bluuebunny/crossref_metadata_2025 huggingface dataset:
Two methods for firn-area and mass-balance monitoring of Svalbard glaciers with SAR satellite images
Authors: König Max , Winther Jan-Gunnar , Kohler Jack , König Florian
Published: November 2007

This paper presents two methods for glacier monitoring on Svalbard using synthetic aperture radar (SAR) satellite images. Both methods were developed on glaciers in the Kongsfjorden area. The first method monitors the firn area extent and the firn line over time by thresholding and filtering the SAR image. Manual detection of the threshold is preferable, but using a constant threshold for all images also gives adequate results. A retreat of the firn-line position is visible, especially on Kongsvegen, corresponding to consecutive years of negative mass balance. The second method applies a k-means classification to three clusters on the glacier surface. The areal extent of the resulting class on the upper part of the glacier correlates remarkably well with the independently measured mass balance of Kongsvegen, having a correlation coefficient of around 0.89 for the various glaciers. This is because the snow from the accumulation area influences the k-means classification. Thus, on glaciers where mass-balance values are available, new mass-balance values can be predicted solely from SAR images. For glaciers where no mass balance is available, the area change cannot be calibrated to absolute mass-balance values, but relative changes can be predicted.
Melting and freezing beneath the Ross ice streams, Antarctica
Authors: Joughin Ian , Tulaczyk Slawek , MacAyeal Douglas R. , Engelhardt Hermann
Published: November 2007

We have estimated temperature gradients and melt rates at the bottom of the ice streams in West Antarctica. Measured velocities were used to include the effects of horizontal advection and strain heating in the temperature model and to determine shear heating at the bed. Our modeled temperatures agree well with measured temperatures from boreholes in regions of steady flow. We find that ice-stream tributaries and the inland ice account for about 87% of the total melt generated beneath the Ross ice streams and their catchments. Our estimates indicate that the ice plains of Whillans Ice Stream and Ice Stream C (even when active) have large areas subject to basal freezing, confirming earlier estimates that import of water from upstream is necessary to sustain motion. The relatively low melt rates on Whillans Ice Stream are consistent with observations of deceleration over the last few decades and suggest a shutdown may take place in the future, possibly within this century. While there are pockets of basal freezing beneath Ice Streams D and E, there are larger areas of basal melt that produce enough melt to more than offset the freezing, which is consistent with inferences of relatively steady flow for these ice streams over the last millennium.
Ice-shelf microtopography observed using satellite thermal imagery
Authors: Anderson P.S.
Published: November 2007

Small anomalies in ice-shelf surface temperature correlate with measured microtopography. Clear-sky thermal infrared (TIR) images of the Brunt Ice Shelf, Antarctica, frequently show persistent patterns of anomalous snow surface temperatures. The anomalous signatures appear as stripes orientated along the ice flowline and are of the order of 5 K in magnitude. The positional persistence of the stripes suggests a topographic mechanism for their formation. In order to test this hypothesis, the TIR stripes are compared to a digital terrain model (DTM) derived from a kinematic global positioning system survey of the ice shelf. Ridges and valleys are seen in the DTM; the ridges correspond to the warmer TIR stripes, the valleys to the colder areas. In order to investigate the mechanism that couples elevation with thermal signature, two comparable but contrasting sets of clear-sky infrared images are presented, along with surface meteorological data. The first shows strong TIR stripes, whilst the second, despite similar snow- and air-temperature profiles, shows a weaker signature and smaller sensible-heat flux, H. Two possible mechanisms are presented which explain the TIR signature: surface elevation mapping onto the vertical air-temperature profile or, alternatively, enhanced surface sensible-heat flux on elevated areas. At present, there is insufficient information to resolve this uncertainty.
Changes in the ice plain of Whillans Ice Stream, West Antarctica
Authors: Bindschadler Robert , Vornberger Patricia , Gray Laurence
Published: November 2007

Data from the mouth of the decelerating Whillans Ice Stream (WIS), West Antarctica, spanning 42 years are reviewed. Deceleration has continued, with local areas of both thinning and thickening occurring. The mean thinning rate is 0.48 ± 0.77 ma–1. No consistent overall pattern is observed. Ice thickens immediately upstream of Crary Ice Rise where deceleration and divergence are strongest, suggesting expanded upstream influence of the ice rise. Thinning is prevalent on the Ross Ice Shelf. Grounding-line advance at a rate of 0.3 km a–1 is detected in a few locations. Basal stresses vary across an ice-stream transect with a zone of enhanced flow at the margin. Marginal shear is felt at the ice-stream center. Mass-balance values are less negative, but larger errors of earlier measurements mask any possible temporal pattern. Comparisons of the recent flow field with flow stripes suggest WIS contributes less ice to the deep subglacial channel carved by Mercer Ice Stream and now flows straighter. The general lack of geometric changes suggests that the regional velocity decrease is due to changing basal conditions.
Spatial and temporal variations in surface velocity and basal drag across the tongue of the polythermal glacier midre Lovénbreen, Svalbard
Authors: Rippin D.M. , Willis I.C. , Arnold N.S. , Hodson A.J. , Brinkhaus M.
Published: November 2007

We present results of a detailed investigation of surface motion across the tongue of a polythermal glacier, midre Lovenbreen, Svalbard, during the 1999 summer. Surface velocities in the warm-based upper tongue increased during periods of enhanced surface melting and rainfall events, and force-balance analysis indicates that these velocity variations were locally forced, probably by fluctuations in subglacial water pressure. Surface speed-ups were also observed on the cold-based lower tongue (which acted as a sticky spot, through which there was minimal subglacial drainage for most of the summer), but these were largely non-locally forced by longitudinal coupling to the faster-moving ice up-glacier. On one occasion, however, a large, rapid input of surface water to the glacier reduced the basal drag beneath the cold-based lower tongue, presumably due to hydraulic jacking. This resulted in locally forced enhanced surface velocities across the entire tongue, accompanied by a breaching of the lower tongue and an outburst of subglacially stored water.
Signal variability in replicate ice cores
Authors: Wolff Eric W. , Cook Eliza , Barnes Piers R.F. , Mulvaney Robert
Published: November 2007

Replicate ice cores have been drilled about 10 m apart for the top 790 m of the ice sheet at Dome C, Antarctica. This provides an opportunity to examine inter-core variation of the signal for identical events, based on dielectric profile (DEP) comparisons. Comparison of the signal from the same core (a section 48 m long), measured 1 year apart, showed good reproducibility, with peak heights varying by around 10% between the two measurements. For the two replicate cores, identical peaks were matched and showed variability between cores of typically a factor 1.5. This can be explained based on the likelihood of significant time periods of missing accumulation in any single core at sites with such low snow accumulation rate. To synchronize core depths by matching peaks, it is essential to use the pattern of peaks, rather than just widely spaced individual strong peaks. To derive a quantitative volcanic index from these low-accumulation rate sites, it will be necessary to combine or average the results from several closely spaced parallel cores.
An experimental study of incongruent dissolution of CaCO3 under analogue glacial conditions
Authors: McGillen Max R. , Fairchild Ian J.
Published: November 2007

This paper addresses the nature of incongruent dissolution of calcite in glacial settings using an experimental approach. Various CaCO3 samples were comminuted using two contrasting techniques (dry machine-milling, and hand-grinding of an ice-water mixture) and dissolved to calcite saturation in both high-pCO2 (10−2 atm) and low-pCO2 (10−3.5 = atmospheric) conditions. Ion yields of Ca, Mg and Sr were determined at the end of the experiments. Leachates displayed enhancement of Mg/Ca and Sr/Ca by a factor of 1.3–8.3 compared with bulk solid carbonate. Lower Mg/Ca and Sr/Ca ratios under higher-pCO2 conditions reflect the percentage dissolution of the carbonate, not the pCO2 per se. The experimental results imply that reported natural incongruent dissolution is readily reproducible in the laboratory and is primarily dependent on the water/rock ratio. Quantitative analysis of new and previous results suggests that the effect relates to the most reactive 0.15–0.25% of the sample, equating volumetrically to the outer few lattice layers, and it is interpreted as an anomalous leaching behaviour of calcite surfaces freshly exposed by crushing. This phenomenon could serve as an index of the relative efficiencies of fragmenting and dissolutional processes in glacial environments.
Accumulation variability over a small area in east Dronning Maud Land, Antarctica, as determined from shallow firn cores and snow pits: some implications for ice-core records
Authors: Karlöf Lars , Isaksson Elisabeth , Winther Jan-Gunnar , Gundestrup Niels , Meijer Harro A.J. , Mulvaney Robert , Pourchet Michel , Hofstede Coen , Lappegard Gaute , Pettersson Rickard , Van Den Broeke Michiel , Van De Wal Roderik S.W.
Published: November 2007

We investigate and quantify the variability of snow accumulation rate around a medium-depth firn core (160 m) drilled in east Dronning Maud Land, Antarctica (75°00′ S, 15°00’ E; 3470 m h.a.e. (ellipsoidal height)). We present accumulation data from five snow pits and five shallow (20 m) firn cores distributed within a 3.5–7 km distance, retrieved during the 2000/01 Nordic EPICA (European Project for Ice Coring in Antarctica) traverse. Snow accumulation rates estimated for shorter periods show higher spatial variance than for longer periods. Accumulation variability as recorded from the firn cores and snow pits cannot explain all the variation in the ion and isotope time series; other depositional and post-depositional processes need to be accounted for. Through simple statistical analysis we show that there are differences in sensitivity to these processes between the analyzed species. Oxygen isotopes and sulphate are more conservative in their post-depositional behaviour than the more volatile acids, such as nitrate and to some degree chloride and methanesulphonic acid. We discuss the possible causes for the accumulation variability and the implications for the interpretation of ice-core records.
Intermediate-depth ice coring of high-altitude and polar glaciers with a lightweight drilling system
Authors: Zagorodnov V. , Thompson L.G. , Ginot P. , Mikhalenko V.
Published: November 2007

A total of 11 ice cores to a maximum depth of 460 m have been obtained over the past 3 years from high-altitude glaciers on the saddle of Mount Bona and Mount Churchill in Alaska (designated B–C), and on Quelccaya ice cap and Nevado Coropuna in Peru. Ice coring was conducted using an intermediate-depth drilling system. The system includes an electromechanical drill (EMD) and an ethanol thermal electric drill (ETED). The EMD permitted an average ice-core production rate (ICPR) of 7.0 m h−1 down to 150 m. An average ICPR of 2 m h−1 to 460 m depth was possible with the ETED. The quality of the B–C ice cores is better than that of cores previously drilled with an EMD and ETED system. A new cutter design, drilling with a lubricant/cutting fluid and a new anti-torque assembly were tested in the laboratory and in glacier boreholes. We examine the performance of the drills in cold and temperate ice and in clean and particle-laden ice. The influence of the ethanol drilling fluid on ice-core isotopic, ionic and dust composition is discussed.
Structure of southeastern Antarctic Peninsula ice shelves and ice tongues from synthetic aperture radar imagery
Authors: Jezek K.C. , Liu H.X.
Published: November 2007

Examination of synthetic aperture radar data collected over the southeastern Antarctic Peninsula shows that features sometimes mapped as ice shelves are more likely composed of numerous ice tongues interspersed within a matrix of fast ice and icebergs. The tongues are formed by the seaward extension of numerous small mountain glaciers that drain from the Antarctic Peninsula. Once afloat, the tongues intermingle with a matrix of fast ice and brash. Examination of 1997 RADARSAT-1 image mosaics shows that southeastern Antarctic Peninsula composite-ice shelves covered an area of about 3500 km2. Like ice tongues around the rest of Antarctica, these features are highly fragmented and likely to be susceptible to mechanical failure. One such composite shelf, located between New Bedford and Wright Inlets, was observed to decrease in area by 1200 km2 between 1997 and 2000.
Model experiments on large tabular iceberg evolution: ablation and strain thinning
Authors: Jansen Daniela , Sandhäger Henner , Rack Wolfgang
Published: November 2007

Antarctic tabular icebergs are important active components of the ice–ocean system. To investigate the relevance of inherent ice dynamics to iceberg evolution, we developed a numerical model based on the fundamental equations of ice-shelf flow and heat transfer, forced by environmental parameters of the ice–ocean–atmosphere system. Model experiments with idealized icebergs of constant density show that the strain thinning rate for a typical iceberg with a thickness of 250 m and a temperature of −15°C is about 1 m a−1. Sensitivity studies for different scenarios of environmental conditions confirmed the reliability of our model. A 5 year simulation of the evolution of iceberg A-38B yielded a mean decrease in thickness from 220 m to 106.3 m, 95% of which was caused by basal melting, 1% by surface melting and 4% by strain thinning. We found iceberg spreading decelerating by about 75%, and ice temperatures being strongly affected by progressive erosion of the relatively warm basal layers and warming in the uppermost part. According to the model results, basal melting is the primary cause of change of iceberg geometry during drift, whereas strain thinning is only relevant in cold areas where basal melting is low.
An exploratory study of ice-cube spikes
Authors: Knight Charles A.
Published: November 2007

It is well known that ice-cube spikes form by the extrusion of water at the surface of a freezing ice cube, driven by the expansion accompanying freezing. The growing spikes are water-filled ice tubes, growing at their tips as the water is expelled. This paper represents an exploration of their formation and the principles behind whether a freezing ice cube grows a spike or not. For this purpose, ice cubes are frozen in one set of conditions to observe what happens when spikes do or do not form. Spike formation requires a nearly steady-state freezing-and-extrusion process at the growing tip, in order to maintain a nearly constant diameter. Most spikes are polycrystals with special orientation relationships that evidently allow the steady state to become established. Thus the orientations of the initial crystals, formed by chance, help to determine whether a spike forms or not. The main alternative to spike formation is flooding over the upper surface of the ice. Spike formation depends upon an effectively non-zero contact angle of water on ice that impedes flooding. The onset of flooding is probably sensitive to the ice growth rate at the water surface, which depends strongly upon the crystal orientation with respect to the water surface where it contacts the crystal.
Characterization and formation of melt layers in polar snow: observations and experiments from West Antarctica
Authors: Das Sarah B. , Alley Richard B.
Published: November 2007

Surface melting rarely occurs across most of the Antarctic ice sheet, away from the warmer coastal regions. Nonetheless, isolated melt features are preserved in the firn and ice in response to infrequent and short-lived melting events. An understanding of the formation and occurrence of these melt layers will help us to interpret records of past melt occurrences from polar ice cores such as the Siple Dome ice-core record from West Antarctica. A search in the near-surface firn in West Antarctica found that melt features are extremely rare, and consist of horizontal, laterally continuous, one to a few millimeter thick, ice layers with few air bubbles. The melt layers found date from the 1992/93 and 1991/92 summers. Field experiments to investigate changes in stratigraphy taking place during melt events reproduced melt features as seen in the natural stratigraphy. Melting conditions of varying intensity were created by passively heating the near-surface air for varying lengths of time inside a clear plastic hotbox. Melt layers formed due entirely to preferential flow and subsequent refreezing of meltwater from the surface into near-surface, fine-grained, crust layers. Continuous melt layers were formed experimentally when positive-degree-day values exceeded 1ºC-day, a value corresponding well with air-temperature records from automatic weather station sites where melt layers formed in the recent past.
Spatial and temporal variability of snow accumulation in East Antarctica from traverse data
Authors: Frezzotti Massimo , Pourchet Michel , Flora Onelio , Gandolfi Stefano , Gay Michel , Urbini Stefano , Vincent Christian , Becagli Silvia , Gragnani Roberto , Proposito Marco , Severi Mirko , Traversi Rita , Udisti Roberto , Fily Michel
Published: November 2007

Recent snow accumulation rate is a key quantity for ice-core and mass-balance studies. Several accumulation measurement methods (stake farm, fin core, snow-radar profiling, surface morphology, remote sensing) were used, compared and integrated at eight sites along a transect from Terra Nova Bay to Dome C, East Antarctica, to provide information about the spatial and temporal variability of snow accumulation. Thirty-nine cores were dated by identifying tritium/b marker levels (1965_66) and non-sea-salt (nss) SO42_ spikes of the Tambora (Indonesia) volcanic event (1816) in order to provide information on temporal variability. Cores were linked by snow radar and global positioning system surveys to provide detailed information on spatial variability in snow accumulation. Stake-farm and ice-core accumulation rates are observed to differ significantly, but isochrones (snow radar) correlate well with ice-core derived accumulation. The accumulation/ablation pattern from stake measurements suggests that the annual local noise (metre scale) in snow accumulation can approach 2 years of ablation and more than four times the average annual accumulation, with no accumulation or ablation for a 5 year period in up to 40% of cases. The spatial variability of snow accumulation at the kilometre scale is one order of magnitude higher than temporal variability at the multi-decadal/secular scale. Stake measurements and firn cores at Dome C confirm an approximate 30% increase in accumulation over the last two centuries, with respect to the average over the last 5000 years
A distributed surface energy-balance model for complex topography and its application to Storglaciären, Sweden
Authors: Hock Regine , Holmgren Björn
Published: November 2007

A grid-based surface energy-balance mass-balance model has been developed to simulate snow- and ice melt in mountainous regions with an hourly resolution. The model is applied to Storglaciären, a valley glacier in Sweden, using a 30 m resolution digital elevation model. Emphasis is directed towards computing the radiation components. These are modelled individually, considering the effects of slope angle, aspect and effective horizon. A new parameterization for snow albedo is suggested, modifying the albedo of the preceding hour as a function of time after snowfall, air temperature and cloudiness. The model is used to provide the meltwater input for discharge modelling and to assess the influence of the individual components on melt. Results are validated by means of observed melt rates, patterns of snow-line retreat and proglacial discharge. In general, simulations are in good agreement with observations. In particular, the diurnal and seasonal fluctuations of discharge are simulated remarkably well.
Decadal-scale variations in ice flow along Whillans Ice Stream and its tributaries, West Antarctica
Authors: Stearns Leigh A. , Jezek Kenneth C. , Van Der Veen C.J.
Published: November 2007

We investigate velocity changes occurring along Whillans Ice Stream (WIS) by comparing velocities derived from repeat aerial photographs acquired in 1985-89 (average date of 1987) to interferometric satellite radar (InSAR) velocities collected in 1997. Three different analysis methods are applied to the velocity data. First, temporal and spatial changes in velocities are correlated to identifiable features (flowlines, shear margins, bed features) visible on the 1997 RADARSAT Antarctic Mapping Project mosaic. Second, we relate velocity gradients to stresses via the flow law and, along with surface topography and ice-thickness data, apply the force-budget technique to determine the relative importance of driving stress, side drag and basal drag over time. Finally, the mass balance of the main part of WIS is determined for 1987 and 1997. Our results are consistent with previous studies that show an overall deceleration resulting in downstream thickening of the ice stream (Whillans and others, 2001; Joughin and others, 2002).
Glaciochemical reconnaissance of a new ice core from Severnaya Zemlya, Eurasian Arctic
Authors: Weiler Karin , Fischer Hubertus , Fritzsche Diedrich , Ruth Urs , Wilhelms Frank , Miller Heinz
Published: November 2007

A deep ice core has been drilled on Akademii Nauk ice cap, Severnaya Zemlya, Eurasian Arctic. High-resolution chemical analysis has been carried out for the upper 53 m of this ice core to study its potential as an atmospheric aerosol archive, despite strong meltwater percolation. These records show that a seasonal atmospheric signal cannot be deduced. However, strong year-to-year variations have allowed the core to be dated, and a mean annual net mass balance of 0.46 m w.e. a-1 was deduced. The chemical signature of an extraordinarily high peak in electrical conductivity at 26 m depth pointed clearly to the eruption of Bezymianny, Kamchatka, in 1956. However, in general, peaks in the electrical conductivity are not necessarily related to deposition of volcanogenic sulphur aerosol. In contrast, maximum sulphate and nitrate concentrations in the ice could be related to maximum SO2 and NOx anthropogenic emissions in the 1970s, probably caused by the nickel- and copper-producing industries in Norilsk and on the Kola peninsula or by industrial combustion processes occurring in the Siberian Arctic. In addition, during recent decades sulphate and nitrate concentrations declined by 80% and 60%, respectively, reflecting a decrease in anthropogenic pollution of the Arctic basin.
Dissipated work, stability and the internal flow structure of granular snow avalanches
Authors: Bartelt Perry , Buser Othmar , Kern Martin
Published: November 2007

We derive work dissipation functionals for granular snow avalanches flowing in simple shear. Our intent is to apply constructive theorems of non-equilibrium thermodynamics to the snow avalanche problem. Snow chute experiments show that a bi-layer system consisting of a non-yielded flow plug overriding a sheared fluidized layer can be used to model avalanche flow. We show that for this type of constitutive behaviour the dissipation functionals are minimum at steady state with respect to variations in internal velocity; however, the functionals must be constrained by subsidiary mass- continuity integrals before the equivalence of momentum balance and minimal work dissipation can be established. Constitutive models that do not satisfy this equivalence are henceforth excluded from our consideration. Fluctuations in plug and slip velocity depend on the roughness of the flow surface and viscosity of the granular system. We speculate that this property explains the transition from flowing avalanches to powder avalanches. Because the temperature can safely be assumed constant, we demonstrate within the context of non-equilibrium thermodynamics that granular snow avalanches are irreversible, dissipative systems, minimizing – in space – entropy production. Furthermore, entropy production is linear both near and far from steady-state non-equilibrium because of the mass-continuity constraint. Finally, we derive thermodynamic forces and conjugate fluxes as well as expressing the corresponding phenomenological Onsager coefficients in terms of the constitutive parameters.
The use of remote-sensing data for mass-balance studies at Mýrdalsjökull ice cap, Iceland
Authors: Jaenicke Julia , Mayer Christoph , Scharrer Kilian , Münzer Ulrich , Gudmundsson Agúst
Published: November 2007

A series of satellite images of Mýrdalsjökull, Iceland, was analyzed in view of their value for mass-balance investigations. A combination of optical satellite images from the ASTER sensor and synthetic aperture radar data from ERS-2 and Envisat ASAR proved very useful. The glacier margin of Mýrdalsjökull was delineated on ASTER images from summer and winter 2004. With a time series of summer ASAR images it was possible to monitor the temporal and spatial development of the transient snowline (TSL) throughout the year 2004, as well as the firn line (FL) at the end of the balance year. An ‘inverse’ function was applied to visually enhance detail in the radar imagery. Winter radar images were not useful for mass-balance observations because of frequent surface melting, which prevented the transparency of the snow cover for C-band microwaves. Interannual mass-balance fluctuations were observed by comparing three radar images acquired in late summer 1998, 1999 and 2004 respectively. These fluctuations follow the same trend as the annual mean air temperature which shows a strong increasing trend between 1999 and 2004. An accumulation-area ratio of <0.43 was determined for 2004, indicating clear negative mass-balance conditions. Monitoring the TSL-FL with radar summer images for mass-balance studies, rather than the equilibrium line (EL), is suggested for large ice caps in maritime climates.
A wireless multi-sensor subglacial probe: design and preliminary results
Authors: Hart Jane K. , Martinez Kirk , Ong Royan , Riddoch Alistair , Rose Kathryn C. , Padhy Paritosh
Published: November 2007

This paper introduces a new way to investigate in situ processes, the wireless multi-sensor probe, as part of an environmental sensor network. Instruments are housed within a ‘probe’ which can move freely and so behave like a clast. These were deployed in the ice and till at Briksdalsbreen, Norway. The sensors measure temperature, resistivity, case stress, tilt angle and water pressure and send their data to a base station on the glacier surface via radio links. These data are then forwarded by radio to a reference station with mains power 2.5 km away, from where they are sent to a web server in the UK. The system deployed during 2004/05 was very successful and a total of 859 probe days worth of data from the ice and till were collected, along with GPS, weather and diagnostic data about the system.
A new fully three-dimensional numerical model for ice dynamics
Authors: Deponti Alberto , Pennati Vincenzo , De Biase Lucia , Maggi Valter , Berta Fabio
Published: November 2007

The problem of describing ice dynamics has been faced by many researchers; in this paper a fully three-dimensional model for ice dynamics is presented and tested. Using an approach followed by other researchers, ice is considered a non-linear incompressible viscous fluid so that a fluid-dynamic approach can be used. The model is based on the full three-dimensional Stokes equations for the description of pressure and velocity fields, on the Saint-Venant equation for the description of the free-surface time evolution and on a constitutive law derived from Glen’s law for the description of ice viscosity. The model computes the complete pressure field by considering both the hydrostatic and hydrodynamic pressure components; it is time-evolutive and uses high-order numerical approximation for equations and boundary conditions. Moreover it can deal with both constant and variable viscosity. Three theoretical tests and two applications to Priestley Glacier, Antarctica, are presented in order to evaluate the performance of the model and to investigate important phenomena of ice dynamics such as the influence of viscosity on pressure and velocity fields, basal sliding and flow over perturbed bedrocks. All these applications demonstrate the importance of treating the complete pressure and stress fields.
Microstructure mapping: a new method for imaging deformation-induced microstructural features of ice on the grain scale
Authors: Kipfstuhl Sepp , Hamann Ilka , Lambrecht Anja , Freitag Johannes , Faria Sérgio H. , Grigoriev Dimitri , Azuma Nobuhiko
Published: November 2007

This work presents a method of mapping deformation-related sublimation patterns, formed on the surface of ice specimens, at microscopic resolution (3–4 μm pixel−1). The method is based on the systematic sublimation of a microtomed piece of ice, prepared either as a thick or a thin section. The mapping system consists of an optical microscope, a CCD video camera and a computer-controlled xy-stage. About 1500 images are needed to build a high-resolution mosaic map of a 4.5 × 9 cm section. Mosaics and single images are used to derive a variety of statistical data about air inclusions (air bubbles and air clathrate hydrates), texture (grain size, shape and orientation) and deformation-related features (subgrain boundaries, slip bands, subgrain islands and loops, pinned and bulged grain boundaries). The most common sublimation patterns are described, and their relevance for the deformation of polar ice is briefly discussed.
Debris characteristics and ice-shelf dynamics in the ablation region of the McMurdo Ice Shelf, Antarctica
Authors: Glasser Neil , Goodsell Becky , Copland Luke , Lawson Wendy
Published: November 2007

This paper presents observations and measurements of debris characteristics and ice-shelf dynamics in the ablation region of the McMurdo Ice Shelf in the Ross Sea sector of Antarctica. Ice-shelf surface processes and dynamics are inferred from a combination of sedimentological descriptions, ground-penetrating radar investigations and through ablation, velocity and ice-thickness measurements. Field data show that in the study area the ice shelf moves relatively slowly (1.5–18.3ma–1), has high ablation rates (43–441 mm during 2003/04 summer) and is thin (6–22 m). The majority of debris on the ice shelf was originally transported into the area by a large and dynamic ice-sheet/ice-shelf system at the Last Glacial Maximum. This debris is concentrated on the ice-shelf surface and is continually redistributed by surface ablation (creating an ice-cored landscape of large debris-rich mounds), ice-shelf flow (forming medial moraines) and meltwater streams (locally reworking material and redistributing it across the ice-shelf surface). A conceptual model for supraglacial debris transport by contemporary Antarctic ice shelves is presented, which emphasizes these links between debris supply, surface ablation and ice-shelf motion. Low-velocity ice shelves such as the McMurdo Ice Shelf can maintain and sequester a debris load for thousands of years, providing a mechanism by which ice shelves can accumulate sufficient debris to contribute to sediment deposition in the oceans.
Steady radial ice-sheet flow with fabric evolution
Authors: Morland Leslie W. , Staroszczyk Ryszard
Published: November 2007

Reorientation of individual crystal glide planes, as isotropic surface ice is deformed during its passage to depth in an ice sheet, creates a fabric and associated anisotropy. We adopt an evolving orthotropic viscous law which was developed to reflect the induced anisotropy arising from the mean rotation of crystal axes during deformation. This expresses the deviatoric stress in terms of the strain rate, strain and three structure tensors based on the principal stretch axes, and involves one fabric response function which determines the strength of the anisotropy. The initial isotropic response enters as a multiplying factor depending on a strain-rate invariant and incorporating a temperature-dependent rate factor. The fabric response function has been constructed by correlations with complete (idealized) uniaxial compression and shearing responses for both ‘cold’ and ‘warm’ ice. The possible effects of such fabric evolution are now illustrated by determining steady radially symmetric flow solutions for an ice sheet with a prescribed temperature distribution and subject to an elevation-dependent surface accumulation/ablation distribution, zero basal melting and a prescribed basal sliding law. Comparisons are made with solutions for the conventional isotropic viscous law, for a flat bed, for a bed with a single modest slope hump and for a bed with a single modest slope hollow, for both cold and warm ice.
Analytical solutions for the effect of topography, accumulation rate and lateral flow divergence on isochrone layer geometry
Authors: Parrenin F. , Hindmarsh R.C.A. , Rémy F.
Published: November 2007

The effect of spatial variations in ice thickness, accumulation rate and lateral flow divergence on radar-detected isochrone geometry in ice sheets is computed using an analytical method, under assumptions of a steady-state ice-sheet geometry, a steady-state accumulation pattern and a horizontally uniform velocity shape function. By using a new coordinate transform, we show that the slope of the isochrones (with a normalized vertical coordinate) depends on three terms: a principal term which determines the sign of the slope, and two scale factors which can modify only the amplitude of the slope. The principal term depends only on a local characteristic time (ice thickness divided by accumulation rate minus melting rate) between the initial and final positions of the ice particle. For plug flow, only the initial and final values have an influence. Further applications are a demonstration of how the vertical velocity profile can be deduced from sharp changes in isochrone slopes induced by abrupt steps in bedrock or mass balance along the ice flow. We also demonstrate ways the new coordinate system may be used to test the accuracy of numerical flow models.
The development of brittle structures in an alpine valley glacier: Pasterzenkees, Austria, 1887–1997
Authors: Herbst Paul , Neubauer Franz , Schöpfer Martin PJ.
Published: November 2007

Brittle structures exposed in the ablation area of Pasterzenkees, Austria, were interpreted using aerial photographs and maps covering a period of 100 years. The most common structural features observed in aerial photographs are: (1) normal faults, which are particularly well developed along the lateral margins of the glacier and at the terminus; (2) large-scale tension gashes and Riedel shears that develop along the northeastern lateral margin of the glacier and between ice-flow units; (3) thrust faults, which develop at the terminus and cross-cut the full width of the glacier; and (4) band ogives. Longitudinal and transverse topographic profiles are available for the period covered in this study, and ice-flow velocity data are available from 1927. These data provide a means for interpreting the variations of observed structures in terms of ice-flow velocity. Thrust faults predominantly develop during periods of glacier retreat, when the glacier snout becomes an obstacle. Normal faults typically develop in areas of high glacier surface relief and are interpreted as gravity collapse structures. The orientation of sub-vertical, wide open crevasses along the lateral margin of Pasterzenkees varied. These variations are interpreted as reflecting changes of the flow regime and indicate a transition from simple shearing to transtension during a period of ice-flow deceleration.
Bed radar reflectivity across the north margin of Whillans Ice Stream, West Antarctica, and implications for margin processes
Authors: Raymond C.F. , Catania Ginny A. , Nereson Nadine , Van Der Veen C.J.
Published: November 2007

Surface-based ice-penetrating radar profiles were made across the active north margin (the Snake) of the upper part of Whillans Ice Stream (formerly Ice Stream B, branch B2), West Antarctica, at three locations. Low frequency (about 2 MHz) and the ground deployment of the radar allowed penetration through the near-surface zone of fracturing to detect internal layering and bed reflection characteristics on continuous profiles spanning from the slow-moving ice of Engelhardt Ridge well into the chaotic zone of the shear margin. Internal layers were tracked beneath the chaotic zone, where they are warped but remain continuous. The energy returned from internal layers showed no systematic changes associated with the transition from the undisturbed surface of the slow-moving ice into the fractured surface of the shear margin, thus indicating little effect from the surface crevasses on the penetration of the radar signal. Based on this calibration of the near-surface effects and corrections for path length, spreading and attenuation, we examine the spatial variation of bed reflectivity. Low bed reflectivity found under Engelhardt Ridge extends under the chaotic zone of the margin into fast-moving ice. We argue that the fast motion in a band along the margin is mediated by processes other than deformation of thick dilated till that is the source of lubrication allowing fast motion in the interior of the ice stream.
Glacier recession on Cerro Charquini (16° S), Bolivia, since the maximum of the Little Ice Age (17th century)
Authors: Rabatel Antoine , Machaca Abraham , Francou Bernard , Jomelli Vincent
Published: November 2007

Cerro Charquini, Bolivia (Cordillera Real, 5392 ma.s.l.) was selected as a site to reconstruct glacier recession since the maximum of the Little Ice Age (LIA) in the central Andes. Five glaciers, located on differently exposed slopes, present comprehensive and well-preserved morainic systems attributed to former centuries. The moraines were dated by lichenometry and show a consistent organization on the different slopes. The past geometry of the glaciers was reconstructed using ground topography and aerophotogrammetry. Lichenometric dating shows that the LIA maximum occurred in the second half of the 17th century, after which the glaciers have receded nearly continuously. Over the last decades of the 20th century (1983–97), recession rates increased by a factor of four. On the northern and western slopes, glaciers receded more than on the southern and eastern slopes (by 78% and 65% of their LIA maximum area, respectively). The mean equilibrium-line altitude (ELA) rose by about 160 m between the LIA maximum and 1997. Recession rates were analysed in terms of climatic signal, suggesting that glacier recession since the LIA maximum was mainly due to a change in precipitation and that the 19th century may have been drier. For the 20th century, a temperature rise of about 0.6°C appears to be the main cause of glacier recession. Recent climatic conditions from 1983 to 1997 correspond to a mass deficit of about 1.36m w.e.a–1. If such conditions persist, the small glaciers below 5300ma.s.l. in the Cordillera Real should disappear completely in the near future.
Optimal filtering for incompletely measured polynomial states over linear observations
Authors: Basin Michael , Calderon‐Alvarez Dario , Skliar Mikhail
Published: November 2007

In this paper, the optimal filtering problem for polynomial system states over linear observations with an arbitrary, not necessarily invertible, observation matrix is treated proceeding from the general expression for the stochastic Ito differential of the optimal estimate and the error variance. As a result, the Ito differentials for the optimal estimate and error variance corresponding to the stated filtering problem are first derived. A transformation of the observation equation is introduced to reduce the original problem to the previously solved one with an invertible observation matrix. The procedure for obtaining a closed system of the filtering equations for any polynomial state over linear observations is then established, which yields the explicit closed form of the filtering equations in the particular case of a third‐order state equation. In the example, performance of the designed optimal filter is verified against a conventional extended Kalman–Bucy filter. Copyright © 2007 John Wiley & Sons, Ltd.
Cover Picture: Excitonic and Vibrational Properties of Single‐Walled Semiconducting Carbon Nanotubes (Adv. Funct. Mater. 17/2007)
Authors: Kilina S. , Tretiak S.
Published: November 2007

The cover picture illustrates excited state dynamics of semiconducting single‐walled carbon nanotubes studied theoretically. Sergai Tretiak and Svetlana Kilina report on p. 3405 that absorption of the light quantum leads to spatially delocalized photoexcitation, which can be described as tightly bound excitons and characterized by 2D plots of transition density. The photoexcitation is coupled to the vibrational degrees of freedom, leading to complex exciton‐phonon dynamics, which can be monitored experimentally using ultrafast spectroscopic probes.

We review quantum‐chemical studies of the excited‐state electronic structure of finite‐size semiconducting single‐walled carbon nanotubes (SWCNTs) using methodologies previously successfully applied to describe conjugated polymers and other organic molecular materials. The results of our simulations are in quantitative agreement with available spectroscopic data and show intricate details of excited‐state properties and photoinduced vibrational dynamics in carbon nanotubes. We analyze in detail the nature of strongly bound first and second excitons in SWCNTs for a number of different tubes, emphasizing emerging size‐scaling laws. Characteristic delocalization properties of excited states are identified by the underlying photoinduced changes in charge densities and bond orders. Due to the rigid structure, exciton–phonon coupling is much weaker in SWCNTs compared to typical molecular materials. Yet we find that, in the ground state, a SWCNT's surface experiences the corrugation associated with electron–phonon interactions. Vibrational relaxation following photoexcitation reduces this corrugation, leading to a local distortion of the tube surface, which is similar to the formation of self‐trapped excitons in conjugated polymers. The calculated associated Stokes shift increases with enlargement of the tube diameters. Such exciton vibrational phenomena are possible to detect experimentally, allowing for better understanding of photoinduced electronic dynamics in nanotube materials.
Inside Front Cover: Surfactant‐Assisted Preparation of Novel Layered Silver Bromide‐Based Inorganic/Organic Nanosheets by Pulsed Laser Ablation in Aqueous Media (Adv. Funct. Mater. 17/2007)
Authors: He C. , Sasaki T. , Zhou Y. , Shimizu Y. , Masuda M. , Koshizaki N.
Published: November 2007

Novel layered AgBr‐based inorganic/organic nanosheets can be prepared by pulsed laser ablation of Ag in an aqueous solution of cetyltrimethylammonium bromide (CTAB), report Takeshi Sasaki, Naoto Koshizaki, and co‐workers on p. 3554. The green image shows the distribution of carbon in a nanosheet deposited on a holey carbon film. The nanosheet structure is composed of anionic bromine‐rich AgBr inorganic layers and cationic surfactant layers (CTA+) with an interdigitated bilayer arrangement.

A novel layered AgBr‐based inorganic/organic nanocomposite was prepared by pulsed laser ablation (PLA) of Ag in aqueous media in the presence of cetyltrimethylammonium bromide (CTAB), and the formation mechanism of two‐dimensional nanosheet was discussed. TEM observations indicate that the obtained AgBr‐based inorganic/organic nanocomposite possesses a well‐defined two‐dimensional shape and that the size of the nanosheet can be changed with the surfactant concentration in the solution. X‐ray diffraction (XRD) pattern was composed of a series of peaks that could be indexed to (00l) reflections of a layered structure, and the basal spacing of 20.0 Å indicated that the surfactant was included between the AgBr interlayers in an interdigitated bilayer arrangement. In contrast, a layered inorganic/organic nanocomposite cannot be formed at a CTAB concentration lower than the critical micelle concentration (CMC). Based on our detailed investigation, we proposed the nanocomposite formation process, that is, that negatively charged inorganic AgBr was produced by a strong reaction between the ablated Ag species and the bromide ions, which are concurrently assembling with cationic surfactant molecules controlled by the charge‐matching mechanism.
Rhodium‐Catalyzed Allylation of Aldehydes with Homoallylic Alcohols by Retroallylation and Isomerization to Saturated Ketones with Conventional or Microwave Heating
Authors: Sumida Yuto , Takada Yuko , Hayashi Sayuri , Hirano Koji , Yorimitsu Hideki , Oshima Koichiro
Published: November 2007

The treatment of an aldehyde with a tertiary homoallylic alcohol at 100–250 °C in the presence of cesium carbonate and a rhodium catalyst leads to allyl transfer from the homoallylic alcohol to the aldehyde. The process includes Rh‐mediated retroallylation to form an allyl rhodium species as the key intermediate. The homoallylic alcohol formed initially through allyl transfer is converted under the reaction conditions into the corresponding saturated ketone when bulky ligands are used. Microwave heating at 250 °C accelerates the reaction significantly.
A performance‐oriented adaptive scheduler for dependent tasks on grids
Authors: Bittencourt Luiz F. , Madeira Edmundo R. M.
Published: November 2007

A scheduler must consider the heterogeneity and communication delays when scheduling dependent tasks on a grid. The task‐scheduling problem is NP‐Complete in general, which led us to the development of a heuristic for the associated optimization problem. In this work we present a dynamic adaptive approach to schedule dependent tasks onto a grid based on the Xavantes grid middleware. The developed dynamic approach is applied to the Path Clustering Heuristic, and introduces the concept of rounds, which take turns sending tasks to execution and evaluating the performance of the resources. The adaptive extension changes the size of rounds during the process execution, taking task attributes and resources performance as parameters, and it can be adopted in other task schedulers. The experiments show that the dynamic round‐based and adaptive schedule can minimize the effects of performance losses while executing processes on the grid. Copyright © 2007 John Wiley & Sons, Ltd.