Relating polarimetric radar measurements of ice fabric to ice-flow enhancement of Rutford Ice Stream

2020 ◽  
Author(s):  
Thomas Jordan ◽  
Alex Brisbourne ◽  
Carlos Martin ◽  
Rebecca Schlegel ◽  
Dustin Schroeder ◽  
...  
Keyword(s):  
Polarimetric Radar ◽  
Ice Streams ◽  
Ice Flow ◽  
Near Surface ◽  
Flow Models ◽  
Relative Contribution ◽  
Ice Stream ◽  
Glacier Ice ◽  
Flow Enhancement ◽  
The Impact

<p>Lateral shear margins provide resistance to ice flow within ice streams and play an important role in the overall dynamics of ice sheets. The strength and location of shear margins are known to be influenced by both subglacial factors (e.g. bed roughness, meltwater availability) and ice rheology (ice temperature, ice fabric, and damage). Assessing the relative contribution of these factors upon ice-stream flow is complex but can be aided by geophysical measurements (e.g. radar-sounding and seismic imaging) of the ice-stream subsurface. There are, however, ongoing challenges in obtaining geophysical information in an appropriate form to be incorporated into ice-flow models. This is true of ice fabric, and its direction-dependent effect upon ice viscosity is typically neglected in models of ice streams.</p><p>Here we develop a framework to relate ice fabric measurements from polarimetric radar sounding to ice-flow enhancement within ice streams. First, we extend a `polarimetric coherence’ radar method to automate the extraction of ice fabric using quad-polarized data.  Second, using a previously developed anisotropic flow-law formulation, we relate the radar fabric measurements to direction-dependent enhancement factors of glacier ice. We demonstrate the approach using a radar ground survey, collected by the British Antarctic Survey, which traverses between the centre and shear margin of Rutford Ice Stream. The data indicate that a vertical girdle fabric is present in the near-surface of the ice stream (approximately the top 300 m) which azimuthally rotates and strengthens toward the shear margin. We then assess the effect that the girdle fabric has upon shear and compression and the impact upon ice-flow models of Rutford Ice Stream.</p><p> </p><p> </p><p> </p>

scholarly journals The impact of subglacial hydrology on the force balance of a physically modelled ice stream

2013 ◽  
Vol 54 (63) ◽  
pp. 333-342 ◽  
Author(s):  
Benjamin M. Wagman ◽  
Ginny A. Catania
Keyword(s):  
Water Distribution ◽  
Water Discharge ◽  
Water Layer ◽  
Flow Speed ◽  
Force Balance ◽  
Ice Streams ◽  
Basal Resistance ◽  
Ice Flow ◽  
Ice Stream ◽  
The Impact

AbstractWe use a physical model to investigate how changes in subglacial hydrology affect ice motion of Antarctic ice streams. Ice streams are modelled using silicone polymer placed over a thin water layer to mimic ice flow dominated by basal sliding. The model ice-stream force balance is calculated and compared directly to the observed force balance of Whillans Ice Stream (WIS). Dynamic similarity between the model and WIS is achieved when their force balances are equivalent. The WIS force balance has evolved over time owing to increased basal resistance. We test two hypotheses: (1) the subglacial water distribution influences the ice-flow speed and thus the force balance; (2) shear margins are locations where transitions in water layer thickness occur. We find that the velocity and force balance are sensitive to pulsed water discharge events and changes in lubrication that result in sticky spots, and that model shear margins tend to overlie water lubrication boundaries. We conclude that local changes in basal lubrication near margins (possibly as a result of the presence of sticky spots or subglacial lakes) influence the stability of ice-stream margin position and may be responsible for large and rapid shifts in margin location.

scholarly journals Radar surveys of the Rutford Ice Stream onset zone, West Antarctica: indications of flow (in)stability?

2009 ◽  
Vol 50 (51) ◽  
pp. 57-62 ◽  
Author(s):  
John Woodward ◽  
Edward C. King
Keyword(s):  
Accumulation Rate ◽  
Flow Direction ◽  
Cross Flow ◽  
Radar Data ◽  
West Antarctica ◽  
Ice Streams ◽  
Ice Flow ◽  
Near Surface ◽  
West Antarctic ◽  
Ice Stream

AbstractWe present 1 and 100 MHz ground-based radar data from the onset region of Rutford Ice Stream, West Antarctica, which indicate the form and internal structure of isochrones. In the flow-parallel lines, modelled isochrone patterns reproduce the gross pattern of the imaged near-surface layers, assuming steady-state flow velocity from GPS records and the current accumulation rate for the last 200 years. We interpret this as indicating overall stability in flow in the onset region of Rutford Ice Stream throughout this period. However, in the cross-flow lines some local variability in accumulation is seen in areas close to the ice-stream margin where a number of tributaries converge towards the ice-stream onset zone. Episodic surface lowering events are observed followed by rapid fill episodes. The fill events indicate deposition towards the northwest, most likely generated by storm winds, which blow at an oblique angle to ice flow. More problematic is explaining the generation of episodic surface lowering in this area. We speculate this may be due to: changing ice-flow direction in the complex tributary area of the onset zone; a change in basal sediments or sedimentary landforms; a change in basal melt rates or water supply; or episodic lake drainage events in the fjord systems of the Ellsworth Subglacial Highlands. The study highlights the difficulty of assessing flow stability in the complex onset regions of West Antarctic ice streams.

scholarly journals The role of subglacial hydrology in ice streams with elevated geothermal heat flux

2020 ◽  
Vol 66 (256) ◽  
pp. 303-312
Author(s):  
Silje Smith-Johnsen ◽  
Basile de Fleurian ◽  
Kerim H. Nisancioglu
Keyword(s):  
Heat Flux ◽  
Ice Sheet ◽  
Complex Surface ◽  
Surface Velocity ◽  
Ice Streams ◽  
Geothermal Heat ◽  
Ice Flow ◽  
Ice Stream ◽  
The Impact

AbstractThe spatial distribution of geothermal heat flux (GHF) under ice sheets is largely unknown. Nonetheless, it is an important boundary condition in ice-sheet models, and suggested to control part of the complex surface velocity patterns observed in some regions. Here we investigate the effect of including subglacial hydrology when modelling ice streams with elevated GHF. We use an idealised ice stream geometry and a thermomechanical ice flow model coupled to subglacial hydrology in the Ice Sheet System Model (ISSM). Our results show that the dynamic response of the ice stream to elevated GHF is greatly enhanced when including the interactive subglacial hydrology. On the other hand, the impact of GHF on ice temperature is reduced when subglacial hydrology is included. In conclusion, the sensitivity of ice stream dynamics to GHF is likely to be underestimated in studies neglecting subglacial hydrology.

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 Thermomechanical modelling of the Scandinavian ice sheet: implications for ice-stream formation

1999 ◽  
Vol 28 ◽  
pp. 83-89 ◽  
Author(s):  
A. J. Payne ◽  
D.J. Baldwin
Keyword(s):  
Three Dimensional ◽  
Ice Sheet ◽  
Ice Streams ◽  
Ice Flow ◽  
Low Viscosity ◽  
Ice Stream ◽  
Scandinavian Ice Sheet ◽  
The Baltic ◽  
Stream Formation ◽  
Initial Results

AbstractThis work attempts to explain the fan-like landform assemblages observed in satellite images of the area covered by the former Scandinavian ice sheet (SIS). These assemblages have been interpreted as evidence of large ice streams within the SIS. If this interpretation is correct, then it calls into doubt current theories on the formation of ice streams. These theories regard soft sediment and topographic troughs as being the key determinants of ice-stream location. Neither can be used to explain the existence of ice streams on the flat, hard-rock area of the Baltic Shield. Initial results from a three-dimensional, thermomechanical ice-sheet model indicate that interactions between ice flow, form and temperature can create patterns similar to those mentioned above. The model uses a realistic, 20 km resolution gridded topography and a simple parameterization of accumulation and ablation. It produces patterns of maximum ice-sheet extent, which are similar to those reconstructed from the area’s glacial geomorphology. Flow in the maximum, equilibrium ice sheet is dominated by wedges of warm, low-viscosity, fast-flowing ice. These are separated by areas of cold, slow-flowing ice. This patterning appears to develop spontaneously as the modelled ice sheet grows.

Ribbed bedforms, markers of palaeo-ice stream margins, basal meltwater drainage and ice flow dynamic

2021 ◽  
Author(s):  
Jean Vérité ◽  
Édouard Ravier ◽  
Olivier Bourgeois ◽  
Stéphane Pochat ◽  
Thomas Lelandais ◽  
...  
Keyword(s):  
Flow Direction ◽  
Local Stress ◽  
Sediment Surface ◽  
Shear Stresses ◽  
Ice Streams ◽  
Ice Flow ◽  
Ice Stream ◽  
Physical Experiments ◽  
Stream Beds ◽  
Ice Water

<p>Over the three last decades, great efforts have been undertaken by the glaciological community to characterize the behaviour of ice streams and better constrain the dynamics of ice sheets. Studies of modern ice stream beds reveal crucial information on ice-meltwater-till-bedrock interactions, but are restricted to punctual observations limiting the understanding of ice stream dynamics as a whole. Consequently, theoretical ice stream landsystems derived from geomorphological and sedimentological observations were developed to provide wider constraints on those interactions on palaeo-ice stream beds. Within these landsystems, the spatial distribution and formation processes of subglacial periodic bedforms transverse to the ice flow direction – ribbed bedforms – remain unclear. The purpose of this study is (i) to explore the conditions under which these ribbed bedforms develop and (ii) to constrain their spatial organisation along ice stream beds.  </p><p>We performed physical experiments with silicon putty (to simulate the ice), water (to simulate the meltwater) and sand (to simulate a soft sedimentary bed) to model the dynamics of ice streams and produce analog subglacial landsystems. We compare the results of these experiments with the distribution of ribbed bedforms on selected examples of palaeo-ice stream beds of the Laurentide Ice Sheet. Based on this comparison, we can draw several conclusions regarding the significance of ribbed bedforms in ice stream contexts:</p><ul><li>Ribbed bedforms tend to form where the ice flow undergoes high velocity gradients and the ice-bed interface is unlubricated. Where the ribs initiate, we hypothesize that high driving stresses generate high basal shear stresses, accommodated through bed deformation of the active uppermost part of the bed.</li> <li>Ribbed bedforms can develop subglacially from a flat sediment surface beneath shear margins (i.e., lateral ribbed bedforms) and stagnant lobes (i.e., submarginal ribbed bedforms) of ice streams, while they do not develop beneath surging lobes.</li> <li>The orientation of ribbed bedforms reflects the local stress state along the ice-bed interface, with transverse bedforms formed by compression beneath ice lobes and oblique bedforms formed by transgression below lateral shear margins.</li> <li>The development of ribbed bedforms where the ice-bed interface is unlubricated reveals distinctive types of discontinuous basal drainage systems below shear and lobe margins: linked-cavities and efficient meltwater channels respectively.</li> </ul><p>Ribbed bedforms could thus constitute convenient geomorphic markers for the reconstruction of palaeo-ice stream margins, palaeo-ice flow dynamics and palaeo-meltwater drainage characteristics.</p>

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 Migration of the Siple Dome ice divide, West Antarctica

1998 ◽  
Vol 44 (148) ◽  
pp. 643-652 ◽  
Author(s):  
N. A. Nereson ◽  
C. F. Raymond ◽  
E. D. Waddington ◽  
R. W. Jacobel
Keyword(s):  
Migration Rate ◽  
Internal Layer ◽  
Internal Layers ◽  
West Antarctica ◽  
Ice Streams ◽  
Ice Flow ◽  
Flow Models ◽  
The Past ◽  
Siple Dome ◽  
Local Accumulation

AbstractThe non-linearity of the ice-flow law or a local accumulation low over an ice divide can cause isochrones (internal layers) to be shallower under the divide relative to the flanks, forming a “divide bump” in the internal layer pattern. This divide signature is analyzed using ice-flow models and inverse techniques to detect and quantify motion of the Siple Dome ice divide, West Antarctica. The principal feature indicating that migration has occurred is a distinct tilt of the axis of the peaks of the warped internal layers beneath the divide. The calculated migration rate is 0.05-0.50 m a−1 toward Ice Stream D and depends slightly on whether the divide bump is caused by the non-linearity of ice flow or by a local accumulation low. Our calculations also suggest a strong south-north accumulation gradient of 5-10 x 10−6 a−1 in a narrow zone north of the divide. A consequence of divide migration is that pre-Holocene ice is thickest about 0.5 km south of the present divide position. Divide motion indicates that non-steady processes, possibly associated with activity of the bounding ice streams, are affecting the geometry of Siple Dome. The migration rate is sufficiently slow that the divide bump is maintained in the internal layer pattern at all observable depths. This suggests that major asynchronous changes in the elevation or position of the bounding ice streams are unlikely over at least the past 103-104 years.

scholarly journals Is Ice-Stream Evolution Revealed By Satellite Imagery?

1990 ◽  
Vol 14 ◽  
pp. 273-277 ◽  
Author(s):  
S.N. Stephenson ◽  
R.A. Bindschadler
Keyword(s):  
Stream Flow ◽  
Temporal Evolution ◽  
Thematic Mapper ◽  
Landsat Thematic Mapper ◽  
West Antarctica ◽  
Ice Streams ◽  
Ice Flow ◽  
Ice Shelves ◽  
Ice Stream ◽  
Grounding Line

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.

scholarly journals Is Ice-Stream Evolution Revealed By Satellite Imagery?

1990 ◽  
Vol 14 ◽  
pp. 273-277 ◽  
Author(s):  
S.N. Stephenson ◽  
R.A. Bindschadler
Keyword(s):  
Stream Flow ◽  
Temporal Evolution ◽  
Thematic Mapper ◽  
Landsat Thematic Mapper ◽  
West Antarctica ◽  
Ice Streams ◽  
Ice Flow ◽  
Ice Shelves ◽  
Ice Stream ◽  
Grounding Line

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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