scholarly journals Ice flow around large obstacles as indicated by basal ice exposed at the margin of the Greenland ice sheet

1994 ◽  
Vol 40 (135) ◽  
pp. 359-367 ◽  
Author(s):  
Peter G. Knight ◽  
David E. Sugden ◽  
Christopher D. Minty
Keyword(s):  
Empirical Model ◽  
Basal Layer ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Ice Flow ◽  
Subglacial Environment ◽  
Basal Ice ◽  
Convergence And Divergence ◽  
Basal Melting ◽  
Marginal Environment

AbstractSpatial variations in the debris-bearing basal ice layer exposed at the ice-sheet margin in West Greenland reflect the geography of basal melting and ice flow around large obstacles close to the margin. This paper demonstrates the character of the basal ice layer, which comprises fine material incorporated in an interior, subglacial environment and coarser material entrained in an ice-marginal environment. We develop an empirical model of ice flow close to a lobate margin of the ice sheet in which ice convergence and divergence, and limited subglacial melting affect the character and distribution of the basal ice at the margin. There is a tendency for the convergence and divergence to thicken the basal layer in lobate areas and to thin it in inter-lobate areas. Under certain circumstances, basal melting may remove much of the layer from beneath the snouts of larger lobes, thus causing the basal layer to be thickest in an intermediate location.

scholarly journals Ice flow around large obstacles as indicated by basal ice exposed at the margin of the Greenland ice sheet

1994 ◽  
Vol 40 (135) ◽  
pp. 359-367 ◽  
Author(s):  
Peter G. Knight ◽  
David E. Sugden ◽  
Christopher D. Minty
Keyword(s):  
Empirical Model ◽  
Basal Layer ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Ice Flow ◽  
Subglacial Environment ◽  
Basal Ice ◽  
Convergence And Divergence ◽  
Basal Melting ◽  
Marginal Environment

AbstractSpatial variations in the debris-bearing basal ice layer exposed at the ice-sheet margin in West Greenland reflect the geography of basal melting and ice flow around large obstacles close to the margin. This paper demonstrates the character of the basal ice layer, which comprises fine material incorporated in an interior, subglacial environment and coarser material entrained in an ice-marginal environment. We develop an empirical model of ice flow close to a lobate margin of the ice sheet in which ice convergence and divergence, and limited subglacial melting affect the character and distribution of the basal ice at the margin. There is a tendency for the convergence and divergence to thicken the basal layer in lobate areas and to thin it in inter-lobate areas. Under certain circumstances, basal melting may remove much of the layer from beneath the snouts of larger lobes, thus causing the basal layer to be thickest in an intermediate location.

scholarly journals The age of surface-exposed ice along the northern margin of the Greenland Ice Sheet

2020 ◽  
Vol 66 (258) ◽  
pp. 667-684
Author(s):  
Joseph A. MacGregor ◽  
Mark A. Fahnestock ◽  
William T. Colgan ◽  
Nicolaj K. Larsen ◽  
Kristian K. Kjeldsen ◽  
...  
Keyword(s):  
Late Pleistocene ◽  
Thermal State ◽  
Basal Layer ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Dust Deposition ◽  
Northern Margin ◽  
Basal Ice ◽  
Multispectral Satellite Imagery ◽  
Sentinel 2

AbstractEach summer, surface melting of the margin of the Greenland Ice Sheet exposes a distinctive visible stratigraphy that is related to past variability in subaerial dust deposition across the accumulation zone and subsequent ice flow toward the margin. Here we map this surface stratigraphy along the northern margin of the ice sheet using mosaicked Sentinel-2 multispectral satellite imagery from the end of the 2019 melt season and finer-resolution WorldView-2/3 imagery for smaller regions of interest. We trace three distinct transitions in apparent dust concentration and the top of a darker basal layer. The three dust transitions have been identified previously as representing late-Pleistocene climatic transitions, allowing us to develop a coarse margin chronostratigraphy for northern Greenland. Substantial folding of late-Pleistocene stratigraphy is observed but uncommon. The oldest conformal surface-exposed ice in northern Greenland is likely located adjacent to Warming Land and may be up to ~55 thousand years old. Basal ice is commonly exposed hundreds of metres from the ice margin and may indicate a widespread frozen basal thermal state. We conclude that the ice margin across northern Greenland offers multiple opportunities to recover paleoclimatically distinct ice relative to previously studied regions in southwestern Greenland.

scholarly journals The age of surface-exposed ice along the northern margin of the Greenland Ice Sheet

2020 ◽  
Author(s):  
Joseph MacGregor ◽  
Mark Fahnestock ◽  
William Colgan ◽  
Nicolaj Larsen ◽  
Kristian Kjeldsen ◽  
...  
Keyword(s):  
Late Pleistocene ◽  
Thermal State ◽  
Basal Layer ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Dust Deposition ◽  
Northern Margin ◽  
Basal Ice ◽  
Multispectral Satellite Imagery ◽  
Sentinel 2

<p>Each summer, surface melting of the margin of the Greenland Ice Sheet exposes a distinctive visible stratigraphy that is related to past variability in subaerial dust deposition across the accumulation zone and subsequent ice flow toward the margin. Here we map this surface stratigraphy along the northern margin of the ice sheet using mosaicked Sentinel-2 multispectral satellite imagery from the end of the 2019 melt season and finer-resolution WorldView-2/3 imagery for smaller regions of interest. We trace three distinct transitions in apparent dust concentration and the top of a darker basal layer. The three dust transitions have been identified previously as representing late-Pleistocene climatic transitions, allowing us to develop a coarse margin chronostratigraphy for northern Greenland. Substantial folding of late-Pleistocene stratigraphy is observed but uncommon. The oldest conformal surface-exposed ice in northern Greenland is likely located adjacent to Warming Land and may be up to ~55 thousand years old. Basal ice is commonly exposed hundreds of meters from the ice margin and may indicate a widespread frozen basal thermal state. We conclude that the ice margin across northern Greenland offers multiple compelling opportunities to recover paleoclimatically valuable ice relative to previously studied regions in southwestern Greenland.</p>

scholarly journals Origin and significance of ‘dispersed facies’ basal ice: Svínafellsjökull, Iceland

2011 ◽  
Vol 57 (204) ◽  
pp. 710-720 ◽  
Author(s):  
Simon J. Cook ◽  
Darrel A. Swift ◽  
David J. Graham ◽  
Nicholas G. Midgley
Keyword(s):  
Significant Contribution ◽  
Ice Flow ◽  
Subglacial Environment ◽  
Basal Ice ◽  
Tectonic Processes ◽  
Glacial Processes ◽  
Glacier Flow ◽  
Bed Slope ◽  
Basal Melting ◽  
Tectonic Origin

AbstractDispersed facies basal ice – massive (i.e. structureless) ice with dispersed debris aggregates – is present at the margins of many glaciers and, as a product of internal glacial processes, has the potential to provide important information about the mechanisms of glacier flow and the nature of the subglacial environment. The origin of dispersed facies is poorly understood, with several hypotheses having been advanced for its formation, and there is disagreement as to whether it is largely a sedimentary or a tectonic feature. We test these established hypotheses at the temperate glacier Svínafellsjökull, Iceland, and find that none fully account for dispersed facies characteristics at this location. Instead, dispersed facies physical, sedimentological and stable-isotope (δ18O, δD) characteristics favour a predominantly tectonic origin that we suggest comprises the regelation and strain-induced metamorphism of debris-rich basal ice that has been entrained into an englacial position by tectonic processes operating at the base of an icefall. Further thickening of the resultant dispersed facies may also occur tectonically as a result of ice flow against the reverse bed slope of a terminal overdeepening. Lack of efficient subglacial drainage in the region of the overdeepening may limit basal melting and thus favour basal ice preservation, including the preservation of dispersed facies. Despite the relatively low sediment content of dispersed facies ( ∼1.6% by volume), its thickness (up to 25 m) and ubiquity at Svínafellsjökull results in a significant contribution to annual sediment discharge (1635–3270 m3 a−1) that is ∼6.5 times that contributed by debris-rich stratified facies basal ice.

scholarly journals A first constraint on basal melt-water production of the Greenland ice sheet

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Nanna B. Karlsson ◽  
Anne M. Solgaard ◽  
Kenneth D. Mankoff ◽  
Fabien Gillet-Chaulet ◽  
Joseph A. MacGregor ◽  
...  
Keyword(s):  
Mass Loss ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Recent Change ◽  
The Arctic ◽  
Water Production ◽  
Ice Flow ◽  
Fjord Circulation ◽  
Current Mass ◽  
Basal Melting

AbstractThe Greenland ice sheet has been one of the largest sources of sea-level rise since the early 2000s. However, basal melt has not been included explicitly in assessments of ice-sheet mass loss so far. Here, we present the first estimate of the total and regional basal melt produced by the ice sheet and the recent change in basal melt through time. We find that the ice sheet’s present basal melt production is 21.4 +4.4/−4.0 Gt per year, and that melt generated by basal friction is responsible for about half of this volume. We estimate that basal melting has increased by 2.9 ± 5.2 Gt during the first decade of the 2000s. As the Arctic warms, we anticipate that basal melt will continue to increase due to faster ice flow and more surface melting thus compounding current mass loss trends, enhancing solid ice discharge, and modifying fjord circulation.

scholarly journals Uppermost crustal structure regulates the flow of the Greenland Ice Sheet

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
G. A. Jones ◽  
A. M. G. Ferreira ◽  
B. Kulessa ◽  
M. Schimmel ◽  
A. Berbellini ◽  
...  
Keyword(s):  
Basal Slip ◽  
Seismic Velocity ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Velocity Model ◽  
Upper Crust ◽  
Geothermal Heat ◽  
Ice Flow ◽  
The Past ◽  
Basal Ice

AbstractThe flow of the Greenland Ice Sheet is controlled by subglacial processes and conditions that depend on the geological provenance and temperature of the crust beneath it, neither of which are adequately known. Here we present a seismic velocity model of the uppermost 5 km of the Greenlandic crust. We show that slow velocities in the upper crust tend to be associated with major outlet glaciers along the ice-sheet margin, and elevated geothermal heat flux along the Iceland hotspot track inland. Outlet glaciers particularly susceptible to basal slip over deformable subglacial sediments include Jakobshavn, Helheim and Kangerdlussuaq, while geothermal warming and softening of basal ice may affect the onset of faster ice flow at Petermann Glacier and the Northeast Greenland Ice Stream. Interactions with the solid earth therefore control the past, present and future dynamics of the Greenland Ice Sheet and must be adequately explored and implemented in ice sheet models.

scholarly journals Subglacial roughness of the Greenland Ice Sheet: relationship with contemporary ice velocity and geology

2019 ◽  
Vol 13 (11) ◽  
pp. 3093-3115 ◽  
Author(s):  
Michael A. Cooper ◽  
Thomas M. Jordan ◽  
Dustin M. Schroeder ◽  
Martin J. Siegert ◽  
Christopher N. Williams ◽  
...  
Keyword(s):  
Flow Direction ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Flow Speed ◽  
Surface Velocity ◽  
Ice Flow ◽  
Systematic Analysis ◽  
Subglacial Environment ◽  
Topographic Roughness ◽  
Ice Surface

Abstract. The subglacial environment of the Greenland Ice Sheet (GrIS) is poorly constrained both in its bulk properties, for example geology, the presence of sediment, and the presence of water, and interfacial conditions, such as roughness and bed rheology. There is, therefore, limited understanding of how spatially heterogeneous subglacial properties relate to ice-sheet motion. Here, via analysis of 2 decades of radio-echo sounding data, we present a new systematic analysis of subglacial roughness beneath the GrIS. We use two independent methods to quantify subglacial roughness: first, the variability in along-track topography – enabling an assessment of roughness anisotropy from pairs of orthogonal transects aligned perpendicular and parallel to ice flow and, second, from bed-echo scattering – enabling assessment of fine-scale bed characteristics. We establish the spatial distribution of subglacial roughness and quantify its relationship with ice flow speed and direction. Overall, the beds of fast-flowing regions are observed to be rougher than the slow-flowing interior. Topographic roughness exhibits an exponential scaling relationship with ice surface velocity parallel, but not perpendicular, to flow direction in fast-flowing regions, and the degree of anisotropy is correlated with ice surface speed. In many slow-flowing regions both roughness methods indicate spatially coherent regions of smooth beds, which, through combination with analyses of underlying geology, we conclude is likely due to the presence of a hard flat bed. Consequently, the study provides scope for a spatially variable hard- or soft-bed boundary constraint for ice-sheet models.

Extreme melt season traction variations recorded on the western Greenland Ice Sheet

2020 ◽  
Author(s):  
Nathan Maier ◽  
Neil Humphrey ◽  
Joel Harper ◽  
Toby Meierbachtol
Keyword(s):  
Conceptual Model ◽  
Dynamic Models ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Late Summer ◽  
Force Balance ◽  
Surface Velocity ◽  
Ice Flow ◽  
Multi Scale ◽  
Basal Ice

<p>Basal traction is fundamental to the dynamics of glaciers and ice sheets. On the Greenland Ice Sheet meltwater delivery to the bed and evolving drainage efficiency and connectivity modulate traction producing a characteristic seasonal velocity response. While numerical modelling and basal pressure observations have linked these velocity variations to evolving subglacial drainage, a high-fidelity record of basal traction is needed to constrain the timing and magnitude of traction changes that modulate summer ice flow.  We present a continuous summertime record of basal traction, basal ice deformation, and surface velocity measured at a densely instrumented field site in western Greenland. We use a five-station GPS network and englacial measurements of shearing and ice temperature to directly estimate the basal traction using the force balance method at the site-scale (100s of meters). Localized traction variations (10s of meters) are inferred via variations in the near-basal deformation field recorded by inclinometers installed directly above the basal interface. Combined, the data give a multi-scale perspective on how the basal traction changes during summer and relates to the conceptual model of melt season flow. Our results show the basal traction migrates between extremes during the melt season, with magnitudes greater than three times the average winter traction and near zero. The basal traction extremes correspond with the spring event, the inferred transition to efficient drainage, and the late summer velocity decline. The rapid strengthening and weakening of the basal interface show the complicated interaction of local and regional forcing that modulate melt season sliding. The near-basal deformation variations allow us to constrain the stress configuration and drainage state during each extreme traction period. Overall, the results allow us to refine the conceptual model for melt season traction changes and provide measured estimates of traction variations which can be used as quantitative targets for coupled drainage – ice dynamic models.</p>

scholarly journals Subglacial roughness of the Greenland Ice Sheet: relationship with contemporary ice velocity and geology

2019 ◽  
Author(s):  
Michael A. Cooper ◽  
Thomas M. Jordan ◽  
Dustin M. Schroeder ◽  
Martin J. Siegert ◽  
Christopher N. Williams ◽  
...  
Keyword(s):  
Flow Direction ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Flow Speed ◽  
Surface Velocity ◽  
Ice Flow ◽  
Systematic Analysis ◽  
Subglacial Environment ◽  
Topographic Roughness ◽  
Ice Surface

Abstract. The subglacial environment of the Greenland Ice Sheet (GrIS) is poorly constrained, both in its bulk properties, for example geology, presence of sediment, and of water, and interfacial conditions, such as roughness and bed rheology. There is, therefore, limited understanding of how spatially heterogeneous subglacial properties relate to ice-sheet motion. Here, via analysis of two decades worth of radio-echo sounding data, we present a new systematic analysis of subglacial roughness beneath the GrIS. We use two independent methods to quantify subglacial roughness: first, the variability of along- track topography—enabling an assessment of roughness anisotropy from pairs of orthogonal transects aligned perpendicular and parallel to ice flow; and second, from bed-echo scattering—enabling assessment of fine-scale bed characteristics. We establish the spatial distribution of subglacial roughness and quantify its relationship with ice flow speed and direction. Overall, the beds of fast-flowing regions are observed to be rougher than the slow-flowing interior. Topographic roughness exhibits an exponential scaling relationship with ice surface velocity parallel, but not perpendicular, to flow direction in fast-flowing regions, and the degree of anisotropy is correlated with ice surface speed. In many slow-flowing regions both roughness methods indicate spatially coherent regions of smooth bed, which, through combination with analyses of underlying geology, we conclude is likely due to the presence of a hard flat bed. Consequently, the study provides scope for a spatially variable hard bed/soft bed boundary constraint for ice-sheet models.

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