THE THERMAL REGIME OF SUBPOLAR GLACIERS MAPPED BY MULTIFREQUENCY RADIO-ECHO SOUNDING

Radio-echo soundings provide an effective tool for mapping the thermal regimes of polythermal glaciers on a regional scale. Radar signals of 320-370 MHz penetrate ice at sub-freezing temperatures but are reflec ted from the top of layers of ice which are at the melting point and c ontain water. Radar signals of 5-20 MHz, on the other hand, see throug h both the cold and the temperate ice down to the glacier bed. Radio-e cho soundings at these frequencies have been used to investigate the t hermal regimes of four polythermal glaciers in Svalbard: Kongsvegen, U versbreen, Midre Lovenbreen and Austre Broggerbreen. In the ablation a rea of Kongsvegen, a cold surface layer (50-160 m thick) was underlain by a warm basal layer which is advected from the temperate accumulati on area. The surface ablation of this cold layer may be compensated by freezing at its lower cold-temperate interface. This requires that th e free water content in the ice at the freezing interface is about 1% of the volume. The cold surface layer is thicker beneath medial morain es and where cold-based hanging glaciers enter the main ice stream. On Uversbreen the thermal regime was similar to that of Kongsvegen. A te mperate hole was found in the otherwise cold surface layer of the abla tion area in a surface depression between Kongsvegen and Uversbreen wh ere meltwater accumulates during the summer (near the subglacial lake Setevatnet, 250 m a.s.l.). Lovenbreen was frozen to the bed at the sno ut and along all the mountain slopes but beneath the central part of t he glacier a warm basal layer (up to 50 m thick) was fed by temperate ice from two cirques. On Austre Broggerbreen, a temperate basal layer was not detected by radio-echo soundings but the basal ice was observe d to be at the melting point in two boreholes.