme strain heating (where the ice sheet is grounded). An influx of water is necessary to provide nutrients, material and biota to support subglacial lake ecosystems but has not been detected previously. Freezing is the dominant observed basal process at over 60% of the surface area above the lake. The total volume of accreted ice above the lake surface is estimated as 50-60 km3, roughly 25-30% of the 200 ± 40 km3 estimated lake volume. Estimated rates of melting and freezing are very similar, ±2-6 mm a-1. The apparent net freezing may reflect the present-day response of Lake Concordia to cooling associated with the Last Glacial Maximum, or a large influx of water either via a subglacial hydrological system or from additional melting of the ice sheet. Lake Concordia is an excellent candidate for subglacial exploration given active basal processes, proximity to the Dome Concordia ice core and traverse resupply route.
Modelling the evolution of supraglacial lakes on the West Greenland ice-sheet margin
Authors: Lüthje M. , Pedersen L.T. , Reeh N. , Greuell W.
Published: November 2007

We present a model study investigating the summer evolution of supraglacial lakes on the Greenland ice margin. Using a one-dimensional (1-D) model we calculate the surface ablation for a bare ice surface and beneath supraglacial lakes for 30 days in the summers of 1999 and 2001. The surface ablation beneath the lake was enhanced by 110% in 1999 and 170% in 2001 compared with the ablation for bare ice. We then use the results from the 1-D model to further model the vertical and horizontal evolution of the supraglacial lakes, the results of which are compared with satellite images. Within the region of the ice sheet where supraglacial lakes presently occur, the area covered by supraglacial lakes is found to be more or less independent of the summer melt rate but controlled by topography. We therefore predict that, inside this region, the area covered by supraglacial lakes will remain constant even in a warmer climate. However, in a warmer climate, surface melting will occur higher on the ice sheet where small surface slopes favour formation of large supraglacial lakes. Enhanced surface melting beneath such lakes is a hitherto overlooked feedback mechanism related to climate warming.
Using electron backscatter diffraction patterns to examine recrystallization in polar ice sheets
Authors: Obbard Rachel , Baker Ian , Sieg Katherine
Published: November 2007

The fabric of polycrystalline ice is typically described using the c-axis orientation alone, but this is insufficient for a full description of grain orientations in this hexagonal material. Electron backscatter diffraction (EBSD) provides full c- and a-axis orientation of individual grains, and is used here to study Greenland Ice Sheet Project 2 (GISP2) ice specimens from depths of 1551, 1642 and 1745 m. Complete orientation data are used to compare nearest-neighbor relationships to overall fabric and to differentiate between recrystallization mechanisms. Changes in orientation between grains and subgrains in GISP2 specimens were correlated with the appearance of grain boundaries on thin sections and used to identify grain sets that were probably produced by polygonization. Comparison of grain misorientations that take into account both c- and a-axis differences with those derived from c-axis directions alone reveals the presence of polygonization and illustrates the usefulness of this technique.
Time-dependent basal stress conditions beneath Black Rapids Glacier, Alaska, USA, inferred from measurements of ice deformation and surface motion
Authors: Amundson Jason M. , Truffer Martin , Lüthi Martin P.
Published: November 2007

Observations of surface motion and ice deformation from 2002–03 were used to infer mean stress fields in a cross-section of Black Rapids Glacier, Alaska, USA, over seasonal timescales. Basal shear stresses in a well-defined zone north of the center line (orographic left) were approximately 7% and 16% lower in spring and summer, respectively, than in winter. Correspondingly higher stresses were found near the margins. These changes in the basal shear stress distribution were sufficiently large to cause mean surface velocities to be 1.2 and 1.5 times larger in spring and summer than in winter. These results were inferred with a simple inverse finite-element flow model that can successfully reproduce bulk surface velocities and tiltmeter data. Stress redistribution between the well-defined zone and the margins may also occur over much shorter time periods as a result of rapidly changing basal conditions (ice–bed decoupling or enhanced till deformation), thereby causing large variations in surface velocity and strongly influencing the glacier’s net motion during summer.
Local response of a glacier to annual filling and drainage of an ice-marginal lake
Authors: Walder Joseph S. , Trabant Dennis C. , Cunico Michelle , Fountain Andrew G. , Anderson Suzanne P. , Anderson Robert S. , Malm Andrew
Published: November 2007

Ice-marginal Hidden Creek Lake, Alaska, USA, outbursts annually over the course of 2–3 days. As the lake fills, survey targets on the surface of the ‘ice dam’ (the glacier adjacent to the lake) move obliquely to the ice margin and rise substantially. As the lake drains, ice motion speeds up, becomes nearly perpendicular to the face of the ice dam, and the ice surface drops. Vertical movement of the ice dam probably reflects growth and decay of a wedge of water beneath the ice dam, in line with established ideas about jökulhlaup mechanics. However, the distribution of vertical ice movement, with a narrow (50–100 m wide) zone where the uplift rate decreases by 90%, cannot be explained by invoking flexure of the ice dam in a fashion analogous to tidal flexure of a floating glacier tongue or ice shelf. Rather, the zone of large uplift-rate gradient is a fault zone: ice-dam deformation is dominated by movement along high-angle faults that cut the ice dam through its entire thickness, with the sense of fault slip reversing as the lake drains. Survey targets spanning the zone of steep uplift gradient move relative to one another in a nearly reversible fashion as the lake fills and drains. The horizontal strain rate also undergoes a reversal across this zone, being compressional as the lake fills, but extensional as the lake drains. Frictional resistance to fault-block motion probably accounts for the fact that lake level falls measurably before the onset of accelerated horizontal motion and vertical downdrop. As the overall fault pattern is the same from year to year, even though ice is lost by calving, the faults must be regularly regenerated, probably by linkage of surface and bottom crevasses as ice is advected toward the lake basin.
Improved method to determine radio-echo sounding reflector depths from ice-core profiles of permittivity and conductivity
Authors: Eisen Olaf , Wilhelms Frank , Steinhage Daniel , Schwander Jakob
Published: November 2007

We present a technique that modifies and extends down-hole target methods to provide absolute measures of uncertainty in radar-reflector depth of origin. We use ice-core profiles to model wave propagation and reflection, and then cross-correlate the model results with radio-echo sounding (RES) data to identify the depth of reflector events. Stacked traces recorded with RES near the EPICA drill site in Dronning Maud Land, Antarctica, provide reference radargrams, and dielectric properties along the deep ice core form the input data to a forward model of wave propagation that produces synthetic radargrams. Cross-correlations between synthetic and RES radargrams identify differences in propagation wave speed. They are attributed to uncertainties in pure-ice permittivity and are used for calibration. Removing conductivity peaks results in the disappearance of related synthetic reflections and enables the unambiguous relation of electric signatures to RES features. We find that (i) density measurements with g-attenuation or dielectric profiling are too noisy below the firn–ice transition to allow clear identification of reflections, (ii) single conductivity peaks less than 0.5 m wide cause the majority of prominent reflections beyond a travel time of about 10 µs (~900m depth) and (iii) some closely spaced conductivity peaks within a range of 1–2m cannot be resolved within the RES or synthetic data. Our results provide a depth accuracy to allow synchronization of age–depth profiles of ice cores by RES, modeling of isochronous internal structures, and determination of wave speed and of pure-ice properties. The technique successfully operates with dielectric profiling and electrical conductivity measurements, suggesting that it can be applied at other ice cores and drill sites.
In situ measurements of till deformation and water pressure
Authors: Truffer Martin , Harrison William D.
Published: November 2007

A newly developed hammer was used to insert two autonomous probes 0.8 m and 2.1 m into clast-rich subglacial till under Black Rapids Glacier, Alaska, USA. Both probes were instrumented with a dual-axis tilt sensor and a pore-water pressure transducer. The data are compared to a 75 day record of surface velocities. Till deformation at depth was found to be highly seasonal: it is significant during an early-season speed-up event, but during long periods thereafter measured till deformation rates are negligible. Both tilt records show rotation around the probe axis, which indicates a change in tilt direction of about 30°. The tilt records are very similar, suggesting spatial homogeneity on the scale of the probe separation (4 m horizontal and 3.3 m vertical). There is evidence that during much of the year sliding of ice over till or deformation of a thin till layer (<20 cm) accounts for at least two-thirds of total basal motion. Basal motion accounts for 50–70% of the total surface motion. The inferred amount of ice–till sliding is larger than that found at the same location in a previous study, when surface velocities were about 10% lower. We suggest that variations in ice–till coupling account for the observed variations in mean annual speed.
Dynamics of tidewater glaciers: comparison of three models
Authors: Nick F.M. , Oerlemans J.
Published: November 2007

A minimal model of a tidewater glacier based solely on mass conservation is compared with two one-dimensional numerical flowline models, one with the calving rate proportional to water depth, and the other with the flotation criterion as a boundary condition at the glacier terminus. The models were run with two simplified bed geometries and two mass-balance formulations. The models simulate the full cycle of length variations and the equilibrium states for a tidewater glacier. This study shows that the branching of the equilibrium states depends significantly on the bed geometry. The similarity between the results of the three models indicates that if there is a submarine undulation at the terminus of a tidewater glacier, any model in which the frontal ice loss is related to the water depth yields qualitatively the same non-linear behaviour. For large glaciers extending into deep water, the flotation model causes unrealistic behaviour.
Thermo-erosional notch development at fresh-water-calving Tasman Glacier, New Zealand
Authors: Röhl Katrin
Published: November 2007

Controls on glacier calving rates are receiving increased scientific interest. At fresh-watercalving glaciers, limnological factors might be more important than glaciological ones. Measurements of thermo-erosional notch development at the calving ice cliff of Tasman Glacier, New Zealand, suggest that the calving rates at this glacier are directly controlled by the rate of thermal undercutting. Notch formation rates typically vary between 10 and 30 cm d–1 (maximum rate 65cmd–1) in summer, corresponding to an average calving rate of 34 m a–1. Notch formation is slower than waterline melt and is controlled by water temperatures and circulation, cliff geometry, debris supply and water-level fluctuations. The latter shift the position of undercutting, resetting the level of the notch formation process and thereby slowing it. The geometry of the notch and the debris supply determine the extent of influence of the lake on notch water temperatures and circulation. Hence, water temperatures in the lake are not necessarily indicative of the rate of notch formation. The prediction of rate of notch formation from far-field variables is hampered by the complex interaction of the influencing factors. The significance of thermal undercutting as a calving rate-controlling process decreases with increasing ice velocities, calving rates and surface gradients.
Glacier change (1958–1998) in the North Cascades National Park Complex, Washington, USA
Authors: Granshaw Frank D. , G. Fountain Andrew
Published: November 2007

The spatial characteristics for all glaciers in the North Cascades National Park Complex, USA, were estimated in 1958 and again in 1998. The total glacier area in 1958 was 117.3 ± 1.1 km2; by 1998 the glacier area had decreased to 109.1 ± 1.1 km2, a reduction of 8.2 ± 0.1 km2 (7%). Estimated volume loss during the 40 year period was 0.8 ± 0.1 km3 of ice. This volume loss contributes up to 6% of the August–September stream-flow and equals 16% of the August–September precipitation. No significant correlations were found between magnitude of glacier shrinkage and topographic characteristics of elevation, aspect or slope. However, the smaller glaciers lost proportionally more area than the larger glaciers and had a greater variability in fractional change than larger glaciers. Most of the well-studied alpine glaciers are much larger than the population median, so global estimates of glacier shrinkage, based on these well-studied glaciers, probably underestimate the true magnitude of regional glacier change.