bluuebunny/crossref_metadata_2025 huggingface dataset:
Basal-crevasse-fill origin of laminated debris bands at Matanuska Glacier, Alaska, U.S.A.
Authors: Ensminger Staci L. , Alley Richard B. , Evenson Edward B. , Lawson Daniel E. , Larson Grahame J.
Published: November 2007

The numerous debris bands in the terminus region of Matanuska Glacier, Alaska, U.S.A., were formed by injection of turbid meltwaters into basal crevasses. The debris bands are millimeter(s)-thick layers of silt-rich ice cross-cutting older, debris-poor englacial ice. The sediment grain-size distribution of the debris bands closely resembles the suspended load of basal waters, and of basal and proglacial ice grown from basal waters, but does not resemble supraglacial debris, till or the bedload of subglacial streams. Most debris bands contain anthropogenic tritium (3H) in concentrations similar to those of basal meltwater and ice formed from that meltwater, but cross-cut englacial ice lacking tritium. Stable-isotopic ratios (δ18O and δD) of debris-band ice are consistent with freezing from basal waters, but are distinct from those in englacial ice. Ice petrofabric data along one debris band lack evidence of active shearing. High basal water pressures and locally extensional ice flow associated with overdeepened subglacial basins favor basal crevasse formation.
Greenland ice-sheet mass-balance distribution: a variance analysis of existing field data
Authors: Six Delphine , Letréguilly Anne , Reynaud Louis
Published: November 2007

A large number of mass-balance measurements have been carried out on the Greenland ice sheet, but few of the series obtained are well suited for statistical studies. Rather than looking at the global mass-balance value, this paper deals with the spatial and temporal mass-balance variability on the ice sheet. Two sorts of analysis are possible: direct comparisons of the series measured at a given site, or a broader approach involving comparisons between different sites using variance analysis. For glaciers in the southwest ablation area, a significant interannual variability (around 1.0 m w.e.) is found. This variability is spatially consistent. In the accumulation area, the results are more complex. For example, there is consistent evidence of year-to-year variations on the west–east Expéditions Glaciologiques Internationales au Groenland (EGIG) profile, but other closer sites are weakly correlated. These results emphasize the need for a better coverage of measurements over the entire ice sheet, as well as longer and more continuous measurement series.
A high-altitude snow chemistry record from Amundsenisen, Dronning Maud Land, Antarctica
Authors: Isaksson Elisabeth , Karlén Wibjörn , Mayewski Paul , Twickler Mark , Whitlow Sallie
Published: November 2007

In this paper a detailed record of major ions from a 20 m deep firn core from Amundsenisen, western Dronning Maud Land, Antarctica, is presented. The core was drilled at 75° S, 2° E (2900 m a.s.l.) during austral summer 1991/92. The following ions were measured at 3 cm resolution: Na+, Mg2+, Ca2+, Cl−, NO3−, S042− and CH3SO3H (MSA). The core was dated back to 1865 using a combination of chemical records and volcanic reference horizons. The volcanic eruptions identified in this core are Mount Ngauruhoe, New Zealand (1974–75), Mount Agung, Indonesia (1963), Azul, Argentina (1932), and a broad peak that corresponds in time toTarawera, New Zealand (1886), Falcon Island, South Shetlands, Southern Ocean (1885), and Krakatau, Indonesia (1883). There are no trends in any of the ion records, but the annual to decadal changes are large. The mean concentrations of the measured ions are in agreement with those from other high-altitude cores from the Antarctic plateau. At this core site there may be a correspondence between peaks in the MSA record and major El Niño–Southern Oscillation events.
Frazil-ice entrainment of sediment: large-tank laboratory experiments
Authors: Smedsrud Lars Henrik
Published: November 2007

Laboratory experiments that simulate natural ice-formation processes and sediment entrainment in shallow water are presented. A 10–30 cm s−1 current was forced with impellers in a 20 m long, 1 m deep indoor tank. Turbulence in the flow maintained a suspension of sediments at concentrations of 10–20 mg L−1 at 0.5 m depth. Low air temperatures (∼−15°C) and 5 m s−1 winds resulted in total upward heat fluxes in the range 140–260 W m−2. The cooling produced frazil-ice crystals up to 2 cm in diameter with concentrations up to 4.5 g L−1 at 0.5 m depth. Considerable temporal variability with time-scales of <1 min was documented. A close to constant portion of the smaller frazil crystals remained in suspension. After some hours the larger crystals, which made up most of the ice volume, accumulated as slush at the surface. Current measurements were used to calculate the turbulent dissipation rate, and estimates of vertical diffusion were derived. After 5–8 hours, sediment concentrations in the surface slush were normally close to those of the water. After 24 hours, however, they were 2–4 times higher. Data indicate that sediment entrainment depends on high heat fluxes and correspondingly high frazil-ice production rates, as well as sufficiently strong turbulence. Waves do not seem to increase sediment entrainment significantly.
The surges of Variegated Glacier, Alaska, U.S.A., and their connection to climate and mass balance
Authors: Eisen O. , Harrison W. D. , Raymond C. F.
Published: November 2007

One of the questions still unanswered concerning the surge behavior of glaciers concerns their quasi-periodic occurrence. Some results on the phenomenological connection between local cumulative balance and surge initiation of Variegated Glacier, Alaska, U.S.A., are discussed here. Based on climate data from neighboring weather stations, an empirical relation between precipitation, temperature and local mass balance is established and used to reconstruct the annual balance at a location in the accumulation area back to 1905. Between the last four surges in 1946/47, 1964/65, 1982/83 and 1994/95, the ice-equivalent cumulative balance was 43.5 m on average, with a 1σ error of 1.2 m. Although the existence of a surge level cannot be directly interpreted in physical terms, it explains the variable length of the quiescent periods of Variegated Glacier by variations in the accumulation rate prior to the surge. We use the surge level to hindcast former unobserved surges, to compare the results with other surge datings obtained from photographs and to establish a complete surge history for Variegated Glacier for the 20th century.
Post-stagnation behavior in the upstream regions of Ice Stream C, West Antarctica
Authors: Price S. F. , Bindschadler R. A. , Hulbe C. L. , Joughin I. R.
Published: November 2007

The region where two active tributaries feed into the now stagnant Ice Stream C (ISC), West Antarctica, is thickening. In this region, we observe a correlation between faster ice flow (the tributaries) and elevated topography. We conclude that stagnation of ISC resulted in compression and thickening along the tributaries, eventually forming a “bulge” on the ice-sheet surface. Modern hydraulic potential gradients would divert basal meltwater from ISC to Ice Stream B (ISB). These gradients are primarily controlled by the bulge topography, and so likely formed subsequent to trunk stagnation. As such, we argue against “water piracy” as being the cause for ISC’s stagnation. Kinematic-wave theory suggests that thickness perturbations propagate downstream over time, but that kinematic-wave speed decreases near the stagnant trunk. This and modest diffusion rates combine to trap most of the tributary-fed ice in the bulge region. Using interfero-metric synthetic aperture radar velocity measurements, we observe that half of the ice within ISC’s southern tributary flows into ISB. That flow pattern and other observations of non-steady flow in the region likely result from stagnation-induced thickening along upper ISC combined with a longer period of thinning on upper ISB. If current trends in thickness change continue, more ice from upper ISC will be diverted to ISB.
Dynamical processes involved in the retreat of marine ice sheets
Authors: Hindmarsh Richard C.A. , Meur E. Le
Published: November 2007

Marine ice sheets with mechanics described by the shallow-ice approximation by definition do not couple mechanically with the shelf. Such ice sheets are known to have neutral equilibria. We consider the implications of this for their dynamics and in particular for mechanisms which promote marine ice-sheet retreat. The removal of ice-shelf buttressing leading to enhanced flow in grounded ice is discounted as a significant influence on mechanical grounds. Sea-level rise leading to reduced effective pressures under ice streams is shown to be a feasible mechanism for producing postglacial West Antarctic ice-sheet retreat but is inconsistent with borehole evidence. Warming thins the ice sheet by reducing the average viscosity but does not lead to grounding-line retreat. Internal oscillations either specified or generated via a MacAyeal–Payne thermal mechanism promote migration. This is a noise-induced drift phenomenon stemming from the neutral equilibrium property of marine ice sheets. This migration occurs at quite slow rates, but these are sufficiently large to have possibly played a role in the dynamics of the West Antarctic ice sheet after the glacial maximum. Numerical experiments suggest that it is generally true that while significant changes in thickness can be caused by spatially uniform changes, spatial variability coupled with dynamical variability is needed to cause margin movement.
Longitudinal variations in cross-section morphology along a glacial valley: a case-study from the Tien Shan, China
Authors: Yingkui Li , Gengnian Liu , Zhijiu Cui
Published: November 2007

A new model, the variable width/depth ratio (VWDR) model, is used to analyze longitudinal variations in cross-section morphology along glacial valleys. In the VWDR, the cross-sectional shape of a valley is expressed as a function of the width/depth ratio at various heights above the valley floor. Two parameters, m, a measure of the breadth of the valley floor, and n, a measure of the steepness of the valley sides, appear in the model. We have used the VWDR model to study morphological variations of cross-sections along glacial valleys in the middle Tien Shan mountains, China, and find that: (1) in valleys without tributaries, m increases (the valley floor becomes wider) and n becomes more negative (valley sides become steeper) from the head of the valley to the equilibrium line, and then m decreases and n becomes less negative to the end of the valley; (2) in valleys with tributaries, a similar pattern is observed, with an oscillating maximum in m and minimum in n in those sections where a tributary enters the main valley. These characteristics are believed to reflect a maximization of glacial erosion potential in the vicinity of the equilibrium line and in locations of confluence.
Glacier-climate interaction at low latitudes
Authors: Kaser Georg
Published: November 2007

In the low latitudes there is an absence of major thermal seasonality, yet there are three different climate regimes related to global circulation patterns and their seasonal oscillation: the humid inner tropics, the dry subtropics and, intermediate between these two, the outer tropics. For the respective glacier regimes the vertical profiles of specific mass balance (VBPs) are modeled considering vertical gradients of accumulation, air temperature and albedo, the duration of the ablation period and a factor for the ratio between melting and sublimation. The model is first calibrated with data from Hintereisferner, Austrian Alps, and is then applied to tropical conditions. The simulated VBP matches well the measured profiles from Irian jaya and Mount Kenya. Due to lack of field evidence, the subtropical VBP cannot be verified directly. However, application of the respective model versions separately to the humid and dry seasons of the outer-tropical Glaciar Uruashraju, Cordillera Blanca, Peru, provides reasonable results. Glaciers in the humid inner tropics are considered to be most sensitive to variations in air temperature, while dry subtropical glaciers are most sensitive to changes in air humidity. The two seasons of the outer tropics have to be viewed from these different perspectives.
A new ice-core record from Lomonosovfonna, Svalbard: viewing the 1920–97 data in relation to present climate and environmental conditions
Authors: Isaksson Elisabeth , Pohjola Veijo , Jauhiainen Tauno , Moore John , Pinglot Jean Francis , Vaikmäe Rein , van De Wal Roderik S.W. , Hagen Jon Ove , Ivask Jüri , Karlöf Lars , Martma Tõnu , Meijer Harro A.J. , Mulvaney Robert , Thomassen Martijn , van den Broeke Michiel
Published: November 2007

In 1997 a 121 m ice core was retrieved from Lomonosovfonna, the highest ice field in Spitsbergen, Svalbard (1250 m a.s.l.). Radar measurements indicate an ice depth of 126.5 m, and borehole temperature measurements show that the ice is below the melting point. High-resolution sampling of major ions, oxygen isotopes and deuterium has been performed on the core, and the results from the uppermost 36 m suggest that quasi-annual signals are preserved. The 1963 radioactive layer is situated at 18.5–18.95 m, giving a mean annual accumulation of 0.36 m w.e. for the period 1963–96. The upper 36 m of the ice core was dated back to 1920 by counting layers provided by the seasonal variations of the ions in addition to using a constant accumulation rate, with thinning by pure shear according to Nye (1963). The stratigraphy does not seem to have been obliterated by meltwater percolation, in contrast to most previous core sites on Svalbard. The anthropogenic influence on the Svalbard environment is illustrated by increased levels of sulphate, nitrate and acidity. Both nitrate and sulphate levels started to increase in the late 1940s, remained high until the late 1980s and have decreased during the last 15 years. The records of δ18O, MSA (methane-sulphonic acid), and melt features along the core agree with the temperature record from Longyearbyen and the sea-ice record from the Barents Sea at a multi-year resolution, suggesting that this ice core reflects local climatic conditions.
Cold firn and ice of high-altitude glaciers in the Alps: measurements and distribution modelling
Authors: Suter Stephan , Laternser Martin , Haeberli Wilfried , Frauenfelder Regula , Hoelzle Martin
Published: November 2007

The thermal regime of high-altitude accumulation areas in the Swiss Alps was systematically investigated on the Jungfraufirn, Bernese Alps, on the Breithornplateau, Valais Alps, and on Grenzgletscher, Valais Alps. In 1991, 1992 and 1994, temperatures were measured in a deep hole (120 m deep) and in several shallow holes (14–30 m deep). Whereas the wide névé of the Jungfraufirn at 3400–3600 m a.s.l. and the 3800 m high Breithornplateau seems to be predominantly temperate, cold firn and ice temperatures were measured throughout on Grenzgletscher (3900–4450 m a.s.l.). Mean firn temperatures on Grenzgletscher vary strongly and range between −3° and −14°C. A comparison between the measured temperature profiles and a one-dimensional heat-conduction calculation shows that the release of latent heat by penetrating and refreezing meltwater decisively influences the thermal pattern of the firn pack. A multiple linear regression model, based on measured firn temperatures from the European Alps and the parameters altitude and aspect, yields aspect-dependent lower boundaries for the occurrence of cold firn ranging between 3400 (northerly aspects) and 4150 m a.s.l. (southerly aspects). A total of 120 glaciers with cold-firn areas are found when applying the model to glacier inventory data from the European Alps.
Prevention of outburst floods from periglacial lakes at Grubengletscher, Valais, Swiss Alps
Authors: Haeberli Wilfried , Kääb Andreas , Mühll Daniel Vonder , Teysseire Philip
Published: November 2007

Flood and debris-flow hazards at Grubengletscher near Saas Balen in the Saas Valley, Valais, Swiss Alps, result from the formation and growth of several lakes at the glacier margin and within the surrounding permafrost. In order to prevent damage related to such hazards, systematic investigations were carried out and practical measures taken. The evolution of the polythermal glacier, the creeping permafrost within the large adjacent rock glacier and the development of the various periglacial lakes were monitored and documented for the last 25 years by photogrammetric analysis of annually flown high-resolution aerial photographs. Seismic refraction, d.c. resistivity and gravimetry soundings were performed together with hydrological tracer experiments to determine the structure and stability of a moraine dam at a proglacial lake. The results indicate a maximum moraine thickness of > 100 m; extremely high porosity and even ground caverns near the surface may have resulted from degradation of sub- and periglacial permafrost following 19th/20th-century retreat of the partially cold glacier tongue. The safety and retention capacity of the proglacial lake were enhanced by deepening and reinforcing the outlet structure on top of the moraine complex. The water level of an ice-dammed lake was lowered and a thermokarst lake artficially drained.
Snow grain-size measurements in Antarctica
Authors: Gay Michel , Fily Michel , Genthon Christophe , Frezzotti Massimo , Oerter Hans , Winther Jan-Gunnar
Published: November 2007

Grain-size is an important but not well-known characteristic of snow at the surface of Antarctica. In the past, grain-size has been reported using various methods, the reliability, reproducibility and intercomparability of which is not warranted. In this paper, we present and recommend, depending on available logistical support, three techniques of snow-grain sampling and/or imaging in the field as well as an original digital image-processing method, which we have proved provides reproducible and intercomparable measures of a snow grain-size parameter, the mean convex radius. Results from more than 500 samples and 3000 images of snow grains are presented, which yield a still spatially limited yet unprecedentedly wide picture of near-surface snow grain-size distribution from fieldwork in Antarctica. In particular, except at sites affected by a very particular meteorology, surface grains in the interior of the ice sheet are uniformly small (0.1–0.2 mm). The climate-related increase of grain-size with depth through metamorphism is, as expected, not spatially uniform. Our Antarctic snow grain-size database will continue to grow as field investigations bring new samples, images and measures of snow grain.
Stable-isotopic composition of precipitation over the northern slope of the central Himalaya
Authors: Kang Shichang , Kreutz Karl J. , Mayewski Paul A. , Qin Dahe , Yao Tandong
Published: November 2007

Stable-water-isotope data (δD and δ18O) from three groups of samples (fresh-snow and snow-pit samples collected on Qomolangma (Mount Everest) and Xixa-bangma during field seasons 1997, 1998 and 2001, and precipitation samples collected at Tingri station during summer 2000) are presented and used to survey the isotopic composition of precipitation over the northern slope of the central Himalaya. Multi-year snow-pit samples on Qomolangma have a local meteoric water-line (slope = 8) close to the global value. Deuterium excess (d = δD – 8δ18O) values at Tingri are much lower than those in fresh snow from Qomolangma, probably due to differences in moisture source and air-mass trajectories as well as local weather conditions. There is no obvious seasonal trend for d values in the Qomolangma region. A negative relationship exists between δ18O and d values in both fresh snow on Qomolangma and precipitation at Tingri. Fresh-snow samples collected from different altitudes on Xixabangma allow us to investigate the altitude effect on δ18O values in snow. Of four storm events, only one has an obvious altitude effect on δ18O variation and a very low gradient of −0.1 % per 100 m elevation.
Ice-dynamic conditions of Shirase Glacier, Antarctica, inferred from ERS SAR interferometry
Authors: Pattyn Frank , Derauw Dominique
Published: November 2007

The surface velocity field of Shirase Glacier, a fast-flowing East Antarctic outlet glacier, is determined from ERS synthetic aperture radar (SAR) images by means of speckle tracking using phase correlation, a technique which matches small image kernels of two complex SAR images by maximization of the local coherence. Velocity estimates are used to calculate surface strain rates, which are then used to calculate the large-scale, vertically integrated force balance and to determine the major stress components resisting the driving stress. For the whole glacier system, the driving stress is largely balanced by the basal drag, but with contributions from lateral drag up to 15% of the driving stress at the grounding line. Longitudinal stress gradients have only local importance to the balance of forces, limited to an area of a few square kilometers near the grounding line, where they resist the driving stress. In the grounded part of the glacier, >90% of the total ice velocity is due to basal sliding. Comparison with a balance-flux distribution of the Antarctic ice sheet suggests that the glacier in the downstream part of the Shirase drainage basin is close to equilibrium, showing a slight negative imbalance.
A borehole camera system for imaging the deep interior of ice sheets
Authors: Carsey Frank , Behar Alberto , Lonne Lane A. , Realmuto Vince , Engelhardt Hermann
Published: November 2007

The design and first deployment is described for the Jet Propulsion Laboratory–California Institute of Technology ice borehole camera system for acquisition of down-looking and side-looking images in a borehole made by a hot-water drill. The objective of the system is to acquire images in support of studies of the basal dynamics and thermodynamics of West Antarctic ice streams. A few sample images, obtained during the 2000/01 Antarctic field season, are shown from the basal layers of Ice Stream C.
Flow mechanism of the Des Moines lobe of the Laurentide ice sheet
Authors: Hooyer Thomas S. , Iverson Neal R.
Published: November 2007

Rapid flow of the Des Moines lobe of the Laurentide ice sheet may have been related to its unlithified substrate. New reconstructions of the lobe, based on moraine elevations, sediment subsidence during moraine deposition, and flow-direction indicators, indicate that the lobe may have been ∼3 times thicker than in previous reconstructions. Nevertheless, implied basal shear stresses are <15 kPa, so internal ice deformation was not significant. Instead, the lobe likely moved by a combination of sliding, plowing of particles through the bed surface, and bed shear. Consolidation tests on basal till yield preconsolidation stresses of 125–300 kPa, so effective normal stresses on the bed were small. A model of sliding and plowing indicates that at such stresses most particles gripped by the ice may have plowed easily through the till bed, resulting in too small a shear traction on the bed to shear it at depth. Consistent with this prediction, measurements of orientations of clasts in basal till yield a weak fabric, implying pervasive bed shear strain less than ∼2, although some stronger fabrics have been reported by others. We infer, tentatively, that movement was principally at the bed surface by plowing.
Healed cracks in iceberg ice
Authors: Barrette Paul D. , Jordaan Ian J.
Published: November 2007

A noticeable trait of iceberg ice is the presence of several sets of planar features cross-cutting each other at various angles. Close-up views of these features show that they consist of an array of individual air inclusions that differ in size, shape and spatial distribution. In none of the cases resolvable at the scale of our observations were these inclusions physically linked to form a continuous fracture plane, although they may have originated as such. All postdate the formation of ice veins.
On the representation of ice-shelf grounding zones in SAR interferograms
Authors: Rabus Bernhard T. , Lang Oliver
Published: November 2007

We investigate limitations of the one-dimensional elastic-beam model to detect grounding line and thickness of an ice shelf from a differential interferogram. Spatial limitations due to grounding-line curvature and variable ice thickness are analyzed by comparison with two-dimensional plate flexure. Temporal limitations from the tide-dependent shift of the grounding line are analyzed by superpositions of four tidal flexure profiles representing differential interferograms. (i) At scales greater than one ice thickness, seaward protrusions of the grounding line are well represented by the elastic-beam model, while landward embayments of the same scale produce significant misplacements >10% of the ice thickness. (ii) For reasonable spatial variations of shelf thickness, the elastic-beam model gives reliable estimates of grounding-line position and unfractured mean ice thickness near the grounding line. (iii) For about 20% of superpositions of four tidal flexure profiles, the resulting grounding-line misplacements exceed the physical tidal shift of the grounding line by factors >2. For differential tide levels <10% of a 1 m tide dynamics, a physical shift of the grounding line of 0.3 km per metre of tide can lever misplacements of >2 km. Examples of real interferometric profiles from West Antarctic ice shelves corroborate our results.
Patterning mechanisms in subglacial carbonate dissolution and deposition
Authors: NG Felix , Hallet Bernard
Published: November 2007

Deglaciated bedrock surfaces in limestone areas often exhibit extensive patterning by solutional furrows and carbonate deposits that occur in close association with undulations in the bed topography. These features clearly result from subglacial dissolution and precipitation of calcite on the bed — induced, for instance, by melting and freezing in a regelation water film — but little is known about the observed morphology. In particular, it is intriguing that (i) the solutional furrows, whose formation requires explanation, are collectively organized into arcuate patterns, with characteristic spacing, and (ii) a fluted or “spiculed” surface texture is ubiquitous on the calcite deposits. Herein, we propose specific mechanisms for such patterning based on a theory where chemical processes in the water film are coupled to regelation physics. Solutional furrows reflect locally enhanced dissolution along stoss surfaces, where CO2-rich bubbles advected in the ice from up-glacier come into contact with the bed. The bubbles form as CO2is exsolved from freezing film water at the lee of bed bumps. The flutings on the deposit are inherently the manifestation of a spatial instability at the interface where calcite precipitation occurs. Complex interactions underlie some of the striking glacier-bed features shaped by subglacial chemical processes.
Transient glacier response with a higher-order numerical ice-flow model
Authors: Pattyn Frank
Published: November 2007

In this paper, a higher-order numerical flowline model is presented which is numerically stable and fast and can cope with very small horizontal grid sizes (<10 m). The model is compared with the results from Blatter and others (1998) on Haut Glacier d’Arolla, Switzerland, and with the European Ice-Sheet Modelling Initiative benchmarks (Huybrechts and others, 1996). Results demonstrate that the significant difference between calculated basal-drag and driving-stress profiles in a fixed geometry disappears when the glacier profile is allowed to react to the surface mass-balance conditions and reaches a steady state. Dynamic experiments show that the mass transfer in higher-order models occurs at a different speed in the accumulation and ablation areas and that the front position is more sensitive to migration compared to the shallow-ice approximation.
Last Glacial Maximum equilibrium-line altitudes and paleoclimate, northern Uinta Mountains, Utah, U.S.A.
Authors: Munroe Jeffrey S. , Mickelson David M.
Published: November 2007

Nineteen former valley glaciers were reconstructed for their Last Glacial Maximum (LGM) extents in the northern Uinta Mountains, Utah, U.S.A. Mean equilibrium-line altitudes (ELAs) calculated by four methods (accumulation–area ratio, toe–headwall altitude ratio, lateral moraines and cirque floors) range from 3050 to 3300 m a.s.l. Modern mean summer temperatures (Ts) at the ELAs range from 8.7° to 11.2°C, while modern winter precipitation (P) ranges from 354 to 590 mm snow water equivalent (SWE). Based on the difference in elevation of mean ELAs across the range, LGM P values must have ranged from 940 to 3040 mm SWE, assuming the modern summer lapse rate was the same during the LGM. A Ts depression of 5.5°C is required for these precipitation values to plot in the range of modern ELA values. The reconstructed increase in P at the western end of the range is 10 times the modern increase, reflecting the influence of pluvial Lake Bonneville. Assuming ELA depression (ΔELA) resulted from this P increase and a uniform 5.5°C Ts decrease, the regional LGM ΔELA was approximately 900 m.
Discharge of debris from ice at the margin of the Greenland ice sheet
Authors: Knight Peter G. , Waller Richard I. , Patterson Carrie J. , Jones Alison P. , Robinson Zoe P.
Published: November 2007

Sediment production at a terrestrial section of the ice-sheet margin in West Greenland is dominated by debris released through the basal ice layer. The debris flux through the basal ice at the margin is estimated to be 12–45 m3 m−1 a−1. This is three orders of magnitude higher than that previously reported for East Antarctica, an order of magnitude higher than sites reported from in Norway, Iceland and Switzerland, but an order of magnitude lower than values previously reported from tidewater glaciers in Alaska and other high-rate environments such as surging glaciers. At our site, only negligible amounts of debris are released through englacial, supraglacial or subglacial sediment transfer. Glaciofluvial sediment production is highly localized, and long sections of the ice-sheet margin receive no sediment from glaciofluvial sources. These findings differ from those of studies at more temperate glacial settings where glaciofluvial routes are dominant and basal ice contributes only a minor percentage of the debris released at the margin. These data on debris flux through the terrestrial margin of an outlet glacier contribute to our limited knowledge of debris production from the Greenland ice sheet.
A method for monitoring glacier mass balance using satellite albedo measurements: application to Vatnajökull, Iceland
Authors: De Ruyter De Wildt Martijn S. , Oerlemans Johannes , Björnsson Helgi
Published: November 2007

We compare satellite albedo images of Vatnajökull, Iceland, with mass-balance measurements for the years 1991–99. We find that the equilibrium line is mostly not visible when it is located above its position of the previous year(s). Equilibrium-line detection is further hindered by clouds and a gradual transition between ice and firn or snow. Consequently, firn-line elevation at the end of the melting season is not particularly useful for estimating the annual mass balance. Instead, we propose to study the mean albedo of the entire ice cap throughout the melting season so that all available information about the surface albedo is taken into account. The mean net potential global radiation, which can be estimated from the mean surface albedo alone, both depends on and influences summer melt. It also depends on winter precipitation and, integrated over the melting season, is found to relate linearly to the specific mass balanceB(r= 0.87 and 0.94 for different outlets of Vatnajökull).Bcan be estimated quantitatively when this relation is known and qualitatively when it is not. The uncertainty in the satellite-derived value ofBis 0.5–0.8 m w.e., which for Vatnajökull corresponds to about 27% of the interannual variability ofB.
Measurement of snow density and microstructure using computed tomography
Authors: Lundy Chris C. , Edens Michael Q. , Brown Robert L.
Published: November 2007

In the past, two-dimensional images of internal snow structure have been obtained through plane surface sections or thin sections. These techniques are time-consuming and necessarily destroy the snow specimen. Computed tomography (CT) allows similar images to be obtained, but in a more efficient and non-destructive manner. To demonstrate the methodology, a CT scanner was used to obtain cross-sectional images over time of a snow sample undergoing kinetic-growth metamorphism. Densities calculated from the CT images correlated well to density measured using a traditional method. A procedure was developed that allows the CT image to be converted to binary in an objective manner. Employing innovative stereological software, the microstructural properties (grain diameter, bond diameter, neck length and intercept length) of the snow were then measured from the two-dimensional CT images. The presented methodology provides significant improvements over previous techniques, requiring less time and labor to obtain high-quality microstructural data.
Formation of band ogives and associated structures at Bas Glacier d’Arolla, Valais, Switzerland
Authors: Goodsell Becky , Hambrey Michael J. , Glasser Neil F.
Published: November 2007

Structural glaciological, sedimentological and geophysical techniques are used to provide new insight concerning the formation of band ogives and associated structures at Bas Glacier d’Arolla, Switzerland. Sedimentary stratification, crevasse traces and transverse foliation are identified as planar structures in the lower icefall and glacier tongue. Stratification and crevasse traces are progressively deformed into, and enhance, the transverse foliation found in the glacier tongue. Three-dimensional geometry has been defined using ground-penetrating radar, which portrays four main characteristics: (i) deep reflectors interpreted as the ice/bed interface, (ii) alternating reflection-rich and reflection-poor zones interpreted as ogives, (iii) up-glacier-dipping reflectors, interpreted as planar structures, and (iv) down-glacier-dipping reflectors of uncertain origin. At the glacier surface, each band ogive consists of a light and dark band. The dark bands contain more intense foliation which, on differential weathering, traps fine debris. Clasts and clear ice of basal character within dark ogive bands suggest that basal ice has been raised to the glacier surface. The most applicable model for the formation of band ogives at Bas Glacier d’Arolla is a refinement of Posamentier’s (1978) “reverse faulting” hypothesis. In this context, multiple shear zones are formed, through which basal ice is uplifted to the glacier surface to produce the dark, foliated ogive bands. This model fits observations reported from other glaciers with band ogives.
Force balance along an inland tributary and onset to Ice Stream D, West Antarctica
Authors: Price Stephen F. , Bindschadler Robert A. , Hulbe Christina L. , Blankenship Donald D.
Published: November 2007

The transition from inland- to streaming-style ice flow near to and upstream from the onset to Ice Stream D, West Antarctica, is investigated using the force-balance technique. Basal drag provides the majority of the flow resistance over the study area but is substantially modified by non-local stress gradients. Lateral drag increases with distance downstream, balancing ∼50–100% of the driving stress at the onset. Longitudinal stress gradients (LSG) are also found to be significant, an observation that distinguishes ice flow in this region from the inland- and streaming-flow regimes that bound it, in which LSG are usually negligible. LSG decrease the spatial variability in basal drag and sliding speed and increase the area of the bed over which frictional melting occurs. Overall, LSG decrease the resistive influence of basal stress concentrations and increase the spatial uniformity of basal sliding. These observations suggest that streaming flow develops as an integrated response to the physical interaction between the ice and its bed over an extended region upstream from the onset, rather than being solely due to changes in basal characteristics at the onset. An implication is that non-steady-flow behavior upstream from the onset may ultimately propagate downstream and result in non-steady behavior at the onset.
Performance and applications of an automated c-axis ice-fabric analyzer
Authors: Hansen Dirk P. , Wilen Larry A.
Published: November 2007

A completely automated instrument for ice-fabric analysis has been built, tested and applied. The analysis of a thin section is performed in two steps. First, the section is loaded into the machine and is automatically imaged in about 16 min. Then, at a later time and convenient location, the grains in the section are identified from a stored image (either automatically or by the user), and the c-axis orientation of each grain is calculated at a rate of 1200 grains h−1. Comparison of automated results with manual Rigsby stage results shows excellent agreement. Diagnostics indicate that the accuracy of the new technique is 0.5° in the c-axis orientation. Two specific applications are demonstrated: (1) the ability to measure internal grain orientations that may reveal internal stress, and (2) the ability to measure very thin thin sections. The latter, combined with the ability to zoom in on small regions of a section, will be useful for examining the fabric of fine-grained (< 1 mm size grains) ice.
A century reconstruction of the mass balance of Glacier de Sarennes, French Alps
Authors: Torinesi Olivier , Letréguilly Anne , Valla François
Published: November 2007

The 50 year time series of mass balance on Glacier de Sarennes is one of the longest in the French Alps, and so is often used as a reference for glacier variations in the French Alps. Meteorological data can be used to extend the series backwards in time. Martin (1978) proposed such a reconstruction for the 1882–1977 period. With 50 years of observations, we show that the classical method used by Martin is too dependent on the calibration period. We therefore try to improve the accuracy of this reconstruction using the Vincent and Vallon (1997) method which takes into account the albedo change of the surface during the ablation period (this is called the daily method). This new method appears to be stable in time. Once calibrated, the daily method is applied to reconstruct the 1881–1949 period. The new reconstruction is compared to a volumetric balance between two maps from 1906 and 1981. It appears that both reconstructions (classical and daily) fail to render the trend correctly over a long period of time. The cumulative centred mass balance correlates well (r2 = 0.62) with the hydrological mass-balance series of Aletschgletscher, Switzerland.
Direct measurement of basal motion at a hard-bedded, temperate glacier: Glacier de Tsanfleuron, Switzerland
Authors: Hubbard Bryn
Published: November 2007

A rigid mast, equipped with five multi-turn potentiometers, was bolted to the bedrock floor of a frontal cavity at Glacier de Tsanfleuron, Switzerland. Each potentiometer was linked by a thin cord to an anchor emplaced in the adjacent ice wall, enabling basal ice motion to be measured continually over a 6 day period in the late summer. Results indicate that basal ice velocity increased quasi-linearly away from the glacier bed, rising from 10.66 to 11.82 mm d−1 between 25 and ∼ 265 mm above the ice– bed interface. Extrapolation of this gradient indicates that ∼ 10.55 mm d−1 may have been accommodated by pure slip between the ice and the glacier bed. Basal ice motion is temporally variable, generally being characterized by tens of minutes to hours of little or no motion interspersed with rapid-motion “events” lasting for between < 120 s (a single measurement interval) and 360 s. These motion events were responsible for speeds of up to > 400 mm d−1 over individual measurement intervals. The magnitude–frequency distribution of these events is consistent with a pattern of infrequent and large “slips” initiated at the ice–bed interface, that are manifested as more frequent and smaller motion events some tens of cm above that interface.
Basal conditions and glacier motion during the winter/spring transition, Worthington Glacier, Alaska, U.S.A.
Authors: Harper Joel T. , Humphrey Neil F. , Greenwood Mark C.
Published: November 2007

Observations of the motion and basal conditions of Worthington Glacier, Alaska, U.S.A., during late-winter and spring melt seasons revealed no evidence of a relationship between water pressure and sliding velocity. Measurements included borehole water levels (used as a proxy for basal water pressure), surface velocity, englacial deformation, sliding velocity, and time-lapse videography of subglacial water flow and bed characteristics. The boreholes were spaced 10–15 m apart; six were instrumented in 1997, and five in 1998. In late winter, the water-pressure field showed spatially synchronous fluctuations with a diurnal cycle. The glacier’s motion was relatively slow and non-cyclic. In spring, the motion was characterized by rapid, diurnally varying sliding. The basal water pressure displayed no diurnal signal, but showed high-magnitude fluctuations and often strong gradients between holes. This transition in character of the basal water-pressure field may represent a seasonal evolution of the drainage system from linked cavities to a network of isolated patches and conduits. These changes occurred as the glacier was undergoing a seasonal-velocity peak. The apparent lack of correlation between sliding velocity and water pressure suggests that local-scale water pressure does not directly control sliding during late winter or early in the melt season.
Altitudinal variation of the stable isotopes of snow in regions of high relief
Authors: Holdsworth Gerald , Roy Krouse H.
Published: November 2007

A major discontinuity in the variation of δ18O (δD) with altitude in high mountains was first seen in data from Mount Logan, YukonTerritory, Canada (Holdsworth and others, 1991). The profile of δ vs altitude revealed three well-defined regions: (1) a lower, monotonic, fractionation sequence below ∼3 km; (2) a middle layer, typically 1–2 km thick, within which δ values are nearly constant or stepped with altitude, and (3) part of another fractionation sequence in the “quasi-geostrophic flow region” above ∼5.3 km. The middle region was inferred to be a “mixed layer”, combining moisture from regions (1) and (3). This type of structure is now seen to occur on other high-altitude mountains, including Cerro Aconcagua, Argentina, where observations reach almost 7 km. The new observations confirm the general occurrence of a multi-layered atmosphere during precipitation at high-altitude glacier sites. This structure is linked to synoptic-scale polar cyclones, where the middle layer is identified as being the signature of the warm-front zone. These results have implications for the common practice of using a specific, spatially derived, isotopic thermometer in the time domain for the paleoclimatic interpretation of high-altitude ice-core δ records.
Computing marine-ice thickness at an ice-shelf base
Authors: Holland David M.
Published: November 2007

The freezing of sea water to the base of an ice shelf can give rise to large patches of accumulated ice, a phenomenon known as marine ice. In this study a numerical method is presented for calculating the thickness of the marine-ice layer using an ice- shelf-ocean model. The present-day modeling paradigm of ice-shelf–ocean interaction usually involves the fixed specification of the ice-shelf geometry while the ocean circulation in the cavity beneath the ice shelf evolves freely. This approach relies on several assumptions, such as steady-state ice-shelf thickness and ice-shelf flow fields, in order to make reasonable quantitative estimates of the thermodynamic exchange processes occurring at the ice-shelf base. This paper discusses the impact of these and other assumptions on the estimation of the thickness of the marine-ice layer. Model simulation results are presented for an idealized ice-shelf–ocean configuration as a demonstration of the feasibility of the numerical method. A sensitivity analysis is given so as to quantify the relative uncertainty in the marine-ice thickness that arises from uncertainties in the model input parameters, these being principally the ice-shelf flow field, the basal accumulation rate and the ice-shelf thickness field.
Self-similarity in glacier surface characteristics
Authors: Arnold Neil S. , Rees W. Gareth
Published: November 2007

Catchment-wide information on glacier snow-cover depth, surface albedo and surface roughness is important input data for distributed models of glacier energy balance. In this study, we investigate the small-scale (mm to 100 m) spatial variability in these properties, with a view to better simulating this variability in such models. Data were collected on midre Lovénbreen, a 6 km2 valley glacier in northwest Svalbard. The spatial variability of all three properties was found to be self-similar over the range of scales under investigation. Snow depth and albedo exhibit a correlation length within which measurements were spatially autocorrelated. Late-winter and summer properties of snow depth differed, with smaller depths in summer due to melt, and shorter correlation lengths. Similar correlation lengths for snow depth and surface albedo may suggest that snow-depth variation is an important control on the small-scale spatial variability of glacier surface albedo. For surface roughness, the data highlight a possible problem in energy-balance studies which use microtopographic surveys to calculate aerodynamic roughness, in that the scale of the measurements made affects the calculated roughness value. This suggests that further investigations of the relationships between surface form and aerodynamic roughness of glacier surfaces are needed.
New velocity map and mass-balance estimate of Mertz Glacier, East Antarctica, derived from Landsat sequential imagery
Authors: Berthier Etienne , Raup Bruce , Scambos Ted
Published: November 2007

Automatic feature tracking on two Landsat images (acquired inJanuary 2000 and December 2001) generates a complete and accurate velocity field of Mertz Glacier, East Antarctica. This velocity field shows two main tributaries to the ice stream. Between the tributaries, a likely obstruction feature in the bedrock results in a slow-down of the flow. A third Landsat image, acquired in 1989 and combined with the 2000 image, permits the determination of the glacier mean velocity during the 1990s. Although some parts of the Mertz Glacier system show evidence of slight speed increase, we conclude that the Mertz flow speed is constant within our uncertainty (35 m a−1). Using this complete velocity field, new estimates of the ice discharge flux, 17.8 km3 a−1 (16.4 Gt a−1), and of the basal melting of the tongue, 11 m a−1 of ice, are given. Our results lead to an apparent imbalance of the drainage basin (ice discharge 3.5 km3 a−1 lower than the accumulation). Considering previous studies in the Mertz Glacier area, we then discuss the uncertainty of this imbalance and the problems with accumulation mapping for this region.