Interpreting englacial layer deformation in the presence of complex ice flow history with synthetic radargrams

AbstractFast ice flow on the Antarctic continent constitutes much of the mass loss from the ice sheet. However, geophysical methods struggle to constrain ice flow history at depth, or separate the signatures of topography, ice dynamics and basal conditions on layer structure. We develop and demonstrate a methodology to compare layer signatures in multiple airborne radar transects in order to characterize ice flow at depth, or improve coverage of existing radar surveys. We apply this technique to generate synthetic, along-flow radargrams and compare different deformation regimes to observed radargram structure. Specifically, we investigate flow around the central sticky spot of Whillans Ice Stream, West Antarctica. Our study suggests that present-day velocity flowlines are insufficient to characterize flow at depth as expressed in layer geometry, and streaklines provide a better characterization of flow around a basal sticky spot. For Whillans Ice Stream, this suggests that ice flow wraps around the central sticky spot, supported by idealized flow simulations. While tracking isochrone translation and rotation across survey lines is complex, we demonstrate that our approach to combine radargram interpretation and modeling can reveal critical details of past ice flow.

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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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Kamb Ice Stream flow history and surge potential

Annals of Glaciology ◽

10.3189/2013aog63a535 ◽

2013 ◽

Vol 54 (63) ◽

pp. 287-298 ◽

Cited By ~ 22

Author(s):

Hermann Engelhardt ◽

Barclay Kamb

Keyword(s):

Video Camera ◽

Hot Water ◽

West Antarctica ◽

Time Intervals ◽

Ice Flow ◽

Ice Stream ◽

History Of ◽

Fast Ice ◽

Fast Motion ◽

Kamb Ice Stream

AbstractA basal zone, tens of meters thick, of debris-laden ice was observed in Kamb Ice Stream, West Antarctica, using a video camera lowered into boreholes made by hot-water drilling. The debris content varies, sometimes abruptly, forming a sequence of layers that reflect the complex history of fast ice flow and bed interaction. In most parts, the concentration of debris is low, a few percent by weight, with particles, often mud clots, dispersed in a matrix of clear ice. The nature of the debris distribution can be interpreted in terms of specific time intervals in the history of fast motion of Kamb Ice Stream including processes leading up to the termination of its streaming behavior and possible reactivation.

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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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Decadal-scale variations in ice flow along Whillans Ice Stream and its tributaries, West Antarctica

Journal of Glaciology ◽

10.3189/172756505781829610 ◽

2005 ◽

Vol 51 (172) ◽

pp. 147-157 ◽

Cited By ~ 27

Author(s):

Leigh A. Stearns ◽

Kenneth C. Jezek ◽

C.J. Van Der Veen

Keyword(s):

Aerial Photographs ◽

West Antarctica ◽

Ice Flow ◽

Velocity Gradients ◽

Ice Stream ◽

Spatial Changes ◽

Decadal Scale ◽

Whillans Ice Stream ◽

Satellite Radar ◽

Velocity Changes

AbstractWe investigate velocity changes occurring along Whillans Ice Stream (WIS) by comparing velocities derived from repeat aerial photographs acquired in 1985-89 (average date of 1987) to interferometric satellite radar (InSAR) velocities collected in 1997. Three different analysis methods are applied to the velocity data. First, temporal and spatial changes in velocities are correlated to identifiable features (flowlines, shear margins, bed features) visible on the 1997 RADARSAT Antarctic Mapping Project mosaic. Second, we relate velocity gradients to stresses via the flow law and, along with surface topography and ice-thickness data, apply the force-budget technique to determine the relative importance of driving stress, side drag and basal drag over time. Finally, the mass balance of the main part of WIS is determined for 1987 and 1997. Our results are consistent with previous studies that show an overall deceleration resulting in downstream thickening of the ice stream (Whillans and others, 2001; Joughin and others, 2002).

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Englacial stratigraphy in Ellsworth Subglacial Highlands, West Antarctica

10.5194/egusphere-egu21-3366 ◽

2021 ◽

Author(s):

Felipe Napoleoni ◽

Neil Ross ◽

Michael J. Bentley ◽

Stewart S.R. Jamieson ◽

Andrew M. Smith ◽

...

Keyword(s):

Internal Structure ◽

Ice Sheet ◽

Glacial History ◽

West Antarctica ◽

Large Area ◽

Ice Dynamics ◽

Ice Flow ◽

Surface Mass ◽

West Antarctic ◽

Ice Stream

Airborne ice-penetrating radar surveys around the Ellsworth Subglacial Highlands (ESH) have mapped and dated englacial ice sheet layers, hereafter referred to as &#8216;Internal Reflection Horizons&#8217; (IRHs). The geometry and internal structure of IRHs can reveal the cumulative effects of surface mass balance, strain, basal melt and ice dynamics, to improve understanding of the glacial history of West Antarctic Ice Sheet (WAIS). Despite the airborne-surveyed IRHs however, international efforts to develop a continental-wide scale coverage of IRHs (i.e. AntArchitecture), are limited by a lack of data in the critical regions between the upper reach of Pine Island Glacier (PIG), Rutford Ice Stream (RIS) and Institute Ice Stream (IIS). This region is important because any significant collapse of WAIS or reorganisation of ice flow would likely be felt in the ESH because it hosts deep subglacial troughs (Ellsworth Trough and CECs Trough), that represent a potential connection between the Weddell and Amundsen Seas. Using an extensive ground-based ice radar dataset acquired by Centro de Estudios Cient&#237;ficos (CECs) we bridge this regional gap by mapping IRHs across the Amundsen-Weddell divide of the WAIS. This work links airborne-derived IRH datasets across PIG and IIS, to develop an extensive layer characterisation across a large area of West Antarctica. We present the regional internal structure of the ice sheet, gridded paleo ice surfaces, and identify areas with complex IRH structures, and evaluate the possible glaciological processes responsible. We then compare our results with modelled outputs of ice sheet geometry and outline our current understanding of the past ice flow behaviour of the ESH, and the implications for WAIS glacial history. We consider our results in the context of the characterisation of &#8216;old-ice&#8217; in WAIS and in relation to the upcoming plans for accessing subglacial Lake CECs in order to determine its history.

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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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Streamlined bedrock terrain and fast ice flow, Jakobshavns Isbrae, West Greenland: implications for ice stream and ice sheet dynamics

Boreas ◽

10.1080/03009480510012818 ◽

2005 ◽

Vol 34 (1) ◽

pp. 25-42 ◽

Cited By ~ 71

Author(s):

David H. Roberts ◽

Antony J. Long

Keyword(s):

Ice Sheet ◽

Ice Flow ◽

West Greenland ◽

Ice Stream ◽

Fast Ice ◽

Ice Sheet Dynamics

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Pathways and timescales of connectivity around the Antarctic continental shelf

10.5194/egusphere-egu21-14011 ◽

2021 ◽

Author(s):

Hannah Dawson ◽

Adele Morrison ◽

Veronica Tamsitt ◽

Matthew England

Keyword(s):

Continental Shelf ◽

Ross Sea ◽

Coastal Current ◽

West Antarctica ◽

Source Regions ◽

Freshwater Forcing ◽

Antarctic Continental Shelf ◽

Antarctic Continent ◽

Antarctic Slope Current ◽

The Antarctic

The Antarctic margin is surrounded by two westward flowing currents: the Antarctic Slope Current and the Antarctic Coastal Current. The former influences key processes near the Antarctic margin by regulating the flow of heat and nutrients onto and off the continental shelf, while together they advect nutrients, biological organisms, and temperature and salinity anomalies around the coastline, providing a connective link between different shelf regions. However, the extent to which these currents transport water from one sector of the continental shelf to another, and the timescales over which this occurs, remain poorly understood. Concern that crucial water formation sites around the Antarctic coastline could respond to non-local freshwater forcing from ice shelf meltwater motivates a more thorough understanding of zonal connectivity around Antarctica. In this study, we use daily velocity fields from a global high-resolution ocean-sea ice model, combined with the Lagrangian tracking software Parcels, to investigate the pathways and timescales connecting different regions of the Antarctic continental shelf with a view to understanding the timescales of meltwater transport around the continent. Virtual particles are released over the continental shelf, poleward of the 1000 metre isobath, and are tracked for 20 years. Our results show a strong seasonal cycle connecting different sectors of the Antarctic continent, with more particles arriving further downstream during winter than during summer months. Strong advective links exist between West Antarctica and the Ross Sea while shelf geometry in some other regions acts as barriers to transport. We also highlight the varying importance of the Antarctic Slope Current and Antarctic Coastal Current in connecting different sectors of the coastline. Our results help to improve our understanding of circum-Antarctic connectivity and the timescales of meltwater transport from source regions to downstream shelf locations. Furthermore, the timescales and pathways we present provide a baseline from which to assess long-term changes in Antarctic coastal circulation due to local and remote forcing.

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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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