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Determination of the Flow Properties at Dye 3, South Greenland, by Bore-Hole-Tilting Measurements and Perturbation Modelling

  • Dorthe Dahl-Jensen (a1)

Abstract

The ice-flow properties at great depths have a strong impact on the depth–age relationship of the strata in the Greenland ice sheet. Previous attempts to calculate this relationship have used Glen′s flow law with a temperature-dependent flow-law parameter. The data from the bore-hole-tilting at Dye 3, south Greenland, make it possible to calculate the flow-law parameter versus depth. The flow-law parameter shows a big change at the Holocene/Pleistocene transition with a mean flow enhancement factor of 3 for the ice deposited during the last glaciation. This high enhancement factor is believed to be due to high concentrations of dust and other impurities and small ice-crystal size in the Pleistocene ice. The derived flow-law parameter and the up-stream basal topography are used in a two-dimensional, multi-layer, first-order perturbation model to calculate the surface undulations and strain-rates. The results compare favourably with the observations. The results depend strongly on the flow-law parameter profile, which can therefore be estimated in other areas by perturbation-model calculations along flow lines, if the surface and base topographies are known.

Résumé

Les propriétés du fluage de la glace à grande profondeur ont un fort impact sur la relation profondeur–âge pour la calotte de glace du Groenland. Des essais antérieurs de calcul de cette relation ont untilisé une loi de fluage de Glen avec un paramétre de fluage dépendant de la température. Les données issues de l′inclinométrie à Dye 3, Groenland du sud, rendent possible un calcul du paramètre de la loi de fluage en fonction de la profondeur. Ce paramètre montre une importante variation lors de la transition Holocène/Pléistocène en triplant sa valeur pour la glace formée au cours de la dernière glaciation. Cette forte augmentation est interprêtée comme étant la conséquence d′importantes concentrations de poussières et autres impuretés, ainsi que de la faible taille des cristaux de glace au cours du Pléistocène. La paramètre de la loi d′écoulement ainsi déterminé et la topographie du lit sont utilisés dans un modèle de perturbation du premier ordre à deux dimensions, multi couches pour calculer les ondulations de surface et les taux de déformation. Les résultats sont en bon accord avec les observations. Ils sont très sensibles aux variations due profil du paramètre de la loi de déformation, qui peut cependant être estimé dans d′autres zones par le modèle de perturbations le long des lignes d′écoulement lorsque les topographies en surface et sur le lit sont connues.

Zusammenfassung

Die Fliessverhältnisse des Eises in grosser Tiefe haben starken Einfluss auf die Beziehung zwischen Tiefe und Alter der Eisschichten im grönländischen Inlandeis. Frühere Versuche zur Berechnung dieser Beziehung benutzten das Glen′sche Fliessgesetz mit einem temperaturabängigen Parameter. Die Daten der Bohrlochneigungen bei Dye 3 in Südgrönland ermöglichen eine Berechnung des Fliessgesetzparameters in der Tiefe. Dieser weist eine grosse Änderung am Übergang zwischen Holozän und Pleistozän auf, wobei der mittlere Fluss für das während der letzten Vereisung abgelagerte Eis um den Faktor 3 vergrössert erscheint. Ein derartiger Anstieg wird auf die hohen Konzentrationen von Staub und anderen Verunreinigungen sowie auf die geringe Grösse der Eiskristalle zurückgeführt. Der gewonnene Fliessgesetzparameter und die Topographie des Untergrundes werden in einem zweidimensionalen Vielschichten-Störungsmodell erster Ordnung zur Berechnung der Undulationen und Verformungsraten an der Oberfläche herangezogen. Die Ergebnisse stimmen mit den Beobachtungen überein; sie hängen stark vom Profil des Fliessgesetzparameters ab, das deshalb in anderen Gebieten aus der Berechnung des Störungsmodells entlang von Stromlinien abgeschätzt werden kann, wenn die Gestalt der Oberfläche und des Untergrundes bekannt ist.

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References

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Determination of the Flow Properties at Dye 3, South Greenland, by Bore-Hole-Tilting Measurements and Perturbation Modelling

  • Dorthe Dahl-Jensen (a1)

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