Is Ice-Stream Evolution Revealed By Satellite Imagery?

Ten Landsat Thematic Mapper images together show Ice Streams E, D and most of Ice Stream C on Siple Coast, West Antarctica. The images are interpreted to reveal aspects of both spatial and temporal evolution of the ice streams. Onset of ice-stream flow appears to occur at distributed sites within the ice-stream catchment, and the apparent enhanced flow continues in channels until they join, forming the main ice stream. Most crevassing on these ice streams is associated with features of horizontal dimensions between 5 and 20 km. We suggest these features are caused by bed structures which may be an important source of restraint to ice flow, similar to ice rumples on ice shelves. A pattern of features near the grounding line of the now-stagnant Ice Stream C are interpreted as having formed because there was a period of reduced flux before the ice stream stopped.

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Grounding-zone flow variability of Priestley Glacier, Antarctica, in a diurnal tidal regime

10.5194/egusphere-egu21-15197 ◽

2021 ◽

Author(s):

Reinhard Drews ◽

Christian Wild ◽

Oliver Marsh ◽

Wolfgang Rack ◽

Todd Ehlers ◽

...

Keyword(s):

Spatial Scales ◽

Ice Shelf ◽

Ice Streams ◽

Ice Flow ◽

Ice Shelves ◽

Ice Stream ◽

Grounding Line ◽

Temporal And Spatial ◽

Gnss Measurements ◽

Natural Laboratory

<p>Dynamics of polar outlet glaciers vary with ocean tides, providing a natural laboratory to understand basal processes beneath ice streams, ice rheology and ice-shelf buttressing. We apply Terrestrial Radar Interferometry to close the spatiotemporal gap between localized, temporally well-resolved GNSS and area-wide but sparse satellite observations. Three-hour flowfields collected over an eight day period at Priestley Glacier, Antarctica, validate and provide the spatial context for concurrent GNSS measurements. Ice flow is fastest during falling tides and slowest during rising tides. Principal components of the timeseries prove upstream propagation of tidal signatures $>$ 10 km away from the grounding line. Hourly, cm-scale horizontal and vertical flexure patterns occur $>$6 km upstream of the grounding line. Vertical uplift upstream of the grounding line is consistent with ephemeral re-grounding during low-tide impacting grounding-zone stability. On the freely floating ice shelves, we find velocity peaks both during high- and low-tide suggesting that ice-shelf buttressing varies temporally as a function of flexural bending from tidal displacement. Taken together, these observations identify tidal imprints on ice-stream dynamics on new temporal and spatial scales providing constraints for models designed to isolate dominating processes in ice-stream and ice-shelf mechanics.</p>

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New and improved determinations of velocity of Ice Streams B and C, West Antarctica

Journal of Glaciology ◽

10.3189/s0022143000016373 ◽

1993 ◽

Vol 39 (133) ◽

pp. 483-590 ◽

Cited By ~ 6

Author(s):

I. M. Whillans ◽

C.J. Van Der Veen

Keyword(s):

Stream Flow ◽

West Antarctica ◽

Ice Shelf ◽

Ice Streams ◽

Ice Flow ◽

Ice Stream ◽

Fast Ice ◽

Ice Stream B ◽

Shelf Flow ◽

Streaming Flow

AbstractMeasurements of velocity have been made on and next to Ice Streams Β and C, West Antarctica. The results are more precise than previous work and constitute a 93% increase in the number of values. These velocities are used to describe the confluence of flow into the ice streams and the development of fast ice-stream flow. The onset of fast-streaming flow occurs in many separate tributaries that coalesce down-glacier into the major ice streams. For those inter-stream ridges that have been studied, the flow is consistent with steady state. Along Ice Stream B, gradients in longitudinal stress offer little resistance to the ice flow. The transition from basal-drag control to ice-shelf flow is achieved through reduced drag at the glacier base and increased resistance associated with lateral drag. Velocities in the trunk of Ice Stream C are nearly zero but those at the up-glacial head are similar to those at the head of Ice Stream B.

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New and improved determinations of velocity of Ice Streams B and C, West Antarctica

Journal of Glaciology ◽

10.1017/s0022143000016373 ◽

1993 ◽

Vol 39 (133) ◽

pp. 483-590 ◽

Cited By ~ 87

Author(s):

I. M. Whillans ◽

C.J. Van Der Veen

Keyword(s):

Stream Flow ◽

West Antarctica ◽

Ice Shelf ◽

Ice Streams ◽

Ice Flow ◽

Ice Stream ◽

Fast Ice ◽

Ice Stream B ◽

Shelf Flow ◽

Streaming Flow

Abstract Measurements of velocity have been made on and next to Ice Streams Β and C, West Antarctica. The results are more precise than previous work and constitute a 93% increase in the number of values. These velocities are used to describe the confluence of flow into the ice streams and the development of fast ice-stream flow. The onset of fast-streaming flow occurs in many separate tributaries that coalesce down-glacier into the major ice streams. For those inter-stream ridges that have been studied, the flow is consistent with steady state. Along Ice Stream B, gradients in longitudinal stress offer little resistance to the ice flow. The transition from basal-drag control to ice-shelf flow is achieved through reduced drag at the glacier base and increased resistance associated with lateral drag. Velocities in the trunk of Ice Stream C are nearly zero but those at the up-glacial head are similar to those at the head of Ice Stream B.

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Persistence and variability of ice-stream grounding lines on retrograde bed slopes

The Cryosphere ◽

10.5194/tc-10-1883-2016 ◽

2016 ◽

Vol 10 (4) ◽

pp. 1883-1896 ◽

Cited By ~ 12

Author(s):

Alexander A. Robel ◽

Christian Schoof ◽

Eli Tziperman

Keyword(s):

Stream Flow ◽

Flow Line ◽

Reverse Direction ◽

West Antarctica ◽

Ice Streams ◽

Basal Resistance ◽

Physical Mechanisms ◽

Ice Stream ◽

Grounding Line ◽

Transient Variations

Abstract. In many ice streams, basal resistance varies in space and time due to the dynamically evolving properties of subglacial till. These variations can cause internally generated oscillations in ice-stream flow. However, the potential for such variations in basal properties is not considered by conventional theories of grounding-line stability on retrograde bed slopes, which assume that bed properties are static in time. Using a flow-line model, we show how internally generated, transient variations in ice-stream state interact with retrograde bed slopes. In contrast to predictions from the theory of the marine ice-sheet instability, our simulated grounding line is able to persist and reverse direction of migration on a retrograde bed when undergoing oscillations in the grounding-line position. In turn, the presence of a retrograde bed may also suppress or reduce the amplitude of internal oscillations in ice-stream state. We explore the physical mechanisms responsible for these behaviors and discuss the implications for observed grounding-line migration in West Antarctica.

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Persistence and Variability of Ice Stream Grounding Lines on Retrograde Bed Slopes

10.5194/tc-2016-18 ◽

2016 ◽

Author(s):

Alexander A. Robel ◽

Christian Schoof ◽

Eli Tziperman

Keyword(s):

Stream Flow ◽

Reverse Direction ◽

Line Position ◽

West Antarctica ◽

Ice Streams ◽

Basal Resistance ◽

Physical Mechanisms ◽

Ice Stream ◽

Grounding Line ◽

Transient Variations

Abstract. In many ice streams, basal resistance varies in space and time due to the dynamically-evolving properties of subglacial till. These variations can cause internally-generated oscillations in ice stream flow. However, the potential for such variations in basal properties are not considered by conventional theories of grounding line stability on retrograde bed slopes, which assume that bed properties are static in time. Using a numerical flowline model, we show how internally-generated, transient variations in ice stream state interact with retrograde bed slopes. In contrast to predictions from the theory of the marine ice sheet instability, our simulated grounding line is able to persist and reverse direction of migration on a retrograde bed when undergoing oscillations in the grounding line position. In turn, the presence of a retrograde bed may also suppress or reduce the amplitude of internal oscillations in ice stream state. We explore the physical mechanisms responsible for these behaviors and discuss the implications for observed grounding line migration in West Antarctica.

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Ice-stream demise dynamically conditioned by trough shape and bed strength

Science Advances ◽

10.1126/sciadv.aau1380 ◽

2019 ◽

Vol 5 (4) ◽

pp. eaau1380 ◽

Cited By ~ 6

Author(s):

Tom Bradwell ◽

David Small ◽

Derek Fabel ◽

Rachel K. Smedley ◽

Chris D. Clark ◽

...

Keyword(s):

Stream Flow ◽

Three Dimensional ◽

Step Change ◽

Ice Streams ◽

Ice Shelves ◽

Ice Stream ◽

Grounding Line ◽

The Last Glacial Maximum ◽

The Last Glacial

Ice sheet mass loss is currently dominated by fast-flowing glaciers (ice streams) terminating in the ocean as ice shelves and resting on beds below sea level. The factors controlling ice-stream flow and retreat over longer time scales (>100 years), especially the role of three-dimensional bed shape and bed strength, remain major uncertainties. We focus on a former ice stream where trough shape and bed substrate are known, or can be defined, to reconstruct ice-stream retreat history and grounding-line movements over 15 millennia since the Last Glacial Maximum. We identify a major behavioral step change around 18,500 to 16,000 years ago—out of tune with external forcing factors—associated with the collapse of floating ice sectors and rapid ice-front retreat. We attribute this step change to a marked geological transition from a soft/weak bed to a hard/strong bed coincident with a change in trough geometry. Both these factors conditioned and ultimately hastened ice-stream demise.

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Of isbræ and ice streams

Annals of Glaciology ◽

10.3189/172756403781816347 ◽

2003 ◽

Vol 36 ◽

pp. 66-72 ◽

Cited By ~ 88

Author(s):

Martin Truffer ◽

Keith A. Echelmeyer

Keyword(s):

Ice Sheet ◽

Shear Zones ◽

West Antarctica ◽

Ice Streams ◽

Ice Flow ◽

Bed Topography ◽

Ice Stream ◽

Fast Ice ◽

Fast Flow ◽

End Members

AbstractFast-flowing ice streams and outlet glaciers provide the major avenues for ice flow from past and present ice sheets. These ice streams move faster than the surrounding ice sheet by a factor of 100 or more. Several mechanisms for fast ice-stream flow have been identified, leading to a spectrum of different ice-stream types. In this paper we discuss the two end members of this spectrum, which we term the “ice-stream” type (represented by the Siple Coast ice streams in West Antarctica) and the “isbræ” type (represented by Jakobshavn Isbræ in Greenland). The typical ice stream is wide, relatively shallow (∼1000 m), has a low surface slope and driving stress (∼10 kPa), and ice-stream location is not strongly controlled by bed topography. Fast flow is possible because the ice stream has a slippery bed, possibly underlain by weak, actively deforming sediments. The marginal shear zones are narrow and support most of the driving stress, and the ice deforms almost exclusively by transverse shear. The margins seem to be inherently unstable; they migrate, and there are plausible mechanisms for such ice streams to shut down. The isbræ type of ice stream is characterized by very high driving stresses, often exceeding 200 kPa. They flow through deep bedrock channels that are significantly deeper than the surrounding ice, and have steep surface slopes. Ice deformation includes vertical as well as lateral shear, and basal motion need not contribute significantly to the overall motion. The marginal shear zone stend to be wide relative to the isbræ width, and the location of isbræ and its margins is strongly controlled by bedrock topography. They are stable features, and can only shut down if the high ice flux cannot be supplied from the adjacent ice sheet. Isbræs occur in Greenland and East Antarctica, and possibly parts of Pine Island and Thwaites Glaciers, West Antarctica. In this paper, we compare and contrast the two types of ice streams, addressing questions such as ice deformation, basal motion, subglacial hydrology, seasonality of ice flow, and stability of the ice streams.

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Deformation in Rutford Ice Stream, West Antarctica: measuring shear-wave anisotropy from icequakes

Annals of Glaciology ◽

10.3189/2013aog64a033 ◽

2013 ◽

Vol 54 (64) ◽

pp. 105-114 ◽

Cited By ~ 14

Author(s):

S.R. Harland ◽

J.-M. Kendall ◽

G.W. Stuart ◽

G.E. Lloyd ◽

A.F. Baird ◽

...

Keyword(s):

Shear Wave ◽

Flow Direction ◽

Transversely Isotropic ◽

Shear Wave Splitting ◽

Ice Crystal ◽

West Antarctica ◽

Ice Streams ◽

Ice Flow ◽

Ice Stream ◽

Wave Splitting

Abstract Ice streams provide major drainage pathways for the Antarctic ice sheet. The stress distribution and style of flow in such ice streams produce elastic and rheological anisotropy, which informs ice-flow modelling as to how ice masses respond to external changes such as global warming. Here we analyse elastic anisotropy in Rutford Ice Stream, West Antarctica, using observations of shear-wave splitting from three-component icequake seismograms to characterize ice deformation via crystal-preferred orientation. Over 110 high-quality measurements are made on 41 events recorded at five stations deployed temporarily near the ice-stream grounding line. To the best of our knowledge, this is the first well-documented observation of shear-wave splitting from Antarctic icequakes. The magnitude of the splitting ranges from 2 to 80 ms and suggests a maximum of 6% shear-wave splitting. The fast shear-wave polarization direction is roughly perpendicular to ice-flow direction. We consider three mechanisms for ice anisotropy: a cluster model (vertical transversely isotropic (VTI) model); a girdle model (horizontal transversely isotropic (HTI) model); and crack-induced anisotropy (HTI model). Based on the data, we can rule out a VTI mechanism as the sole cause of anisotropy – an HTI component is needed, which may be due to ice crystal a-axis alignment in the direction of flow or the alignment of cracks or ice films in the plane perpendicular to the flow direction. The results suggest a combination of mechanisms may be at play, which represent vertical variations in the symmetry of ice crystal anisotropy in an ice stream, as predicted by ice fabric models.

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The location of the grounding zone of Evans Ice Stream, Antarctica, investigated using SAR interferometry and modelling

Annals of Glaciology ◽

10.3189/172756409789624292 ◽

2009 ◽

Vol 50 (52) ◽

pp. 35-40 ◽

Cited By ~ 17

Author(s):

Helena J. Sykes ◽

Tavi Murray ◽

Adrian Luckman

Keyword(s):

Stream Flow ◽

Elastic Beam ◽

Tidal Height ◽

Sar Interferometry ◽

Beam Model ◽

West Antarctica ◽

Tidal Model ◽

Tidal Forcing ◽

Ice Stream ◽

Grounding Line

AbstractEvans Ice Stream, West Antarctica, has five tributaries and a complex grounding zone. The grounding zone of Evans Ice Stream, between the landward and seaward limits of tidal flexing, was mapped using SAR interferometry. The width of the mapped grounding zone was compared with that derived from an elastic beam model, and the tidal height changes derived from interferometry were compared with the results of a tidal model. Results show that in 1994 and 1996 the Evans grounding zone was located up to 100 km upstream of its location in the BEDMAP dataset. The grounding line of Evans Ice Stream is subjected to 5 m vertical tidal forcing, which would clearly affect ice-stream flow.

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