bluuebunny/crossref_metadata_2025_split huggingface dataset:
Recumbent folding in ice sheets: a core-referential study
Authors: Paul Jacobson H. , Waddington Edwin D. 
Published: November 2007

AbstractTo better understand apparent stratigraphic disturbances in ice cores such as Greenland Ice Sheet Project 2 (GISP2), we examine how ice-sheet flow can transform gentle open folds into order-disturbing recumbent folds. The initial disturbances in the stratigraphy have their roots in transient dynamic processes and local rheological inhomogeneities, but the kinematics of even a simple ice-flow model can deform these disturbances enough to alter paleoclimatic interpretation of an ice core. The local vorticity number suggests which structures can be passively overturned, but analyzing the finite strain along particle paths gives a more complete picture, especially when taken relative to a hypothetical core location. Core-relative isochrones, or “pre-cores”, predict which stratigraphic disturbances will appear as obviously overturned layers in a core. The deformation-gradient tensor along particle paths allows us to calculate the rotation of segments of various reference slopes. These calculations suggest that observed 20° dips in the GISP2 core are rotating on a time-scale of a few hundred years and could result from distortions with much smaller slopes produced upstream. The time during which they can be recognized to be overturning is short because the rotation rate is high. Once overturned they are flattened further and may be hard to recognize, especially in the small cross-section of a core.
The flow of Fireweed rock glacier, Alaska, U.S.A.
Authors: Bucki Adam K. , Echelmeyer Keith A. 
Published: November 2007

AbstractFireweed rock glacier is a large rock glacier in south central Alaska, U.S.A. It flows relatively fast, with velocities up to 3.5 ma–1, and exhibits both seasonal and annual velocity variations, some of which are related to periodic terminus calving and increased rainfall. Our analysis reveals that motion is likely concentrated in a pseudo-rectangular channel within the larger parabolic channel with a “shear plane” at ~27 m depth. There is likely motion along the shear plane as well as internal deformation above it. We estimate that the ice—rock mixture is up to seven times softer than clean glacier ice with a temperature of –2°C. Calving at the terminus is an important component of the mass balance of this rock glacier.
Evolution of cryoconite holes and their contribution to meltwater runoff from glaciers in the McMurdo Dry Valleys, Antarctica
Authors: Fountain Andrew G. , Tranter Martyn , Nylen Thomas H. , Lewis Karen J. , Mueller Derek R. 
Published: November 2007

AbstractCryoconite holes are water-filled holes in the surface of a glacier caused by enhanced ice melt around trapped sediment. Measurements on the ablation zones of four glaciers in Taylor Valley, Antarctica, show that cryoconite holes cover about 4–6% of the ice surface. They typically vary in diameter from 5 to 145 cm, with depths ranging from 4 to 56 cm. In some cases, huge holes form with 5 m depths and 30 m diameters. Unlike cryoconite holes elsewhere, these have ice lids up to 36 cm thick and melt from within each spring. About one-half of the holes are connected to the near-surface hydrologic system and the remainder are isolated. The duration of isolation, estimated from the chloride accumulation in hole waters, commonly shows ages of several years, with one hole of 10 years. The cryoconite holes in the McMurdo Dry Valleys create a near-surface hydrologic system tens of cm below the ice surface. The glacier surface itself is generally frozen and dry. Comparison of water levels between holes a few meters apart shows independent cycles of water storage and release. Most likely, local freeze–thaw effects control water passage and therefore temporary storage. Rough calculations indicate that the holes generate at least 13% of the observed runoff on the one glacier measured. This hydrologic system represents the transition between a melting ice cover with supraglacial streams and one entirely frozen and absent of water.
Characteristics and potential climatic significance of “miniature ice caps” (crest- and cornice-type low-altitude ice archives)
Authors: Haeberli Wilfried , Frauenfelder Regula , Kääb Andreas , Wagner Stephan 
Published: November 2007

AbstractLong-term ice-core records of Alpine glaciers are usually taken from cold-firn areas at high altitudes, as on Mont Blanc and Monte Rosa. Perennial ice bodies at lower altitudes, however, also bear information about the past. Recent findings from the remains of such ice (the Oetztal iceman found in Austria; wooden bows at Lötschen Pass, Switzerland) clearly indicate the hitherto little-recognized fact that small, more-or-less static perennial ice bodies which are cold and frozen to the underlying bedrock may contain very old ice and, hence, important palaeoclimatic information about warm periods with minimum ice extent in the Alps. Since autumn 1998, investigations have been initiated on a crest-type location or “miniature ice cap” at Piz Murtèl, Engadine, Swiss Alps. First results from shallow drilling, temperature data-logging, geodetic surveying, visual observation, finite-element modelling of simplified basic two-dimensional configurations and comparison to earlier measurements at similar sites provide promising perspectives concerning a little-studied phenomenon with considerable scientific-environmental research potential. Specific characteristics of the investigated site, and probably of many other comparable mountain sites, are: cold ice (about –4°C at 10 m depth), no basal sliding, small mass turnover, striking lack of a firn zone, accumulation mainly by superimposed ice, and direct access to old layers (centuries, millennia?) at the ice/bedrock interface.
Genesis of dewatering structures and its implications for melt-out till identification
Authors: Carlson Anders E. 
Published: November 2007

AbstractDewatering structures are a common feature used to identify melt-out till, and the lack of such structures in till could preclude deposition by melt-out. To assess the conditions under which melt-out till can be deposited without forming dewatering structures, I use geotechnical data and a quasi-two-dimensional model of geothermal melt-out. Critical discharge determined from geotechnical data suggests that low-hydraulic-conductivity till can transport up to 1.3 m3water a–1m–2without forming dewatering structures, which is two to three orders of magnitude greater than the volume of meltwater produced at the base of glaciers. The model indicates that debris content of the ice and the ability of the till to drain govern effective pressure during melt-out. If the drainage system is poorly developed or the till comes from debris-poor ice, effective pressure is below zero, the condition under which dewatering structures could form. However, till from relatively debris-rich ice (>40% debris) with a well-developed drainage system (channels every 10 m) can dewater without forming dewatering structures. This suggests that the lack of dewatering structures in till does not necessarily imply deposition by lodgement or deforming bed.
Strain-induced phase changes within cold basal ice from Taylor Glacier, Antarctica, indicated by textural and gas analyses
Authors: Samyn Denis , Fitzsimons Sean J. , Lorrain Reginald D. 
Published: November 2007

AbstractThis paper reports detailed textural and gas measurements conducted in cold basal ice (–17°C from the margin of Taylor Glacier, an outlet glacier of the East Antarctic ice sheet. The analyzed samples were retrieved from a basal ice sequence excavated at the end of a subglacial tunnel dug near the glacier snout. The basal sequence exhibits two contrasting ice facies, defined as the englacial and stratified facies. On the one hand, analysis of ice crystal textures from the basal ice sequence provides evidence for localized ductile deformation, especially within the stratified facies where significant dynamic recrystallization was detected. On the other hand, high-resolution gas analyses reveal that strong changes in gas composition occurred at the structural interfaces of the stratified facies. These gas composition changes are typical of melting–refreezing processes but are not associated with any significant loss of gas volume. Given the specific subglacial thermal conditions at the margin of Taylor Glacier, we interpret this phenomenon as resulting from microscopic phase changes involving selective gas redistribution through the pre-melt phase. It is argued that such processes may play an important role in the post-genetic geochemical evolution of cold debris-laden ice and may be enhanced through intense strain conditions.
Using remote-sensing data to determine equilibrium-line altitude and mass-balance time series: validation on three French glaciers, 1994–2002
Authors: Rabatel Antoine , Dedieu Jean-Pierre , Vincent Christian 
Published: November 2007

AbstractAlpine glaciers are very sensitive to climate fluctuations, and their mass balance can be used as an indicator of regional-scale climate change. Here, we present a method to calculate glacier mass balance using remote-sensing data. Snowline measurements from remotely sensed images recorded at the end of the hydrological year provide an effective proxy of the equilibrium line. Mass balance can be deduced from the equilibrium-line altitude (ELA) variations. Three well-documented glaciers in the French Alps, where the mass balance is measured at ground level with a stake network, were selected to assess the accuracy of the method over the 1994–2002 period (eight mass-balance cycles). Results obtained by ground measurements and remote sensing are compared and show excellent correlation (r2 > 0.89), both for the ELA and for the mass balance, indicating that the remote-sensing method can be applied to glaciers where no ground data exist, on the scale of a mountain range or a given climatic area. The main differences can be attributed to discrepancies between the dates of image acquisition and field measurements. Cloud cover and recent snowfalls constitute the main restrictions of the image-based method.
Identifying microstructural deformation mechanisms in snow using discrete-element modeling
Authors: Johnson Jerome B. , Hopkins Mark A. 
Published: November 2007

AbstractA dynamic model of dry snow deformation is developed using a discrete-element technique to identify microstructural deformation mechanisms and simulate creep densification processes. The model employs grain-scale force models, explicit geometric representations of individual ice grains, and snow microstructure using assemblies of grains. Ice grains are randomly oriented cylinders of random length with hemispherical ends. Particle contacts are detected using a novel and efficient method based on the dilation operation in mathematical morphology. Grain-scale ice interaction algorithms, based on observed snow and ice microscale behavior, are developed and implemented in the model. These processes include grain contact sintering, grain boundary sliding and rotation at contacts, and grain contact deformation in tension, compression, shear, torsion and bending. Grain-scale contact force algorithms are temperature- and rate-dependent, with both elastic and viscous components. Grain bonds rupture when elastic stresses exceed ice tensile or shear strengths, after which intergranular friction and particle rearrangement control deformation until the snow compacts to its critical density. Simulations of creep settlement using 1000-grain model snow samples indicate the bulk viscosity of snow is controlled by the grain contact viscosity and area, grain packing and the increased number of frozen bonds that form during settlement. A linear relationship between contact viscosity and bulk snow viscosity at any specified density indicates that the linear model parameters can be accurately scaled, allowing simulations to be conducted for a broad range of dynamic and viscous creep deformation problems.
Global variations of local asymmetry in glacier altitude: separation of north–south and east–west components
Authors: Evans Ian S. , Cox Nicholas J. 
Published: November 2007

AbstractNorth–south and east–west differences in firn-line altitude, equilibrium-line altitude or middle altitudes of glaciers can be separated by regression on the cosine and sine of glacier aspect (accumulation area azimuth). Allowing for regional trends in altitude, the north–south differences expected from radiation and shade effects can be reliably quantified from World Glacier Inventory (WGI) data. The north–south differences are greater in sunnier climates, mid-latitudes and steeper relief. Local altitude differences between north- and south-facing glaciers are commonly 70–320 m. Such asymmetry is near-universal, although weak in the Arctic and tropics. East–west contrasts are less, and found mainly in the tropics and areas most exposed to strong winds. Altitude, latitude, glacier gradient and height range, calculable from most of the WGI data, are potential controls on the degree of north–south contrast, as well as surrogates for climatic controls (temperature, precipitation, radiation and cloudiness). An asymmetric sine–cosine power model is developed to describe the variation of north–south contrast with latitude. Multiple regression over 51 regions shows altitude and latitude to be the strongest controls of this contrast. Aspect–altitude analysis for former glaciers provides new evidence of cloudiness.
Analysis of meteorological data and the surface energy balance of McCall Glacier, Alaska, USA
Authors: Klok E.J. , Nolan M. , Van Den Broeke M.R. 
Published: November 2007

AbstractWe report on analysis of meteorological data for the period 27 May–20 August 2004, from two automatic weather stations on McCall Glacier, Alaska, USA, aimed at studying the relationship between climate and ablation. One station is located on a mountain ridge and the other in the ablation area where we also analyzed the energy balance. The weather station on the glacier measured an average temperature of 5.3°C (at 2 m height above surface) and wind speed of 3.1 m s−1 (at 3 m height). A sonic height ranger and ablation stakes indicate a specific mass balance of –1.94 ± 0.09 m w.e between 15 June and 20 August. The specific mass balance calculated from the surface energy balance, –2.06 ± 0.18 m w.e., is in close correspondence to this. The latter is the sum of 0.12 m w.e. of snowfall, 0.003 m w.e. of deposition and –2.18 m w.e. of melt. Net radiation contributes 74% of the melt energy. Compared to ablation measurements in the early 1970s, summer ablation was large. This increase is explained by a combination of a relatively higher net radiation, a lower albedo and larger turbulent heat fluxes that led to more energy being available for melting. No single meteorological variable can be isolated as being the principal reason for the high ablation, however. The lower ice albedo (0.19) is possibly due to ash deposits from forest fires.
The effect of particles on dynamic recrystallization and fabric development of granular ice during creep
Authors: Song Min , Baker Ian , Cole David M. 
Published: November 2007

AbstractThe mechanical behavior and microstructural evolution of laboratory-prepared, particle-free fresh-water ice and ice with 1 wt.% (~0.43 vol.%) silt-sized particles were investigated under creep with a stress level of 1.45 MPa at −10°C. The particles were present both within the grains and along the grain boundaries. The creep rates of specimens with particles were always higher than those of particle-free ice. Dynamic recrystallization occurred for both sets of specimens, with new grains nucleating along grain boundaries in the early stages of creep. The ice with particles showed a higher nucleation rate. This resulted in a smaller average grain-size for the ice with particles after a given creep strain. Fabric studies indicated that ice with particles showed a more random orientation of c axes after creep to ~10% strain than the particle-free ice.
Fabric and texture at Siple Dome, Antarctica
Authors: Diprinzio C.L. , Wilen L.A. , Alley R.B. , Fitzpatrick J.J. , Spencer M.K. , Gow A.J. 
Published: November 2007

AbstractPreferred c-axis orientations are present in the firn at Siple Dome, West Antarctica, and recrystallization begins as shallow as 200 m depth in ice below –20°C, based on digital analysis of c-axis fabrics, grain-sizes and other characteristics of 52 vertical thin sections prepared in the field from the kilometer-long Siple Dome ice core. The shallowest section analyzed, from 22 m, shows clustering of c axes toward the vertical. By 200 m depth, girdle fabric and other features of recrystallized ice are evident in layers (or regions), separated by layers (regions) of typically finer-grained ice lacking evidence of recrystallization. Ice from about 700–780m depth, which was deposited during the last ice age, is especially fine-grained, with strongly vertical c axes, but deeper ice shows much larger crystals and strong evidence of recrystallization. Azimuthal asymmetry of some c-axis fabrics, trends in grain-size, and other indicators reveal additional information on processes and history of ice flow at Siple Dome.
Creep of granular ice with and without dispersed particles
Authors: Song Min , Cole David M. , Baker Ian 
Published: November 2007

AbstractThe effects of silt-sized particles (average diameter of 50 μm) on the compressive creep of polycrystalline ice have been studied at stress levels from 0.1 to 1.45 MPa and temperatures of –12ºC and –10°C. Dislocation densities during creep have been estimated using a dislocation-based model of anelasticity. The results indicate that at low concentrations (up to 4wt.%), particles increase the minimum creep rate. Power-law behavior with an exponent of 3 was observed for both particle-free ice and ice with 1 wt.% particles when the stress was >0.3 MPa. In contrast, linear behavior was observed when the stress was <0.3 MPa. Calculations show that the linear behavior is associated with a constant dislocation density, and the power-law behavior is associated with increasing dislocation densities with increasing stress.
Kinematics of Spruce Creek rock glacier, Colorado, USA
Authors: Leonard Eric M. , Staab Peter , Weaver Stephen G. 
Published: November 2007

AbstractA 14 year kinematic survey of Spruce Creek rock glacier, Colorado, USA, provides information on rates and controls of surface strain. Steel-tape measurements of differential movement yield data of cm-scale accuracy, sufficient to assess strain patterns over small portions of the very slowly deforming rock glacier. Flow rates are typically <10cma–1, and measured strain rates range from 1.0 × 10–5a–1 to 1.5 × 10–3a–1. The primary control on longitudinal strain is changing surface slope, with extending flow occurring in areas of down-valley increase in slope, and compressing flow in areas of down-valley slope decrease. Relatively high strain rates are associated with higher flow velocities and with the impingement of faster-flowing up-valley portions of the rock glacier on the slower-moving lower portion. Overall strain rates decreased through the study period, probably as a result of a slowing of the upper part of the rock glacier. Transverse ridges are associated with longitudinal shortening, caused by either slope changes or impingement of faster-moving sections of the rock glacier on slower-moving sections, but transverse ridges do not occur in every area of strong longitudinal shortening.
Recumbent folding of divide arches in response to unsteady ice-divide migration
Authors: Jacobson H. Paul , Waddington Edwin D. 
Published: November 2007

AbstractArches in stratigraphic layers directly under a flow divide (Raymond bumps) are predicted by models of steady ice-sheet flow, and have been observed in several ice domes. Here, we model the evolution of these layers when a formerly stationary divide migrates rapidly to a new position, then again becomes stationary, leaving the arched layers in a flank position. As they are then carried downstream with the flow, these abandoned arches can develop into recumbent folds. These folds can occur over a wide range of divide migration speeds. The shearing flow that produces these recumbent folds also distributes the folded layers over a wide distance downstream from the original divide location. If the divide offset is abrupt, ‘pre-cores’, or material lines comprising core-relative isochrones, can be used to quickly identify portions of an abandoned Raymond bump that would be overturned at any future ice-core site downstream. If, as appears to be the case in Greenland, the divide is never stable long enough to produce a mature arch, folds of this type would not occur. The most likely place to find such folds might be the flank of an ice ridge bounded by unsteady ice streams.
Flow dynamics and iceberg calving rates of Devon Ice Cap, Nunavut, Canada
Authors: Burgess David O. , Sharp Martin J. , Mair Douglas W.F. , Dowdeswell Julian A. , Benham Toby J. 
Published: November 2007

AbstractThe surface velocity field of Devon Ice Cap, Nunavut, Canada, was mapped using interferometric synthetic aperture radar (InSAR). Ascending European Remote-sensing Satellite 1 and 2 (ERS-1/-2) tandem mode data were used for the western and southeast sectors, and 3 day repeat pass ERS-1 imagery for the northeast sector. Speckle-tracking procedures were used with RADARSAT 1 imagery to obtain surface velocities over the terminus of Belcher Glacier (a major calving front) where decorrelation between ERS data occurred. The InSAR data highlight a significant contrast in ice-flow dynamics between the east and west sides of the ice cap. Ice movement west of the main north–south divide is dominated by relatively uniform ‘sheet’ flow, but three fast-flowing outlet glaciers that extend 14–23km beyond the ice-cap margin also drain this region. Several outlet glaciers that extend up to 60 km inland from the eastern margin drain the eastern side of the ice cap. The dominant ice-flow regimes were classified based on the relationship between the driving stress (averaged over a length scale of ten ice thicknesses) and the ratio of surface velocity to ice thickness. The mapped distribution of flow regimes appears to depict the spatial extent of basal sliding across the ice cap. This is supported by a close relationship between the occurrence of flow stripes on the ice surface and flow regimes where basal sliding was found to be an important component of the glacier motion. Iceberg calving rates were computed using measured surface velocities and ice thicknesses derived from airborne radio-echo sounding. The volume of ice calved between 1960 and 1999 was estimated to be 20.5 ± 4.7 km3 (or 0.57 km3 a–1). Approximately 89% of this loss occurred along the eastern margin. The largest single source is Belcher Glacier, which accounts for ~50% of the total amount of ice calved.