Basal conditions beneath a soft-bedded polythermal surge-type glacier: Bakaninbreen, Svalbard

Bakaninbreen, Svalbard is a subpolar glacier that began a prolonged surge in 1985. A steep surge-front propagated downglacier at rates of up to 470 cm day−1 over the period 1985–1995. During 1994 and 1995 the glacier was densely instrumented with borehole sensors to measure mechanical, hydrological and thermal conditions in the region of the surge-front; these sensors included ploughmeters, sliding sensors, thermistors and pressure transducers. Most instruments were run over the period 1994–2000. Samples of the bed were also retrieved. Together with penetration testing, these show the glacier to be underlain by subglacial sediments at least 0.2 m in thickness. These sediments are a mixture of glaciomarine muds and glacial till. Results from the borehole sensors allow estimation of the properties of basal sediments, including the yield strength, which is an important parameter for models of glaciers overlying soft-beds. The mean yield strength upglacier of the surge front is 82 kPa. Downglacier of the surge-front the mean yield strength is estimated to be more than 241 kPa. Our interpretation is that the sediments downglacier of the surge-front are frozen. Results from thermistors, ground-penetrating radar and seismic surveys support this interpretation. Thus, the surge-front has halted coincident with a thermal boundary between cold and warm bed. We suggest two possible interpretations: (i) the cold bed has caused the cessation of the glacier surge or (ii) the surge-front has been propagating coincident with this thermal boundary throughout the surge. We support the latter and propose a soft-bed, thermally regulated surge mechanism.