scholarly journals Bed topography and lubrication inferred from surface measurements on fast-flowing ice streams

2003 ◽  
Vol 49 (167) ◽  
pp. 481-490 ◽  
Author(s):  
Throstur Thorsteinsson ◽  
Charles F. Raymond ◽  
G. Hilmar Gudmundsson ◽  
Robert A. Bindschadler ◽  
Paul Vornberger ◽  
...  
Keyword(s):  
Measurement Errors ◽  
Synthetic Data ◽  
Ice Thickness ◽  
West Antarctica ◽  
Horizontal Scale ◽  
Ice Streams ◽  
Bed Topography ◽  
Ice Stream ◽  
Flow Disturbances ◽  
Surface Measurements

AbstractObservations of surface elevation (s) and horizontal velocity components (u and v) are inverted to infer the topography (b) and lubrication (c) at the bed of an ice stream, based on a linearized perturbation theory of the transmission of flow disturbances through the ice thickness. Synthetic data are used to illustrate non-uniqueness in the inversion, but also demonstrate that effects of b and c can be separated when s, u and v are specified, even with added noise to simulate measurement errors. We have analyzed prominent short-horizontal-scale (∼2 km) features in topography and velocity pattern in a local 64 km by 32 km area of the surface of Ice Stream E,West Antarctica. Our preferred interpretation of bed conditions beneath the most prominent features on the surface identifies a deep trough in the basal topography with low lubrication in the base of the trough.

scholarly journals Of isbræ and ice streams

2003 ◽  
Vol 36 ◽  
pp. 66-72 ◽  
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.

scholarly journals Radar reflections reveal a wet bed beneath stagnant Ice Stream C and a frozen bed beneath ridge BC, West Antarctica

1998 ◽  
Vol 44 (146) ◽  
pp. 149-156 ◽  
Author(s):  
C. R. Bentley ◽  
N. Lord ◽  
C. Liu
Keyword(s):  
Radar Data ◽  
Ice Thickness ◽  
West Antarctica ◽  
Ice Streams ◽  
Ice Stream ◽  
Crossover Analysis ◽  
Automated Processing ◽  
Basal Reflection ◽  
Field Season ◽  
Ice Stream B

AbstractDigital airborne radar data were collected during the 1987-88 Antarctic field season in nine gridded blocks covering the downstream portions of Ice Stream B (6km spacing) and Ice Stream C (11 km spacing), together with a portion of ridge BC between them. An automated processing procedure was used for picking onset times of the reflected radar pulses, converting travel times to distances, interpolating missing data, converting pressure transducer readings, correcting navigational drift, performing crossover analysis, and zeroing rémanent crossover errors. Interpolation between flight-lines was carried out using the minimum curvature method.Maps of ice thickness (estimated accuracy 20 m) and basal-reflection strength (estimated accuracy 1 dB) were produced. The ice-thickness map confirms the characteristics of previous reconnaissance maps and reveals no new features. The reflection-strength map shows pronounced contrasts between the ice streams and ridge BC and between the two ice streams themselves. We interpret the reflection strengths to mean that the bed of Ice Stream C, as well as that of Ice Stream B, is unfrozen, that the bed of ridge BC is frozen and that the boundary between the frozen bed of ridge BC and the unfrozen bed of Ice Stream C lies precisely below the former shear margin of the ice stream.

scholarly journals Ice-Thickness Map of the West Antarctic Ice Streams by Radar Sounding

1988 ◽  
Vol 11 ◽  
pp. 126-136 ◽  
Author(s):  
S. Shabtaie ◽  
C. R. Bentley
Keyword(s):  
Transient Behavior ◽  
Ice Thickness ◽  
Ice Streams ◽  
Cross Sectional ◽  
Ross Ice Shelf ◽  
Bed Topography ◽  
West Antarctic ◽  
Ice Stream ◽  
Adjacent Part ◽  
Thickness Variations

Extensive radar ice-thickness sounding of ice streams A, B, and C, and the ridges between them, has been carried out. Closely spaced flight lines, as well as ties to numerous ground stations, have enabled us to compile a detailed ice-thickness map of the area. The map reveals a highly complex pattern of ice-thickness variations, which, because they are much larger than the surface relief, largely reflect the subglacial topography. Several cross-sectional profiles across the ice streams and ridges are shown, and a new configuration for Ice Stream A is presented. Ice Stream A is connected to Reedy Glacier and Horlick Ice Stream by subglacial troughs that converge down-stream. The single trough continues, at a depth of more than 1000 m below sea-level, beneath the entire length of the ice stream and adjacent part of Ross Ice Shelf. Ridge AB (part of which may be a remanent ice stream) overlies a deep bed with pronounced troughs at its headward end; the bed shoals rapidly down-stream to a height more than 500 m above the beds of the adjacent ice streams. Ice stream B1 overlies a subglacial trough that is deep inland and also shoals markedly toward the grounding line. Near its head. Ice Stream B2 is as much as 1000 m thinner than Ice Stream Bl, but then remains much more nearly constant in thickness along its length. Ridge BC is characterized by a smoother bed and less variation in bed depth than ridge AB. Ice Stream C, which is inactive, is particularly marked by uncorrelated maxima and minima in surface and bed topography. There are no distinct topographical steps that demarcate the transition from sheet to streaming flow at the head of the ice streams, and the ice streams are placed asymmetrically in some places with respect to their subglacial troughs. This may reflect a relative impermanence or transient behavior of the “Ross” ice streams.

scholarly journals Ice stream or not? Radio-echo sounding of Carlson Inlet, West Antarctica

2011 ◽  
Vol 5 (4) ◽  
pp. 907-916 ◽  
Author(s):  
E. C. King
Keyword(s):  
West Antarctica ◽  
Ice Streams ◽  
Basal Resistance ◽  
Bed Topography ◽  
Ice Stream ◽  
Time Period ◽  
Adjacent Part ◽  
Radio Echo Sounding ◽  
History Of ◽  
Fast Flow

Abstract. The Antarctic Ice Sheet loses mass to the surrounding ocean mainly by drainage through a network of ice streams: fast-flowing glaciers bounded on either side by ice flowing one or two orders of magnitude more slowly. Ice streams flow despite low driving stress because of low basal resistance but are known to cease flowing if the basal conditions change, which can take place when subglacial sediment becomes dewatered by freezing or by a change in hydraulic pathways. Carlson Inlet, Antarctica has been interpreted as a stagnated ice stream, based on surface and basal morphology and shallow radar reflection profiling. To resolve the question of whether the flow history of Carlson Inlet has changed in the past, I conducted a ground-based radar survey of Carlson Inlet, the adjacent part of Rutford Ice Stream, and Talutis Inlet, West Antarctica. This survey provides details of the internal ice stratigraphy and allows the flow history to be interpreted. Tight folding of isochrones in Rutford Ice Stream and Talutis Inlet is interpreted to be the result of lateral compression during convergent flow from a wide catchment into a narrow, fast-flowing trunk. In contrast, the central part of Carlson Inlet has gently-folded isochrones that drape over the bed topography, suggestive of local accumulation and slow flow. A 1-D thermo-mechanical model was used to estimate the age of the ice. I conclude that the ice in the centre of Carlson Inlet has been near-stagnant for between 3500 and 6800 yr and that fast flow has not occurred there during that time period.

scholarly journals Ice stream or not? Radio-echo sounding of Carlson Inlet, West Antarctica

2011 ◽  
Vol 5 (2) ◽  
pp. 1219-1238
Author(s):  
E. C. King
Keyword(s):  
West Antarctica ◽  
Ice Streams ◽  
Basal Resistance ◽  
Bed Topography ◽  
Ice Stream ◽  
Time Period ◽  
Adjacent Part ◽  
Radio Echo Sounding ◽  
History Of ◽  
Fast Flow

Abstract. The Antarctic Ice Sheet loses mass to the surrounding ocean mainly by drainage through a network of ice streams: fast-flowing glaciers bounded on either side by ice flowing one or two orders of magnitude more slowly. Ice streams flow despite low driving stress because of low basal resistance but are known to cease flowing if the basal conditions change, which can take place when subglacial sediment becomes dewatered by freezing or by a change in hydraulic pathways. Carlson Inlet, Antarctica has been interpreted as a stagnated ice stream, based on surface and basal morphology and shallow radar reflection profiling. To resolve the question of whether the flow history of Carlson Inlet has changed in the past, I conducted a ground-based radar survey of Carlson Inlet, the adjacent part of Rutford Ice Stream, and Talutis Inlet, West Antarctica. This survey provides details of the internal ice stratigraphy and allows the flow history to be interpreted. Tight folding of isochrones in Rutford Ice Stream and Talutis Inlet is interpreted to be the result of lateral compression during convergent flow from a wide catchment into a narrow, fast-flowing trunk. In contrast, the central part of Carlson Inlet has gently-folded isochrones that drape over the bed topography, suggestive of local accumulation and slow flow. A 1-D thermo-mechanical model was used to estimate the age of the ice. I conclude that the ice in the centre of Carlson Inlet has been near-stagnant for between 3500 and 6800 years and that fast flow has not occurred there during that time period.

scholarly journals Ice-Thickness Map of the West Antarctic Ice Streams by Radar Sounding

1988 ◽  
Vol 11 ◽  
pp. 126-136 ◽  
Author(s):  
S. Shabtaie ◽  
C. R. Bentley
Keyword(s):  
Transient Behavior ◽  
Ice Thickness ◽  
Ice Streams ◽  
Cross Sectional ◽  
Ross Ice Shelf ◽  
Bed Topography ◽  
West Antarctic ◽  
Ice Stream ◽  
Adjacent Part ◽  
Thickness Variations

Extensive radar ice-thickness sounding of ice streams A, B, and C, and the ridges between them, has been carried out. Closely spaced flight lines, as well as ties to numerous ground stations, have enabled us to compile a detailed ice-thickness map of the area. The map reveals a highly complex pattern of ice-thickness variations, which, because they are much larger than the surface relief, largely reflect the subglacial topography. Several cross-sectional profiles across the ice streams and ridges are shown, and a new configuration for Ice Stream A is presented.Ice Stream A is connected to Reedy Glacier and Horlick Ice Stream by subglacial troughs that converge down-stream. The single trough continues, at a depth of more than 1000 m below sea-level, beneath the entire length of the ice stream and adjacent part of Ross Ice Shelf. Ridge AB (part of which may be a remanent ice stream) overlies a deep bed with pronounced troughs at its headward end; the bed shoals rapidly down-stream to a height more than 500 m above the beds of the adjacent ice streams. Ice stream B1 overlies a subglacial trough that is deep inland and also shoals markedly toward the grounding line. Near its head. Ice Stream B2 is as much as 1000 m thinner than Ice Stream Bl, but then remains much more nearly constant in thickness along its length. Ridge BC is characterized by a smoother bed and less variation in bed depth than ridge AB. Ice Stream C, which is inactive, is particularly marked by uncorrelated maxima and minima in surface and bed topography.There are no distinct topographical steps that demarcate the transition from sheet to streaming flow at the head of the ice streams, and the ice streams are placed asymmetrically in some places with respect to their subglacial troughs. This may reflect a relative impermanence or transient behavior of the “Ross” ice streams.

scholarly journals Radar reflections reveal a wet bed beneath stagnant Ice Stream C and a frozen bed beneath ridge BC, West Antarctica

1998 ◽  
Vol 44 (146) ◽  
pp. 149-156 ◽  
Author(s):  
C. R. Bentley ◽  
N. Lord ◽  
C. Liu
Keyword(s):  
Radar Data ◽  
Ice Thickness ◽  
West Antarctica ◽  
Ice Streams ◽  
Ice Stream ◽  
Crossover Analysis ◽  
Automated Processing ◽  
Basal Reflection ◽  
Field Season ◽  
Ice Stream B

AbstractDigital airborne radar data were collected during the 1987-88 Antarctic field season in nine gridded blocks covering the downstream portions of Ice Stream B (6km spacing) and Ice Stream C (11 km spacing), together with a portion of ridge BC between them. An automated processing procedure was used for picking onset times of the reflected radar pulses, converting travel times to distances, interpolating missing data, converting pressure transducer readings, correcting navigational drift, performing crossover analysis, and zeroing rémanent crossover errors. Interpolation between flight-lines was carried out using the minimum curvature method.Maps of ice thickness (estimated accuracy 20 m) and basal-reflection strength (estimated accuracy 1 dB) were produced. The ice-thickness map confirms the characteristics of previous reconnaissance maps and reveals no new features. The reflection-strength map shows pronounced contrasts between the ice streams and ridge BC and between the two ice streams themselves. We interpret the reflection strengths to mean that the bed of Ice Stream C, as well as that of Ice Stream B, is unfrozen, that the bed of ridge BC is frozen and that the boundary between the frozen bed of ridge BC and the unfrozen bed of Ice Stream C lies precisely below the former shear margin of the ice stream.

scholarly journals On the limit to resolution and information on basal properties obtainable from surface data on ice streams

2008 ◽  
Vol 2 (2) ◽  
pp. 167-178 ◽  
Author(s):  
G. H. Gudmundsson ◽  
M. Raymond
Keyword(s):  
Flow Line ◽  
Estimation Method ◽  
Optimal Estimation ◽  
Surface Flow ◽  
Ice Thickness ◽  
Ice Streams ◽  
Statistical Estimates ◽  
Surface Data ◽  
Highly Sensitive ◽  
Surface Measurements

Abstract. An optimal estimation method for simultaneously determining both basal slipperiness and basal topography from variations in surface flow velocity and topography along a flow line on ice streams and ice sheets is presented. We use Bayesian inference to update prior statistical estimates for basal topography and slipperiness using surface measurements along a flow line. Our main focus here is on how errors and spacing of surface data affect estimates of basal quantities and on possibly aliasing/mixing between basal slipperiness and basal topography. We find that the effects of spatial variations in basal topography and basal slipperiness on surface data can be accurately separated from each other, and mixing in retrieval does not pose a serious problem. For realistic surface data errors and density, small-amplitude perturbations in basal slipperiness can only be resolved for wavelengths larger than about 50 times the mean ice thickness. Bedrock topography is well resolved down to horizontal scale equal to about one ice thickness. Estimates of basal slipperiness are not significantly improved by accurate prior estimates of basal topography. However, retrieval of basal slipperiness is found to be highly sensitive to unmodelled errors in basal topography.

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

2013 ◽  
Vol 54 (64) ◽  
pp. 105-114 ◽  
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.

scholarly journals Surface Features of Ice Stream B, Marie Byrd Land, West Antarctica

1986 ◽  
Vol 8 ◽  
pp. 168-170 ◽  
Author(s):  
P.L. Vornberger ◽  
I.M. Whillans
Keyword(s):  
Aerial Photographs ◽  
Relative Importance ◽  
West Antarctica ◽  
Ice Streams ◽  
Longitudinal Compression ◽  
West Antarctic Ice Sheet ◽  
Surface Features ◽  
West Antarctic ◽  
Ice Stream ◽  
Ice Stream B

Aerial photographs have been obtained of Ice Stream B, one of the active ice streams draining the West Antarctic Ice Sheet. A sketch map made from these photographs shows two tributaries. The margin of the active ice is marked by curved crevasses and intense crevassing occurs just inward of them. Transverse crevasses dominate the center of the ice streams and diagonal types appear at the lower end. A “suture zone” originates at the tributary convergence and longitudinal surface ridges occur at the downglacier end. The causes of these surface features are discussed and the relative importance of four stresses in resisting the driving stress is assessed. We conclude that basal drag may be important, longitudinal compression is probably important at the lower end, and longitudinal tension is probably most important near the head of the ice stream. Side drag leads to shearing at the margins, but does not restrain much of the ice stream.

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