From steady streaming to oscillations: the role of subglacial drainage and temperate ice in ice-stream dynamics

2021 ◽  
Author(s):  
Marianne Haseloff ◽  
Ian Hewitt ◽  
Richard Katz
Keyword(s):  
Heat Dissipation ◽  
Lateral Strain ◽  
Hydraulic Permeability ◽  
Ice Streams ◽  
Steady Streaming ◽  
Ice Stream ◽  
Fast Flow ◽  
Stream Bed ◽  
Subglacial Drainage

<p>The majority of Antarctic ice is discharged through fast-flowing ice streams. Some of these ice streams exhibit variations in velocities and ice stream discharge on decadal to centennial time scales, but the factors controlling these variations are still insufficiently understood.  Using computational models of ice flow and hydrology, we predict the existence of two dynamical regimes: stable ice streaming associated with high hydraulic permeability of the bed, and ‘binge-purge’ oscillations associated with low permeability.</p><p>Observations indicate that the fast-flow of ice streams is enabled by meltwater lubricating the ice stream bed, and models suggest that this lubrication is the result of a positive feedback between fast flow, heat dissipation at the ice stream bed and meltwater production within the ice. In particular, recent studies have highlighted that heat dissipation in temperate ice stream margins, which are regions of high lateral strain, can contribute significantly to the subglacial water balance. However, the role of this meltwater flux in ice stream dynamics remains unclear. Here, we investigate the roles of subglacial drainage and feedbacks between fast flow and heat dissipation in ice-stream evolution. </p><p>The ice is modelled as a vertically uniform plug flow. Water flow at the bed is modelled as a Darcian system whose hydraulic transmissivity increases with decreasing effective pressure. Dynamical feedbacks in the energy balance include both frictional heating along the bed and lateral shear heating. Within our model, two distinct dynamic regimes can be identified: if the hydraulic permeability of the bed is sufficiently high to evacuate all meltwater produced at the ice stream bed and in its margins, a moderately-fast steady ice stream forms. Conversely,`binge-purge’ oscillations between fast and stagnant flow emerge when the hydraulic permeability of the bed is too low to evacuate the meltwater produced within the ice stream. Topographic controls can suppress this oscillatory behaviour, while the formation of temperate ice in ice stream margins amplifies it.</p>

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 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 The relationship between sticky spots and radar reflectivity beneath an active West Antarctic ice stream

2014 ◽  
Vol 55 (67) ◽  
pp. 29-38 ◽  
Author(s):  
David W. Ashmore ◽  
Robert G. Bingham ◽  
Richard C.A. Hindmarsh ◽  
Hugh F.J. Corr ◽  
Ian R. Joughin
Keyword(s):  
Free Water ◽  
Radar Data ◽  
Force Balance ◽  
Ice Streams ◽  
West Antarctic ◽  
Ice Stream ◽  
Surface Data ◽  
Fast Flow ◽  
The Relationship ◽  
Modelling Approach

AbstractIsolated areas of high basal drag, or ‘sticky spots’, are important and poorly understood features in the force balance and dynamics of West Antarctic ice streams. Characterizing sticky spots formed by thin or drying subglacial till using ice-penetrating radar is theoretically possible, as high radar bed-returned power (BRP) is commonly related to an abundance of free water at the ice/bed interface, provided losses from englacial attenuation can be estimated. In this study we use airborne radar data collected over Evans Ice Stream to extract BRP profiles and test the sensitivity of BRP to the adopted englacial attenuation correction. We analyse 11 ~ 2 0 km profiles in four fast-flow areas where sticky spots have been inferred to exist on the basis of model and surface data inversions. In the majority of profiles we note that the increase in basal drag is accompanied by a decrease in BRP and suggest that this is evidence both for the presence of a sticky spot in those locations and that local variations in subglacial hydrology are responsible for their existence. A comparison is made between empirical and numerical modelling approaches for deriving englacial attenuation, and our findings generally support previous studies that advocate a modelling approach.

scholarly journals A groove-ploughing theory for the production of mega-scale glacial lineations, and implications for ice-stream mechanics

2003 ◽  
Vol 49 (165) ◽  
pp. 240-256 ◽  
Author(s):  
Chris D. Clark ◽  
Slawek M. Tulaczyk ◽  
Chris R. Stokes ◽  
Miquel Canals
Keyword(s):  
Qualitative Theory ◽  
Transverse Strain ◽  
Supporting Evidence ◽  
Ice Streams ◽  
Flow Acceleration ◽  
High Magnitude ◽  
Ice Stream ◽  
Roughness Elements ◽  
Stream Beds ◽  
Fast Flow

AbstractMega-scale glacial lineations (MSGLs) are longitudinally aligned corrugations (ridge–groove structures 6–100 km long) in sediment produced subglacially. They are indicators of fast flow and a common signature of ice-stream beds. We develop a qualitative theory that accounts for their formation, and use numerical modelling, and observations of ice-stream beds to provide supporting evidence. Ice in contact with a rough (scale of 10–103 m) bedrock surface will mimic the form of the bed. Because of flow acceleration and convergence in ice-stream onset zones, the ice-base roughness elements experience transverse strain, transforming them from irregular bumps into longitudinally aligned keels of ice protruding downwards. Where such keels slide across a soft sedimentary bed, they plough through the sediments, carving elongate grooves, and deforming material up into intervening ridges. This explains MSGLs and has important implications for ice-stream mechanics. Groove ploughing provides the means to acquire new lubricating sediment and to transport large volumes of it downstream. Keels may provide basal drag in the force budget of ice streams, thereby playing a role in flow regulation and stability. We speculate that groove ploughing permits significant ice-stream widening, thus facilitating high-magnitude ice discharge.

scholarly journals Signature of ice streaming in Bjørnøyrenna, Polar North Atlantic, through the Pleistocene and implications for ice-stream dynamics

2009 ◽  
Vol 50 (52) ◽  
pp. 17-26 ◽  
Author(s):  
Karin Andreassen ◽  
Monica Winsborrow
Keyword(s):  
Barents Sea ◽  
Extensional Flow ◽  
Three Dimensional ◽  
Ice Streams ◽  
Ice Dynamics ◽  
Ice Flow ◽  
Ice Stream ◽  
Geomorphic Features ◽  
Common Behaviour ◽  
Fast Flow

AbstractThe geomorphology of palaeo-ice-stream beds and the internal structure of underlying tills can provide important information about the subglacial conditions during periods of fast flow and quiescence. This paper presents observations from three-dimensional seismic data, revealing the geomorphology of buried beds of the Bjørnøyrenna (Bear Island Trough) ice stream, the main drainage outlet of the former Barents Sea ice sheet. Repeated changes in ice dynamics are inferred from the observed successions of geomorphic features. Megablocks, aligned in long chains parallel to inferred ice-stream flowlines, and forming dipping plates that are thrust one on top of another, are taken as evidence for conditions of compressive ice flow. Mega-scale glacial lineations (MSGL) and pull-apart of underlying sediment blocks suggest extensional flow. The observed pattern of megablocks and rafts overprinted by MSGL indicates a change in ice dynamics from a compressional to an extensional flow regime. Till stiffening, due to subglacial freezing, is the favoured mechanism for creating switches in sub-ice-stream conditions. The observed pattern of geomorphic features indicates that periods of slowdown or quiescence were commonly followed by reactivation and fast flow during several glaciations, suggesting that this may be a common behaviour of marine ice streams.

scholarly journals The role of subtemperate slip in thermally driven ice stream margin migration

2018 ◽  
Vol 12 (8) ◽  
pp. 2545-2568 ◽  
Author(s):  
Marianne Haseloff ◽  
Christian Schoof ◽  
Olivier Gagliardini
Keyword(s):  
Migration Rate ◽  
Large Scale ◽  
Heat Dissipation ◽  
Layer Structure ◽  
Ice Sheet ◽  
Scale Model ◽  
Ice Streams ◽  
Ice Stream ◽  
Bed Temperature ◽  
Thermally Driven

Abstract. The amount of ice discharged by an ice stream depends on its width, and the widths of unconfined ice streams such as the Siple Coast ice streams in West Antarctica have been observed to evolve on decadal to centennial timescales. Thermally driven widening of ice streams provides a mechanism for this observed variability through melting of the frozen beds of adjacent ice ridges. This widening is driven by the heat dissipation in the ice stream margin, where strain rates are high, and at the bed of the ice ridge, where subtemperate sliding is possible. The inflow of cold ice from the neighboring ice ridges impedes ice stream widening. Determining the migration rate of the margin requires resolving conductive and advective heat transfer processes on very small scales in the ice stream margin, and these processes cannot be resolved by large-scale ice sheet models. Here, we exploit the thermal boundary layer structure in the ice stream margin to investigate how the migration rate depends on these different processes. We derive a parameterization of the migration rate in terms of parameters that can be estimated from observations or large-scale model outputs, including the lateral shear stress in the ice stream margin, the ice thickness of the stream, the influx of ice from the ridge, and the bed temperature of the ice ridge. This parameterization will allow the incorporation of ice stream margin migration into large-scale ice sheet models.

scholarly journals Basal melt rates beneath Whillans Ice Stream, West Antarctica

2010 ◽  
Vol 56 (198) ◽  
pp. 647-654 ◽  
Author(s):  
Lucas H. Beem ◽  
Ken C. Jezek ◽  
C.J. Van Der Veen
Keyword(s):  
West Antarctica ◽  
Ice Streams ◽  
Stress Regime ◽  
Basal Resistance ◽  
West Antarctic ◽  
Ice Stream ◽  
Whillans Ice Stream ◽  
Fast Flow ◽  
Insight Into ◽  
Streaming Flow

AbstractBasal water lubricates and enables the fast flow of the West Antarctic ice streams which exist under low gravitational driving stress. Identification of sources and rates of basal meltwater production can provide insight into the dynamics of ice streams and the subglacial hydrology, which remain insufficiently described by glaciological theory. Combining measurements and analytic modeling, we identify two regions where basal meltwater is produced beneath Whillans Ice Stream, West Antarctica. Downstream of the onset of shear crevasses, strong basal melt (20–50 mm a−1) is concentrated beneath the relatively narrow shear margins. Farther upstream, melt rates are consistently 3–7 mm a−1 across the width of the ice stream. We show that the transition in melt-rate patterns is coincident with the onset of shear margin crevassing and streaming flow and related to the development of significant lateral shear resistance, which reorganizes the resistive stress regime and induces a concentration of basal resistance adjacent to the shear margin. Finally, we discuss how downstream freeze-on in the ice-stream center coupled with melt beneath the shear margin might result in a slowing but widening ice stream.

scholarly journals Subglacial hydrological control on flow of an Antarctic Peninsula palaeo-ice stream

2019 ◽  
Vol 13 (6) ◽  
pp. 1583-1596 ◽  
Author(s):  
Robert D. Larter ◽  
Kelly A. Hogan ◽  
Claus-Dieter Hillenbrand ◽  
James A. Smith ◽  
Christine L. Batchelor ◽  
...  
Keyword(s):  
Continental Shelf ◽  
Antarctic Peninsula ◽  
Dynamic Behaviour ◽  
Stream Channels ◽  
Ice Streams ◽  
Deep Basin ◽  
Diverse Range ◽  
Ice Stream ◽  
Fast Flow ◽  
The Antarctic

Abstract. Basal hydrological systems play an important role in controlling the dynamic behaviour of ice streams. Data showing their morphology and relationship to geological substrates beneath modern ice streams are, however, sparse and difficult to collect. We present new multibeam bathymetry data that make the Anvers-Hugo Trough west of the Antarctic Peninsula the most completely surveyed palaeo-ice stream pathway in Antarctica. The data reveal a diverse range of landforms, including streamlined features where there was fast flow in the palaeo-ice stream, channels eroded by flow of subglacial water, and compelling evidence of palaeo-ice stream shear margin locations. We interpret landforms as indicating that subglacial water availability played an important role in facilitating ice stream flow and controlling shear margin positions. Water was likely supplied to the ice stream bed episodically as a result of outbursts from a subglacial lake located in the Palmer Deep basin on the inner continental shelf. These interpretations have implications for controls on the onset of fast ice flow, the dynamic behaviour of palaeo-ice streams on the Antarctic continental shelf, and potentially also for behaviour of modern ice streams.

scholarly journals The role of subtemperate slip in thermally-driven ice stream margin migration

2018 ◽  
Author(s):  
Marianne Haseloff ◽  
Christian Schoof ◽  
Olivier Gagliardini
Keyword(s):  
Migration Rate ◽  
Large Scale ◽  
Heat Dissipation ◽  
Layer Structure ◽  
Ice Sheet ◽  
Scale Model ◽  
Ice Streams ◽  
Ice Stream ◽  
Bed Temperature ◽  
Thermally Driven

Abstract. The amount of ice discharged by an ice stream depends on its width, and the widths of unconfined ice streams such as the Siple Coast ice streams in West Antarctica have been observed to evolve on decadal to centennial timescales. Thermally-driven widening of ice streams provides a mechanism for this observed variability through melting of the frozen beds of adjacent ice ridges. This widening is driven by the heat dissipation in the ice stream margin, where strain rates are high, and at the bed of the ice ridge, where subtemperate sliding is possible. The inflow of cold ice from the neighboring ice ridges impedes ice stream widening. Determining the migration rate of the margin requires resolving conductive and advective heat transfer processes on very small scales in the ice stream margin, and these processes cannot be resolved by large scale ice sheet models. Here, we exploit the thermal boundary layer structure in the ice stream margin to investigate how the migration rate depends on these different processes. We derive a parameterization of the migration rate in terms of parameters that can be estimated from observations or large scale model outputs, including the lateral shear stress in the ice stream margin, the ice thickness of the stream, the influx of ice from the ridge, and the bed temperature of the ice ridge. This parameterization will allow the incorporation of ice stream margin migration into large-scale ice sheet models.

scholarly journals The role of sliding in ice stream formation

2021 ◽  
Vol 477 (2248) ◽  
Author(s):  
Christian Schoof ◽  
Elisa Mantelli
Keyword(s):  
Pattern Formation ◽  
Temperature Dependent ◽  
Ice Streams ◽  
Thermomechanical Model ◽  
Positive Feedbacks ◽  
Ice Stream ◽  
Basal Sliding ◽  
Analytical Criterion ◽  
Stream Formation

Ice streams are bands of fast-flowing ice in ice sheets. We investigate their formation as an example of spontaneous pattern formation, based on positive feedbacks between dissipation and basal sliding. Our focus is on temperature-dependent subtemperate sliding, where faster sliding leads to enhanced dissipation and hence warmer temperatures, weak- ening the bed further, and on a similar feedback driven by basal melt water production. Using a novel thermomechanical model, we show that formation of a steady pattern of fast and slow flow can occur through the downstream amplification of noise in basal conditions. This process can lead to the establishment of a clearly defined ice stream separated from slowly flowing, cold-based ice ridges by narrow shear margins. Our model is also able to predict the downstream widening of ice streams due to dissipation and heat transport in these margins. We also show that downward advection of cold ice induced by accelerated sliding is the primary stabilizing mechanism that can suppress ice steam formation altogether, and give an approximate, analytical criterion for pattern formation.

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