GLACIOHYDRAULIC SUPERCOOLING AND BASAL ICE IN TEMPERATE GLACIERS OF ICELAND

Glaciohydraulic supercooling has been demonstrated within the subglacial environment of Matanuska Glacier in Alaska, and hypothesized to be the predominant mechanism entraining debris in the glacier’s basal zone. Although supercooling has been recognized in subglacial conduits at other large glaciers such as the Malaspina and Bering glaciers in Alaska, its occurrence in the presence of a debris-rich basal zone has not been demonstrated anywhere except Matanuska Glacier until now. We report here on recent observations documenting supercooling within subglacial discharges of warm temperate glaciers in Iceland that have debris-rich basal ice similar in characteristic to that of the Matanuska Glacier.

Summer observations, when air temperatures are always above freezing, at Skeidararjokull, Skaftafellsjokull and Kviarjokull - outlet glaciers from Vatnajokull, Iceland's largest Ice cap - show abundant ice growth in and around discharge vents of subglacial waters, producing free frazil crystals, frazil aggregates and actively growing anchor ice terraces. These features demonstrate that glaciohydraulic supercooling is occurring. Winter observations of up-thrusted segments along the ice margin also reveal porous masses of debris-rich secondary ice developed around these vents, as well as 0.5 to 2 m thick sequences of stratified, debris-rich basal ice that is extremely similar to that observed at the Matanuska Glacier.

The sedimentary characteristics of Icelandic stratified basal ice sequences vary only in texture from those at Matanuska, while the frazil ice features are virtually identical to those at Matanuska, Bering and Malaspina glacier. These observations are consistent with theory that supercooling and basal-ice accretion occur wherever sufficient basal water flows out of a sufficiently steep overdeepening (adverse bed slope >1.2-1.7 times the magnitude of the surface slope). We therefore also infer that supercooling, ice growth, and debris entrainment similarly occurred along appropriately overdeepened margins of the former Laurentide and Scandinavian ice sheets, contributing to formation of the sedimentary deposits of those ice sheets. Our initial observations are the starting point for a more quantitative analysis. We anticipate that analysis of dD, d180, and 3H in basal ice of Icelandic glaciers will support our hypothesis that supercooling is responsible for its origin.