HYDROLOGIC MONITORING OF SUPERCOOLED DISCHARGE FROM ICELANDIC GLACIERS: HYDRODYNAMIC AND SEDIMENTARY SIGNIFICANCE

Glaciers and ice sheets entrain huge quantities of sediment. Knowledge of glacial sediment entrainment processes is crucial to understanding glacier movement, sediment flux and the sedimentary record. Glaciohydraulic supercooling has been recognised as an active englacial sediment entrainment mechanism at glaciers in Alaska. Recent work also suggests that supercooling is an active glaciohydraulic process at several temperate Icelandic glaciers. Glaciohydraulic supercooling occurs when the rate of basal meltwater ascent outstrips the rate at which meltwater temperature can rise by heat convection due to latent heat of fusion as frictional ice melt occurs. Frazil ice propagates during melwater depressurisation, trapping sediment as it accretes.

We present measurements of glaciohydraulic supercooling and evidence of associated basal ice formation from Icelandic glaciers Skeiðarárjökull, Skaftafellsjökull and Svinafellsjökull. Fieldwork undertaken in March and July 2002 reveals that large terraces of frazil ice are forming around turbid subglacial artesian vents at all three glaciers. Records of supercooled meltwater discharge from the Súla and Skeiðará outlets of Skeiðarárjökull demonstrate that discharge is deeply supercooled. Radio-echo sounding profiles from Skaftafellsjökull confirm that the western flank sits in an overdeepening close to the margin, corroborating measurements of supercooled meltwater exiting Skaftafellsjökull. We also communicate measurements of supercooling during a small flood in Skeiðará in March 2002. Preliminary analysis of ice samples shows that bomb-produced tritium is found only in basal ice, confirming its relative youth. Initial co-isotopic analysis indicates that basal ice contains significant traces of meteoric water, suggesting that basal ice is not related to regelation of older glacier ice associated with isotopic fractionation within a closed system. This information is vital for i) assessing the pervasiveness of supercooling as a hydraulic process and a sediment entrainment mechanism in modern glacial environments, and ii) for decoding the landform and sediment record of past glaciers and ice sheets.