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

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

Annals of Glaciology ◽

10.3189/172756409789097469 ◽

2009 ◽

Vol 50 (51) ◽

pp. 57-62 ◽

Cited By ~ 12

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.

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

10.5194/egusphere-egu21-9526 ◽

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

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 &#8211; ribbed bedforms &#8211; 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. &#160;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:<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>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.

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Is Ice-Stream Evolution Revealed By Satellite Imagery?

Annals of Glaciology ◽

10.3189/s0260305500008740 ◽

1990 ◽

Vol 14 ◽

pp. 273-277 ◽

Cited By ~ 5

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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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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Is Ice-Stream Evolution Revealed By Satellite Imagery?

Annals of Glaciology ◽

10.1017/s0260305500008740 ◽

1990 ◽

Vol 14 ◽

pp. 273-277 ◽

Cited By ~ 28

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

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Seismic-Reflection Profiling of a Widespread Till Beneath Ice Stream B, West Antarctica (Abstract)

Annals of Glaciology ◽

10.1017/s0260305500006704 ◽

1988 ◽

Vol 11 ◽

pp. 210 ◽

Cited By ~ 3

Author(s):

Sean T. Rooney ◽

D. D. Blankenship ◽

R. B. Alley ◽

C. R. Bentley

Keyword(s):

Seismic Reflection ◽

Ross Sea ◽

High Porosity ◽

West Antarctica ◽

Ice Streams ◽

Ice Flow ◽

West Antarctic ◽

Ice Stream ◽

Seismic Reflection Profiling ◽

Ice Stream B

Seismic-reflection profiling has previously shown that, at least at one location. Ice Stream Β in West Antarctica rests on a layer of till a few meters thick (Blankenship and others 1986). Analyses of both compressional- and shear-wave seismic reflections from the ice–till boundary confirm the results of those earlier studies, which showed that the till is water-saturated and has a high porosity and low differential pressure. We conclude that this till is basically homogeneous, at least on a scale of tens of kilometers, though some evidence that its properties vary laterally can be discerned in these data. We propose that the till is widespread beneath Ice Stream Β and probably also beneath the other West Antarctic ice streams. Our seismic profiling shows that the till is essentially continuous beneath Ice Stream Β over at least 12 km parallel to ice flow and 8 km transverse to flow. Beneath these profiles the till averages about 6.5 m thick and is present everywhere except possibly on isolated bedrock ridges parallel to ice flow. The till thickness on these bedrock ridges falls to less than 2 m, the limit of our seismic resolution, but there is evidence that the ridges do not impede ice flow substantially. The bedrock beneath the till is fluted parallel to flow, with flutes that are 10–13 m deep by 200–1000 m wide; we believe these flutes are formed by erosion beneath a deforming till. We also observe an angular unconformity at the base of the till, which is consistent with the idea that erosion is occurring there. The sedimentary record in the Ross Embayment looks very similar to that beneath Ice Stream B, i.e. a few meters of till resting unconformably (the Ross Sea unconformity) on lithified sedimentary rock, and we postulate that the Ross Sea unconformity was generated by erosion beneath a grounded ice sheet by a deforming till.

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Paleo ice flow and subglacial meltwater dynamics in Pine Island Bay, West Antarctica

The Cryosphere Discussions ◽

10.5194/tcd-6-4267-2012 ◽

2012 ◽

Vol 6 (5) ◽

pp. 4267-4304 ◽

Cited By ~ 2

Author(s):

F. O. Nitsche ◽

K. Gohl ◽

R. Larter ◽

C.-D. Hillenbrand ◽

G. Kuhn ◽

...

Keyword(s):

West Antarctica ◽

Ice Streams ◽

Channel Network ◽

Amundsen Sea ◽

Ice Flow ◽

Swath Bathymetry ◽

Ice Stream ◽

Seabed Topography ◽

Ice Sheet Dynamics ◽

Subglacial Meltwater

Abstract. Increasing evidence for an elaborate subglacial drainage network underneath modern Antarctic ice sheets suggests that basal meltwater has an important influence on ice stream flow. Swath bathymetry surveys from previously glaciated continental margins display morphological features indicative of subglacial meltwater flow in inner shelf areas of some paleo ice stream troughs. Over the last few years several expeditions to the Eastern Amundsen Sea embayment (West Antarctica) have investigated the paleo ice streams that extended from the Pine Island and Thwaites glaciers. A compilation of high-resolution swath bathymetry data from inner Pine Island Bay reveals details of a rough seabed topography including several deep channels that connect a series of basins. This complex basin and channel network is indicative of meltwater flow beneath the paleo-Pine Island and Thwaites ice streams, along with substantial subglacial water inflow from the east. This meltwater could have enhanced ice flow over the rough bedrock topography. Meltwater features diminish with the onset of linear features north of the basins. Similar features have previously been observed in several other areas, including the Dotson-Getz Trough (Western Amundsen Sea embayment) and Marguerite Bay (SW Antarctic Peninsula), suggesting that these features may be widespread around the Antarctic margin and that subglacial meltwater drainage played a major role in past ice-sheet dynamics.

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Seismic-Reflection Profiling of a Widespread Till Beneath Ice Stream B, West Antarctica (Abstract)

Annals of Glaciology ◽

10.3189/s0260305500006704 ◽

1988 ◽

Vol 11 ◽

pp. 210-210

Author(s):

Sean T. Rooney ◽

D. D. Blankenship ◽

R. B. Alley ◽

C. R. Bentley

Keyword(s):

Seismic Reflection ◽

Ross Sea ◽

High Porosity ◽

West Antarctica ◽

Ice Streams ◽

Ice Flow ◽

West Antarctic ◽

Ice Stream ◽

Seismic Reflection Profiling ◽

Ice Stream B

Seismic-reflection profiling has previously shown that, at least at one location. Ice Stream Β in West Antarctica rests on a layer of till a few meters thick (Blankenship and others 1986). Analyses of both compressional- and shear-wave seismic reflections from the ice–till boundary confirm the results of those earlier studies, which showed that the till is water-saturated and has a high porosity and low differential pressure. We conclude that this till is basically homogeneous, at least on a scale of tens of kilometers, though some evidence that its properties vary laterally can be discerned in these data. We propose that the till is widespread beneath Ice Stream Β and probably also beneath the other West Antarctic ice streams.Our seismic profiling shows that the till is essentially continuous beneath Ice Stream Β over at least 12 km parallel to ice flow and 8 km transverse to flow. Beneath these profiles the till averages about 6.5 m thick and is present everywhere except possibly on isolated bedrock ridges parallel to ice flow. The till thickness on these bedrock ridges falls to less than 2 m, the limit of our seismic resolution, but there is evidence that the ridges do not impede ice flow substantially. The bedrock beneath the till is fluted parallel to flow, with flutes that are 10–13 m deep by 200–1000 m wide; we believe these flutes are formed by erosion beneath a deforming till. We also observe an angular unconformity at the base of the till, which is consistent with the idea that erosion is occurring there. The sedimentary record in the Ross Embayment looks very similar to that beneath Ice Stream B, i.e. a few meters of till resting unconformably (the Ross Sea unconformity) on lithified sedimentary rock, and we postulate that the Ross Sea unconformity was generated by erosion beneath a grounded ice sheet by a deforming till.

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A note on the glacial geology and postglacial emergence of the Lake Harbour region, Baffin Island, N.W.T.

Canadian Journal of Earth Sciences ◽

10.1139/e85-197 ◽

1985 ◽

Vol 22 (12) ◽

pp. 1864-1871 ◽

Cited By ~ 6

Author(s):

Peter Clark

Keyword(s):

Large Scale ◽

Flow Direction ◽

Baffin Island ◽

Ice Streams ◽

Ice Flow ◽

The North ◽

West Antarctic ◽

Ice Stream ◽

Flow Directions ◽

Late Wisconsinan

Ice-flow indicators in the Lake Harbour region of northern Hudson Strait define two flow directions affecting this area during the late Wisconsinan glaciation. A pronounced southward flow direction indicated by medium- and large-scale erosional and depositional features represents ice flow from an ice dome centered to the north, perhaps Foxe Basin and (or) Amadjuak Lake. Carbonate-rich till and striations represent eastward–southeastward ice flow down the axis of Hudson Strait. Convergence of ice-sheet flow with a rapidly moving ice stream has been observed and modelled for West Antarctic ice streams and involves sharp bending of flow lines at the point of convergence. A similar scenario is proposed for the Lake Harbour region to explain the two contrasting ice-flow patterns. Impingement of an ice stream in Hudson Strait onto the southern coast of Baffin Island suggests the influence of northerly flowing ice, perhaps from the Ungava plateau.Radiocarbon dates on marine shells and archeological samples are used to reconstruct the postglacial emergence of the Lake Harbour region. The marine limit (90 m aht) and deglaciation are dated by extrapolation at ca. 8300 years BP. Postglacial emergence is characterized by an initial uplift rate of 4.4 m/100 years, which decreased to 0.2 m/100 years over the last 3900 years. The initial rate (4.4 m/100 years) is nearly 50% lower than rates calculated elsewhere in the Hudson Strait region and is interpreted to reflect the influence of an ice load centered over Amadjuak Lake directly north of the Lake Harbour region.

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