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The Dynamics of Ice-Sheet Outlets

  • N. F. Mcintyre (a1)

Abstract

A 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.

Résumé

Une comparaison des données d’altimétrie aérienne, des images de Landsat et de sondages radar a montré des topographies caractéristiques de surface associé à la calotte et l’écoulement. La transition entre les deux est abrupte et intervient à l’occasion d’un décalage de la topographie sous-glaciaire. Ceci marque le début du glissement basal et des vitesses élevées dues à l’eau sous glaciaire et a pour conséquence un bassin crevassé en amphithéatre au début de l’émissaire. C’est aussi la zone du maximum des contraintes, au delà elles diminuent rapidement en même temps que les vitesses croissent. Cette transition abrupte semble être controlée par la topographie puisque la température à la base est un point de fusion bien plus loin à l’intérieur que le changement de régime. Néanmoins, les courants de glace de la Marie Byrd Land révèlent des différences qualitatives avec les autres émissaires de la calotte. Le passage à des états de contraintes inférieures est beaucoup plus progressif et intervient plusieurs centaines de kilomètres à l’intérieur. Tels courants de glace ont en particulier de faibles pentes de surface et ressemble en forme et écoulement à des shelfs guidés plutôt qu’à de la glace reposant sur le sol. L’association répétée d’un passage à un glissement rapide avec des caractéristiques distinctes à la base du glacier implique un effet stabilisateur en débit par des émissaires. L’accélération de la glace est soumise à une marche dans le lit et la propagation de ces vitesses élevées à l’intérieur est improbable. La possibilité de surges pour la calotte ne semble pas vraisemblable pour de tels émissaires dans les conditions actuelles. De rapides écoulment de glace dans des fossés sous-glaciaires peuvent aussi assurer une association assez permanente de ces fossés par l’érosion. Ceci est concentré au début des émissaires dans des régions où un débitage conséquent à la base intervient. Ceci peut être le mécanisme d’approfondissement des fiords à leur bord continental et le développement de leurs fronts très abrupts. L’association des régimes d’écoulement et de la topographie peut se révéler utile à l’explication des dépôts des anciennes calottes de glace.

Zusammenfassung

Ein Vergleich zwischen Daten aus Flugzeug-Profilmessungen sowie aus Landsat- und Radar-Echoaufnahmen hat Bildern charakteristische Oberflächenformen, verbunden mit dem Eisfluss in Schilden und Strömen, gezeigt. Der Übergang zwischen beiden ist abrupt und tritt an einer Stufe im Untergrund ein. Er markiert das Einsetzen des Gleitens am Untergrund und hoher Geschwindigkeiten infolge subglazialen Wassers und führt zu spaltendurchsetzten, amphitheatralischen Becken rund um die Ursprünge von Auslassgletschern. Dort ist auch die Zone grösster Treibkraft, unterhalb derer die Werte bei zunehmenden Fliessgeschwindigkeiten schnell abfallen. Dieser abrupte Übergang scheint topographisch bedingt zu sein, da die Temperaturen am Untergrund schon weit landein vor dem Verhaltenswechsel sich auf dem Druckschmelzpunkt befinden. Jedoch zeigen die Eisströme in Marie Byrd Land qualitative Unterschiede zu anderen Auslass-strömen des Eisschildes; der Wechsel zu geringeren Treibkräften ist viel stärker abgestuft und tritt schon einige hundert Kilometer landeinwärts ein. Sie haben ausnehmend geringe Oberflächenneigungen und scheinen in Form und Fluss mehr begrenzten Schelfeisen zu gleichen als aufsitzendem Eis. Die immer wieder anzutreffende Verbindung des Übergangs zu schnellem Gleiten mit einer bestimmten subglazialen Erscheinung hat eine stabilisierende Wirkung auf den Abfluss durch Auslassgletscher zur Folge. Die Beschleunigung des Eises ist mit einer subglazialen Stufe verknüpft; die Ausbreitung hoher Geschwindigkeiten landein von dieser Erscheinung erscheint unwahrscheinlich. Schneller Eisfluss durch subglaziale Gräben dürfte ebenfalls eine relative dauerhafte Verbindung durch Verstärkung der Erscheinung infolge Erosion sicherstellen. Dies konzentriert sich gegen die Ursprünge von Auslassgletschern hin, stromaufwärts von jenem Gebiet, wo signifikante Ablösung vom Untergrund auftritt. Man kann darin einen Mechanismus für die Übertiefung von Fjorden an ihren rückwärtigen Enden und für die Entwicklung sehr steiler Fjordrückwände sehen. Die Verknüpfung des Eisflussverhaltens mit der Geländeform mag zur Aufklärung des Abflusses von früheren Eisschilden von Nutzen sein.

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The Dynamics of Ice-Sheet Outlets

  • N. F. Mcintyre (a1)

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