bluuebunny/crossref_metadata_2025_split huggingface dataset:
Diffusive mixing between shearing granular layers: constraints on bed deformation from till contacts
Authors: Hooyer Thomas S. , Iverson Neal R. 
Published: November 2007

AbstractShearing of subglacial till has been invoked widely as a mechanism of glacier motion and sediment transport, but standard indicators for determining shear strain from the geologic record are not adequate for estimating the very high strains required of the bed-deformation model. Here we describe a laboratory study of mixing between shearing granular layers that allows an upper limit to be placed on bed shear strain in the vicinity of till contacts. Owing to random vertical motions of particles induced by shearing, mixing can be modeled as a linearly diffusive process, and so can be characterized with a single mixing coefficient, D. Ring-shear experiments with equigranular beads and lithologically distinct tills provide the value of D, although in experiments with till D decreases systematically with strain to a minimum value of 0.0045 mm2. Kinetic gas theory provides an estimate of the dimensionless mixing coefficient which is within an order of magnitude of laboratory values. Knowing the minimum value of D, the distribution of index lithologies measured across till contacts in the geologic record can be used to estimate the maximum shear strain that has occurred across till contacts. Application of this technique to the contact between the Des Moines and Superior Lobe tills in east-central Minnesota, U.S.A., indicates that shear strain did not exceed 15 000 at the depth of the contact.
A millennium of variable ice flow recorded by the Ross Ice Shelf, Antarctica
Authors: Fahnestock M. A. , Scambos T. A. , Bindschadler R. A. , Kvaran G. 
Published: November 2007

AbstractAn enhanced composite Advanced Very High Resolution Radiometer (AVHRR) image is used to map flow stripes and rifts across the Ross Ice Shelf, Antarctica. The patterns of these flow-related features reveal a history of discharge variations from the ice streams feeding the eastern part of the shelf. The most profound variations are visible in the track of rifts downstream of Crary Ice Rise, flow-stripe bends to the west of this ice rise and adjacent to Steershead ice rise, and changes in the northern margin of Ice Stream B. The track of rifts downstream of Crary Ice Rise indicates that the ice rise has existed for at least 700 years. The character of this track changes about 350 km downstream, indicating a rearrangement of flow patterns about 550 years ago. The large bulge in the flow stripes to the west of Crary Ice Rise is shown in detail, with bent flow stripes extending for several hundred kilometers along flow; this feature formed from the south, possibly due to a change in the discharge of Ice Stream A. The AVHRR image documents a complex history associated with the shutdown of Ice Stream C, with changes in the margins of Ice Stream C and the northern margin of Ice Stream B, and the grounding of Steershead ice rise with an associated bending and truncation of flow stripes. Landsat imagery shows a region that appears to be actively extending just downstream of the ice rise, as the shelf continues to respond to recent changes in ice-stream discharge. We present a four-stage flow history which accounts for the features preserved in the ice shelf.
Sliding of ice past an obstacle at Engabreen, Norway
Authors: Cohen Denis , Hooke Roger LeB. , Iverson Neal R. , Kohler Jack 
Published: November 2007

AbstractAt Engabreen, Norway, an instrumented panel containing a decimetric obstacle was mounted flush with the bed surface beneath 210 m of ice. Simultaneous measurements of normal and shear stresses, ice velocity and temperature were obtained as dirty basal ice flowed past the obstacle. Our measurements were broadly consistent with ice thickness, flow conditions and bedrock topography near the site of the experiment. Ice speed 0.45 m above the bed was about 130 mm d−1, much less than the surface velocity of 800 mm d−1. Average normal stress on the panel was 1.0–1.6 MPa, smaller than the expected ice overburden pressure. Normal stress was larger and temperature was lower on the stoss side than on the lee side, in accord with flow dynamics and equilibrium thermodynamics. Annual differences in normal stresses were correlated with changes in sliding speed and ice-flow direction. These temporal variations may have been caused by changes in ice rheology associated with changes in sediment concentration, water content or both. Temperature and normal stress were generally correlated, except when clasts presumably collided with the panel. Temperature gradients in the obstacle indicated that regelation was negligible, consistent with the obstacle size. Melt rate was about 10% of the sliding speed. Despite high sliding speed, no significant ice/bed separation was observed in the lee of the obstacle. Frictional forces between sediment particles in the ice and the panel, estimated from Hallet’s (1981) model, indicated that friction accounted for about 5% of the measured bed-parallel force. This value is uncertain, as friction theories are largely untested. Some of these findings agree with sliding theories, others do not.
Coupled ice–till deformation near subglacial channels and cavities
Authors: Ng Felix S. L. 
Published: November 2007

AbstractPrevious models of ice–till deformation near subglacial channels or cavities neglect the fact that the motions of the two materials are coupled, and thus the interface between ice and till may not remain stationary. Here, we analyze in succession two models which address the effect of such coupling via specification of appropriate continuity conditions for stress and velocity across the interface. The modelled scenario is that of a shallow channel–cavity, with its long axis parallel to the principal ice-flow direction, overlying actively deforming till sediments. By applying asymptotic techniques, we investigate how the pattern and velocity of the creep flow depend generally on the ratio between the ice and till viscosities, and on the deforming-till thickness. A more sophisticated, non-linear rheology for till sediments is then introduced. It reveals that the two-way interaction between water percolation and deformation in the till will enhance the localization of sediment flow near the channel margins. The length scale over which transition of effective stress in the till takes place — from its relatively high, far-field value to the low, channel value — is found to depend critically on a dimensionless permeability parameter (Λ). In any case, coupled deformation causes sediment (and ice) flow towards the channel, subsidence of the ice–till interface just outside the channel, and extension of the area over which the ice is in contact with till. Apart from having direct implications for subglacial sediment transport, these results indicate that coupled deformation can contribute significantly to the spatial evolution of stress distribution under a glacier, and thus its incorporation into future sliding and drainage theories for a soft bed should be considered essential.
Mass balance of the Lambert Glacier–Amery Ice Shelf system, East Antarctica: a comparison of computed balance fluxes and measured fluxes
Authors: Fricker Helen A. , Warner Roland C. , Allison Ian 
Published: November 2007

AbstractWe combine European Remote-sensing Satellite (ERS-1) radar altimeter surface elevations (Fricker and others, 2000) with six different accumulation distributions to compute balance fluxes for the Lambert Glacier–Amery Ice Shelf drainage system. These interpolated balance fluxes are compared with fluxes derived from in situ measurements of ice thickness and velocity at 73 stations of the Lambert Glacier basin traverse and at 11 stations further downstream, to assess the system’s state of balance. For the upstream line we obtain a range of imbalance estimates, from −23.8% to +19.9% of the observed flux, reflecting the sensitivity to the accumulation distributions. For some of the accumulation distributions the imbalance estimates vary significantly between different parts of the line. Imbalance estimates for the downstream line range from −17.7% to +70.2%, with four of the estimates exceeding +30%, again reflecting the sensitivity of the result to input accumulation, and strongly suggesting that the mass balance of the region between the two lines is positive. Our results confirm the importance of accurate estimates of accumulation in ice-sheet mass-balance studies. Furthermore, they suggest that it is not possible to accurately determine the state of balance of large Antarctic drainage basins on the basis of currently available accumulation distributions.
A numerical investigation of ice-lobe–permafrost interaction around the southern Laurentide ice sheet
Authors: Cutler Paul M. , MacAyeal Douglas R. , Mickelson David M. , Parizek Byron R. , Colgan Patrick M. 
Published: November 2007

AbstractPermafrost existed around and under marginal parts of the southern Laurentide ice sheet during the Last Glacial Maximum. The presence of permafrost was important in determining the extent, form and dynamics of ice lobes and the landforms they produced because of influences on resistance to basal motion and subglacial hydrology. We develop a two-dimensional time-dependent model of permafrost and glacier-ice dynamics along a flowline to examine: (i) the extent to which permafrost survives under an advancing ice lobe and how it influences landform development and hydrology, and (ii) the influence of permafrost on ice motion and surface profile. The model is applied to the Green Bay lobe, which terminated near Madison, Wisconsin, during the Last Glacial Maximum. Simulations of ice advance over permafrost indicate that the bed upstream of the ice-sheet margin was frozen for 60–200 km at the glacial maximum. Permafrost remained for centuries to a few thousand years under advancing ice, and penetrated sufficiently deep (tens of meters) into the underlying aquifer that drainage of basal meltwater became inefficient, likely resulting in water storage beneath the glacier. Our results highlight the influence of permafrost on subglacial conditions, even though uncertainties in boundary conditions such as climate exist.
Delineation of a complexly dipping temperate glacier bed using short-pulse radar arrays
Authors: Moran M. L. , Greenfield R. J. , Arcone S. A. , Delaney A. J. 
Published: November 2007

AbstractWe have defined the complex bed topography for a section of a small temperate glacier using 50 MHz monostatic short-pulse radar data and a synthetic-aperture array-processing method. The data were collected on a 100 m by 340 m array grid in the upper stem of Gulkana Glacier, central Alaska, U.S.A. The array processing was based on a modified three-dimensional (3-D) Kirchhoff migration integral and implemented with a synthetic-aperture approach that uses sequences of overlapping sub-arrays to generate depth images in vertical planes. Typical sub-array beam patterns are generally <5° at the −6 dB level, giving a flashlight-like searching capability without distorting the wavelet shape. The bed topography was constructed using normal reflections picked from 3-D array depth images. In some instances reflections were imaged outside the data-cover-age area. The bed surface dips steeply, both parallel and transverse to the direction of ice flow. The maximum observed depth is roughly 140 m. The 3-D method resolved bed dips up to 45°. In regions of steepest dip, it improved depth accuracy by 36% compared with raw data, and by 15% compared with standard two-dimensional (2-D) migration. Over 12 dB of signal-to-noise improvement and improved spatial resolution was achieved compared to raw data and 2-D migration. False bottom layering seen in the raw data and in 2-D migrations is not observed in the 3-D array results. Furthermore, loss of bottom reflections is shown by the 3-D migration to be attributable to the dip and curvature of the reflector, and not scattering losses or signal clutter from englacial inclusions.
Glacier motion dominated by processes deep in underlying till
Authors: Truffer M. , Harrison W. D. , Echelmeyer K. A. 
Published: November 2007

AbstractBlack Rapids Glacier is a 40 km long surge-type glacier in the central Alaska Range. In spring 1997 a wireline drill rig was set up at a location where the measured surface velocities are high and seasonal and annual velocity variations are large. The drilling revealed a layer of subglacial “till”, up to 7 m thick, that is believed to be water-saturated. At one location a string of instruments, containing three dual-axis tiltmeters and one piezometer, was successfully introduced into the till. The tiltmeters monitored the inclination of the borehole at the ice–till interface and at 1 and 2 m into the till, for 410 days. They showed that no significant deformation occurred in the upper 2 m of the till layer, and no significant amount of the basal motion was due to sliding of the ice over the till. The measured surface velocity at the drill site is about 60 m a−1, of which 20–30 m a can be accounted for by ice deformation. Almost the entire amount of basal motion, 30–40 m a−1, was taken up at a depth of > 2 m in the till, possibly in discrete shear layers, or as sliding of till over the underlying bedrock. We propose that the large-scale mobilization of such till layers is a key factor in initiating glacier surges.
Effect of summer accumulation on glacier mass balance on the Tibetan Plateau revealed by mass-balance model
Authors: Fujita Koji , Ageta Yutaka 
Published: November 2007

AbstractThe characteristics and sensitivities of a cold-based glacier on the Tibetan Plateau, where the summer monsoon provides most of the mass input to glaciers, are discussed using an energy-balance model incorporating the process of water refreezing. The model accurately represents the observational results related to the mass balance of Xiao Dongkemadi glacier on the central plateau during 1992/93. Our data revealed that the mass balance of cold glaciers cannot simply be described by the surface mass/heat balances, because about 20% of infiltrated water is refrozen and thus does not run off from the glacier. Model calculations demonstrate that glaciers in an arid environment can maintain their mass since the monsoon provides precipitation during the melting season. Snowfall in summer keeps surface albedo high and largely restrains ablation. Nevertheless, the calculations also make clear that glaciers on the plateau are more vulnerable than those of other regions because of summer accumulation. In the monsoon climate, warming would cause not only a decrease in accumulation, but also a drastic increase in ablation in combination with surface-albedo lowering. Therefore, although glaciers on and around the plateau can be sustained by summer accumulation, they are more vulnerable to warming than winter-accumulation-type glaciers.
Glacier mass-balance determination by remote sensing and high-resolution modelling
Authors: Hubbard Alun , Willis Ian , Sharp Martin , Mair Douglas , Nienow Peter , Hubbard Bryn , Blatter Heinz 
Published: November 2007

AbstractAn indirect methodology for determining the distribution of mass balance at high spatial resolution using remote sensing and ice-flow modelling is presented. The method, based on the mass-continuity equation, requires two datasets collected over the desired monitoring interval: (i) the spatial pattern of glacier surface-elevation change, and (ii) the mass-flux divergence field. At Haut Glacier d’Arolla, Valais, Switzerland, the mass-balance distribution between September 1992 and September 1993 is calculated at 20 m resolution from the difference between the pattern of surface-elevation change derived from analytical photogrammetry and the mass-flux divergence field determined from three-dimensional, numerical flow modelling constrained by surface-velocity measurements. The resultant pattern of mass balance is almost totally negative, showing a strong dependence on elevation, but with large localized departures. The computed distribution of mass balance compares well (R2 = 0.91) with mass-balance measurements made at stakes installed along the glacier centre line over the same period. Despite the highly optimized nature of the flow-modelling effort employed in this study, the good agreement indicates the potential this method has as a strategy for deriving high spatial and temporal-resolution estimates of mass balance.
A flow model for the polar caps of Mars
Authors: Nye J. F. 
Published: November 2007

AbstractA mechanical model with circular symmetry is examined to test the hypothesis that the Martian ice caps are composed of flowing water ice, together with some rock debris. In contrast with most or all previous models, no assumption of a steady state is made. Instead the accumulation and ablation is assumed to be insignificant, and it is suggested that after a sufficient time the profile would have settled down to a particular collapsing form calculated by Halfar (1983). Higher modes of flow would have decayed relatively quickly. To calculate the time constant, it is necessary to consider carefully the distribution of temperature with depth. The time constant is sensitive to the grain-size, which is assumed to be 1–10 mm and is a significant unknown, as is also the effect of preferred crystal orientation. Apart from this, the main uncertainty is the value of the upward heat flux. With a heat flux of 30 mW m−2, the water-ice hypothesis is consistent with an age of about 107 years for both the north and the south polar caps, the north cap being the younger by a factor of about 7.
Non-temperate glacier flow over wavy sloping ground
Authors: Weber Jan Erik 
Published: November 2007

AbstractThe mean, steady-state particle velocity in gravity-driven glacial flow over sinusoidal, sloping ground is computed using a Lagrangian description of motion. A Newtonian viscous fluid approximation is used for the ice. The glacier surface is free to move and is not subject to any stresses. At the bottom, the ice is frozen to the ground. The non-linear interaction between the basic downslope Poiseuille flow and the bottom corrugations yields a mean Lagrangian perturbation velocity that is always directed in the upslope direction near the ground. The requirement of mass balance imposes a mean negative surface slope in the corrugated region and an associated downslope perturbation flow in the upper part of the glacier. The no-slip condition at the wavy bottom induces a strong velocity shear in the ice, and particularly at the base. Analysis shows that the shear heating associated with shortwave perturbations could, in the case of a marginally frozen ground, lead to melting and subsequent sliding at wave crests along the bottom, while the ice stays frozen at the troughs. It is suggested that for glaciers the resulting high strain rates could lead to crevassing.
Comparison of SAR-interferometric and surveyed velocities on a mountain glacier: Black Rapids Glacier, Alaska, U.S.A.
Authors: Rabus B. T. , Fatland D. R. 
Published: November 2007

AbstractAn interferogram reflecting the motion of Black Rapids Glacier, Alaska, U.S.A., was obtained from two European Remote-sensing Satellite (ERS-1) syntheticaperture radar (SAR) images, acquired on 22 and 25 January 1992. We investigate whether the interferometric data are quantitatively consistent with terrestrial velocity measurements along three transverse profiles. These terrestrial data are from different years (1987, 1990, 1996) and cover different periods (6–28 April, 23 May–7 July and a whole year) than the SAR interferogram. Terrestrial ice velocity at the date of the SAR imagery is obtained via seasonal and annual corrections that are calculated from other terrestrial velocity measurements available at higher time resolution for selected sites on the glacier. Interferometric and terrestrial velocity are in excellent agreement if a (terrestrially measured) surface-normal velocity component (v⊥) is properly accounted for. This suggests that both the interferometric velocities and the conversions of terrestrial data to the winter period are reliable. The terrestrial velocity measurements show that ice flow in the upper ablation area (14–16 km sites) changes from longitudinal compression in mid-winter (v⊥ = + 0.82 cm d−1) to moderate longitudinal extension during summer (v⊥ ≈ –0.25 cm d−1). In the lower ablation area, the seasonal variations of the longitudinal strain rate are much smaller: +1.0 and about +0.85 cm d−1 for the respective mid-winter and summer values of v⊥ at the 20 km site.
Relating glacier mass balance to meteorological data by using a seasonal sensitivity characteristic
Authors: Oerlemans J. , Reichert B. K. 
Published: November 2007

AbstractWe propose to quantify the climate sensitivity of the mean specific balance B of a glacier by a seasonal sensitivity characteristic (SSC). The SSC gives the dependence of B on monthly anomalies in temperature and precipitation. It is calculated from a mass-balance model. We show and discuss examples for Franz-Josef Glacier (New Zealand), Nigardsbreen (Norway), Hintereisferner (Austria), Peyto Glacier (Canadian Rockies), Abramov Glacier (Kirghizstan) and White Glacier (Canadian Arctic). With regard to the climate sensitivity of B, the SSCs clearly show that summer temperature is the most important factor for glaciers in a dry climate. For glaciers in a wetter climate, spring and fall temperatures also make a significant contribution to the overall sensitivity. The SSC is a 2 × 12 matrix. Multiplying it with monthly perturbations of temperature and precipitation for a particular year yields an estimate of the balance for that year. We show that, with this technique, mass-balance series can be (re)constructed from long meteorological records or from output of atmospheric models.
Changes in the margin of Ice Stream C, Antarctica
Authors: Jacobel R. W. , Scambos T. A. , Nereson N. A. , Raymond C. F. 
Published: November 2007

AbstractWe present results from satellite imagery, ice-motion surveys and ice-penetrating radar studies of part of the north margin of Ice Stream C, one of the ice streams draining the West Antarctic ice sheet to the “Ross Embayment”. Our studies suggest that the shutdown of Ice Stream C about 150 years ago was not a single event, but a sequence involving stagnation of ice and migration of the ice-stream boundary. Ground-based studies confirm the inference from imagery that a series of former shear zones exist, decreasing in age towards the ice-stream center. A region of ice-stream trunk, including a former margin, lies sheared and folded between the (recent) inner and (older) outer margins of the area. ice-motion and topographic surveys give some constraint on the time of shutdown of the outer margin. The results provide a forum for discussing shutdown mechanisms. Possible causes for the stepwise migration of the north margin of Ice Stream C include a gradual decrease in ice flux, a reduction in the available water or hydrostatic pressure in the basal till, or a freezing of the till layers on the northern side.