Melting-refreezing at the Glacier Sole and the Isotopic Composition of the Ice

A model for the isotopic composition in .50 and .5 180 of ice formed by refreezing at the glacier sole is developed. This model predicts relatively well the distribution of points representing samples from basal layers of an Arctic a nd an Alpine glac ier on a .50.5 180 diagram . The frozen fraction which is the part of the liquid that refreezes can be determined for each basal ice layer. This may have implications on the study of the ice-water system a t the icerock interface. R ESUME. Fusion et regel it la base d'lIll glacier et composition isotopique de la g lace. Un modele pour I'etude de la composition isotopique en .50 et en .5 180 de la glace formee par regel a la base du glacier est develop pe dans cet article. Ce modele predit relativement bien la distribution des points represen ta nt des echantillons de glace basale d 'un glacier pola ire et d'un glacier a lpin sur un diagramme .50 _.5 180. La fraction gelee qui est la part d 'eau qui regeie pour constituer une couche de glace basale peut etre determinee dans c haque cas. Ceci est susceptible d'avoir des implications sur les etudes du comportement de la phase liquide a I'interface glacierrocher. ZUSAMMENFASSUNG. Regelation am Gletscherbell und die Isotopen-ZusammensetzlIllg des Eises. Es wird ein Modell fUr die Zusammensetzung von Eis, das durch Regela tion am Gletscherbett entstanden ist, a us den Isotopen .50 und .5 180 entwickell. Oieses Modell beschreibt verhaltnismassig gut die Vertei lung von Stellen mit Proben aus grundnahen Schichten arktischer oder alpiner Gletscher auf einem .50 .5 180-Oiagramm. Oer Anteil an wiedergefrorener Flussigkeit kann fUr jede Eisschicht a m Untergrund bestimmt werden. Dies kann fUr das Studium des EisWasser-Systems am Gletscherbett von Bedeutung sein. INTRODUCTION Regelation phenomena play a great role in theories explaining glacier basal sliding (Kamb and LaChapelle, 1964 ; Weertman, 1964; Kamb, 1970; Nye, 1970; Lliboutry, 1976). Melting and refreezing at the glacier sole is thus an important process in glacier studies. Such phase changes can result from pressure fluctuations in a glacier which is at the pressure-melting point but no substantial ice formation can be produced by such a process. Accretion by refreezing is possible because of the contribution of cold air even in the central part of the glacier or because of the presence of cold patches (Robin, 1976) or permafrost in the down-glacier direction (Weertman,196l). Water at the glacier bed can be in the form of a film which primarily accommodates the local transport of melt water associated with regelation sliding (Hallet, 1979). Through-flowing water appears to drain through a different hydraulic system, distinct channels at the icerock interface (Rothlisberger, 1972; Shreve, 1972). These waters produce a basal ice layer when they refreeze. This refreezing can be incomplete or total. It is of interest to know which situation occurs. The aim of this paper is to provide a framework based on isotopic study for a better understanding of the meltingrefreezing process at the interface. A model of the isotopic composition of ice developed by such a mechanism is described and tested against field evidence. The basal ice layers from an Arctic and an Alpine glacier were sampled in that context. SAMPLING PROCEDURE AND SAMPLE ANALYSIS Sampling procedure began with the removal of the upper 20 cm of ice exposed to the air. An