Abstract
Abstract. Cold glacier beds, i.e., where the ice is frozen to its base, are widespread in polar regions. Common theories state that stable permafrost should exist under glacier beds on shorter timescales, varying from years to decades. Presently, only a few direct measurements of both subglacial permafrost and the processes influencing its thermal regime exist. Here, we present subglacial permafrost and active layer measurements obtained from within the basal drainage systems of two cold-based glaciers on Svalbard during the summer melt season. Temperature observations were obtained from subglacial sediment that was accessed through the drainage systems of the two glaciers in the previous winters. The temperature records cover the periods from spring to autumn in 2016 and 2019 at the glaciers Larsbreen and Tellbreen in central Svalbard. The ground temperature below Larsbreen indicates colder ground conditions, whereas the temperatures of the Tellbreen drainage system show considerably warmer conditions, close to the freezing point. We suggest the latter is due to the presence of liquid water all year round inside the Tellbreen drainage system. Both drainage systems investigated show an increase in subglacial sediment temperatures after the disappearance of snow bridges and the subsequent connection to surface meltwater supply at the start of the summer melt season. Temperature records show influence of sudden summer water supply events, when heavy melt and rain left their signatures on the thermal regime and the erosion of the glacier bed. Observed vertical erosion can reach up to 0.9 m d−1 at the base of basal drainage channels during summer. We also show that the thermal regime under the subglacial drainage systems is not stable during summer but experiences several freeze–thaw cycles driven by weather events. Our results show the direct importance of heavy melt events and rain on the thermal regime of subglacial permafrost and the erosion of the glacier bed in the vicinity of subglacial drainage channels. Increased precipitation and surface melt, as expected for future climate, will therefore likely lead to increased degradation of subglacial permafrost, as well as higher subglacial erosion of available sediment around the preferential hydrological paths. This in turn might have significant impacts on proglacial and fjord ecosystems due to increased sediment and nutrient input.
Highlights
Cold-based glaciers and polythermal glaciers with coldbased margins are common and widespread in the polar regions
During wintertime the drainage system is decoupled from the atmosphere due to snow infilling of crevasses and channels, closing off the entrances and blocking air and water flow from the surface, which would else allow heat exchange with the atmosphere. During this phase the thermal regime of the subglacial drainage system is controlled by the ice and ground temperatures and the heat flux therein
The fact that the thermal regime of the subglacial drainage system and its surrounding borders are coupled to the surface air temperature leads to short-lived surface events playing a major role for the thermal regime of the subglacial drainage system
Summary
Cold-based glaciers and polythermal glaciers with coldbased margins are common and widespread in the polar regions. Interactions between glaciers and permafrost have, received only little attention in scientific literature, and subglacial permafrost is not included into the estimations of permafrost extent. The reason is that subglacial permafrost was long believed to either be not persistent or to shutdown basal processes, which have relevance for ice dynamics and geomorphological processes (e.g., Etzelmüller and Hagen, 2005; Haeberli, 2005; Harris and Murton, 2005; Waller et al, 2012). Permafrost is generally thought to occur as thin layers in the periphery of cold-based glaciers.
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