Glacier sliding, seismicity and sediment entrainment

ABSTRACTThe evolution of glaciers and ice sheets depends on processes in the subglacial environment. Shear seismicity along the ice–bed interface provides a window into these processes. Such seismicity requires a rapid loss of strength that is typically ascribed to rate-weakening friction, i.e., decreasing friction with sliding or sliding rate. Many friction experiments have investigated glacial materials at the temperate conditions typical of fast flowing glacier beds. To our knowledge, however, these studies have all found rate-strengthening friction. Here, we investigate the possibility that rate-weakening rock-on-rock friction between sediments frozen to the bottom of the glacier and the underlying water-saturated sediments or bedrock may be responsible for subglacial shear seismicity along temperate glacier beds. We test this ‘entrainment-seismicity hypothesis’ using targeted laboratory experiments and simple models of glacier sliding, seismicity and sediment entrainment. These models suggest that sediment entrainment may be a necessary but not sufficient condition for the occurrence of basal shear seismicity. We propose that stagnation at the Whillans Ice Stream, West Antarctica may be caused by the growth of a frozen fringe of entrained sediment in the ice stream margins. Our results suggest that basal shear seismicity may indicate geomorphic activity.

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Basal melt beneath Whillans Ice Stream and Ice Streams A and C, West Antarctica

Annals of Glaciology ◽

10.3189/172756403781816130 ◽

2003 ◽

Vol 36 ◽

pp. 257-262 ◽

Cited By ~ 39

Author(s):

Ian R. Joughin ◽

Slawek Tulaczyk ◽

Hermann F. Engelhardt

Keyword(s):

Temperature Gradients ◽

Shear Stresses ◽

Temperature Profiles ◽

West Antarctica ◽

Ice Streams ◽

Advection Diffusion Equation ◽

Ice Stream ◽

Single Temperature ◽

Whillans Ice Stream ◽

Basal Shear

AbstractWe have used a recently derived map of the velocity of Whillans Ice Stream and Ice Streams A and C, West Antarctica, to help estimate basal melt. Ice temperature was modeled with a simple vertical advection–diffusion equation,“tuned” to match temperature profiles. We find that most of the melt occurs beneath the tributaries, where larger basal shear stresses and thicker ice favor greater melt (e.g. 10–20mm a−1). The occurrence of basal freezing is predicted beneath much of the ice plains of Ice Stream C andWhillans Ice Stream. Modeled melt rates for when Ice Stream C was active suggest there was enough meltwater generated in its tributaries to balance basal freezing on its ice plain. Net basal melt for Whillans Ice Stream is greater due to less steep basal temperature gradients. Modeled temperatures on Whillans Ice Stream, however, were constrained by a single temperature profile at UpB. Basal temperature gradients for Whillans branch 1 and Ice Stream A may have conditions more similar to those beneath Ice Streams C and D, in which case, there may not be sufficient melt to sustain motion. This would be consistent with the steady deceleration of Whillans Ice Stream over the last few decades.

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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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Interpretation of radar-detected internal layer folding in West Antarctic ice streams

Journal of Glaciology ◽

10.3189/s0022143000016427 ◽

1993 ◽

Vol 39 (133) ◽

pp. 528-537 ◽

Cited By ~ 1

Author(s):

W. Jacobel Robert ◽

M. Gades Anthony ◽

L. Gottschling David ◽

M. Hodge Steven ◽

L. Wright David

Keyword(s):

Flow Direction ◽

Low Frequency ◽

Internal Layers ◽

West Antarctica ◽

Ice Streams ◽

West Antarctic ◽

Ice Stream ◽

High Resolution Images ◽

Basal Shear ◽

Inland Ice

AbstractLow-frequency surface-based radar-profiling experiments on Ice Streams Β and C, West Antarctica, have yielded high-resolution images which depict folding of the internal layers that can aid in the interpretation of ice-stream dynamics. Unlike folding seen in most earlier radar studies of ice sheets, the present structures have no relationship to bedrock topography and show tilting of their axial fold planes in the flow direction. Rather than being standing waves created by topography or local variations in basal shear stress, the data show that these folds originate upstream of the region of streaming flow and are advected into the ice streams. The mechanism for producing folds is hypothesized to be changes in the basal boundary conditions as the ice makes the transition from inland ice to ice-stream flow. Migration of this transition zone headward can then cause folds in the internal layering to be propagated down the ice streams.

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Basal mechanics of ice streams: Insights from the stick-slip motion of Whillans Ice Stream, West Antarctica

Journal of Geophysical Research Atmospheres ◽

10.1029/2008jf001035 ◽

2009 ◽

Vol 114 (F1) ◽

Cited By ~ 63

Author(s):

J. Paul Winberry ◽

Sridhar Anandakrishnan ◽

Richard B. Alley ◽

Robert A. Bindschadler ◽

Matt A. King

Keyword(s):

West Antarctica ◽

Ice Streams ◽

Stick Slip ◽

Ice Stream ◽

Whillans Ice Stream ◽

Stick Slip Motion ◽

Slip Motion

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Thickness changes on Whillans Ice Stream and Ice Stream C, West Antarctica, derived from laser altimeter measurements

Journal of Glaciology ◽

10.3189/172756503781830683 ◽

2003 ◽

Vol 49 (165) ◽

pp. 223-230 ◽

Cited By ~ 13

Author(s):

Vandy Blue Spikes ◽

Beáta M. Csatho ◽

Gordon S. Hamilton ◽

Ian M. Whillans

Keyword(s):

Average Rate ◽

West Antarctica ◽

Laser Altimeter ◽

Equivalent Thickness ◽

Thickness Change ◽

Airborne Laser ◽

Ice Stream ◽

Whillans Ice Stream ◽

Elevation Changes ◽

Global Sea Level

AbstractRepeat airborne laser altimeter measurements are used to derive surface elevation changes on parts of Whillans Ice Stream and Ice Stream C, West Antarctica. Elevation changes are converted to estimates of ice equivalent thickness change using local accumulation rates, surface snow densities and vertical bedrock motions. The surveyed portions of two major tributaries of Whillans Ice Stream are found to be thinning almost uniformly at an average rate of ∼1 m a−1. Ice Stream C has a complicated elevation-change pattern, but is generally thickening. These results are used to estimate the contribution of each surveyed region to the current rate of global sea-level rise.

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Melting and freezing beneath the Ross ice streams, Antarctica

Journal of Glaciology ◽

10.3189/172756504781830295 ◽

2004 ◽

Vol 50 (168) ◽

pp. 96-108 ◽

Cited By ~ 75

Author(s):

Ian Joughin ◽

Slawek Tulaczyk ◽

Douglas R. MacAyeal ◽

Hermann Engelhardt

Keyword(s):

Steady Flow ◽

Temperature Model ◽

West Antarctica ◽

Ice Streams ◽

Horizontal Advection ◽

Ice Stream ◽

Whillans Ice Stream ◽

Shear Heating ◽

Strain Heating ◽

Inland Ice

AbstractWe 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.

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Surface velocity and mass balance of Ice Streams D and E, West Antarctica

Journal of Glaciology ◽

10.3189/s0022143000003452 ◽

1996 ◽

Vol 42 (142) ◽

pp. 461-475 ◽

Cited By ~ 4

Author(s):

Robert Bindschadler ◽

Patricia Vornberger ◽

Donald Blankenship ◽

Ted Scambos ◽

Robert Jacobel

Keyword(s):

Mass Balance ◽

Surface Velocity ◽

Random Errors ◽

West Antarctica ◽

Velocity Measurements ◽

Ice Streams ◽

Ice Stream ◽

Data Density ◽

Basal Shear ◽

Collection Methods

AbstractOver 75 000 surface-velocity measurements are extracted from sequential satellite imagery of Ice Streams D and E to reveal a complex pattern of flow not apparent from previous measurements. Horizontal and vertical strain rates, calculated from surface velocity, indicate that the bed experiences larger basal shear where the surface of these ice streams is rougher. Ten airborne-radar profiles and one surface-based radar profile of ice thickness make possible the calculation of mass balance for longitudinal sections of each ice stream. Improved data-collection methods increase data density, substantially reducing random errors in velocity. However, systematic errors continue to limit the ability of the flux-differencing technique used here to resolve local variations in mass balance. Nevertheless, significant local variations in mass balance are revealed, while, overall, Ice Streams D and E are in approximate equilibrium. An earlier estimate of the net mass balance for Ice Stream D is improved.

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Continued deceleration of Whillans Ice Stream, West Antarctica

Geophysical Research Letters ◽

10.1029/2005gl024319 ◽

2005 ◽

Vol 32 (22) ◽

pp. n/a-n/a ◽

Cited By ~ 58

Author(s):

I. Joughin ◽

R. A. Bindschadler ◽

M. A. King ◽

D. Voigt ◽

R. B. Alley ◽

...

Keyword(s):

West Antarctica ◽

Ice Stream ◽

Whillans Ice Stream

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Scale independence of till rheology

Journal of Glaciology ◽

10.3189/172756506781828601 ◽

2006 ◽

Vol 52 (178) ◽

pp. 377-380 ◽

Cited By ~ 37

Author(s):

Slawek Tulaczyk

Keyword(s):

Characteristic Length ◽

West Antarctica ◽

Ice Flow ◽

Length Scales ◽

Ice Stream ◽

In Situ Experiments ◽

Whillans Ice Stream ◽

Speed Up

AbstractRepresentation of till rheology in glaciological models of ice motion over deformable sediments has, until now, focused largely on two end-member cases: (1) linear, or mildly non-linear, viscous rheology and (2) (nearly) plastic rheology. Most laboratory and in situ experiments support the latter model. Hindmarsh (1997) and Fowler (2002, 2003) proposed that experimental results represent the behavior of small till samples (characteristic length scales of ~0.1 to ~1 m) but that till behaves viscously over length scales that are relevant to determination of ice-flow rates in glaciers and ice sheets (~1 km or more). Observations of short speed-up events on the ice plain of Whillans Ice Stream, West Antarctica, provide an opportunity to compare the in situ rheology of this till, integrated over ~10–100 km, with the rheology of till from beneath the same ice stream determined on small laboratory samples and in local borehole experiments. This comparison indicates that the rheology of the subglacial till beneath Whillans Ice Stream is independent of scale.

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