bluuebunny/crossref_metadata_2025_split huggingface dataset:
The distribution of basal motion beneath a High Arctic polythermal glacier
Authors: Copland Luke , Sharp Martin J. , Nienow Peter , Bingham Robert G. 
Published: November 2007

AbstractThe longitudinal pattern of surface velocity of a large, predominantly cold, polythermal glacier (John Evans Glacier, Ellesmere Island, Canada) was measured over summer and winter periods. In the accumulation and upper ablation areas, where ice is predominantly cold-based, summer velocities were slightly higher than overwinter velocities. Predicted velocities due to ice deformation alone in these areas closely matched these observations in the winter, with limited basal motion likely in the summer. In the lower ablation area, where ice is likely warm-based, measured summer velocities were up to double overwinter velocities. Predicted ice deformation could not account for all of these measured velocities in either summer or winter. This suggests that basal motion occurs throughout the year over at least part of the lower ablation area. This finding is supported by radio-echo sounding, subglacial drainage reconstructions and analyses of early-summer meltwater chemistry, which suggest that subglacial water is present throughout the year in this region. In summer, basal motion may account for up to 75% of the total surface velocity throughout the lower ablation area. The inferred rate of basal motion increases sharply directly below a set of moulins by which most surface meltwater reaches the glacier bed.
A simple visualization method for distinguishing subglacial-bed and side-wall returns in radio-echo records from outlet and valley glaciers
Authors: Benham Toby J. , Dowdeswell Julian A. 
Published: November 2007

AbstractRadar returns from valley side-walls may complicate the identification of the glacier bed when examining airborne radio-echo sounding (RES) records. We describe simple visualization techniques for aiding analysis of RES data with the aim of resolving these ambiguities, making use of high-resolution satellite imagery. An application of the technique is illustrated using RES data obtained from part of Stygge Glacier, Ellesmere Island, Nunavut, Canada.
A laboratory study of the retarding effects of braking mounds on snow avalanches
Authors: Hákonardóttir Kristín Martha , Hogg Andrew J. , Jóhannesson Tómas , Tómasson Gunnar G. 
Published: November 2007

AbstractA series of laboratory experiments in a 6 m long chute using glass particles of mean diameter 100 μm were performed to investigate the interaction of a supercritical, granular flow with obstacles. It was found that the collision of the flow with a row of mounds led to the formation of a jet, whereby a large fraction of the flow was launched from the top of the mounds and subsequently landed back on the chute. The retarding effect of the mounds was investigated quantitatively by direct measurements of the velocity of the flow, its runout length and the geometry of the jet. The effects of several aspects of the layout of the mounds on their retarding effects were examined. It was observed that a row of steep mounds with an elongated shape in the transverse direction to the flow and with a height several times the flow depth led to dissipation of a large proportion of the kinetic energy of the flow.
A groove-ploughing theory for the production of mega-scale glacial lineations, and implications for ice-stream mechanics
Authors: Clark Chris D. , Tulaczyk Slawek M. , Stokes Chris R. , Canals Miquel 
Published: November 2007

AbstractMega-scale glacial lineations (MSGLs) are longitudinally aligned corrugations (ridge–groove structures 6–100 km long) in sediment produced subglacially. They are indicators of fast flow and a common signature of ice-stream beds. We develop a qualitative theory that accounts for their formation, and use numerical modelling, and observations of ice-stream beds to provide supporting evidence. Ice in contact with a rough (scale of 10–103 m) bedrock surface will mimic the form of the bed. Because of flow acceleration and convergence in ice-stream onset zones, the ice-base roughness elements experience transverse strain, transforming them from irregular bumps into longitudinally aligned keels of ice protruding downwards. Where such keels slide across a soft sedimentary bed, they plough through the sediments, carving elongate grooves, and deforming material up into intervening ridges. This explains MSGLs and has important implications for ice-stream mechanics. Groove ploughing provides the means to acquire new lubricating sediment and to transport large volumes of it downstream. Keels may provide basal drag in the force budget of ice streams, thereby playing a role in flow regulation and stability. We speculate that groove ploughing permits significant ice-stream widening, thus facilitating high-magnitude ice discharge.
New insights into the subglacial and periglacial hydrology of Vatnajökull, Iceland, from a distributed physical model
Authors: Flowers Gwenn E. , Björnsson Helgi , Pálsson Finnur 
Published: November 2007

AbstractWe apply a time-dependent distributed glaciohydraulic model to Vatnajökull ice cap, Iceland, aiming to determine the large-scale subglacial drainage structure, the importance of basally derived meltwater, the influence of a permeable glacier bed and Vatnajökull’s discharge contribution to major rivers in Iceland. The model comprises two coupled layers that represent the subglacial horizon perched on a subsurface aquifer in the western sector and bedrock in the eastern sector. To initialize and drive the simulations, we use digital elevation models of the ice surface and bed, the 1999/2000 measured mass balance and an estimate of subglacial geothermal heat fluxes. The modelled subglacial flow field differs substantially from that derived by hydraulic-potential calculations, and the corresponding distribution of basal effective pressure shows a strong correlation between low effective pressure and surge-prone areas in northeastern and southern sectors of Vatnajökull. Simulations suggest that geothermally derived basal melt may account for up to ∼5% of the annual glacial discharge, and buried aquifers may evacuate up to ∼30% of subglacialwater.Time-dependent tests yield estimates of the glacial discharge component in various outlet rivers and suggest a possible seasonal migration of subglacial hydraulic divides. This study of present-day Vatnajökull hydrology forms the starting point for investigations of its future evolution.
On the Clague–Mathews relation for jökulhlaups
Authors: NG Felix , Björnsson Helgi 
Published: November 2007

AbstractIn the empirical study of jökulhlaups, the peak discharge, Qmax, and water volume drained by the ice-dammed lake during the floods, Vt, appear to follow a power-law relation , where K are b are constants determined from field data. First identified by Clague and Mathews (1973), this relation is a useful reference for predicting flood magnitude, but its physical origin remains unclear. Here, we develop the theory that connects it to contemporary models for simulating the flood hydrograph. We explain how the function Qmax = f(Vt) arises from Nye’s (1976) theory of time-dependent water flow in a subglacial channel coupled to a lake, and we describe how discharge–volume data record the (monotonically increasing) form of this function so long as the lake is not emptied in the floods. The Grímsvötn jökulhlaups present an example where, because of partial draining of the lake, agreement between the model-derived f and data is excellent. It is documented that other lake systems drain completely, but we explain how the exponent b ≈ 2/3 observed for them collectively is due primarily to a scaling effect related to their size, modified by other factors such as the flood initiation process.
Catastrophic ice-shelf break-up by an ice-shelf-fragment-capsize mechanism
Authors: MacAyeal Douglas R. , Scambos Ted A. , Hulbe Christina L. , Fahnestock Mark A. 
Published: November 2007

AbstractTwo disintegration events leading to the loss of Larsen A and B ice shelves, Antarctic Peninsula, in 1995 and 2002, respectively, proceeded with extreme rapidity (order of several days) and reduced an extensive, seemingly integrated ice shelf to ajumble of small fragments. These events strongly correlate with warming regional climate and accumulation of surface meltwater, supporting a hypothesis that meltwater-induced propagation of pre-existing surface crevasses may have initiated ice-shelf fragmentation.We address here an additional, subsequent mechanism that may sustain and accelerate the ice-shelf break-up once it begins. The proposed mechanism involves the coherent capsize of narrow (less than thickness) ice-shelf fragments by rolling 90° in a direction toward, or away from, the ice front. Fragment capsize liberates gravitational potential energy, forces open ice-shelf rifts and contributes to further fragmentation of the surrounding ice shelf.
Triaxial tests on snow at low strain rate. Part II. Constitutive behaviour
Authors: Scapozza Carlo , Bartelt Perry 
Published: November 2007

AbstractFine-grained, dry snow with a density range of 190–435 kg m−3 was tested in triaxial compression at −12°C with confining pressures varying between 0 and 40 kPa. The tests were strain-rate controlled, with strain rates ranging between 7.4 × 10−7 s−1 and 6.6 × 10−5 s−1. The analysis of the test results revealed that the relationship between yield stress and viscous strain rate is best given by a power law, similar to polycrystalline ice. However, the power-law exponent n is a function of density and varies between 1.8 (low-density snow, ρ < 200 kg m−3) and 3.6 (high-density snow, ρ > 320 kg m−3). The tests also showed that lower-density snow displays a significant non-linear stress–strain response before yielding. Two further aspects of the constitutive behaviour of snow were identified: (1) the strainrate independence of the post-yield work-hardening behaviour in compression and (2) the independence of the axial yield stress in relation to the confining pressure. The experimental observations are discussed with respect to the mechanical properties of polycrystalline ice, which is the constituent material of the load-bearing ice skeleton.
Glacier changes since the Little Ice Age maximum in the western Qilian Shan, northwest China, and consequences of glacier runoff for water supply
Authors: Shiyin Liu , Wenxin Sun , Yongping Shen , Gang Li 
Published: November 2007

AbstractBased on aerial photographs, topographical maps and the Landsat-5 image data, we have analyzed fluctuations of glaciers in the western Qilian Shan, north-west China, from the Little Ice Age (LIA) to 1990. The areas and volumes of glaciers in the whole considered region decreased 15% and 18%, respectively, from the LIA maximum to 1956. This trend of glacier shrinkage continued and accelerated between 1956 and 1990. These latest decreases in area and volume were about 10% in 34 years. The recent shrinkage may be due either to a combination of higher temperatures and lower precipitation during the period 1956–66, or to continuous warming in the high glacierized mountains from 1956 to 1990. As a consequence, glacier runoff from ice wastage between 1956 and 1990 has increased river runoff by 6.2 km3 in the four river basins under consideration. Besides, the equilibrium-line altitude (ELA) rise estimated from the mean terminus retreat of small glaciers <1 km long is 46 m, which corresponds to a 0.3°C increase of mean temperatures in warm seasons from the LIA to the 1950s.
Evidence for abundant subglacial meltwater beneath the paleo-ice sheet in Pine Island Bay, Antarctica
Authors: Lowe Ashley L. , Anderson John B. 
Published: November 2007

AbstractMarine-geological and -geophysical data collected from the continental shelf in Pine Island Bay, Antarctica, reveal a complex paleo-subglacial drainage system controlled by bedrock topography and subglacial meltwater discharge. Significant amounts of freely flowing meltwater existed beneath former ice sheets in Pine Island Bay. Subglacial drainage is characterized by descriptions of glacial landforms imaged on the sea floor and sedimentary deposits collected in piston cores. Bedrock geology is characterized using seismic data. Large-scale landforms on the shelf include channels and cavities incised into impermeable crystalline bedrock. There is a transition from randomly oriented channels on the inner shelf to a dendritic pattern of elongate channels on the middle shelf. On the outer shelf, a change in basal conditions occurs where sedimentary deposits bury crystalline bedrock. No evidence for flowing meltwater exists on sedimentary substrates. Instead, meltwater formed at the ice–sediment contact was incorporated into the sediments, contributing to development of a deforming bed, which was sampled in piston cores. Characterization of subglacial meltwater processes that occurred in the past may aid in understanding the role meltwater plays in stability of the West Antarctic ice sheet today.
Spatial and temporal variations in albedo on Storglaciären, Sweden
Authors: Jonsell Ulf , Hock Regine , Holmgren Björn 
Published: November 2007

AbstractExtensive albedo data from Storglaciären, Sweden, during nine summers are analyzed, focusing on the effect of surface slope on measurements and on the influence of clouds on albedo of both snow and ice surfaces. On clear-sky days, albedo continuously dropped throughout the day by > 0.3 when derived from measurements in a horizontal plane over the slightly sloping surface. When we correct for the tilt effect, over frozen surfaces the apparent decrease in albedo largely disappeared, while over melting surfaces it became significantly less pronounced. This emphasizes the need to correct horizontally measured data for slope when they are used, for example, as ground truth for satellite-derived albedo, or in high-resolution glacier melt runoff models. Albedo varied by > 0.1 due to cloud fluctuations. To quantify cloud effects, albedo changes between successive (half-)hourly and daily data were regressed against corresponding changes in cloudiness expressed by the ratio of global radiation to top-of-atmosphere solar radiation. The resulting relationship over snow explains 66% of oberved albedo changes using hourly, and 42% using daily, time-steps. Over ice, the corresponding values are 28% and 0%. Hence, cloudiness is a dominating factor in determining short-term variability of albedo over snow, whereas over ice the observed large day-to-day variability ( > 20%) is mainly attributed to changes in the physical characteristics of the weathering crust. Results suggest that cloud effects should be incorporated into models treating snow and ice surfaces separately.
Ice dynamics preceding catastrophic disintegration of the floating part of Jakobshavn Isbræ, Greenland
Authors: Johnson Jesse V. , Prescott Paul R. , Hughes Terence J. 
Published: November 2007

AbstractThe floating terminal of Jakobshavn Isbræ, the fastest Greenland ice stream, has disintegrated since 2002, resulting in a doubling of ice velocity and rapidly lowering inland ice elevations. Conditions prior to disintegration were modeled using control theory in a plane-stress solution, and the Missoula model of ice-shelf flow. Both approaches pointed to a mechanism that inhibits ice flow and that is not captured by either approach. Jamming of flow, an inherent property of granular materials passing through a constriction (Jakobshavn Isfjord), is postulated as the mechanism. Rapid disintegration of heavily crevassed floating ice accompanies break-up of the ice jam.
A study of the surface mass balance in Dronning Maud Land, Antarctica, using automatic weather stationS
Authors: Van Den Broeke Michiel R. , Reijmer Carleen H. , Van De Wal Roderik S.W. 
Published: November 2007

AbstractWe use data from four automatic weather stations (AWSs) in Dronning Maud Land, East Antarctica, to study the surface mass balance and its components. Distinct differences were found between the moisture climates of the high plateau, the katabatic wind zone and the coastal ice shelves: significant undersaturation occurs year-round in the katabatic wind zone, while on the high plateau and on the coastal ice shelf the air is usually close to saturation. In summer, absorption of shortwave radiation at the snow surface enhances surface sublimation at all sites, removing 3-9% of the annual solid precipitation. Significant summer melting is an equally important ablation term near the coast, but vanishes inland. Vertically integrated column drifting-snow sublimation was estimated using two different methods. This process appears to be similar to or greater in magnitude than surface sublimation. Because intervals between significant precipitation events may last as long as several months, sublimation and melt cause extended periods of surface ablation in summer. In summer, all ablation processes together remove 15-56% of the solid precipitation, or 6-27% on an annual basis.
Influence of subglacial Vostok lake on the regional ice dynamics of the Antarctic ice sheet: a model study
Authors: Pattyn Frank , De Smedt Bert , Souchez Roland 
Published: November 2007

AbstractWe applied a newly developed three-dimensional, time-dependent, thermomechanical icesheet model including higher-order stress gradients, to simulate the ice flow across subglacial Vostok lake, East Antarctica. Simulations of both ‘lake’ and ‘no lake’ conditions (by treating the ice/lake interface as a stress-free surface, similar to an ice shelf) demonstrate the effects of the subglacial lake, such as pronounced surface flattening and ice-flow turning, on the overall ice dynamics in the vicinity, although subglacial lake dynamics are not treated explicitly. When buoyancy forces and hydrostatic equilibrium of the ice above the lake are taken into account, the along-lake surface slope is preserved and the ice-flow pattern is in accord with sparse observations. Model experiments point to a local ice speed-up in the northern part of the lake, which can be associated with the onset of an enhanced ice-flow feature, more precisely the onset of the Totten Glacier catchment.
A level-set method for modeling the evolution of glacier geometry
Authors: Pralong Antoine , Funk Martin 
Published: November 2007

AbstractA level-set method is proposed for modeling the evolution of a glacier surface subject to a prescribed mass balance. This leads to a simple and versatile approach for computing the evolution of glaciers: the description of vertical fronts and overriding phenomena presents no difficulties, topological changes are handled naturally and steady-state solutions can be calculated without integration over time. A numerical algorithm is put forth as a means of solving the proposed model of glacier surface evolution. It is evaluated by comparing different numerical solutions of the model with analytical and published numerical solutions. The level-set method appears to be a reliable approach for dealing with different glaciological problems.
Short-term variations in glacier flow controlled by subglacial water pressure at Lauteraargletscher, Bernese Alps, Switzerland
Authors: Sugiyama Shin , Gudmundsson G. Hilmar 
Published: November 2007

AbstractShort-term variations in horizontal and vertical surface motion were studied with high temporal resolution during the ablation season in Lauteraargletscher, Bernese Alps, Switzerland. Horizontal surface flow speed oscillated diurnally, showing a correlation with the water level in a borehole. Flow speed increased as a function of the water level, with an asymptote at the ice overburden level. This observation implied that the flow variations were principally controlled by the local water pressure which enhanced basal motions. Detailed examination of the diurnal variations, however, showed that the speed was larger when the pressure was increasing than when it was decreasing. Greater speed with increasing pressure was interpreted by subglacial water-cavity opening and/or longitudinal stress coupling with the upper reaches of the glacier. Upward surface movements were observed when the glacier flow speed increased. Simultaneous measurement of internal vertical strain in a borehole showed that the uplift had two different sources: vertical straining of ice and volume increase of subglacial water cavities. The vertical surface movement was largely affected by the vertical strain, and the uplift events could not be simply attributed to cavity opening.
Comparison of geodetic and glaciological mass-balance techniques, Gulkana Glacier, Alaska, U.S.A.
Authors: Cox Leif H. , March Rod S. 
Published: November 2007

AbstractThe net mass balance on Gulkana Glacier, Alaska, U.S.A., has been measured since 1966 by the glaciological method, in which seasonal balances are measured at three index sites and extrapolated over large areas of the glacier. Systematic errors can accumulate linearly with time in this method. Therefore, the geodetic balance, in which errors are less time-dependent, was calculated for comparison with the glaciological method. Digital elevation models of the glacier in 1974, 1993 and 1999 were prepared using aerial photographs, and geodetic balances were computed, giving – 6.0 ± 0.7 m w.e. from 1974 to 1993 and -11.8 ± 0.7 m w.e. from 1974 to 1999. These balances are compared with the glaciological balances over the same intervals, which were – 5.8 ± 0.9 and -11.2 ± 1.0 m w.e. respectively; both balances show that the thinning rate tripled in the 1990s. These cumulative balances differ by <6%. For this close agreement, the glaciologically measured mass balance of Gulkana Glacier must be largely free of systematic errors and be based on a time-variable area-altitude distribution, and the photography used in the geodetic method must have enough contrast to enable accurate photogrammetry.
Dynamics of crystal formation in the Greenland NorthGRIP ice core
Authors: Mathiesen Joachim , Ferkinghoff-Borg Jesper , Jensen Mogens H. , Levinsen Mogens , Olesen Poul , Dahl-Jensen Dorthe , Svensson Anders 
Published: November 2007

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