Rapid Ice Flow Research Articles

Subglacial decoupling at the sediment/bedrock interface: a new mechanism for rapid flowing ice

On millennial or even centennial time scales, the activity of rapid flowing ice can affect climate variability and global sea level through release of meltwater into the ocean and positive feedback loops to the climate system. At the surge-type glacier Brúarjökull, an outlet of the Vatnajökull ice cap, eastern Iceland, extremely rapid ice flow was sustained by overpressurized water causing decoupling beneath a thick sediment sequence that was coupled to the glacier. This newly discovered mechanism has far reaching consequences for our understanding of fast-flowing ice and its integration with sediment discharge and meltwater release.

Granular flow in the marginal ice zone

The region of sea ice near the edge of the sea ice pack is known as the marginal ice zone (MIZ), and its dynamics are complicated by ocean wave interaction with the ice cover, strong gradients in the atmosphere and ocean and variations in sea ice rheology. This paper focuses on the role of sea ice rheology in determining the dynamics of the MIZ. Here, sea ice is treated as a granular material with a composite rheology describing collisional ice floe interaction and plastic interaction. The collisional component of sea ice rheology depends upon the granular temperature, a measure of the kinetic energy of flow fluctuations. A simplified model of the MIZ is introduced consisting of the along and across momentum balance of the sea ice and the balance equation of fluctuation kinetic energy. The steady solution of these equations is found to leading order using elementary methods. This reveals a concentrated region of rapid ice flow parallel to the ice edge, which is in accordance with field observations, and previously called the ice jet. Previous explanations of the ice jet relied upon the existence of ocean currents beneath the ice cover. We show that an ice jet results as a natural consequence of the granular nature of sea ice.

Laurentide ice streaming on the Canadian Shield: A conflict with the soft-bedded ice stream paradigm?

Elucidating the controls on the location and vigor of ice streams is crucial to understanding the processes that lead to fast disintegration of ice flows and ice sheets. In the former North American Laurentide ice sheet, ice stream occurrence appears to have been governed by topographic troughs or areas of soft-sediment geology. This paper reports robust evidence of a major paleo–ice stream over the northwestern Canadian Shield, an area previously assumed to be incompatible with fast ice flow because of the low relief and relatively hard bedrock. A coherent pattern of subglacial bedforms (drumlins and megascale glacial lineations) demarcates the ice stream flow set, which exhibits a convergent onset zone, a narrow main trunk with abrupt lateral margins, and a lobate terminus. Variations in bedform elongation ratio within the flow set match theoretical expectations of ice velocity. In the center of the ice stream, extremely parallel megascale glacial lineations tens of kilometers long with elongation ratios in excess of 40:1 attest to a single episode of rapid ice flow. We conclude that while bed properties are likely to be influential in determining the occurrence and vigor of ice streams, contrary to established views, widespread soft-bed geology is not an essential requirement for those ice streams without topographic control. We speculate that the ice stream acted as a release valve on ice-sheet mass balance and was initiated by the presence of a proglacial lake that destabilized the ice-sheet margin and propagated fast ice flow through a series of thermomechanical feedbacks involving ice flow and temperature.

High geothermal heat flow, Basal melt, and the origin of rapid ice flow in central Greenland.

Age-depth relations from internal layering reveal a large region of rapid basal melting in Greenland. Melt is localized at the onset of rapid ice flow in the large ice stream that drains north off the summit dome and other areas in the northeast quadrant of the ice sheet. Locally, high melt rates indicate geothermal fluxes 15 to 30 times continental background. The southern limit of melt coincides with magnetic anomalies and topography that suggest a volcanic origin.

Ice‐stream‐related patterns of ice flow in the interior of northeast Greenland

Ice flow in the interior of the NE quadrant of the Greenland ice sheet is focused on the large ice stream draining the north side of the summit dome. The rapid ice flow in the stream is apparent in the surface features in the stream and at the margins, in the broad scale topography that drives ice flow, and in satellite‐derived ice motion information. The patterns of ice flow in the upper half of the stream are remarkable for their level of organization, simple geometry, and effects on local surface topography. The stream begins less than 100 km from the ice divide as a current 15 km wide and then broadens symmetrically downstream by the addition of ice from the sides to a width of more than 60 km. Elevation data and visible‐band imagery show that the stream has marginal troughs tens of meters deep in its upper reach which are coincident with regions of high shear strain rate. The topography of the margins and undulating surface of the stream is generated by the ice flow; the surface undulations in the stream are fixed in location and shape over the 5 year period from 1994 to 1999. The enhanced flow presents a challenge for researchers trying to understand the history of ice discharge from a significant area in the interior of the ice sheet.

Old Ice Streams

GEOLOGY The size and behavior of past Antarctic ice sheets are essential elements in the quest to understand how and why climate changed in the past and what the consequences may be in the future. Modern observations have shown that Antarctic ice flow is slow and diffuse over most of the interior of the ice sheet but focuses into relatively rapid streams near the edges of the sheet. Canals et al. have found a set of parallel to subparallel ridges and grooves up to 100 kilometers (km) in length at a water depth of 1000 m in the Western Bransfield Basin off Antarctica. They identify this formation, called a “bundle” structure, as material deposited by a giant ice stream during the last glacial maximum. This finding illustrates the importance of rapid ice flow during past glacial maxima that are related to present ice sheet movements. The minimum ice thickness necessary to have formed this bundle is 1000 m. This single stream, which was probably about 340-km long, would have transported approximately 8000 km3 of ice to the ocean—enough to raise sea level slightly more than 2 cm.— HJS Geology 28 , 31 (2000)

Ice-sheet surging and ice-stream formation

A simplified model of ice-sheet behaviour is described. It combines the assumptions of rapid ice flow, high viscous activation energy and realistic sediment-based sliding dynamics to form a non-linear diffusion-type equation which can display relaxation oscillations analogous to those of surging glaciers, and which may be relevant to large-scale surges of the Hudson Strait and Cabot Strait ice streams of the Laurentide ice sheet.When the physics of this model is applied to a laterally extensive unidirectional ice flow, such as that in the Siple Coast of Antarctica, an appropriate mechanism may exist for the spontaneous generation of ice streams.

Ice-sheet surging and ice-stream formation

A simplified model of ice-sheet behaviour is described. It combines the assumptions of rapid ice flow, high viscous activation energy and realistic sediment-based sliding dynamics to form a non-linear diffusion-type equation which can display relaxation oscillations analogous to those of surging glaciers, and which may be relevant to large-scale surges of the Hudson Strait and Cabot Strait ice streams of the Laurentide ice sheet. When the physics of this model is applied to a laterally extensive unidirectional ice flow, such as that in the Siple Coast of Antarctica, an appropriate mechanism may exist for the spontaneous generation of ice streams.

Deformation till in the Great Lakes region: implications for rapid flow along the south-central margin of the Laurentide Ice Sheet

Structural and lithologic data indicate that, while deposited under actively moving ice, considerable portions of three muddy calcareous subglacial tills in the Great Lakes region probably experienced some component of ductile deformation. Viscous till flow and ductile shear are invoked to explain a combination of features such as recumbent isoclinal folds, unlithified sediment clasts, mixed ostracode shells, reversed stone lee ends, girdle and transverse fabrics, irregular stone pavements, fine striae following stone curves, and inconsistent stone striae and a-axes. Deforming, fine-textured subglacial till is considered as a subhorizontal shear zone, rheologically layered with associated structures (in descending order): ductile (e.g., isoclinal folds), brittle–ductile (e.g., fissility), and brittle (e.g., till wedges). Rheology would be controlled mainly by till pore water content, matrix texture, and stone content. Spatial and temporal superposition of rheologies and subglacial processes probably occurred while some fine tills were forming. Fine deformation till may be especially common around areas of the Great Lakes region where proglacial mud and weak bedrock were remoulded as ice travelled along major basins and troughs. In such areas, under a wet-based glacier, resulting till would have been too weak to sustain a large shear stress or inhibit rapid ice flow over it. Instead, in these places, the till was probably water saturated, accounting for most of the glacial flow, and protected the substrate from extensive deformation while effectively acting as a lubricant to overriding ice. Areas of fine deformation till probably represent areas of former low subglacial fluid conductivity and rapid glacial flow. In other areas, subglacial sheet flow of meltwater may have accelerated glacial flow. These two types of areas may have been connected at times under zones of ice streaming and (or) surging.

Lobal interactions and rheologic superposition in subglacial till near Bradtville, Ontario, Canada

Structural, stratigraphic, and lithologic data from a section 69 m long of Catfish Creek drift (north shore of Lake Erie) tell a complex story of two competing glacial lobes. Stone surface features and orientations indicate that stones rotated in viscously deforming, fine‐medium textured subglacial till prior to final emplacement. Fractures, shears, and attenuated sediment lenses in tills reveal that they experienced some brittle shear superposed on ductile shear during till dewatering and stiffening. The Huron‐Georgian Bay lobe advanced first from the northwest, deforming interstadial sediments and depositing subglacial till. Next, southward confluent flow of the Huron, Georgian Bay, and Erie lobes carved subglacial troughs into sediments and deposited (then deformed) bouldery deformation till by squeeze flow. The northwest flowing Erie lobe then prevailed, depositing deformation till, subglacial aquatic sediments, and mudflows. Finally, a pavement‐bearing, hybrid deformation‐lodgement till covered the section. Till formation was mainly by subglacial viscous flow with minor lodgement superposed as water content decreased and some fines were probably winnowed. This implies that till deformation probably accounted for much of the glacier movement. Therefore, rapid ice flow could have occurred over the section, along the southern margin of the Laurentide Ice Sheet.

Studying the Process of Dissipation of Energy of Motion in Highly Fractured Glaciers using Remote-Sensing Techniques (Abstract)

New aspects of the physics of ice movement in glaciers are presented. An analysis of the equation for the energy of a moving ice mass shows that the energy which is dissipated into heat (internal friction in a glacier) occurs not only because of average ice-velocity profiles, as it is commonly considered when simulating glaciers by continuum laminar flow. It also turns out that a significant part of the internal friction is associated with the dissipation of motion energy on internal macroheterogeneities in a glacier: at places where fractures appear and, in the case of a block structure, at points of contact between individual blocks, on ice dams, ice bridges, etc. This mechanism seems to be dominant during rapid ice flow in highly fractured glaciers.The process was previously examined on the surging Medvezhiy Glacier in the central Pamir Mountains. On Medvezhiy Glacier the above phenomenon was particularly distinctive because of anomalously high ice-movement velocities during a calm period; that is, between consecutive surges (ice velocity amounted to 3–5 m d−1) and because of the block structure of the glacier against the background of its fracturing. Simple measurements of “instant” velocities of a large number of bench marks on the glacier surface, located across the middle flow course of the glacier, established that velocities at all points are of a fluctuating character, which outwardly resemble velocity fluctuations seen in turbulent liquid flow. It turns out that deviations of the “instant” velocity from the average velocity can amount to more than 50% of the latter. Using data obtained from periodic recordings of the “instant” velocity for fixed pairs of bench marks, which were located at different distances (so that it was possible to evaluate the difference in instant velocities), a structural function was constructed. This structural function meets the well-known, so-called Kholmogorov two-thirds law For local isotropic turbulence. This indicates that, as with turbulence, the energy of middle movement is transferred along a cascade of ice conglomerates of decreasing size down to the smallest, limited only by the dimensions of fundamental blocks. At the points of contact at dams and junctions, motion energy dissipates into heat. It is postulated that this phenomenon may have a much broader application; it may also be characteristic of the movement of any fractured mass of solid material.These findings may be used to formulate a theory which describes the mechanism of auto-oscillations in a surging glacier. According to this theory, the pseudoturbulent character of internal friction during a quiescent period (between surges) is one of the factors which prevents the appearance of a surge as long as possible, thus ensuring the growth of a new active glacier after each ice catastrophe.These studies also conclude research on the general concept of auto-oscillations of mountain glaciers — the so-called surging glaciers, originally formulated in the course of research on the cause of a surge on Medvezhiy Glacier in 1963. This concept was subsequently adopted and is now widely used in glaciology.

Studying the Process of Dissipation of Energy of Motion in Highly Fractured Glaciers using Remote-Sensing Techniques (Abstract)

New aspects of the physics of ice movement in glaciers are presented. An analysis of the equation for the energy of a moving ice mass shows that the energy which is dissipated into heat (internal friction in a glacier) occurs not only because of average ice-velocity profiles, as it is commonly considered when simulating glaciers by continuum laminar flow. It also turns out that a significant part of the internal friction is associated with the dissipation of motion energy on internal macroheterogeneities in a glacier: at places where fractures appear and, in the case of a block structure, at points of contact between individual blocks, on ice dams, ice bridges, etc. This mechanism seems to be dominant during rapid ice flow in highly fractured glaciers. The process was previously examined on the surging Medvezhiy Glacier in the central Pamir Mountains. On Medvezhiy Glacier the above phenomenon was particularly distinctive because of anomalously high ice-movement velocities during a calm period; that is, between consecutive surges (ice velocity amounted to 3–5 m d−1) and because of the block structure of the glacier against the background of its fracturing. Simple measurements of “instant” velocities of a large number of bench marks on the glacier surface, located across the middle flow course of the glacier, established that velocities at all points are of a fluctuating character, which outwardly resemble velocity fluctuations seen in turbulent liquid flow. It turns out that deviations of the “instant” velocity from the average velocity can amount to more than 50% of the latter. Using data obtained from periodic recordings of the “instant” velocity for fixed pairs of bench marks, which were located at different distances (so that it was possible to evaluate the difference in instant velocities), a structural function was constructed. This structural function meets the well-known, so-called Kholmogorov two-thirds law For local isotropic turbulence. This indicates that, as with turbulence, the energy of middle movement is transferred along a cascade of ice conglomerates of decreasing size down to the smallest, limited only by the dimensions of fundamental blocks. At the points of contact at dams and junctions, motion energy dissipates into heat. It is postulated that this phenomenon may have a much broader application; it may also be characteristic of the movement of any fractured mass of solid material. These findings may be used to formulate a theory which describes the mechanism of auto-oscillations in a surging glacier. According to this theory, the pseudoturbulent character of internal friction during a quiescent period (between surges) is one of the factors which prevents the appearance of a surge as long as possible, thus ensuring the growth of a new active glacier after each ice catastrophe. These studies also conclude research on the general concept of auto-oscillations of mountain glaciers — the so-called surging glaciers, originally formulated in the course of research on the cause of a surge on Medvezhiy Glacier in 1963. This concept was subsequently adopted and is now widely used in glaciology.

The Dynamics of Ice-Sheet Outlets

AbstractA comparison of data from aircraft altimetry, Landsat imagery, and radia echo-sounding has shown characteristic surface topographies associated with sheet and stream flow. The transition between the two is abrupt and occurs at a step in the subglacial topography. This marks the onset of basal sliding and high velocities caused by subglacial water; it results in crevassed amphitheatre-like basins round the head of outlet glaciers. It is also the zone of maximum driving stress beyond which values decline rapidly as velocities increase. This abrupt transition appears to be topographically controlled since basal temperatures are at the pressure-melting point well inland of the change in regime. The Marie Byrd Land ice streams exhibit qualitative differences from other ice-sheet outlets, however; the change to lower driving stresses is much more gradual and occurs several hundred kilometres inland. Such ice streams have particularly low surface slopes and appear in form and flow regime to resemble confined ice shelves rather than grounded ice. The repeated association of the transition to rapid sliding with a distinct subglacial feature implies a stabilizing effect on discharge through outlet glaciers. Acceleration of the ice is pinned to a subglacial step and propagation of high velocities inland of this feature seems improbable. Rapid ice flow through subglacial trenches may also ensure a relatively permanent trough through accentuation of the feature by erosion. This is concentrated towards the heads of outlet glaciers up-stream of the region where significant basal decoupling occurs. This may be a mechanism for the overdeepening of fjords at their inland ends and the development of very steep fjord headwalls.

The Dynamics of Ice-Sheet Outlets

AbstractA comparison of data from aircraft altimetry, Landsat imagery, and radia echo-sounding has shown characteristic surface topographies associated with sheet and stream flow. The transition between the two is abrupt and occurs at a step in the subglacial topography. This marks the onset of basal sliding and high velocities caused by subglacial water; it results in crevassed amphitheatre-like basins round the head of outlet glaciers. It is also the zone of maximum driving stress beyond which values decline rapidly as velocities increase. This abrupt transition appears to be topographically controlled since basal temperatures are at the pressure-melting point well inland of the change in regime. The Marie Byrd Land ice streams exhibit qualitative differences from other ice-sheet outlets, however; the change to lower driving stresses is much more gradual and occurs several hundred kilometres inland. Such ice streams have particularly low surface slopes and appear in form and flow regime to resemble confined ice shelves rather than grounded ice. The repeated association of the transition to rapid sliding with a distinct subglacial feature implies a stabilizing effect on discharge through outlet glaciers. Acceleration of the ice is pinned to a subglacial step and propagation of high velocities inland of this feature seems improbable. Rapid ice flow through subglacial trenches may also ensure a relatively permanent trough through accentuation of the feature by erosion. This is concentrated towards the heads of outlet glaciers up-stream of the region where significant basal decoupling occurs. This may be a mechanism for the overdeepening of fjords at their inland ends and the development of very steep fjord headwalls.