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A Reconsideration of the Mass Balance of a Portion of the Ross Ice Shelf, Antarctica *

  • Kenneth C. Jezek (a1) and Charles R. Bentley (a1)

Abstract

The identification of a small region of grounded ice in the north-western sector of the Ross Ice Shelf has forced a re-evaluation of the mass-balance calculations carried out by Thomas and Bentley (1978). Those authors concluded that the Ross Ice Shelf up-stream of Crary Ice Rise was thickening, but they did not take into account the effects on the velocity field of grounded ice (of which they were unaware), which is located near the input gate to their volume element. Reasonable estimates of the degree to which the ice velocity just up-stream of the grounded ice is diminished indicate that it is no longer possible to conclude that the ice shelf is thickening using Thomas and Bentley’s original flow band. Therefore, a new flow band was chosen which was grid east of Thomas and Bentley’s band and unaffected by any nearby grounded areas. The mass balance in this flow band was found to be zero within experimental error; a difference exceeding about 0.2 m a−1 in magnitude between the thickening and bottom freeze-on rates is unlikely.

Résumé

La découverte d’une petite région de glace reposant sur le sol dans le secteur nord-ouest du Ross Ice Shelf nous a contraints à réestimer les calculs du bilan de masse effectués par Thomas et Bentley (1978). Ces auteurs concluaient à un épaississement du Ross Ice Shelf à l’amont de la ride de glace Crary, Ice Rise, mais ils n’ont pas tenu compte des effets du champ de vitesse de la glace reposant sur le sol (ce dont ils n’avaient pas connaissance) qui est situé prés de l’entrée de leur élément de volume. Des estimations raisonnables du degré de diminution de la vitesse juste à l’amont de la zone de glace reposant sur le sol indiquent qu’il n’est plus possible de conclure à un épaississement à l’aide de la bande d’écoulement originale de Thomas et Bentley. C’est pourquoi une nouvelle bande d’écoulement, située plus à l’est que celle de Thomas et Bentley et non influencée par quelque zone de glace reposant sur le sol a été choisie. Le bilan de masse de cette bande d’écoulement n’est pas significativement différent de zéro; une différence de vitesse entre l’épaississement et l’accrétion à la base dépassant environ 0,2 m a −1 n’est pas vraisemblable

Zusammeneassung

Die Feststellung eines kleinen Gebietes aufsitzenden Eises im Nordwestsektor des Ross Ice Shelf zwang zu einer Neubewertung der Massenbilanzberechnungen von Thomas und Bentley (1978). Die genannten Autoren schlossen, dass sich das Ross lce Shelf oberhalb des Crary Ice Rise verdickt, aber sie berücksichtigen dabei nicht die Einwirkungen des ihnen unbekannten aufsitzenden Eises nahe dem Einlass zu ihrem Volumenelement auf das Geschwindigkeitsfeld. Vernünftige Abschätzungen des Grades der Geschwindigkeitsabnahme gerade oberhalb des aufsitzenden Eises ergaben, dass es nicht mehr möglich ist, aus dem ursprünglichen Fliess-Streifen von Thomas und Bentley auf eine Dickenzunahme des Schelfeises zu schliessen. Es wurde deshalb ein neuer Fliess-Streifen weiter östlich gewählt, der durch kein nahegelegenes Gebiet aüfsitzenden Eises beeinflusst wird. Die Massenbilanz in diesem Fliess-Streifen ergab sich unmessbar wenig verschieden von Null; eine Differenz zwischen der Verdickung und der Menge anfrierenden Eises an der Unterseite von mehr als etwa 0,2 m pro Jahr ist unwahrscheinlich.

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Copyright

Footnotes

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page 381 note

Present address: U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire 03755, U.S.A.

page 381 note *

Geophysical and Polar Research Center Contribution No. 408.

Footnotes

References

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Bentley, C.R., and others. 1979. Ice-thickness patterns and the dynamics of the Ross Ice Shelf, Antarctica, by Bentley, C.R., Clough, J.W., Jezek, K.C., and Shabtaie, S.. Journal of Glaciology, Vol. 24, No. 90, p. 28794.
Jezek, K.C., and Bentley, C.R. 1983. Field studies of bottom crevasses in the Ross Ice Shelf, Antarctica. Journal of Glaciology, Vol. 29, No. 101, p. 11826.
Thomas, R.H. 1973. The creep of ice shelves: theory. Journal of Glaciology, Vol. 12, No. 64, p. 4553.
Thomas, R.H., and Bentley, C.R. 1978. The equilibrium state of the eastern half of the Ross Ice Shelf. Journal of Glaciology, Vol. 20, No. 84, p. 50918.
Thomas, R.H., and others. In press. Glaciological studies on the Ross Ice Shelf, Antarctica, 1973‒1978, by Thomas, R.H., MacAyeal, D.R., Eilers, D.H., and Gaylord, D.R.. (In Hayes, D., and Bentley, C.R., ed The Ross Ice Shelf: glaciology and geophysics. Washington, D.C., American Geophysical Union. (Antarctic Research Series, Vol. 40.))
U.S. Geological Survey. 1972. Map of the Ross Ice Shelf. Washington, D.C., U.S. Geological Survey and National Science Foundation. [Scale 1 : 2 188 800.]

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