Glaciohydraulic Supercooling and Basal Ice Formation at Outlet Glaciers of Vatnajökull and Oraefajokull, Iceland

The hydrology of glaciers is affected by the subglacial environment and in particular overdeepenings that may result in supercooling of subglacial waters and the entrainment of debris into basal ice. Warm-based outlet glaciers of Vatnajökull and Oraefajökull, Iceland, are often characterized by overdeepenings in their terminal regions and up to 100-m long exposures of meters-thick, debris-laden (4 to 35% by vol.) basal zones. During the period 2002 to 2005, we collected bulk samples (300g) of the basal ice, englacial ice, melt water and frazil ice from a number of these outlet glaciers to evaluate probable origins of the basal zone and thus the role of basal topography and hydrology in its generation. Analyses of the 18O and D content show delta 18O and D values for the basal ice generally plot below the meteoric water line and are less negative then values for englacial ice. The difference between the mean delta 18O values of the basal and englacial ice is approximately 2.41 per mil, while the mean delta D values differ by about 11.12 per mil. The delta 18O and delta D values of frazil ice also generally plot below the meteoric water line and overlap those of the basal ice. Mean delta 18O values of frazil ice and meltwater differ by approximately 1.93 per mil, while the mean deuterium values differ by about 13.16 per mil. Little difference exists between the mean delta 18O and D values of meltwater and englacial ice. Tritium measured in basal ice ranges from 1.9 to 6.1 TU and averages 2.9 TU, whereas in englacial ice it is essentially 0. Both the physical properties and isotopic composition of the basal zone ice and debris in the outlet glaciers sampled is consistent with a glaciohydraulic supercooling origin resulting from subglacial discharge out of overdeepenings.