Ice-stream Width Research Articles

Bistatic Radar Tomography of Shear Margins: Simulated Temperature and Basal Material Inversions

A significant portion of the sea level contributions of Antarctica and Greenland comes from ice streams, but the physical processes controlling ice stream width are poorly understood, especially when topographic controls are absent. Recent modeling studies have indicated that ice stream width may be controlled by elevated temperatures inside ice stream shear margins. While radio echo sounders can provide measurements of englacial water storage and subglacial conditions, existing radar-sounding techniques cannot measure temperature profiles at the scale required to test this hypothesis. We propose using a wide-angle radar survey and tomographic inversion to resolve temperature profiles, gradients, and anomalies at the scale required to study the thermophysical controls on shear margins. Recent work produced a bistatic radar system capable of obtaining the long offsets required for well-constrained inversions; however, shear-margin-specific temperature inversion techniques have not been developed for this system. In this article, we develop Newton’s method and alternating direction method of multipliers’ inversions for estimating temperature distribution and basal material across ice stream shear margins. We evaluate the performance of these inversion techniques on simulated bistatic radar-sounding data. Our results suggest that bistatic radar tomography experiments should be able to produce temperature maps on 50 m <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times50$ </tex-math></inline-formula> m grids with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.83~^{\circ} \text{C}~\pm ~0.084~^{\circ} \text{C}$ </tex-math></inline-formula> mean temperature error, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.58~^{\circ} \text{C}~\pm ~0.20~^{\circ} \text{C}$ </tex-math></inline-formula> maximum temperature error, and an error in relative basal permittivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.63~\pm ~0.08$ </tex-math></inline-formula> for a 4-km transect.

Glacial sedimentation, fluxes and erosion rates associated with ice retreat in Petermann Fjord and Nares Strait, north-west Greenland

Abstract. Petermann Fjord is a deep (&gt;1000 m) fjord that incises the coastline of north-west Greenland and was carved by an expanded Petermann Glacier, one of the six largest outlet glaciers draining the modern Greenland Ice Sheet (GrIS). Between 5 and 70 m of unconsolidated glacigenic material infills in the fjord and adjacent Nares Strait, deposited as the Petermann and Nares Strait ice streams retreated through the area after the Last Glacial Maximum. We have investigated the deglacial deposits using seismic stratigraphic techniques and have correlated our results with high-resolution bathymetric data and core lithofacies. We identify six seismo-acoustic facies in more than 3500 line kilometres of sub-bottom and seismic-reflection profiles throughout the fjord, Hall Basin and Kennedy Channel. Seismo-acoustic facies relate to bedrock or till surfaces (Facies I), subglacial deposition (Facies II), deposition from meltwater plumes and icebergs in quiescent glacimarine conditions (Facies III, IV), deposition at grounded ice margins during stillstands in retreat (grounding-zone wedges; Facies V) and the redeposition of material downslope (Facies IV). These sediment units represent the total volume of glacial sediment delivered to the mapped marine environment during retreat. We calculate a glacial sediment flux for the former Petermann ice stream as 1080–1420 m3 a−1 per metre of ice stream width and an average deglacial erosion rate for the basin of 0.29–0.34 mm a−1. Our deglacial erosion rates are consistent with results from Antarctic Peninsula fjord systems but are several times lower than values for other modern GrIS catchments. This difference is attributed to fact that large volumes of surface water do not access the bed in the Petermann system, and we conclude that glacial erosion is limited to areas overridden by streaming ice in this large outlet glacier setting. Erosion rates are also presented for two phases of ice retreat and confirm that there is significant variation in rates over a glacial–deglacial transition. Our new glacial sediment fluxes and erosion rates show that the Petermann ice stream was approximately as efficient as the palaeo-Jakobshavn Isbræ at eroding, transporting and delivering sediment to its margin during early deglaciation.

Controls on the early Holocene collapse of the Bothnian Sea Ice Stream

AbstractNew high‐resolution multibeam data in the Gulf of Bothnia reveal for the first time the subglacial environment of a Bothnian Sea Ice Stream. The geomorphological record suggests that increased meltwater production may have been important in driving rapid retreat of Bothnian Sea Ice during deglaciation. Here we apply a well‐established, one‐dimensional flow line model to simulate ice flow through the Gulf of Bothnia and investigate controls on retreat of the ice stream during the post‐Younger Dryas deglaciation of the Fennoscandian Ice Sheet. The relative influence of atmospheric and marine forcings are investigated, with the modeled ice stream exhibiting much greater sensitivity to surface melting, implemented through surface mass balance and hydrofracture‐induced calving, than to submarine melting or relative sea level change. Such sensitivity is supported by the presence of extensive meltwater features in the geomorphological record. The modeled ice stream does not demonstrate significant sensitivity to changes in prescribed ice stream width or overall bed slope, but local variations in basal topography and ice stream width result in nonlinear retreat of the grounding line, notably demonstrating points of short‐lived retreat slowdown on reverse bed slopes. Retreat of the ice stream was most likely governed by increased ice surface meltwater production, with the modeled retreat rate less sensitive to marine forcings despite the marine setting.

Determining conditions that allow a shear margin to coincide with a Röthlisberger channel

AbstractThe mass loss from the West Antarctic Ice Sheet is dominated by numerous rapidly flowing ice streams, which are separated from stagnant ice in the adjacent ridges by zones of concentrated deformation known as shear margins. Because the discharge from a single ice stream depends sensitively on the ice stream width, determining the physical processes that control shear margin location is crucial to a full understanding of ice stream dynamics. Previous work has shown that the transition from a deforming to an undeforming bed within a shear margin concentrates large stresses on the undeforming bed beneath the ridge. In this paper we investigate how the presence of a drainage channel collocated with the transition from a deforming to an undeforming bed perturbs the stress field within the shear margin. We show that the channel limits the maximum shear stress on the undeforming bed and alters the yield strength of the till by changing the normal stress on the ice‐till interface. By comparing the maximum stress with the till strength, we show that the transition from a deforming to an undeforming bed can occur across a channel whenever the water flux in the channel exceeds a critical value. This critical flux is sensitive to the rheology and loading of the shear margin, but we conclude that there are some scenarios where the transition from a deforming to an undeforming bed can be collocated with a drainage channel, though this configuration is probably not typical.

Ice-stream stability on a reverse bed slope

Marine ice streams whose beds deepen inland are thought to be inherently unstable. Numerical modelling of the Maguerite Bay ice-stream retreat in West Antarctica since the Last Glacial Maximum suggests that an ice stream can stabilize on an inland-sloping bed owing to increased lateral drag where the ice stream narrows. Marine-based ice streams whose beds deepen inland are thought to be inherently unstable1,2,3. This instability is of particular concern because significant portions of the marine-based West Antarctic and Greenland ice sheets are losing mass and their retreat could contribute significantly to future sea-level rise4,5,6,7. However, the present understanding of ice-stream stability is limited by observational records that are too short to resolve multi-decadal to millennial-scale behaviour or to validate numerical models8. Here we present a dynamic numerical simulation of Antarctic ice-stream retreat since the Last Glacial Maximum (LGM), constrained by geophysical data, whose behaviour is consistent with the geomorphological record. We find that retreat of Marguerite Bay Ice Stream following the LGM was highly nonlinear and was interrupted by stabilizations on a reverse-sloping bed, where theory predicts rapid unstable retreat. We demonstrate that these transient stabilizations were caused by enhanced lateral drag as the ice stream narrowed. We conclude that, as well as bed topography, ice-stream width and long-term retreat history are crucial for understanding decadal- to centennial-scale ice-stream behaviour and marine ice-sheet vulnerability.

The seismic architecture and geometry of grounding-zone wedges formed at the marine margins of past ice sheets

Grounding-zone wedges form where halts of decades to centuries during ice-sheet retreat across polar shelves allow sediment buildup at the grounding zone through delivery of deforming basal debris from fast-flowing ice streams. Thirty grounding-zone wedges were identified on two-dimensional seismic-reflection profiles across the continental shelves of NE and NW Greenland. Grounding-zone wedges close to the seafloor are probably of late Weichselian age; others are on paleoshelves, buried beneath thick prograding glacial sediments. Several Greenland-shelf grounding-zone wedges occur on topographic highs that provide stabilizing pinning points in relatively shallow water. Grounding-zone wedges are asymmetric in the ice-flow direction with steeper ice-distal sides. Typical grounding-zone wedges are approximately 5–20 km long and 50–100 m thick, with a lateral width of several tens of kilometers. Forty grounding-zone wedges from the Greenland, Norwegian, and Antarctica margins show considerable variability about these values. Grounding-zone wedge dimensions are controlled by sediment flux, the duration of halts in ice retreat, subice cavity shape, and ice-stream width. Low-angle ice-shelf cavity roofs immediately beyond the grounding-zone probably restrict vertical accommodation space, preventing formation of high-amplitude ridges. Grounding-zone wedges are mainly transparent or chaotic on seismic profiles, probably resulting from delivery of diamictic debris. Offlapping reflectors represent progradation into an ice-roofed cavity. Reflector truncations of grounding-zone wedge bases indicate erosion during initiation. Channels are present within some grounding-zone wedges; meltwater flow is under high pressure, and V-shaped incisions suggest high-energy flow. Channels are not ubiquitous within Greenland grounding-zone wedges and may represent nonsteady meltwater release, perhaps through lake drainage. Where ice-sheet mass loss is dominated by meltwater runoff, grounding-zone wedges probably contain more sorted sediment than those from Greenland. Simple models of grounding-zone wedge architecture are presented. Grounding-zone wedges indicate episodic rather than catastrophic ice-sheet retreat and are a mechanism for ice-shelf stabilization because wedge growth counteracts collapse induced by ice-sheet thinning and sea-level rise.

Extent and basal characteristics of the M’Clintock Channel Ice Stream

The M’Clintock Channel Palaeo Ice Stream drained the northwestern margin (Victoria Island, Arctic Canada) of the Laurentide Ice Sheet, functioning for 200 years prior to 10 ka BP. From its bed imprint, it is reconstructed as 720×140 km with a cross-sectional area of 100 km 2, comparable in size to the Hudson Strait Ice Stream. It drained an estimated 80,000 km 3 of ice, enough to significantly affect the shrinking ice sheet and drive the Keewatin ice divide south-eastwards. Such discharges likely produced Heinrich events and influenced the temperature and salinity of the Western Arctic Ocean. Ice stream location was controlled by a topographic trough, but glaciological regulation, rather than properties of the bed, controlled margin locations and hence ice stream width. The ice stream eroded into pre-existing unconsolidated sediments and left a pattern of subglacially produced landforms, recording a snapshot view of the bed prior to shutdown. Frictional-shutdown occurred once downcutting through sediments reached bedrock in places. High sediment availability appears critical for ice stream functioning, with a drift thickness of 50 m required to sustain its ∼200 year lifespan. Debris discharge was exceptionally high (73,000 m 3 a −1 m −1). This ice stream demonstrates non-steady response of ice sheets to climate change and their large impact on mass balance and stability of ice sheets.

Bed properties of Siple Dome and adjacent ice streams, West Antarctica, inferred from radio-echo sounding measurements

AbstractWe have used ground-based radio-echo sounding (RES) profiles to reveal the spatial distribution of basal and internal ice properties across Siple Dome, West Antarctica, and under the dormant ice streams on its flanks. The RES-detected bed-reflection power, corrected for the effects of instrumentation and ice-thickness variation, is nearly constant across Siple Dome at a value suggesting spatially homogeneous basal properties of ice frozen to bedrock. Till, if present under the dome, must be thin (&lt;0.1 m). The high basal reflectivity measured under now dormant “Siple Ice Stream” (SIS) and Ice Stream C suggests that they are underlain by either a thin (&lt;0.05 m) water layer or a thick (&gt;1 m) thawed or frozen till layer. The evidence that the dormant SIS is not frozen directly to underlying bedrock (but is separated by a water or till layer) is a further indication that it was once an active ice stream, and suggests that streaming motion may have ceased before the basal layer was frozen. The absence of a thick till layer beneath Siple Dome is consistent with its apparent stability as an inter-ice-stream ridge in the past and may suggest that it will remain as a stable limitation of ice-stream width in the future.

Variations in basal conditions on Rutford Ice Stream, West Antarctica

AbstractSeismic reflection data from two lines on Rutford Ice Streem are presented and are compared with data already published from a third line on the ice stream. The amplitude and phase of the reflections have been used io investigate the properties ol the sub-ice material. Multiple reflections on long record-length data allowed calibration of the reflection coefficient at the ice-bed interlace and determination of the acoustic impedance of the bed material. The characteristics of the bed material vary both along and across the ice stream. The average acoustic impedance of the bed material across the glacier at the upstream line is 3.88 x 106 kg−2 s−1. This decreases to 3.19 x 10−6 kg m−2s−1 52 km further downstream. These values are within the rang which is typical of soft sediments. Using acoustic impedance as an indicator of subgiacial porosity, some areas of the ice-stream bed are interpreted as dilatant water-saturated sediments undergoing pervasive deformation. In other areas, the bed is not deforming and basal sliding may be a more important process. The proportional the ice-stream width over which bed deformation occurs increases downstream.

Variations in basal conditions on Rutford Ice Stream, West Antarctica

AbstractSeismic reflection data from two lines on Rutford Ice Streem are presented and are compared with data already published from a third line on the ice stream. The amplitude and phase of the reflections have been used io investigate the properties ol the sub-ice material. Multiple reflections on long record-length data allowed calibration of the reflection coefficient at the ice-bed interlace and determination of the acoustic impedance of the bed material. The characteristics of the bed material vary both along and across the ice stream. The average acoustic impedance of the bed material across the glacier at the upstream line is 3.88 x 106kg−2s−1. This decreases to 3.19 x 10−6kg m−2s−152 km further downstream. These values are within the rang which is typical of soft sediments. Using acoustic impedance as an indicator of subgiacial porosity, some areas of the ice-stream bed are interpreted as dilatant water-saturated sediments undergoing pervasive deformation. In other areas, the bed is not deforming and basal sliding may be a more important process. The proportional the ice-stream width over which bed deformation occurs increases downstream.

Model experiments on the evolution and stability of ice streams

A simple model is developed based on the notion that on active ice streams the resistance to flow is partitioned between basal drag and lateral drag. The relative roles of these sources of resistance is determined by a friction parameter that effectively describes the strength of the bed under the ice stream. Reduction in the basal strength is caused by meltwater production, taken proportional to the product of basal drag and ice speed. The width of the ice stream is governed by the balance between entrainment or erosion of ice from the slow-moving inter-stream ridges and advection from the ridges into the ice stream. Entrainment of ridge ice is parameterized as a function of the shear stress at the lateral margins, in one case proportional to the lateral shear stress and in the second case scaled to ice-stream width. In the first formulation, the model rapidly becomes unstable but, using the second formulation, a steady state is reached with lateral drag providing all or most of the resistance to flow. The results point to the great importance of achieving an understanding of entrainment. With the second model and a wide range of parameter values, there is no cyclic behavior, with rapid flow being followed by a quiescent phase.

Model experiments on the evolution and stability of ice streams

A simple model is developed based on the notion that on active ice streams the resistance to flow is partitioned between basal drag and lateral drag. The relative roles of these sources of resistance is determined by a friction parameter that effectively describes the strength of the bed under the ice stream. Reduction in the basal strength is caused by meltwater production, taken proportional to the product of basal drag and ice speed. The width of the ice stream is governed by the balance between entrainment or erosion of ice from the slow-moving inter-stream ridges and advection from the ridges into the ice stream. Entrainment of ridge ice is parameterized as a function of the shear stress at the lateral margins, in one case proportional to the lateral shear stress and in the second case scaled to ice-stream width. In the first formulation, the model rapidly becomes unstable but, using the second formulation, a steady state is reached with lateral drag providing all or most of the resistance to flow. The results point to the great importance of achieving an understanding of entrainment. With the second model and a wide range of parameter values, there is no cyclic behavior, with rapid flow being followed by a quiescent phase.

Development of enhanced ice flow at the southern margin of Ice Stream D, Antarctica

A combination of image-based velocity mapping techniques and finite-element modeling has been used to study a part of the southern shear margin of Ice Stream D, Antarctica. The study area is a region over which the margin shows considerable development morphologically, where a new southern margin is forming in response to an abrupt increase in ice-stream width just upstream of the study area. A series of ice-speed profiles perpendicular to the margin was determined by semiautomated displacement measurements of small ice features in sequential Landsat TM images. Transverse speed gradients ∂u/∂y of these profiles were determined by calculating the slope of a high-order polynomial fit to the speed profiles. Maximum ice speed and ∂u/∂y increase dramatically as the margin develops in the downstream direction, from 420 to 670 mal,and from 0.02 to 0.16 a, respectively. Finite-element modeling of the upstream and downstream profiles suggests that a considerable change occurs in the stiffness of the ice in the marginal zone between the two profiles, and in the stiHness or amount of sliding in the basal layer underlying the margin. Ice in the downstream profile appears to have marginal zones of softer ice in which shear strain is concentrated and uniformly low resistance to deformation in the bed. For the upstream profile, modeling suggests that the ice is not softened near the margin and that the bed is stiffer near the margin. Model-based calculations suggest that the bed shear is responsible for 69% of the resistance to flow in the upstream margin area;this value is 51 % in the downstream area.

Development of enhanced ice flow at the southern margin of Ice Stream D, Antarctica

A combination of image-based velocity mapping techniques and finite-element modeling has been used to study a part of the southern shear margin of Ice Stream D, Antarctica. The study area is a region over which the margin shows considerable development morphologically, where a new southern margin is forming in response to an abrupt increase in ice-stream width just upstream of the study area. A series of ice-speed profiles perpendicular to the margin was determined by semiautomated displacement measurements of small ice features in sequential Landsat TM images. Transverse speed gradients ∂u/∂y of these profiles were determined by calculating the slope of a high-order polynomial fit to the speed profiles. Maximum ice speed and ∂u/∂y increase dramatically as the margin develops in the downstream direction, from 420 to 670 mal,and from 0.02 to 0.16 a, respectively. Finite-element modeling of the upstream and downstream profiles suggests that a considerable change occurs in the stiffness of the ice in the marginal zone between the two profiles, and in the stiHness or amount of sliding in the basal layer underlying the margin. Ice in the downstream profile appears to have marginal zones of softer ice in which shear strain is concentrated and uniformly low resistance to deformation in the bed. For the upstream profile, modeling suggests that the ice is not softened near the margin and that the bed is stiffer near the margin. Model-based calculations suggest that the bed shear is responsible for 69% of the resistance to flow in the upstream margin area;this value is 51 % in the downstream area.

Development of enhanced ice flow at the southern margin of Ice Stream D, Antarctica

A combination of image-based velocity mapping techniques and finite-element modeling has been used to study a part of the southern shear margin of Ice Stream D, Antarctica. The study area is a region over which the margin shows considerable development morphologically, where a new southern margin is forming in response to an abrupt increase in ice-stream width just upstream of the study area. A series of ice-speed profiles perpendicular to the margin was determined by semiautomated displacement measurements of small ice features in sequential Landsat TM images. Transverse speed gradients ∂u/∂y of these profiles were determined by calculating the slope of a high-order polynomial fit to the speed profiles. Maximum ice speed and ∂u/∂y increase dramatically as the margin develops in the downstream direction, from 420 to 670 mal,and from 0.02 to 0.16 a, respectively. Finite-element modeling of the upstream and downstream profiles suggests that a considerable change occurs in the stiffness of the ice in the marginal zone between the two profiles, and in the stiHness or amount of sliding in the basal layer underlying the margin. Ice in the downstream profile appears to have marginal zones of softer ice in which shear strain is concentrated and uniformly low resistance to deformation in the bed. For the upstream profile, modeling suggests that the ice is not softened near the margin and that the bed is stiffer near the margin. Model-based calculations suggest that the bed shear is responsible for 69% of the resistance to flow in the upstream margin area;this value is 51 % in the downstream area.