Investigating changes in basal conditions of Variegated Glacier prior to and during its 1982–1983 surge

摘要

Variegated Glacier (Alaska) is known to surge periodically after asufficient amount of cumulative mass balance is reached, but this observationis difficult to link with changes in the basal conditions. Here, using a10-yr dataset, consisting of surface topography and surface velocityobservations along a flow line for 25 dates, we have reconstructed theevolution of the basal conditions prior to and during the 1982–1983 surge. Themodel solves the full-Stokes problem along the central flow line using thefinite element method. For the 25 dates of the dataset, the basal frictionparameter distribution is inferred using the inverse method proposed byArthern and Gudmundsson (2010). This method is here slightly modified by incorporating aregularisation term in the cost function to avoid short wavelength changesin the friction parameter. Our results indicate that dramatic changes in thebasal conditions occurred between 1973 to 1983. Prior to the surge,periodic changes can be observed between winter and summer, with a regularincrease of the sliding from 1973 to 1982. During the surge, the basalfriction decreased dramatically and an area of very low friction moved fromthe upper part of the glacier to its terminus. Using a more complex frictionlaw, these changes in basal sliding are then interpreted in terms of basalwater pressure. Our results support that dramatic changes took place in thesubglacial drainage system of Variegated Glacier, moving from a relativelyefficient drainage system prior to the surge to an inefficient one during thesurge. By reconstructing the water pressure evolution at the base of theglacier it is possible to propose a scenario for the hydrologicalhistory leading to the occurrence of a surge.