The thermal regime of sub-polar glaciers mapped by multi-frequency radio-echo sounding

Abstract

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 reflected from the top of layers of ice which are at the melting point and contain water. Radar signals of 5–20 MHz, on the other hand, see through both the cold and the temperate ice down to the glacier bed. Radio-echo soundings at these frequencies have been used to investigate the thermal regimes of four polythermal glaciers in Svalbard: Kongsvegen, Uvérsbreen, Midre Lovénbreen and Austre Brøggerbreen. In the ablation area of Kongsvegen, a cold surface layer (50–160 m thick) was underlain by a warm basal layer which is advected from the temperate accumulation area. The surface ablation of this cold layer may be compensated by freezing at its lower cold-temperate interface. This requires that the free water content in the ice at the freezing interface is about 1 % of the volume. The cold surface layer is thicker beneath medial moraines and where cold-based hanging glaciers enter the main ice stream. On Uvérsbreen the thermal regime was similar to that of Kongsvegen. A temperate hole was found in the otherwise cold surface layer of the ablation area in a surface depression between Kongsvegen and Uvérsbreen where meltwater accumulates during the summer (near the subglacial lake Setevatnet, 250 m a.s.l.). Lovénbreen w as frozen to the bed at the snout and along all the mountain slopes but beneath the central part of the glacier a warm basal layer (up to 50 m thick) was fed by temperate ice from two cirques. On Austre Brøggerbreen, a temperate basal layer was not detected by radio-echo soundings but the basal ice was observed to be at the melting point in two boreholes.

A sedimentological and isotopic study of the origin of supraglacial debris bands: Kongsfjorden, Svalbard

“…Since then it has ceased to be active and, in contrast to fast-flowing ($700 m a^1) Kronebreen with which it shares a common tidewater terminus, it has a maximum surface velocity of $8 m a^1 (Hagen and others,1991b). Austre BrÖggerbreen and midre Love¤ nbreen are also slow-moving glaciers, with equilibrium-line surface velocities of only a few metres per year (Bjo« rnsson and others, 1996). Close to their neoglacial maxima around 1890 they had vertical terminal cliffs and were considered by LiestÖl (1988) to be surging, an inference that is disputed by Hambrey and Glasser (2003), who cite the equivocal nature of LiestÖl's (1988) evidence.…”

Surge dynamics in the Nathorstbreen glacier system, Svalbard

“…In Svalbard most glaciers are assumed to be polythermal, and temperate conditions are found to prevail in the upper part of the glaciers (e.g. Hagen and Saetrang, 1991;Björnsson et al, 1996;Ødegård et al, 1997;Melvold and Hagen, 1998;Sund and Eiken, 2004). This also applies to NGS, where radio-echo sounding indicates a polythermal regime on Nathorstbreen (Dowdeswell et al, 1984;Fig.…”

Measurement and simulation of snow and superimposed ice at the Kongsvegen glacier, Svalbard (Spitzbergen)

“…The glacier is known to have surged in 1948 but has been in a quiescent stage since then. Borehole temperature measurements and radio echo soundings reveal that a surface cold layer reaches the bedrock in the ablation area where the glacier is partly frozen to the ground [Björnsson et al, 1996]. The ice is mostly temperate in the higher basins of the glacier, where the formation of SI is an essential factor regarding the energy and mass budget.…”

“…In lack of corresponding measurements, some assumptions had to be introduced with respect to the underlying glacier ice. Thus a 10-m-thick ice layer was prescribed a constant base temperature, which was derived from earlier borehole temperature measurements at the Kongsvegen glacier [Björnsson et al, 1996]. A value of À5°C was adopted at first and the potential effects of the inherent uncertainty on the results will be discussed later.…”