bluuebunny/crossref_metadata_2025 huggingface dataset:
A level-set method for modeling the evolution of glacier geometry
Authors: Pralong Antoine , Funk Martin
Published: November 2007

A 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

Short-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

The 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

The 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.
Short-term velocity variations on Hansbreen, a tidewater glacier in Spitsbergen
Authors: Vieli Andreas , Jania Jacek , Blatter Heinz , Funk Martin
Published: November 2007

Spatial and temporal variations of the flow of Hansbreen, a tidewater glacier in southern Spitsbergen, Svalbard, are investigated. During summer 1999, surface flow velocities were measured in the ablation zone of Hansbreen with a temporal resolution of 1—2 hours. Short events with strongly increased surface velocities and a typical duration of 1—2 days were observed. These “speed-up events” are related to periods of strongly increased water input to the glacier, due to rainfall or enhanced surface melt. A close relation is found between the surface velocities and water pressure recorded in a moulin. However, there are indications from a short time lag between velocity and water-pressure peak as well as from observed vertical surface uplifts that basal motion is related to basal water storage rather than directly to basal water pressure. The observed short-term velocity variations and associated processes on Hansbreen are very similar to those observed on land-based valley glaciers and suggest that the relevant mechanisms and physical processes that control the flow and its temporal variations are similar. In contrast to the flow of land-based glaciers, sliding velocities on Hansbreen are observed to be high all year round and velocities increase towards the calving front.
Microstructure-dependent densification of polar firn derived from X-ray microtomography
Authors: Freitag Johannes , Wilhelms Frank , Kipfstuhl Sepp
Published: November 2007

The densification of dry polar snow and firn results in a continuous increase of density with depth accompanied by significant density fluctuations within seasonal layers. Density measurements of high spatial resolution reveal a persistent minimum of density fluctuations in the vicinity of the snow–firn transition (0.55–0.65 g cm-3) in firn-core records. In this study we give an explanation for the fluctuation minimum by applying a new method of X-ray microtomography to obtain three-dimensional (3-D) structural data of a Greenland firn core. At 13 different depths between 10 and 78 m a set of 16 samples of 40 cm total length for each depth interval was measured. A reconstructed firn segment of 40 cm covers 1–2 years of snow accumulation. Using digital image analysis techniques, different structural parameters are estimated including 3-D pore and particle sizes and specific surface areas. It is shown that the densification rates of snow and firn layers consisting of coarse particles are much higher than those consisting of fine particles within the same depth interval. This causes a density crossing of fine- and coarse-grained layers with a minimum of density variations at the crossover point. This crossing-over implies that formerly dense layers in the seasonal density signal are not of the same origin as dense layers in the deeper part of the firn column and that the seasonal density signal will totally change shape with depth. It is speculated that in coarse- and fine-grained firn the dominant mechanism of densification acts over different regimes of density.
Effects of firn ventilation on isotopic exchange
Authors: Neumann Thomas A. , Waddington Edwin D.
Published: November 2007

A new model of isotopic diffusion in the upper few meters of firn tracks the isotopic composition of both the ice matrix and the pore-space vapor through time in two dimensions. Stable isotopes in the vapor phase move through the firn by diffusion along concentration gradients and by advection. Wind-driven ventilation carries atmospheric water vapor into the firn, where it mixes with existing pore-space vapor. Unlike previous models, our model allows disequilibrium between pore-space vapor and the surrounding snow grains. We also calculate the isotopic effects of ventilation-driven sublimation and condensation in the firn. Model predictions of isotopic diffusion in firn compare favorably with existing diffusion models. Model results quantify what other investigators have suggested: isotopic change in the upper few meters is more rapid than can be explained by the Whillans and Grootes (1985) model; isotopic equilibration with atmospheric vapor is an important component of post-depositional isotopic change; and ventilation enhances isotopic exchange by creating regions of relatively rapid sublimation and condensation in the firn.
Firn accumulation records for the past 1000 years on the basis of dielectric profiling of six cores from Dronning Maud Land, Antarctica
Authors: Hofstede Coen M. , Roderik S.W van de Wal , Kaspers Karsten A. , van den Broeke Michiel R. , Karlöf Lars , Winther Jan-Gunnar , Isaksson Elisabeth , Lappegard Gante , Mulvaney Robert , Oerter Hans , Wilhelms Frank
Published: November 2007

This paper presents an overview of firn accumulation in Dronning Maud Land (DML), Antarctica, over the past 1000 years. It is based on a chronology established with dated volcanogenic horizons detected by dielectric profiling of six medium-length firn cores. In 1998 the British Antarctic Survey retrieved a medium-length firn core from western DML. During the Nordic EPICA (European Project for Ice Coring in Antarctica) traverse of 2000/01, a 160 m long firn core was drilled in eastern DML. Together with previously published data from four other medium-length ice cores from the area, these cores yield 50 possible volcanogenic horizons. All six firn cores cover a mutual time record until the 29th eruption. This overlapping period represents a period of approximately 1000 years, with mean values ranging between 43 and 71 mm w.e. The cores revealed no significant trend in snow accumulation. Running averages over 50 years, averaged over the six cores, indicate temporal variations of5%. All cores display evidence of a minimum in the mean annual firn accumulation rate around AD 1500 and maxima around AD 1400 and 1800. The mean increase over the early 20th century was the strongest increase, but the absolute accumulation rate was not much higher than around AD 1400. In eastern DML a 13% increase is observed for the second half of the 20th century.
A new Snow-SVAT to simulate the accumulation and ablation of seasonal snow cover beneath a forest canopy
Authors: Tribbeck Melody J. , Gurney Robert J. , Morris Elizabeth M. , Pearson David W. C.
Published: November 2007

A new snow—soil—vegetation—atmosphere transfer (Snow-SVAT) scheme, which simulates the accumulation and ablation of the snow cover beneath a forest canopy, is presented. The model was formulated by coupling a canopy optical and thermal radiation model to a physically based multi-layer snow model. This canopy radiation model is physically based yet requires few parameters, so can be used when extensive in situ field measurements are not available. Other forest effects such as the reduction of wind speed, interception of snow on the canopy and the deposition of litter were incorporated within this combined model, SNOWCAN, which was tested with data taken as part of the Boreal Ecosystem—Atmosphere Study (BOREAS) international collaborative experiment. Snow depths beneath four different canopy types and at an open site were simulated. Agreement between observed and simulated snow depths was generally good, with correlation coefficients ranging between r2 = 0.94 and r2 = 0.98 for all sites where automatic measurements were available. However, the simulated date of total snowpack ablation generally occurred later than the observed date. A comparison between simulated solar radiation and limited measurements of sub-canopy radiation at one site indicates that the model simulates the sub-canopy downwelling solar radiation early in the season to within measurement uncertainty.
A novel backpackable ice-penetrating radar system
Authors: Matsuoka Kenichi , Saito Ryoji , Naruse Renji
Published: November 2007

We have developed a novel ice-penetrating radar system that can be carried on a backpack. Including batteries for a 3 hour continuous measurement, the total weight is 13 kg. In addition, it operates reliably down to –25°C, has a low power consumption of 24 W, and is semi-waterproof. The system has a built-in-one controller with a high-brightness display for reading data quickly, a receiver with 12-bit digitizing, and a 1 kV pulse transmitter in which the pulse amplitude varies by <0.2%. Optical communications between components provides low-noise data acquisition and allows synchronizing of the pulse transmission with sampling. Measurements with the system revealed the 300 m deep bed topography of a temperate valley glacier in the late ablation season.
Force-perturbation analysis of recent thinning and acceleration of Jakobshavn Isbræ, Greenland
Authors: Thomas Robert H.
Published: November 2007

Observations between 1997 and 2001, of a 30% velocity increase and up to 60 m thinning of downstream parts of Jakobshavn Isbræ, Greenland, immediately following calving of about 4 km of its 15 km floating ice tongue, suggest that acceleration may have been initiated by the calving. Assuming that the force perturbation associated with such weakening is swiftly transmitted far up-glacier, I develop equations to estimate the perturbation. Initially, the observed changes are consistent with the comparatively small perturbation associated with the calving. Thereafter, it was probably sustained by thinning of the remaining ice tongue at rates of about 80 ma–1. Otherwise, the force perturbation would soon have been balanced by reduction in the hydrostatic driving force for longitudinal creep as the glacier thinned, with velocities dropping to their former values. The calculated force perturbation increases to a maximum about 10 km inland of the grounding line, consistent with decreasing weight forces as the glacier thins over bedrock that slopes uphill seawards. Further inland, it progressively decreases, probably because marginal drag increased as the glacier accelerated. Both here and on the floating tongue, marginal ice appears to have been softened by the influence of locally intense shear on ice temperature and/or fabric. More recent observations show continued acceleration and thinning, and most of the remaining ice tongue calved away in April 2003, so thinning is likely to continue.
Seasonal mass-balance gradients in Norway
Authors: Rasmussen L.A. , Andreassen L.M.
Published: November 2007

Previously discovered regularity in vertical profiles of net balance, bn(z), on ten glaciers in Norway also exists in profiles of both winter, bw(z), and summer, bs(z), seasonal balances. All three profiles, unlike those of many glaciers elsewhere in the world, are remarkably linear. Variations of gradients, dbw/dz and dbs/dz, from year to year are small and correlate poorly with glacier-total balances bw and bs. Glacier-to-glacier correlation is weak for both gradients but is strongly positive for bw and bs. There are two direct consequences of these properties of the gradients that apply to both seasonal balances bw and bs. First, because db/dz varies so little from year to year, the difference in balance, ∆b, from year to year is nearly the same over the entire glacier, except near the top and bottom of its altitude range. Therefore, balance at a site near the middle of the altitude range of the glacier correlates very well with glacier-total balance. Second, this correlation, combined with the strong positive correlation of balance from glacier to glacier, is the reason balance at one altitude on one glacier correlates well with glacier-total balance at other nearby glaciers.
Mass changes of the Greenland and Antarctic ice sheets and shelves and contributions to sea-level rise: 1992–2002
Authors: Zwally H. Jay , Giovinetto Mario B. , Li Jun , Cornejo Helen G. , Beckley Matthew A. , Brenner Anita C. , Saba Jack L. , Yi Donghui
Published: November 2007

Changes in ice mass are estimated from elevation changes derived from 10.5 years (Greenland) and 9 years (Antarctica) of satellite radar altimetry data from the European Remote-sensing Satellites ERS-1 and -2. For the first time, the dH/dt values are adjusted for changes in surface elevation resulting from temperature-driven variations in the rate of firn compaction. The Greenland ice sheet is thinning at the margins (–42 ± 2Gta¯1 below the equilibrium-line altitude (ELA)) and growing inland (+53 ± 2Gta-1 above the ELA) with a small overall mass gain (+11 ± 3Gta–1; –0.03 mma–1 SLE (sea-level equivalent)). The ice sheet in West Antarctica (WA) is losing mass (–47 ± 4Gta–1) and the ice sheet in East Antarctica (EA) shows a small mass gain (+16 ± 11 Gta–1) for a combined net change of –31 ± 12 Gta–1 (+0.08mma–1 SLE). The contribution of the three ice sheets to sea level is +0.05±0.03mma–1. The Antarctic ice shelves show corresponding mass changes of –95 ± 11 Gta–1 in WA and +142 ± 10Gta–1 in EA. Thinning at the margins of the Greenland ice sheet and growth at higher elevations is an expected response to increasing temperatures and precipitation in a warming climate. The marked thinnings in the Pine Island and Thwaites Glacier basins of WA and the Totten Glacier basin in EA are probably ice- dynamic responses to long-term climate change and perhaps past removal of their adjacent ice shelves. The ice growth in the southern Antarctic Peninsula and parts of EA may be due to increasing precipitation during the last century.
Statistical techniques to select detection thresholds for peak signals in ice-core data
Authors: Karlöf L. , Ølgård T.A. , Godtliebsen F. , Kaczmarska M. , Fischer H.
Published: November 2007

Five statistical techniques to determine peaks in ice-core time series are presented and compared. The ice-core time series, representing different signal characteristics, comprise electrical conductivity measurements (ECM), dielectric properties (DEP) and sulphate. Three techniques (I–III) utilize all the data in the time series to estimate significant thresholds for identifying peaks. Technique IV applies a moving window and conducts a statistical inference within the defined window. In technique V, a family of smoothed estimates of the ice-core time series is produced, and statistical tests are performed on the significant changes in the derivative of the estimates. The correction of the significance level, α, due to multiple tests is introduced and implemented in techniques II–V. The threshold obtained by techniques I–III is determined by the influence of the error term on the global variance estimate, whereas the threshold of IV is determined by the data within the window. The success of identifying peaks with technique V is dependent on the redundancy in the data, i.e. the sampling rate. It is concluded that techniques II and III are superior to the other techniques due to their simplicity and robustness.
An enhanced temperature-index glacier melt model including the shortwave radiation balance: development and testing for Haut Glacier d’Arolla, Switzerland
Authors: Pellicciotti Francesca , Brock Ben , Strasser Ulrich , Burlando Paolo , Funk Martin , Corripio Javier
Published: November 2007

An enhanced temperature-index glacier melt model, incorporating incoming shortwave radiation and albedo, is presented. The model is an attempt to combine the high temporal resolution and accuracy of physically based melt models with the lower data requirements and computational simplicity of empirical melt models, represented by the ‘degree-day’ method and its variants. The model is run with both measured and modelled radiation data, to test its applicability to glaciers with differing data availability. Five automatic weather stations were established on Haut Glacier d’Arolla, Switzerland, between May and September 2001. Reference surface melt rates were calculated using a physically based energy-balance melt model. The performance of the enhanced temperature-index model was tested at each of the four validation stations by comparing predicted hourly melt rates with reference melt rates. Predictions made with three other temperature-index models were evaluated in the same way for comparison. The enhanced temperature-index model offers significant improvements over the other temperature-index models, and accounts for 90–95% of the variation in the reference melt rate. The improvement is lower, but still significant, when the model is forced by modelled shortwave radiation data, thus offering a better alternative to existing models that require only temperature data input.
Surface mass balance of glaciers in the French Alps: distributed modeling and sensitivity to climate change
Authors: Gerbaux M. , Genthon C. , Etchevers P. , Vincent C. , Dedieu J.P.
Published: November 2007

A new physically based distributed surface mass-balance model is presented for Alpine glaciers. Based on the Crocus prognostic snow model, it resolves both the temporal (1 hour time-step) and spatial (200 m grid-step) variability of the energy and mass balance of glaciers. Mass-balance reconstructions for the period 1981–2004 are produced using meteorological reconstruction from the SAFRAN meteorological model for Glacier de Saint-Sorlin and Glacier d’Argentière, French Alps. Both glaciers lost mass at an accelerated rate in the last 23 years. The spatial distribution of precipitation within the model grid is adjusted using field mass-balance measurements. This is the only correction made to the SAFRAN meteorological input to the glacier model, which also includes surface atmospheric temperature, moisture, wind and all components of downward radiation. Independent data from satellite imagery and geodetic measurements are used for model validation. With this model, glacier sensitivity to climate change can be separately evaluated with respect to a full range of meteorological parameters, whereas simpler models, such as degree-day models, only account for temperature and precipitation. We provide results for both mass balance and equilibrium-line altitude (ELA) using a generic Alpine glacier. The sensitivity of the ELA to air temperature alone is found to be 125 m °C–1, or 160 m °C¯1 if concurrent (Stefan–Boltzmann) longwave radiation change is taken into account.
Surface morphology and internal layer stratigraphy in the downstream end of Kamb Ice Stream, West Antarctica
Authors: Catania G. A. , Conway H. , Raymond C.F. , Scambos T.A.
Published: November 2007

Satellite images of Kamb Ice Stream (formerly Ice Stream C), West Antarctica, reveal several long, curved linear features (lineations) oriented sub-parallel to the ice-flow direction. We use ground-based radar to characterize the internal layer stratigraphy of these lineations and the terrains that they bound. Some lineations are relict ice-stream shear margins, identified by hyperbolic diffractors near the surface (interpreted to be buried crevasses) and highly disturbed internal layers at depth. Satellite images show another set of lineations outside the relict margins that wrap around the ends of the surrounding inter-ice-stream ridges. Internal layers beneath these lineations are downwarped strongly into a syncline shape. The internal stratigraphy of the terrain between these lineations and the relict margins is characterized by deep hyperbolic line diffractors. Our preferred hypothesis for the origin of this terrain is that it was floating sometime in the past; the deep hyperbolas are interpreted to be basal crevasses, and the strongly downwarped internal layers mark the position of a relict grounding line. Our study shows that lineations and intervening terrains have different internal layer characteristics implying different origins. Differentiation between these features is not possible using satellite images alone.
Dating of seasonal snow/firn accumulation layers using pollen analysis
Authors: Nakazawa Fumio , Fujita Koji , Takeuchi Nozomu , Fujiki Toshiyuki , Uetake Jun , Aizen Vladimir , Nakawo Masayoshi
Published: November 2007

Reliable chronologies in ice cores and snow pits from many alpine glaciers in latitudes between 60° N and 60° S are often difficult to establish owing to problems with annual-layer counting. Problems arise from melting, wind erosion and the negligible amount of precipitation in some seasons, all of which tend to obscure the seasonal variations in δ18O and chemical concentrations that are typically used to date ice cores. However, alpine glaciers contain many species of pollen grains that peak at particular times of the year. We used the peaks in Betulaceae, Pinus, Artemisia and a combination of Abies and Picea pollen species to determine the four seasonal layers of a snow pit on Belukha glacier in Russia’s Altai Mountains. Comparing the pollen-dated profiles with wind and precipitation records allows us to determine where a seasonal layer is missing. Thus, the pollen-dating method described here may be a useful tool to measure the annual snow deposition on alpine glaciers, even when some seasonal layers are eroded by wind or missing due to negligible precipitation.
Controls on the basal water pressure in subglacial channels near the margin of the Greenland ice sheet
Authors: Ahlstrøm Andreas P. , Mohr Johan J. , Reeh Niels , Christensen Erik Lintz , Hooke Roger LeB.
Published: November 2007

Assuming a channelized drainage system in steady state, we investigate the influence of enhanced surface melting on the water pressure in subglacial channels, compared to that of changes in conduit geometry, ice rheology and catchment variations. The analysis is carried out for a specific part of the western Greenland ice-sheet margin between 66° N and 66°30′N using new high-resolution digital elevation models of the subglacial topography and the ice-sheet surface, based on an airborne ice-penetrating radar survey in 2003 and satellite repeat-track interferometric synthetic aperture radar analysis of European Remote-sensing Satellite 1 and 2 (ERS-1/-2) imagery, respectively. The water pressure is calculated up-glacier along a likely subglacial channel at distances of 1, 5 and 9 km from the outlet at the ice margin, using a modified version of Röthlisberger’s equation. Our results show that for the margin of the western Greenland ice sheet, the water pressure in subglacial channels is not sensitive to realistic variations in catchment size and mean surface water input compared to small changes in conduit geometry and ice rheology.
Variegated Glacier, Alaska, USA: a century of surges
Authors: Eisen Olaf , Harrison William D. , Raymond Charles F. , Echelmeyer Keith A. , Bender Gary A. , Gorda Jeannette L.D.
Published: November 2007

A 1995 surge of Variegated Glacier, Alaska, USA, is discussed in the context of its six 20th-century predecessors, especially the previous surge in 1982/83 which was studied in detail. The average time between surge initiations is 15 years. The 1995 surge was considerably weaker than its predecessors, having a single phase or at most a very weak second phase. The 1995 surge confirms that there is a seasonal cycle, with surge initiation in winter and termination in the first part of the melt season, and a correlation between weather and both surge termination date and surge extent. Two days of record high temperature correlated with the termination of the 1995 surge. The most obvious issue is the absence of a strong second surge phase (as there was in the 1982/83 surge) culminating in a surge extent more in line with that of the predecessors. This is considered in the light of a simple criterion for surge initiation and re-initiation which depends upon the evolving basal shear stress.
A time constant for hemispheric glacier mass balance
Authors: Greene Arthur M.
Published: November 2007

The notion is developed of a mass-balance time constant applicable to the Northern Hemispheric glacier inventory taken as a whole. Ice dynamics are incorporated only implicitly in its estimation, which follows directly from a consideration of observed mass-balance and hemispheric temperature time series. While such a parameter must certainly be related to the rate at which glacier hypsometry adjusts to variations in climate, as are time constants derived via dynamic considerations, the parameter discussed herein differs with respect to its statistical character. For an ensemble of Northern Hemisphere glaciers a time-constant value on the order of a century is estimated. It is shown that such a value is consistent with the hemispheric near-equilibration of glaciers that prevailed around 1970. A ‘reference climate’ is defined, such that the mass balance in a given year is a function only of the difference between that year’s climate and the reference. This difference was small during the hemispheric near-equilibrium that prevailed around 1970, implying that the glacier wastage of the late 20th century is essentially a response to post-1970 warming. It is shown that precipitation fluctuations play a compensating role in the hemispheric net mass budget, in that they are strongly anticorrelated with fluctuations in temperature-induced melting. However, the contribution of precipitation does not override that of temperature, which remains the dominant influence on hemisphere-wide glacier fluctuations.
Ice friction, wear features and their dependence on sliding velocity and temperature
Authors: Marmo Brett A. , Blackford Jane R. , Jeffree Chris E.
Published: November 2007

Friction processes for ice samples sliding on steel have been determined by examining wear and debris morphology with low-temperature scanning electron microscopy and relating the processes to the velocity and temperature of formation. Friction experiments were carried out over a temperature range of −27 to −0.5°C and velocity range of 0.008–0.37 m s−1. Data were used to develop a friction map. Low friction (µ < 0.1) at high temperature (–3.4ºC)–low velocity (0.02 ms−1), and low temperature (–25.1ºC)–high velocity (0.30 ms−1) is due to the presence of liquid water which lubricates the sliding interface. Diagnostic morphologies for lubricated sliding include the presence of residual liquid in wear grooves and the development of a consolidated mass of debris on the trailing side of the wear surface with distinct grain boundaries and spheroidal air bubbles. High friction (µ > 0.15) at low temperature (−24.5ºC)–low velocity (0.03 m s−1) results from insufficient lubrication at the sliding interface, leading to plastic deformation. Diagnostic morphologies of plastic deformation include scuffing features on the wear surface and the accumulation of sheets of unconsolidated debris on the trailing edge of the wear surface.
Non-climate influences on stable isotopes at Taylor Mouth, Antarctica
Authors: Neumann Thomas A. , Waddington Edwin D. , Steig Eric J. , Grootes Pieter M.
Published: November 2007

The late-Holocene trends in δ18O differ significantly in two ice cores (30 km apart) from the area of Taylor Dome, Antarctica. It is unlikely that the trend in the core from Taylor Mouth (the flank site) is due to a standard δ18O–surface temperature relationship. Assuming that the Taylor Dome (nearsummit) core records local climate variations common to both cores, we assess two leading possible causes for the observed differences: (1) Relative to Taylor Dome, Taylor Mouth may collect snow from more sources with distinct isotopic compositions. (2) Vapor motion during prolonged near-surface exposure may cause post-depositional isotope enrichment at Taylor Mouth, where the accumulation rate is low. Our model of firn pore-space vapor and sublimating ice grains suggests that post-depositional processes can modify δ18O values by several ‰. Isotopic samples from areas with significantly different accumulation rates near Taylor Mouth could differentiate between possibilities (1) and (2).
South Polar in situ radio-frequency ice attenuation
Authors: Barwick S. , Besson D. , Gorham P. , Saltzberg D.
Published: November 2007

We have determined the in situ electric field attenuation length Lα (defined as the length over which the signal amplitude diminishes by a factor 1/e) for radio-frequency signals broadcast vertically through South Polar ice and reflected off the underlying bed. Conservatively assuming a bedrock field reflectivity for f = 380 MHz, and T = –50°C; the errors incorporate uncertainties in R. This value is consistent with previous estimates that the radiofrequency attenuation length exceeds the attenuation length at optical frequencies by an order of magnitude.
Subglacial drainage processes at a High Arctic polythermal valley glacier
Authors: Bingham Robert G. , Nienow Peter W. , Sharp Martin J. , Boon Sarah
Published: November 2007

Dye-tracer experiments undertaken over two summer melt seasons at polythermal John Evans Glacier, Ellesmere Island, Canada, were designed to investigate the character of the subglacial drainage system and its evolution over a melt season. In both summers, dye injections were conducted at several moulins and traced to a single subglacial outflow. Tracer breakthrough curves suggest that supraglacial meltwater initially encounters a distributed subglacial drainage system in late June. The subsequent development and maintenance of a channelled subglacial network are dependent upon sustained high rates of surface melting maintaining high supraglacial inputs. In a consistently warm summer (2000), subglacial drainage became rapidly and persistently channelled. In a cooler summer (2001), distributed subglacial drainage predominated. These observations confirm that supraglacial meltwater can access the bed of a High Arctic glacier in summer, and induce significant structural evolution of the subglacial drainage system. They do not support the view that subglacial drainage systems beneath polythermal glaciers are always poorly developed. They do suggest that the effects on ice flow of surface water penetration to the bed of predominantly cold glaciers may be short-lived.
A non-destructive method for measuring the salinity and solid fraction of growing sea ice in situ
Authors: Notz Dirk , Wettlaufer John S. , Worster M. Grae
Published: November 2007

We describe an instrument developed to make in situ measurements of salinity and solid- fraction profiles in growing sea ice. The vertical resolution of the measurements is up to a few millimeters, with a temporal resolution of up to fractions of a second. The technique is based on impedance measurements between platinum wires around which sea ice grows. Data obtained using this instrument in laboratory experiments are in good agreement with theoretical predictions. In a field test in the Arctic, the bulk salinity of growing sea ice has been measured in situ throughout the whole depth of the ice layer. The data are compared with bulk salinities obtained from ice cores, and confirm the general understanding that the bulk salinity in ice-core studies is significantly underestimated in the lower parts of the cores. The approach can also be used in other glaciological applications and for general studies of two-phase, two-component porous media.
A user-friendly anisotropic flow law for ice-sheet modeling
Authors: Gillet-Chaulet Fabie , Gagliardini Olivier , Meyssonnier Jacques , Montagnat Maurine , Castelnau Olivier
Published: November 2007

For accurate ice-sheet flow modelling, the anisotropic behaviour of ice must be taken fully into account. However, physically based micro-macro (μ-M) models for the behaviour of an anisotropic ice polycrystal are too complex to be implemented easily in large-scale ice-sheet flow models. An easy and efficient method to remedy this is presented. Polar ice is assumed to behave as a linearly viscous orthotropic material whose general flow law (GOLF) depends on six parameters, and its orthotropic fabric is described by an ‘orientation distribution function’ (ODF) depending on two parameters. A method to pass from the ODF to a discrete description of the fabric, and vice versa, is presented. Considering any available μ-M model, the parameters of the GOLF that fit the response obtained by running this μ-M model are calculated for any set of ODF parameters. It is thus possible to tabulate the GOLF over a grid in the space of the ODF parameters. This step is performed once and for all. Ice-sheet flow models need the general form of the GOLF to be implemented in the available code (once), then, during each individual run, to retrieve the GOLF parameters from the table by interpolation. As an application example, the GOLF is tabulated using three different μ-M models and used to derive the rheological properties of ice along the Greenland Icecore Project (GRIP) ice core.
Monitoring glacier variations on Geladandong mountain, central Tibetan Plateau, from 1969 to 2002 using remote-sensing and GIS technologies
Authors: Ye Qinghua , Kang Shichang , Chen Feng , Wang Jinghua
Published: November 2007

Using Geographical Information System (GIS) and remote-sensing technologies, we describe quantitative measurements of glacier variations in the Geladandong mountain region of central Tibet. Data from Landsat images at three different times, 1973–76, 1992 and 2002, are compared with glacier areas digitized from a topographic map based on aerial photographs taken in 1969. We find that while some glaciers have advanced during the past 30 years, others have retreated. The area of retreat is much larger than that of advance. The total glacier area has decreased from 889km2 in 1969 to 847km2 in 2002, a reduction of almost 43 km2 (i.e. 4.8% decrease, or an average of 1.29km2 a–1). The variation of glacier area in the Geladandong mountain region is not as large as in other regions within the Tibetan Plateau. Glacier areas decreased 4.7km2 (i.e. an average of 0.68km2 a–1) during 1969–76, 15.4km2 (0.96km2 a–1) during 1976–92, and 22.4km2 (2.24km2 a–1) during 1992–2002, suggesting accelerated glacier retreat in recent years. The recession rates of glacier termini also increased. It is likely that the increase in summer air temperature is the major reason for glacier shrinkage in the Geladandong mountain region.
Hydraulic impacts of glacier advance over a sediment bed
Authors: Boulton Geoffrey , Zatsepin Sergei
Published: November 2007

A sedimentary sequence of till overlying a gravel aquifer was instrumented with water-pressure transducers prior to a small, anticipated surge of the margin of the glacier Breiðamerkurjökull in Iceland. The records of water pressure at each transducer site show a well-defined temporal sequence of hydraulic regimes that reflect the changing recharge of surface-derived meltwater, the pressure drop along the drainage pathway and the pattern of ice loading. The poroelastic and water-pressure response of glacially overridden sediments to the recharge rate is determined in the frequency domain through an analytic solution. This permits the in situ conductivity, compressibility and consolidation states of subglacial sediments to be derived, and reveals aquifer-scale compressibility that produces an important water-pressure wave associated with the advancing glacier. The model is then used to explore how varying conductivity/compressibility, largely determined by granulometry, can determine drainage states and instabilities that may have a large impact on glacier/ice-sheet dynamics, and how the drainage time of surface water to the bed can determine the frequency response of subglacial groundwater regimes and their influence on subglacial sediment stability. Mismatches between model predictions and specific events in water-pressure records are used to infer processes that are not incorporated in the model: hydrofracturing that changes the hydraulic properties of subglacial sediments; the impact on groundwater pressure of subglacial channel formation; upwelling beyond the glacier margin; and rapid variations in the state of consolidation. The poroelastic model also suggests how seismic methods can be developed further to monitor hydraulic conditions at the base of an ice sheet or glacier.
Basal conditions beneath enhanced-flow tributaries of Slessor Glacier, East Antarctica
Authors: Rippin D.M. , Bamber J.L. , Siegert M.J. , Vaughan D.G. , Corr H.F.J.
Published: November 2007

Radio-echo sounding data are used to investigate bed roughness beneath the three enhanced-flow tributaries of Slessor Glacier, East Antarctica. Slow-moving inter-tributary areas are found to have rough beds, while the bed of the northernmost tributary is relatively smooth. A reconstruction of potential subglacial drainage routing indicates that water would be routed down this tributary, and investigations of basal topography following isostatic recovery reveal that the bed would have been below sea level in preglacial times, so marine sediments may have accumulated here. Together, these factors are further support for the dominance of basal motion in this tributary, reported elsewhere. Conversely, although the other two Slessor tributaries may have water routed beneath them, they would not have been below sea level before the growth of the ice sheet, so cannot be underlain by marine sediments. They are also found to be rough, and, within the range of uncertainties, it is likely that basal motion does not play a major role in the flow of these tributaries. Perhaps the most interesting area, however, is a deep trough where flow rates are currently low but the bed is as smooth as the northern Slessor trough. It is proposed that, although ice deformation currently dominates in this trough, basal motion may have occurred in the past, when the ice was thicker.
Borehole logging with an eight-arm caliper–inclinometer probe
Authors: Schwerzmann Aurel , Funk Martin , Blatter Heinz
Published: November 2007

A newly designed eight-arm caliper–inclinometer probe for borehole logging is presented. The caliper sounding gives eight points of cross-sections of the borehole at 1 cm intervals along its axis. Regression circles and ellipses are calculated and used as obvious hypotheses to be tested in order to obtain information on strain rates in the horizontal planes and isotropic or anisotropic borehole closure. The scratching of identifiable marks into the wall of the borehole enables material displacement to be tracked, in particular in the vertical direction, thus providing information for determining vertical strain rates along the borehole. Measurements in two boreholes drilled on high-altitude glaciers in the Swiss Alps are used to demonstrate both the potential and the limitations of the probe and of the mathematical methods presented.
Radio-wave depolarization and scattering within ice sheets: a matrix-based model to link radar and ice-core measurements and its application
Authors: Fujita Shuji , Maeno Hideo , Matsuoka Kenichi
Published: November 2007

Crystal-orientation fabric (COF) has a large influence on ice-sheet flow. Earlier radar studies have shown that COF-based birefringence occurs within ice sheets. Radio-wave scattering in polar ice results from changing physical properties of permittivity and conductivity that arise from differing values of density, acidity and COF. We present an improved mathematical model that can handle all these phenomena together. We use this matrix-based model to study the two-way propagation of depolarized radio waves that scatter at both isotropic and anisotropic boundaries. Based on numerical simulations, we demonstrate how COF affects the radar signals in terms of radar polarization and frequency. We then compare the simulated features with VHF radar data obtained at two contrasting inland sites in East Antarctica, where COF is known from ice-core studies. These two sites are Dome Fuji, located near a dome summit, and Mizuho, located in a converging ice-flow region. Data at Dome Fuji are dominated by typical features resulting from birefringence. In contrast, both birefringence and anisotropic scattering affect the radar data at Mizuho. We argue that radar methods can be used to determine principal axes and strength of birefringence in the ice sheets.
Glacier change over the past four decades in the middle Chinese Tien Shan
Authors: Li Baolin , Zhu A-Xing , Zhang Yichi , Pei Tao , Qin Chengzhi , Zhou Chenghu
Published: November 2007

Terminus fluctuation and area change in mountain glaciers in the middle Chinese Tien Shan over the past four decades are examined, based on Landsat multispectral scanner (MSS), Enhanced Thematic Mapper Plus (ETM+), Système Probatoire pour l’Observation de la Terre (SPOT) highresolution visible (HRV) imagery, topographical maps and China Glacier Inventory data using Geographical Information System (GIS) and remote-sensing techniques. This study shows that the glaciers in the study area have undergone continuous recession between 1963 and 2000. Over the past 37 years the total glacier area decreased from 55 × 106 m2 to 48 × 106 m2 (a 13% decrease). The glaciers at low elevations decreased more rapidly than those at high elevations. Seven of the ten large valley glaciers in the study area retreated by at least 150 ± 49 m during this period. The average rate of recession for these glaciers was 4–8 m a−1. The recession rate between 1986 and 2000 was larger than between 1963 and 1986. The glacier changes in the study area were similar to those in the Ürümqi river basin in the northern Chinese Tien Shan. It is apparent that temperature rise over the past four decades has had a profound impact on these glaciers.
Calculating ice melt beneath a debris layer using meteorological data
Authors: Nicholson Lindsey , Benn Douglas I.
Published: November 2007

Generalized numerical models of sub-debris ice ablation are preferable to empirical approaches for predicting runoff and glacier response to climate change, as empirical methods are site-specific and strongly dependent upon the conditions prevailing during the measurement period. We present a modified surface energy-balance model to calculate melt beneath a surface debris layer from daily mean meteorological variables. Despite numerous simplifications, the model performs well and modelled melt rates give a good match to observed melt rates, suggesting that this model can produce reliable estimates of ablation rate beneath debris layers several decimetres thick. This is a useful improvement on previous models which are inappropriate for thick debris cover.
Scale independence of till rheology
Authors: Tulaczyk Slawek
Published: November 2007

Representation of till rheology in glaciological models of ice motion over deformable sediments has, until now, focused largely on two end-member cases: (1) linear, or mildly non-linear, viscous rheology and (2) (nearly) plastic rheology. Most laboratory and in situ experiments support the latter model. Hindmarsh (1997) and Fowler (2002, 2003) proposed that experimental results represent the behavior of small till samples (characteristic length scales of ~0.1 to ~1 m) but that till behaves viscously over length scales that are relevant to determination of ice-flow rates in glaciers and ice sheets (~1 km or more). Observations of short speed-up events on the ice plain of Whillans Ice Stream, West Antarctica, provide an opportunity to compare the in situ rheology of this till, integrated over ~10–100 km, with the rheology of till from beneath the same ice stream determined on small laboratory samples and in local borehole experiments. This comparison indicates that the rheology of the subglacial till beneath Whillans Ice Stream is independent of scale.