bluuebunny/crossref_metadata_2025_split huggingface dataset:
A geometric glacier model for sea-level change calculations
Authors: Raper S. C. B. , Brown O. , Braithwaite R. J. 
Published: November 2007

AbstractTowards accounting for the dynamic response of glaciers and ice caps in the estimation of their contribution to sea-level rise due to global warming, a mass-balance degree-day model is coupled to a geometric glacier model. The ice dynamics are treated implicitly in the geometric model by using scaling parameters that have been extensively investigated in the literature. The model is tested by presenting a case-study of the glacier Hintereisferner, Austrian Alps. The results are compatible with geomorphological data and other modelling studies. An estimate is made of the volume decrease due to initial disequilibrium. An extensive sensitivity study using generalized glacier shapes and sizes allows a comparison of results with dynamic theory. According to the geometric model, glaciers with a narrowing channel change more with a change in mass balance than glaciers with a widening channel, due to their shape and the way in which that shape changes with a changing climate. Also their response time is longer. As time progresses after a mass-balance perturbation, the longer response time for continental glaciers compared to glaciers with larger mass turnover offsets the effect of their smaller static sensitivity. Thus, although for the next century we may expect greater changes in volume from alpine glaciers, the equilibrium or committed change is greater for the continental glaciers.
Evaluation of remote-sensing techniques to measure decadal-scale changes of Hofsjökull ice cap, Iceland
Authors: Hall D. K. , Williams R. S. , Barton J. S. , Sigurđsson O. , Smith L. C. , Garvin J. B. 
Published: November 2007

AbstractDynamic surficial changes and changes in the position of the firn line and the areal extent of Hofsjökull ice cap, Iceland, were studied through analysis of a time series (1973–98) of synthetic-aperture radar (SAR) and Landsat data. A digital elevation model of Hofsjökull, which was constructed using SAR interferometry, was used to plot the SAR backscatter coefficient (σ°) vs elevation and air temperature along transects across the ice cap. Seasonal and dailyσ° patterns are caused by freezing or thawing of the ice-cap surface, and abrupt changes inσ° are noted when the air temperature ranges from ∼−5° to 0°C. Late-summer 1997σ° (SAR) and reflectance (Landsat) boundaries agree and appear to be coincident with the firn line and a SARσ° boundary that can be seen in the January 1998 SAR image. In January 1994 through 1998, the elevation of thisσ° boundary on the ice cap was quite stable, ranging from 1000 to 1300 m, while the equilibrium-line altitude, as measured on the ground, varied considerably. Thus the equilibrium line may be obscured by firn from previous years. Techniques are established to measure long-term changes in the elevation of the firn line and changes in the position of the ice margin.
Characteristics of a crater glacier at Ushkovsky volcano, Kamchatka, Russia, as revealed by the physical properties of ice cores and borehole thermometry
Authors: Shiraiwa Takayuki , Murav’yev Yaroslav D. , Kameda Takao , Nishio Fumihiko , Toyama Yoko , Takahashi Akiyoshi , Ovsyannikov Alexander A. , Salamatin Andrey N. , Yamagata Kotaro 
Published: November 2007

AbstractA glacier at the summit of Ushkovsky volcano, Kamchatka peninsula, Russia, was studied in order to obtain information about the physical characteristics of a glacier that fills a volcanic crater. The glacier has a gentle surface and a concave basal profile with a maximum measured depth of 240 m at site K2. The annual accumulation rate was 0.54 m a−1 w.e., and the 10 m depth temperature was −15.8°C. A 211.70 m long ice core drilled at K2 indicates that (1) the site is categorized as a percolation zone, (2) the stress field in the glacier changes at 180 m depth from vertical and longitudinal compression with transversal extension, which is divergent flow, to a shear-dominated stress field, and (3) the frequent occurrence of ash layers can be a good tool for dating the ice core. The borehole temperature profiles were considered to be non-stationary, but the linear profile made it possible to estimate the basal temperature and the geothermal heat flux at K2. Assuming constant surface and the basal boundary conditions, we constructed two depth–age relationships at K2. These predicted that the bottom ages of the ice core were about 511 or 603 years.
Distributed shear of subglacial till due to Coulomb slip
Authors: Iverson Neal R. , Iverson Richard M. 
Published: November 2007

AbstractIn most models of the flow of glaciers on till beds, it has been assumed that till behaves as a viscoplastic fluid, despite contradictory evidence from laboratory studies. In accord with this assumption, displacement profiles measured in subglacial till have been fitted with viscoplastic models by estimating the stress distribution. Here we present a model that illustrates how observed displacement profiles can result from till deformation resisted solely by Coulomb friction. Motion in the till bed is assumed to be driven by brief departures from static equilibrium caused by fluctuations in effective normal stress. These fluctuations result from chains of particles that support intergranular forces that are higher than average and that form and fail at various depths in the bed during shearing. Newton’s second law is used to calculate displacements along slip planes and the depth to which deformation extends in the bed. Consequent displacement profiles are convex upward, similar to those measured by Boulton and colleagues at Breidamerkurjökull, Iceland. The model results, when considered together with the long-term and widespread empirical support for Coulomb models in soils engineering, indicate that efforts to fit viscoplastic flow models to till displacement profiles may be misguided.
Ice-divide flow at Hans Tausen Iskappe, North Greenland, from surface movement data
Authors: Hvidberg C. S. , Keller K. , Gundestrup N. , Jonsson P. 
Published: November 2007

AbstractSurface strain rates around the southeastern dome of Hans Tausen Iskappe in Peary Land, North Greenland (82.5° N, 27.5° W), are determined from global positioning system surveys of a strain net. Average longitudinal surface strain rate increases towards the dome, from (1.4 ± 0.2) × 10−4a−1at 5–10 ice thicknesses from the divide to (2.4 ± 1.0) × 10−4a−1within 1 ice thickness from the divide. Analysis of the data shows that the ice cap is presently building up within the strain net with an average rate of 〈∂H/∂t〉 = + 0.04 ± 0.02 m a−1. Assuming a uniform thickening, the shape factor of the horizontal velocity (the ratio between the vertically averaged horizontal velocity and the horizontal surface velocity) decreases towards the dome, from 0.9 at a distance of 10 ice thicknesses from the dome to 0.5 at the dome based on application of the continuity equation. Our results indicate that a region with anomalous flow is formed around the dome, supporting recent indications reported by Vaughan and others (1999). It is not possible from our data to constrain parameters of the flow law, because there is no independent estimate of the significant present thickening of the central part of the ice cap and its pattern around the dome.
Mechanisms of fast flow in Jakobshavn Isbræ, West Greenland: Part III. Measurements of ice deformation, temperature and cross-borehole conductivity in boreholes to the bedrock
Authors: Lüthi Martin , Funk Martin , Iken Almut , Gogineni Shivaprasad , Truffer Martin 
Published: November 2007

AbstractAt a site on the ice sheet adjacent to the Jakobshavn ice stream in West Greenland, ice deformation rates and temperatures have been measured in boreholes to the bedrock at 830 m depth. Enhanced deformation rates were recorded just below the Holocene–Wisconsin transition at 680 m depth. A 31 m layer of temperate ice and the temperature minimum of −22°C at 520 m depth were detected. The good agreement of these data with results of a two-dimensional thermomechanically coupled flow model implies that the model input is adequate. Discrepancies between modelled and measured temperature profiles on a flowline at the ice-stream centre have been attributed to effects not accounted for by the model. We have suggested that the convergent three-dimensional flow leads to a vertical extension of the basal ice entering the stream. A thick basal layer of temperate and Wisconsin ice would explain the fast flow of this ice stream. As a test of this hypothesis, the new core-borehole conductivity (CBC) method has been used to compare conductivity sequences from the ice stream to those of the adjacent ice sheet. The correlation thus inferred suggests that the lowest 270 m of the ice sheet correspond to the lowermost 1700 m of the stream, and, consequently, that the lower part of the ice stream has experienced a very large vertical extension.
Airborne-radar stratigraphy and electrical structure of temperate firn: Bagley Ice Field, Alaska, U.S.A.
Authors: Arcone Steven A. 
Published: November 2007

AbstractHelicopter-borne 135 MHz short-pulse radar profiles of the Bagley Ice Field, southern Alaska, U.S.A., have been analyzed to determine the nature of radio-wave propagation through crevassed, stratified, wet and draining firn. Diffractions characterize the firn horizons along the trunk, and reflections characterize those of overlying snow-fields. Dynamic stacking was used to form and determine firn depths, and unstacked diffraction analysis to determine firn-layer properties. Refractive indices range from 4.1 to 4.5 for the near-surface, from 1.7 to 2.1 at about 13–17 m depth along the main trunk, and to 2.6 to 58 m depth within a snowfield. Average trunk values correspond with volumetric water contents of about 0.09, which agree with values measured for other glaciers. The analysis of the airborne-recorded diffractions is confirmed by deriving the approximate refractive index of water from a supraglacial lake-bottom diffraction. The lack of snowfield diffractions suggests that crevassing and not firn structure caused the trunk diffractions. The reasonable values of the indices imply that the diffractions originated from single points or edges orthogonal to the profile transect, and they predict low interlayer transmission losses. The snowfield penetration suggests that several hundred meters might be penetrated in uncrevassed accumulation zones with improved system design.
Causes of glacier change in the source regions of the Yangtze and Yellow rivers on the Tibetan Plateau
Authors: Jianping Yang , Yongjian Ding , Rensheng Chen , Shiyin Liu , Anxin Lu 
Published: November 2007

AbstractGlaciers are an important element of the environment in the source regions of the Yangtze and Yellow rivers on the Tibetan Plateau. Using Geographical Information System techniques, we have studied changes in the location of glacier margins in two areas: the Geladandong area in the headwaters of the Yangtze, and the A’nyêmaqên Shan mountains in the headwaters of the Yellow River. Marginal positions during the Little Ice Age (LIA) maximum, in 1969 in the Geladandong area, in 1966 in the A’nyêmaqên Shan, and in 2000 in both areas, were determined using aerial photographs, satellite images, topographical maps and digital elevation models. Extrapolating the results to the entire source regions of the Yangtze and Yellow rivers, we estimate that the total glacierized area decreased about 1.7% between 1969 and 2000 in the Geladandong area and about 17% between 1966 and 2000 in the A’nyêmaqên Shan. Glaciers were stable or advanced slightly between 1969 and 1995 in the Geladandong area, and between 1966 and 1981 in the A’nyêmaqên Shan, but have retreated since the mid-1990s in the former and since the 1980s in the latter. Significant increases in summer air temperature and decreases in annual precipitation are the causes of the present retreat. As a consequence of the retreat, water storage, as ice, in the Yangtze and Yellow river drainages is decreasing by 65–70 × 106 m3 a−1.
Glacier surge mechanisms inferred from ground-penetrating radar: Kongsvegen, Svalbard
Authors: Woodward John , Murray Tavi , Clark Roger A. , Stuart Graham W. 
Published: November 2007

AbstractDeformational structures at the surge-type glacier Kongsvegen, Svalbard, are displayed at the glacier surface and on a grounded cliff section at the terminus. A 300 m × 65 m grid of 200 MHz ground-penetrating radar (GPR) profiles has been collected adjacent to the cliff section in order to identify englacial structure.Two sub-horizontal reflectors have been imaged; the upper is interpreted as the glacier bed, and represents a transition between glacier ice and frozen subglacial sediments; while the lower is interpreted as a transition between frozen and unfrozen subglacial sediment. Dipping reflectors, corresponding to sediment-filled features on the cliff and glacier surface, do not cross the glacier bed. A small number of reflectors, interpreted as thrust faults, are visible below the bed reflector. A model is developed for structural development, which suggests that ice built up in a reservoir zone during quiescence. During the surge, ice propagated rapidly from this reservoir, creating a zone of compression which resulted in thrusting. Subsequently an extensional flow regime resulted in extensive fracture of the ice. We suggest dilated sediment was evacuated into these extensional crevasses from the glacier bed, accelerating surge termination.
Meltwater circulation and permeability of Arctic summer sea ice derived from hydrological field experiments
Authors: Freitag Johannes , Eicken Hajo 
Published: November 2007

AbstractPermeability and meltwater flow have been studied in sea ice in the Siberian and central Arctic during the summers of 1995 and 1996. A bail-test technique has been adapted to allow for measurements of in situ permeability, found to range between 10−11 and 10−8 m2. Permeability varied by about a factor of 2 between 1995 (above-normal melt rates) and 1996 (below-normal melt rates). Release of fluorescent tracers (fluorescein, rhodamine) furthermore allowed the derivation of flow velocities and assessment of the relevant driving forces. Hydraulic gradients in rough ice and wind stress in ponded ice were found to be particularly important, driving meltwater over distances of several meters per day. The mid- to late-summer ice was found to be permeable enough to completely divert meltwater from the surface into the ice interior. It is shown, however, that lower permeabilities of the upper ice layers as well as refreezing of meltwater, particularly during the early melt season, foster the development of surface melt ponds.
Hydraulics of subglacial outburst floods: new insights from the Spring–Hutter formulation
Authors: Clarke Garry K. C. 
Published: November 2007

AbstractUsing a slightly modified form of the Spring–Hutter equations, glacial outburst floods are simulated from three classic sites, “Hazard Lake”, Yukon, Canada, Summit Lake, British Columbia, Canada, and Grímsvötn, Iceland, in order to calibrate the hydraulic roughness associated with subglacial conduits. Previous work has suggested that the Manning roughness of the conduits is remarkably high, but the new calibration yields substantially lower values that are representative of those for natural streams and rivers. The discrepancy can be traced to a poor assumption about the effectiveness of heat transfer at the conduit walls. The simulations reveal behaviour that cannot be inferred from simplified theories: (1) During flood onset, water pressure over much of the conduit can exceed the confining pressure of surrounding ice. (2) Local values of fluid potential gradient can differ substantially from the value averaged over the length of the conduit, contradicting an assumption of simple theories. (3) As the flood progresses, the location of flow constrictions that effectively control the flood magnitude can jump rapidly over large distances. (4) Predicted water temperature at the conduit outlet exceeds that suggested by measurements of exit water temperature.
In situ index-of-refraction measurements of the South Polar firn with the RICE detector
Authors: Kravchenko Ilya , Besson David , Meyers Josh 
Published: November 2007

AbstractUsing the RICE (Radio Ice Cherenkov Experiment) detector at the South Pole, we have estimated the variation in the index of refraction (n) of the firn, as a function of elevation (z) measured from the surface down to z =-150 m. Measurements were made by lowering a dipole transmitter into a dry (5 in (127 mm) caliber) borehole, originally drilled for the RICE experiment in 1998, and determining signal arrival times, as a function of transmitter depth, in the englacial RICE receiver array. We clearly confirm the expected correlation of n(z) with ice density. Our measurements are in fair agreement with previous laboratory characterizations of the dielectric properties of ice cores. These are the first such in situ measurements to be performed at the South Pole.
Ice temperature and high geothermal flux at Siple Dome, West Antarctica, from borehole measurements
Authors: Engelhardt Hermann 
Published: November 2007

AbstractA vertical temperature profile through the West Antarctic ice sheet (WAIS) at the summit of Siple Dome reveals an elevated geothermal flux. This could be the root cause for the existence of a dynamic ice-stream system in the WAIS. Siple Dome is still frozen on its bed, but adjacent ice streams have bed temperatures at the pressure-melting point of ice. Although present-day temperature increases due to climatic change do not have an immediate effect on the basal conditions that control the velocity of the ice, indirect effects like a rapid disintegration of the floating ice shelves or additional melt-water input at the surface could give rise to speed-up of the ice streams with an ensuing rise in sea level. Ongoing melt at the base of the ice and changes at the margins will allow continued rapid flow of the ice streams with a possibility of disintegration, within a relatively short period of time, of at least part of the WAIS.
Stable-isotope time series and precipitation origin from firn-core and snow samples, Altai glaciers, Siberia
Authors: Aizen Vladimir B. , Aizen Elena , Fujita Koji , Nikitin Stanislav A. , Kreutz Karl J. , Takeuchi L. Nozomu 
Published: November 2007

AbstractIn the summers of 2001 and 2002, glacio-climatological research was performed at 4110–4120 m a.s.l. on the Belukha snow/firn plateau, Siberian Altai. Hundreds of samples from snow pits and a 21 m snow/firn core were collected to establish the annual/seasonal/monthly depth–accumulation scale, based on stable-isotope records, stratigraphic analyses and meteorological and synoptic data. The fluctuations of water stable-isotope records show well-preserved seasonal variations. The δ18O and δD relationships in precipitation, snow pits and the snow/firn core have the same slope to the covariance as that of the global meteoric water line. The origins of precipitation nourishing the Belukha plateau were determined based on clustering analysis of δ18O and d-excess records and examination of synoptic atmospheric patterns. Calibration and validation of the developed clusters occurred at event and monthly timescales with about 15% uncertainty. Two distinct moisture sources were shown: oceanic sources with d-excess <12‰, and the Aral–Caspian closed drainage basin sources with d-excess >12‰. Two-thirds of the annual accumulation was from oceanic precipitation, of which more than half had isotopic ratios corresponding to moisture evaporated over the Atlantic Ocean. Precipitation from the Arctic/Pacific Ocean had the lowest deuterium excess, contributing one-tenth to annual accumulation.
Stratigraphic variation within polar firn caused by differential accumulation and ice flow: interpretation of a 400 MHz short-pulse radar profile from West Antarctica
Authors: Arcone Steven A. , Spikes Vandy B. , Hamilton Gordon S. 
Published: November 2007

AbstractWe investigate causes of the stratigraphic variation revealed in a 177 km, 400 MHz short-pulse radar profile of firn from West Antarctica. The profile covers 56 m depth, and its direction was close to those of the ice flow and mean wind. The average, near-surface accumulation rates calculated from the time delays of one radar horizon consistently show minima on leeward slopes and maxima on windward slopes, confirming an earlier study based on stake observations. The stratigraphic variation includes up to 30 m depth variation in individual horizons over tens of km, fold limbs that become progressively steeper with depth, and fold-hinge loci that change direction or propagate down-ice with depth over distances far less than predicted by the ice speeds. We use an accumulation rate model to show how local rate anomalies and the effect of ice speed upon a periodic variation in accumulation rate cause these phenomena, and we reproduce two key features seen in the stratigraphic variations. We conclude that the model provides an explanation of changes in spatial stratigraphy and local measures of accumulation history given the constraints of surface topography, ice and wind velocities, and a general accumulation rate for an area.
Spatial and temporal evolution of rapid basal sliding on Bench Glacier, Alaska, USA
Authors: Macgregor Kelly R. , Riihimaki Catherine A. , Anderson Robert S. 
Published: November 2007

AbstractWe measured the surface velocity field during the summers of 1999 and 2000 on the 7 km long, 185 m thick Bench Glacier, Alaska, USA. In the spring of both years, a short-lived pulse of surface velocity, 2-4 times the annual mean velocity, propagated up-glacier from the terminus at a rate of ~200-250md-1. Displacement attributable to rapid sliding is ~5-10% of the annual surface motion, while the high-velocity event comprised 60-95% of annual basal motion. Sliding during the propagating speed-up event peaked at 6-14 cm d-1, with the highest rates in mid-glacier. Continuous horizontal and vertical GPS measurements at one stake showed divergence and then convergence of the ice surface with the bed as the velocity wave passed, with maximum surface uplift of 8-16 cm. High divergence rates coincided with high horizontal velocities, suggesting rapid sliding on the up-glacier side of bedrock steps. Initiation of the annual speed-up event occurred during the peak in englacial water storage, while the glacier was entirely snow-covered. Basal motion during the propagating speed-up event enlarges cavities and connections among them, driving a transition from a poorly connected hydrologic system to a well-connected linked-cavity system. Sliding is probably halted by the development of a conduit system.