Water-pressure Coupling of Sliding and Bed Deformation: III. Application to Ice Stream B, Antarctica

AbstractBasal motion of a glacier resting on an unconsolidated bed can arise from sliding between ice and bed, ploughing of clasts through the upper layer of the bed, pervasive deformation of the bed, or shearing across discrete planes in the bed. Theoretical analyses and limited observations of soft-bedded glaciers not dominated by supply of channelized melt water from the surface suggest that sliding will be slow if the bed contains abundant clasts in the 1–10 mm size range, and that high velocities by ploughing are unlikely though possible. Pervasive deformation usually will account for 60–100% of the basal velocity, and the strain-rate will be proportional to the basal shear stress and inversely proportional to the square or cube of the effective pressure. These hypotheses are based on results of part I in this series, and allow modeling of Ice Stream B, West Antarctica, in part III of this series.

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The marginal shear stress of Ice Stream B, West Antarctica

Journal of Glaciology ◽

10.3189/s0022143000035000 ◽

1997 ◽

Vol 43 (145) ◽

pp. 415-426 ◽

Cited By ~ 7

Author(s):

Miriam Jackson ◽

Barclay Kamb

Keyword(s):

Shear Stress ◽

Strain Rate ◽

Time Scale ◽

Shear Zones ◽

Hot Water ◽

West Antarctica ◽

Ice Stream ◽

Flow Law ◽

Ice Stream B

AbstractTo ascertain whether the velocity of Ice Stream B, West Antarctica, may be controlled by the stresses in its marginal shear zones (the “Snake” and the “Dragon”), we undertook a determination of the marginal shear stress in the Dragon near Camp Up B by using ice itself as a stress meter. The observed marginal shear strain rate of 0.14 a−1is used to calculate the marginal shear stress from the flow law of ice determined by creep tests on ice cores from a depth of 300 m in the Dragon, obtained by using a hot-water ice-coring drill. The test-specimen orientation relative to the stress axes in the tests is chosen on the basis ofc-axis fabrics so that the test applies horizontal shear across vertical planes parallel to the margin. The resulting marginal shear stress is (2.2 ± 0.3) × 105Pa. This implies that 63–100% of the ice stream’s support against gravitational loading comes from the margins and only 37–0% from the base, so that the margins play an important role in controlling the ice-stream motion. The marginal shear-stress value is twice that given by the ice-stream model of Echelmeyer and others (1994) and the corresponding strain-rate enhancement factors differ greatly (E≈ 1–2 vs 10–12.5). This large discrepancy could be explained by recrystallization of the ice during or shortly after coring. Estimates of the expected recrystallization time-scale bracket the ∼1 h time-scale of coring and leave the likelihood of recrystallization uncertain. However, the observed two-maximum fabric type is not what is expected for annealing recrystallization from the sharp single-maximum fabric that would be expected in situ at the high shear strains involved (γ ∼ 20). Experimental data from Wilson (1982) suggest that, if the core did recrystallize, the prior fabric was a two-maximum fabric not substantially different from the observed one, which implies that the measured flow law and derived marginal shear stress are applicable to the in situ situation. These issues need to be resolved by further work to obtain a more definitive observational assessment of the marginal shear stress.

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In search of ice-stream sticky spots

Journal of Glaciology ◽

10.3189/s0022143000016336 ◽

1993 ◽

Vol 39 (133) ◽

pp. 447-454 ◽

Cited By ~ 7

Author(s):

Richard B. Alley

Keyword(s):

Shear Stress ◽

Large Scale ◽

Water Pressure ◽

Pressure Measurements ◽

West Antarctica ◽

Ice Surface ◽

Ice Stream ◽

Bed Roughness ◽

Basal Shear

AbstractThe basal shear stress of an ice stream may be supported disproportionately on localized regions or “sticky spots”. The drag induced by large bedrock bumps sticking into the base of an ice stream is the most likely cause of sticky spots. Discontinuity of lubricating till can cause sticky spots, but they will collect lubricating water and therefore are unlikely to support a shear stress of more than a few tenths of a bar unless they contain abundant large bumps. Raised regions on the ice-air surface can also cause moderate increases in the shear stress supported on the bed beneath. Surveys of large-scale bed roughness would identify sticky spots caused by bedrock bumps, water-pressure measurements in regions of thin or zero till might reveal whether they were sticky spots, and strain grids across the margins of ice-surface highs would show whether the highs were causing sticky spots. Sticky spots probably are not dominant in controlling Ice Stream Β near the Upstream Β camp, West Antarctica.

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In search of ice-stream sticky spots

Journal of Glaciology ◽

10.1017/s0022143000016336 ◽

1993 ◽

Vol 39 (133) ◽

pp. 447-454 ◽

Cited By ~ 151

Author(s):

Richard B. Alley

Keyword(s):

Shear Stress ◽

Large Scale ◽

Water Pressure ◽

Pressure Measurements ◽

West Antarctica ◽

Ice Surface ◽

Ice Stream ◽

Bed Roughness ◽

Basal Shear

Abstract The basal shear stress of an ice stream may be supported disproportionately on localized regions or “sticky spots”. The drag induced by large bedrock bumps sticking into the base of an ice stream is the most likely cause of sticky spots. Discontinuity of lubricating till can cause sticky spots, but they will collect lubricating water and therefore are unlikely to support a shear stress of more than a few tenths of a bar unless they contain abundant large bumps. Raised regions on the ice-air surface can also cause moderate increases in the shear stress supported on the bed beneath. Surveys of large-scale bed roughness would identify sticky spots caused by bedrock bumps, water-pressure measurements in regions of thin or zero till might reveal whether they were sticky spots, and strain grids across the margins of ice-surface highs would show whether the highs were causing sticky spots. Sticky spots probably are not dominant in controlling Ice Stream Β near the Upstream Β camp, West Antarctica.

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The marginal shear stress of Ice Stream B, West Antarctica

Journal of Glaciology ◽

10.1017/s0022143000035000 ◽

1997 ◽

Vol 43 (145) ◽

pp. 415-426 ◽

Cited By ~ 66

Author(s):

Miriam Jackson ◽

Barclay Kamb

Keyword(s):

Shear Stress ◽

Strain Rate ◽

Time Scale ◽

Shear Zones ◽

Hot Water ◽

West Antarctica ◽

Ice Stream ◽

Flow Law ◽

Ice Stream B

AbstractTo ascertain whether the velocity of Ice Stream B, West Antarctica, may be controlled by the stresses in its marginal shear zones (the “Snake” and the “Dragon”), we undertook a determination of the marginal shear stress in the Dragon near Camp Up B by using ice itself as a stress meter. The observed marginal shear strain rate of 0.14 a−1is used to calculate the marginal shear stress from the flow law of ice determined by creep tests on ice cores from a depth of 300 m in the Dragon, obtained by using a hot-water ice-coring drill. The test-specimen orientation relative to the stress axes in the tests is chosen on the basis ofc-axis fabrics so that the test applies horizontal shear across vertical planes parallel to the margin. The resulting marginal shear stress is (2.2 ± 0.3) × 105Pa. This implies that 63–100% of the ice stream’s support against gravitational loading comes from the margins and only 37–0% from the base, so that the margins play an important role in controlling the ice-stream motion. The marginal shear-stress value is twice that given by the ice-stream model of Echelmeyer and others (1994) and the corresponding strain-rate enhancement factors differ greatly (E≈ 1–2 vs 10–12.5). This large discrepancy could be explained by recrystallization of the ice during or shortly after coring. Estimates of the expected recrystallization time-scale bracket the ∼1 h time-scale of coring and leave the likelihood of recrystallization uncertain. However, the observed two-maximum fabric type is not what is expected for annealing recrystallization from the sharp single-maximum fabric that would be expected in situ at the high shear strains involved (γ ∼ 20). Experimental data from Wilson (1982) suggest that, if the core did recrystallize, the prior fabric was a two-maximum fabric not substantially different from the observed one, which implies that the measured flow law and derived marginal shear stress are applicable to the in situ situation. These issues need to be resolved by further work to obtain a more definitive observational assessment of the marginal shear stress.

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Water-Pressure Coupling of Sliding and Bed Deformation: I. Water System

Journal of Glaciology ◽

10.3189/002214389793701527 ◽

1989 ◽

Vol 35 (119) ◽

pp. 108-118 ◽

Cited By ~ 172

Author(s):

R.B. Alley

Keyword(s):

Shear Stress ◽

Water Supply ◽

Water Pressure ◽

Water System ◽

Effective Pressure ◽

Bed Deformation ◽

Varying Thickness ◽

Melt Water ◽

Basal Shear

AbstractAnalysis of the likely behavior of a water system developed between ice and an unconsolidated glacier bed suggests that, in the absence of channelized sources of melt water, the system will approximate a film of varying thickness. The effective pressure in such a film will be proportional to the basal shear stress but inversely proportional to the fraction of the bed occupied by the film. These hypotheses allow calculation of the sliding and bed-deformation velocities of a glacier from the water supply and basal shear stress, as discussed in the second and third papers in this series.

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Sensitivity of Pine Island Glacier, West Antarctica, to changes in ice-shelf and basal conditions: a model study

Journal of Glaciology ◽

10.3189/172756502781831061 ◽

2002 ◽

Vol 48 (163) ◽

pp. 552-558 ◽

Cited By ~ 47

Author(s):

Marjorie Schmeltz ◽

Eric Rignot ◽

Todd K. Dupont ◽

Douglas R. MacAyeal

Keyword(s):

Shear Stress ◽

Element Model ◽

Shelf Area ◽

West Antarctica ◽

Ice Shelf ◽

Ice Stream ◽

Model Study ◽

Grounding Line ◽

Glacier Velocity ◽

Basal Shear

AbstractWe use a finite-element model of coupled ice-stream/ice-shelf flow to study the sensitivity of Pine Island Glacier, West Antarctica, to changes in ice-shelf and basal conditions. By tuning a softening coefficient of the ice along the glacier margins, and a basal friction coefficient controlling the distribution of basal shear stress underneath the ice stream, we are able to match model velocity to that observed with interferometric synthetic aperture radar (InSAR). We use the model to investigate the effect of small perturbations on ice flow. We find that a 5.5–13% reduction in our initial ice-shelf area increases the glacier velocity by 3.5–10% at the grounding line. The removal of the entire ice shelf increases the grounding-line velocity by > 70%. The changes in velocity associated with ice-shelf reduction are felt several tens of km inland. Alternatively, a 5% reduction in basal shear stress increases the glacier velocity by 13% at the grounding line. By contrast, softening of the glacier side margins would have to be increased a lot more to produce a comparable change in ice velocity. Hence, both the ice-shelf buttressing and the basal shear stress contribute significant resistance to the flow of Pine Island Glacier.

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Ice Stream C, Antarctica, sticky spots detected by microearthquake monitoring

Annals of Glaciology ◽

10.1017/s0260305500016426 ◽

1994 ◽

Vol 20 ◽

pp. 183-186 ◽

Cited By ~ 1

Author(s):

S. Anandakrishnan ◽

R. B. Alley

Keyword(s):

Shear Stress ◽

Stress Drop ◽

Fast Moving ◽

Fault Plane ◽

Spatial Density ◽

Ice Stream ◽

Slow Moving ◽

Ice Stream B ◽

Basal Shear ◽

Plane Area

Microearthquakes at the base of slow-moving Ice Stream C occur many times more frequently than at the base of fast-moving Ice Stream B. We suggest that the microearthquake source sites are so-called “sticky spots”, defined as limited zones of stronger Subglacial material interspersed within a weaker matrix. The fault-plane area of the microearthquakes (O(102m2)) is therefore a measure of the size of the sticky spots. The spatial density of the microearthquakes (O(10 km-2)) is a measure of the distribution of sticky spots.The average stress drop associated with these microearthquakes is consistent with an ice-stream bed model of weak subglacial till interspersed with stronger zones that support much or all of the basal shear stress. We infer a weak inter-sticky-spot material by the large distances (O(103m)), relative to fault radius, to which the microearthquake stress change is transmitted.

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Temporal variations in the flow of a large Antarctic ice-stream controlled by tidally induced changes in the subglacial water system

The Cryosphere Discussions ◽

10.5194/tcd-9-2397-2015 ◽

2015 ◽

Vol 9 (2) ◽

pp. 2397-2429 ◽

Cited By ~ 4

Author(s):

S. H. R. Rosier ◽

G. H. Gudmundsson ◽

J. A. M. Green

Keyword(s):

Water Pressure ◽

Water System ◽

Surface Velocity ◽

Decay Length ◽

Model Parameters ◽

Effective Pressure ◽

Tidal Period ◽

Ice Stream ◽

Highly Nonlinear ◽

Basal Sliding

Abstract. Observations show that the flow of Rutford Ice Stream (RIS) is strongly modulated by the ocean tides, with the strongest tidal response at the 14.77 day tidal period (Msf). This is striking because this period is absent in the tidal forcing. A number of mechanisms have been proposed to account for this effect, yet previous modeling studies have struggled to match the observed large amplitude and decay length scale. We use a nonlinear 3-D viscoelastic full-Stokes model of ice-stream flow to investigate this open issue. We find that the long period Msf modulation of ice-stream velocity observed in data cannot be reproduced quantitatively without including a coupling between basal sliding and tidal subglacial water pressure variations. Furthermore, the subglacial water system must be highly conductive and at low effective pressure, and the relationship between sliding velocity and effective pressure highly nonlinear in order for the model results to match GPS measurements. Hydrological and basal sliding model parameters that produced a best fit to observations were a mean effective pressure N of 105 kPa, subglacial drainage system conductivity K of 7 × 109 m2d-1, with sliding law exponents m = 3 and q =10. Coupled model results show the presence of tides result in a ~ 12% increase in mean surface velocity. Observations of tidally-induced variations in flow of ice-streams provide stronger constraints on basal sliding processes than provided by any other set of measurements.

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Basal Sliding and Bed Separation: Is There a Connection?

Journal of Glaciology ◽

10.1017/s0022143000030045 ◽

1979 ◽

Vol 23 (89) ◽

pp. 407-408 ◽

Cited By ~ 1

Author(s):

Robert Bindschadler

Keyword(s):

Shear Stress ◽

Normal Stress ◽

Water Pressure ◽

Simple Relationship ◽

Sliding Velocity ◽

Separation Parameter ◽

Basal Sliding ◽

Bed Separation ◽

Image Position ◽

Basal Shear

Abstract Analysis of field data from Variegated Glacier supports the conclusion of Meier (1968) that no simple relationship between basal shear stress and sliding velocity can be found. On the other hand, an index of bed separation is defined and evaluated that correlates very well with the longitudinal variation of summer sliding velocity inferred for Variegated Glacier. This bed separation parameter is defined as where τ is the basal shear stress and is proportional to the drop in normal stress on the down-glacier side of bedrock bumps and N eff is the effective normal stress equal to the overburden stress minus the subglacial water pressure. The water-pressure distribution is calculated assuming water flow to be confined in subglacial Röthlisberger conduits. The excellent agreement between the longitudinal profiles of I and sliding velocity suggests that calculations of the variation of bed separation can be used to deduce the variation of sliding velocity in both space and time. Further, it is possible that a functional relationship can be developed that adequately represents the geometric controls on basal sliding to permit accurate predictions of sliding velocities.

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