Lithosphere deformation by continental ice sheets

1981 ◽  
Vol 378 (1775) ◽  
pp. 507-527 ◽  
Keyword(s):  
Steady State ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Viscoplastic Deformation ◽  
Viscoplastic Flow ◽  
Common Assumption ◽  
Maximum Extent ◽  
Proglacial Lakes ◽  
Lithosphere Deformation ◽  
Continental Ice Sheets

In treating problems of mantle deformation, a common assumption of convenience is that deformation is viscous or viscoelastic. An important consequence of this assumption is the unproven prediction that the lithosphere is depressed far beyond the margins of steady-state continental ice sheets at the time of their maximum extent. Depression beyond the margins of shrinking ice sheets is proven by the formation of proglacial lakes along melting margins of retreating late Wisconsin-Weichselian ice sheets. No such lakes existed during the maximum extent of these ice sheets. An investigation of viscoplastic deformation in anisotropic polycrystalline rocks leads to the conclusion that transient creep in the lithosphere is predominantly time-dependent viscous flow, but that slow steady-state creep in the lithosphere is time-independent viscoplastic flow and begins at a viscoplastic yield stress. These results predict lithosphere depression beyond the margin of a growing or shrinking ice sheet, but not beyond the margin of a steady-state ice sheet at its maximum extent.

scholarly journals Suppression of marine ice sheet instability

2018 ◽  
Vol 857 ◽  
pp. 648-680 ◽  
Author(s):  
Samuel S. Pegler
Keyword(s):  
Steady State ◽  
Rapid Assessment ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Stable Steady State ◽  
Ice Shelf ◽  
West Antarctic Ice Sheet ◽  
West Antarctic ◽  
Grounding Line ◽  
The Stability

A long-standing open question in glaciology concerns the propensity for ice sheets that lie predominantly submerged in the ocean (marine ice sheets) to destabilise under buoyancy. This paper addresses the processes by which a buoyancy-driven mechanism for the retreat and ultimate collapse of such ice sheets – the marine ice sheet instability – is suppressed by lateral stresses acting on its floating component (the ice shelf). The key results are to demonstrate the transition between a mode of stable (easily reversible) retreat along a stable steady-state branch created by ice-shelf buttressing to tipped (almost irreversible) retreat across a critical parametric threshold. The conditions for triggering tipped retreat can be controlled by the calving position and other properties of the ice-shelf profile and can be largely independent of basal stress, in contrast to principles established from studies of unbuttressed grounding-line dynamics. The stability and recovery conditions introduced by lateral stresses are analysed by developing a method of constructing grounding-line stability (bifurcation) diagrams, which provide a rapid assessment of the steady-state positions, their natures and the conditions for secondary grounding, giving clear visualisations of global stabilisation conditions. A further result is to reveal the possibility of a third structural component of a marine ice sheet that lies intermediate to the fully grounded and floating components. The region forms an extended grounding area in which the ice sheet lies very close to flotation, and there is no clearly distinguished grounding line. The formation of this region generates an upsurge in buttressing that provides the most feasible mechanism for reversal of a tipped grounding line. The results of this paper provide conceptual insight into the phenomena controlling the stability of the West Antarctic Ice Sheet, the collapse of which has the potential to dominate future contributions to global sea-level rise.

scholarly journals Erosion by Continental Ice Sheets

1979 ◽  
Vol 23 (89) ◽  
pp. 401-402
Author(s):  
I. M. Whillans
Keyword(s):  
Mass Balance ◽  
Short Distance ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Present Theory ◽  
Frictional Heat ◽  
Break Up ◽  
Basal Melting ◽  
Continental Ice Sheets

Abstract Some of the problems with earlier theories for erosion and transport by ice sheets are discussed, and it is noted that those theories cannot simply account for the often-reported finding that most till is derived from bedrock only a few tens of kilometers up-glacier. Considerations of the mass balance of debris in transport lead to the conclusion that ice sheets are capable of transporting most debris only a short distance. The theory that the break-up of bedrock is mostly a preglacial process is developed. The advancing ice sheet collects the debris and then deposits it after a short travel. As the ice sheet first advances over the regolith, debris is frozen onto the base and is carried until basal melting due to geothermal and frictional heat causes lodgment till deposition. Most debris is deposited during the advance of the ice sheet and is carried only a short distance. A generally small amount of debris is carried at higher levels and is deposited during ice standstill and retreat as melt-out and ablation tills. The present theory makes many predictions, among them, that most till units are not traceable over long distances, that thick till sequences represent unstable glacier margins and not necessarily long periods of glacier occupation, and that lodgment tills are to be interpreted in terms of ice advances and ablation tills in terms of ice retreats. This paper is published in full in Journal of Geology, Vol. 86, No. 4, 1978, p. 516–24.

scholarly journals Ice-Sheet Erosion—A Result of Maximum Conditions?

1979 ◽  
Vol 23 (89) ◽  
pp. 402-404 ◽  
Author(s):  
D. E. Sugden
Keyword(s):  
Steady State ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Gradual Transition ◽  
Glacial Erosion ◽  
Mountain Glaciers ◽  
State Models ◽  
Ice Sheet Erosion ◽  
The Relationship ◽  
Sheet Erosion

Abstract Understanding the relationship between the morphology of former ice-sheet beds and glaciological processes is handicapped by the difficulty of establishing which stage of a cycle of ice-sheet growth and decay is responsible for most erosion. Discussions at this conference and in the literature display a variety of opinions, some favouring periods of ice-sheet build up, others periods of fluctuations, and still others steady-state maximum conditions. Here it is suggested that there is geomorphological evidence which points to the dominance of maximum conditions. Along the eastern margins of the Laurentide and Greenland ice sheets there is a sharp discontinuity between Alpine relief which stood above the ice-sheet surface at the maximum and plateau scenery which was covered by the ice sheet. Often the two types of relief are adjacent and yet separated by an altitudinal difference of only 100–200 m. The existence of an abrupt rather than gradual transition from one relief type to the other suggests that most glacial sculpture must have taken place while the ice sheet was at its maximum extent. In other geomorphological situations where high mountains were submerged by ice sheets, the major erosional landforms are frequently found to relate to ice sheets rather than to local mountain glaciers, again suggesting the dominance of erosion during full ice-sheet conditions. Finally, the identification of patterns of glacial erosion on an ice-sheet scale in North America and Greenland points to erosion when the ice sheets were fully expanded, rather than to the variable flow conditions associated with growth or decay. If ice-sheet erosion is accepted as being a result of maximum conditions, then it places certain constraints on glacial theory, for example the need to develop theories of glacial erosion which apply beneath ice thicknesses of several thousand metres. It also suggests that the use of steady-state models of ice sheets is likely to be a profitable way of relating glaciological processes to the morphology of former ice-sheet beds.

Asynchronously coupling the cryosphere and atmosphere in an energy-balance climate model

1997 ◽  
Vol 25 ◽  
pp. 159-164
Author(s):  
Robert S. Steen ◽  
Tamara Shapiro Ledley
Keyword(s):  
Energy Balance ◽  
Sea Ice ◽  
Time Scales ◽  
Climate Model ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Final State ◽  
Coupling Parameters ◽  
Continental Ice Sheets ◽  
Ice Model

A major component of the climate system on the 10 000-100 000 year time-scales is continental ice sheets, yet many of the mechanisms involved in the land-sea-ice processes that affect the ice sheets are poorly understood. In order to examine these processes in more detail, we have developed a coupled energy balance climate-thermodynamic sea-ice—continental-ice-sheet model (CCSLI model). This model includes a hydrologic cycle, a detailed surface energy and mass balance, a thermodynamic sea-ice model, and a zonally averaged dynamic ice-flow model with bedrock depression.Because of the variety of space and time-scales inherent in such a model, we have asynchronously coupled the land—ice model to the other components of the model. In this paper the asynchronous coupling is described and sensitivity studies are presented that determine the values of the asynchronous coupling parameters. Model simulations using these values allow the model to run nearly ten times faster with minimal changes in the final state of the ice sheet.

scholarly journals Existence and stability of steady-state solutions of the shallow-ice-sheet equation by an energy-minimization approach

2011 ◽  
Vol 57 (202) ◽  
pp. 345-354 ◽  
Author(s):  
Guillaume Jouvet ◽  
Jacques Rappaz ◽  
Ed Bueler ◽  
Heinz Blatter
Keyword(s):  
Shear Stress ◽  
Steady State ◽  
Mass Balance ◽  
Energy Minimization ◽  
Existence Of Solutions ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Existence And Stability ◽  
Steady State Solutions ◽  
Minimization Approach

AbstractThe existence of solutions of the non-sliding shallow-ice-sheet equation on a flat horizontal bed with a mass balance linearly depending on altitude is proven for fixed margins. Free-margin solutions for the same mass balance do not exist. Fixed-margin solutions show unbounded shear stress and nonzero mass flux at the margin. Steady-state solutions with realistic margins, vanishing ice flux and vanishing shear stress are found numerically for ice sheets with Weertman-type sliding.

scholarly journals Ice-Sheet Erosion—A Result of Maximum Conditions?

1979 ◽  
Vol 23 (89) ◽  
pp. 402-404
Author(s):  
D. E. Sugden
Keyword(s):  
Steady State ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Gradual Transition ◽  
Glacial Erosion ◽  
Mountain Glaciers ◽  
State Models ◽  
Ice Sheet Erosion ◽  
The Relationship ◽  
Sheet Erosion

AbstractUnderstanding the relationship between the morphology of former ice-sheet beds and glaciological processes is handicapped by the difficulty of establishing which stage of a cycle of ice-sheet growth and decay is responsible for most erosion. Discussions at this conference and in the literature display a variety of opinions, some favouring periods of ice-sheet build up, others periods of fluctuations, and still others steady-state maximum conditions. Here it is suggested that there is geomorphological evidence which points to the dominance of maximum conditions.Along the eastern margins of the Laurentide and Greenland ice sheets there is a sharp discontinuity between Alpine relief which stood above the ice-sheet surface at the maximum and plateau scenery which was covered by the ice sheet. Often the two types of relief are adjacent and yet separated by an altitudinal difference of only 100–200 m. The existence of an abrupt rather than gradual transition from one relief type to the other suggests that most glacial sculpture must have taken place while the ice sheet was at its maximum extent. In other geomorphological situations where high mountains were submerged by ice sheets, the major erosional landforms are frequently found to relate to ice sheets rather than to local mountain glaciers, again suggesting the dominance of erosion during full ice-sheet conditions. Finally, the identification of patterns of glacial erosion on an ice-sheet scale in North America and Greenland points to erosion when the ice sheets were fully expanded, rather than to the variable flow conditions associated with growth or decay.If ice-sheet erosion is accepted as being a result of maximum conditions, then it places certain constraints on glacial theory, for example the need to develop theories of glacial erosion which apply beneath ice thicknesses of several thousand metres. It also suggests that the use of steady-state models of ice sheets is likely to be a profitable way of relating glaciological processes to the morphology of former ice-sheet beds.

scholarly journals Ice-Age Climate and Continental Ice Sheets: Some Experiments with A General Circulation Model

1984 ◽  
Vol 5 ◽  
pp. 100-105 ◽  
Author(s):  
S. Manabe ◽  
A. J. Broccoli
Keyword(s):  
North America ◽  
General Circulation Model ◽  
Northern Hemisphere ◽  
General Circulation ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Circulation Model ◽  
Geological Evidence ◽  
The Difference ◽  
Continental Ice Sheets

The climatic influence of the land ice which existed 18 ka BP is investigated using a climate model developed at the Geophysical Fluid Dynamics Laboratory of the National Oceanic and Atmospheric Administration. The model consists of an atmospheric general circulation model coupled with a static mixed layer ocean model. Simulated climates are obtained from each of two versions of the model: one with the land-ice distribution of the present and the other with that of 18 ka BP.In the northern hemisphere, the difference in the distribution of sea surface temperature (SST) between the two experiments resembles the difference between the SST at 18 ka BP and at present as estimated by CLIMAP Project Members (1981). In the northern hemisphere a substantial lowering of air temperature also occurs in winter, with a less pronounced cooling during summer. The mid-tropospheric flow field is influenced by the Laurentide ice sheet and features a split jet stream straddling the ice sheet and a long wave trough along the east coast of North America. In the southern hemisphere of 18 ka BP, the ice sheet has little influence on temperature. An examination of hemispheric heat balances indicates that this is because only a small change in interhemispheric heat transport exists, as the In situ radiative compensation in the northern hemisphere counterbalances the effective reflection of solar radiation by continental ice sheets.Hydrologic changes in the model climate are also found, with statistically significant decreases in soil moisture occurring in a zone located to the south of the ice sheets in North America and Eurasia. These findings are consistent with some geological evidence of regionally drier climates from the last glacial maximum.

Time-scales and degrees of freedom operating in the evolution of continental ice-sheets

1990 ◽  
Vol 81 (4) ◽  
pp. 371-384 ◽  
Author(s):  
Richard C. A. Hindmarsh
Keyword(s):  
Mass Balance ◽  
Degrees Of Freedom ◽  
Large Scale ◽  
Numerical Models ◽  
Perturbation Expansion ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Plane Flow ◽  
Surface Mass ◽  
Continental Ice Sheets

ABSTRACTComprehensible explanations of the operation of earth climate systems should consist of descriptions of the operation of a few degrees of freedoms. Qualitative interpretations of results from large-scale numerical models generally follow this principle, but do not render formal definitions of the precise nature of such degrees of freedom.At its simplest, ice-sheet kinematics requires knowledge of the evolving height and span. Rheology and surface mass-balance impose different requirements upon the co-evolution of these variables, meaning a two-degree of freedom model is over-prescribed. By means of a perturbation expansion about the analytic similarity solution for viscous spreading, eigenfunctions corresponding to degrees of freedom in the ice-sheet profile are obtained, and are used to decompose mass-balance distributions. Only a few eigenfunctions are needed to replicate numerical models, implying that ice-sheets in plane flow may operate with fewer than ten degrees of freedom.Unstable evolution of ice-sheets can occur, when the operation of a very large number of degrees of freedom can be manifested. Previous work is reviewed and new results for the unstable transformation of valley glaciers into ice-sheets are presented. Phasing of initiation may be an unpredictable phenomenon.

scholarly journals The maximum extent of continental ice sheets at the foot of Ořešník and Poledník in the northern slope of the Jizerské hory Mountains

Geografie ◽  
2006 ◽  
Vol 111 (2) ◽  
pp. 141-151
Author(s):  
Andrzej Traczyk ◽  
Zbyněk Engel
Keyword(s):  
Comparative Analysis ◽  
Ice Sheets ◽  
Rock Weathering ◽  
Northern Slope ◽  
Maximum Extent ◽  
Continental Ice Sheets

Geomorphological observations of the continental ice sheets trim line in the northern part of the Jizerské hory Mountains have been done. The research aims to investigate possibilities of selected techniques in determination of glacial landscape. Continental ice sheets trimline could be reconstructed using comparative analysis of rock weathering variables on the surface of rock landforms. In the Sudetes these methods were applied only in the Pogórze Kaczawskie (Migoń et al. 2002). Results of similar research, which was done at the northern foot of the Ořešník and Poledník Mountains, are presented in this paper.

scholarly journals A case for cold-based continental ice sheets — a transient thermal model

1996 ◽  
Vol 42 (140) ◽  
pp. 37-42 ◽  
Author(s):  
Jan T. Heine ◽  
David F. Mctigue
Keyword(s):  
Heat Flux ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Initial Surface ◽  
Glacial Cycle ◽  
Geothermal Heat ◽  
History Of ◽  
Basal Melting ◽  
Continental Ice Sheets ◽  
Transient Thermal

AbstractA finite-difference numerical model is used to simulate the temperature profile at the center of an ice sheet throughout the course of a glaciation. The ice sheet is gradually built to a thickness of 3000 m over about 10 000 years, starting on permafrost. A geothermal heat flux is applied at large depth. For an initial surface temperature of –12.5°C, our model shows that basal melting occurs 72000 years after the onset of the glaciation. The important parameters determining the basal temperatures are the initial temperature of the ice and substrate, the rate of downward advection of cold ice and, to a lesser extent, the thickness of the ice sheet. The growth history of the ice sheet does not significantly influence the time at which basal melting occurs. Our results show the possibility that the central parts of the continental ice sheets were cold-based for a significant part of their existence. Heating due to the geothermal heat flux cannot account for basal melting during most or all of a glacial cycle. These results may help to explain the existence of preserved land forms under the ice sheets.

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