Sub-diurnal measurements of alpine glacier dynamics: relating basal and surface motion

The recognition that ice dynamics critically influence the response of glaciers to climate change has led to increased efforts to improve our understanding of the fundamental controls on glacier motion. At most temperate glaciers, basal motion contributes the greatest component of motion (typically 60-80%) to overall flow, and there is now growing evidence that basal motion is significant in many of the small to medium-sized ice masses which characterise the High Arctic. A predominant influence on basal motion consists of surface-driven hydrological impacts on basal drag, in particular subglacial water pressures, which commonly vary significantly over sub-diurnal timescales. However, measurements of basal and surface motion over sub-diurnal timescales are rare, so it is unknown whether basal motion varies significantly over such timescales nor whether sub-diurnal variations in basal motion induce similar variations in glacier surface motion. We report here on sub-diurnal measurements of surface and basal motion made over six diurnal cycles at the temperate Glacier de Ferpecle, Valais, Switzerland, during August 2003. The positions of 11 marker stakes drilled into the ice surface across the lower ablation zone were monitored every two hours using standard total station surveying methods, using LEDs attached to the prism targets to locate the stakes at night. The resultant data were processed to produce surface-velocity data over the lower ablation zone with two-hourly resolution. Basal motion was simultaneously monitored using potentiometers installed in two locations in a basal cavity network beneath the lower glacier. The results reveal that surface velocities throughout the lower ablation zone were greatest (average ~ 13 cm day-1) in the late morning (10:00 - 12:00) and slowest (~ 4 cm day-1) in the late evening (22:00 - 00:00), and showed a clear diurnal oscillation. These patterns indicate that meltwater entering moulins during daylight hours diurnally reduced basal drag and the diurnal oscillation in basal motion was transmitted to the surface. However, our records of basal motion display no diurnal oscillation, instead reflecting constant motion of ~ 1 cm day-1. This may indicate either that diurnal variations in internal deformation are significant at this glacier, or that variations in basal motion were not transmitted to the marginal zones represented by the cavity potentiometers.