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Evidence Against and Factors Preventing Major Surges of the Antarctic Ice Sheet

Published online by Cambridge University Press:  30 January 2017

G. de Q. Robin
G. de Q. Robin*
Affiliation:
Scott Polar Research Institute, Lensfield Road, Cambridge CB2 IER, England
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Abstract

Although computer modelling using realistic flow parameters can simulate surging of the Antarctic ice sheet, the present model does not take into account certain factors that make surging less probable. Before discussing these factors, knowledge of the Antarctic ice sheet that might indicate the occurrence of former surging is reviewed. The following studies appear relevant

  • (a) Observed temperature–depth profiles approximate to steady-state solutions, whereas a major surge within the last 10 000 to 20000 years would have produced markedly different temperature–depth profiles at Byrd and Vostok Stations.

  • (b) Isotopic profiles are estimated for steady-state and for surging behaviour of the Antarctic ice sheet. When these are compared with observed profiles no convincing evidence of surging over the past 10 000 to 50 000 years is seen.

  • (c) Studies of flow lines in and around the Ross Ice Shelf do not reveal any surging of discharge glaciers in the past I 000 to 2 000 years.

  • (d) Although mass-balance calculations and balance-velocity calculations on the Antarctic ice sheet are not accurate, ice discharge is generally estimated to be within a factor of two of the total mass accumulation.

Three stabilizing factors that have not been included in computer models and need consideration are

  • (1) It appears unlikely that a surge will be propagated up-stream of any substantial bedrock slope that opposes the ice motion.

  • (2) The very high effective viscosity of great thicknesses of ice at very low temperatures adds considerable rigidity to the ice sheet at the lateral boundaries of any incipient surge.

  • (3) Strong convergence of flow lines towards ice streams and major trunk glaciers apparently provides a stabilizing factor.

Type
Abstracts of Papers Presented at the Symposium but not Published in Full in this volume
Information
Journal of Glaciology , Volume 24 , Issue 90 , 1979 , pp. 485 - 487
Copyright
Copyright © International Glaciological Society 1979

Discussion

T. J. Hughes: You claimed that the hardest part of an ice stream is the zone of converging flow at its head, and this hard ice would keep an ice stream surge from propagating inland. The zone of converging flow should be one of the softest parts of the ice stream, because strong lateral convergence deforms the whole bulk of ice, not just the basal and lateral shear zones. So bulk strain-softening should be a maximum, not a minimum, at the heads of ice streams and the surge could easily propagate inland.

On another point, in your surge paper with Weertman, was not the ablation zone at the glacier snout the means whereby you obtained compressive flow in the snout? If so, compressive flow in ice-stream tongues could be provided if the tongues were imbedded in confined and pinned ice shelves, and your surge mechanism might apply.

G. de Q. Robin: I agree that your first point is of major importance, but you misinterpret my remarks. I was discussing the relative hardness of the lateral shear zone and the zone of converging flow. I believe the former is much softer than the latter, and hence cannot propagate inland as a single shear zone. I agree that the converging zone will be softer than the main ice sheet, but as to how much softer, we still need an answer. Certainly the important key to stability of ice streams lies in this zone, which needs further study.

As regards your second point, your argument does not apply, unless ice-stream discharge stops almost completely above the grounding line and then only if the equilibrium line is much higher.

C. Lorius: Your reconstruction of the D-I 0 profile for the elevation of origin of ice is very attractive. However at the 220 m depth there is a difference of about 1.5 km with the elevations from the ice-core 0 values. This difference decreases to zero in a rather rapid way, which may not be contradictory with the existence of surge.

Robin: I accept your point that surface lowering was quite rapid when ice from 220 to 200 m depth in the present core was being laid down, but any difference between us lies in our definition of a surge. 1 regard a surge as involving some dynamic instability leading to excessive surface lowering, followed by gradual recovery as the surface builds up to levels associated with the prevailing climate, rather than a relatively uniform, though considerable, change in surface level from that appropriate to one climatic regime to that associated with another.

w. F. Budd: Since the paper by Reference AllisonAllison (1979) on the Lambert Glacier basin is not being presented I would like to comment that the interior of the Lambert basin appears to be one large region which has a quite definite positive balance. The region of Lambert Glacier which has apparently had substantial ice lowering of the order of 800 m or more seems to be rising at a high rate (c. 0.2 m/a) which, if continued, would reach the previous thickness in the order of 4000 a.

My second point is that there seems to be a problem in matching the large, relatively rapid changes observed in isotope and gas-content profiles, without substantial increases in velocity, which are difficult to explain in terms of steady state or direct reaction to climate change.

References

Allison, I. 1979. The mass budget of the Lambert Glacier drainage basin, Antarctica. Journal of Glaciology, Vol. 22, No. 87, p. 223–35.CrossRefGoogle Scholar