Dynamics of glacier calving at the ungrounded margin of Helheim Glacier, southeast Greenland

Abstract

During summer 2013 we installed a network of 19 GPS nodes at the ungrounded margin of Helheim Glacier in southeast Greenland together with three cameras to study iceberg calving mechanisms. The network collected data at rates up to every 7 s and was designed to be robust to loss of nodes as the glacier calved. Data collection covered 55 days, and many nodes survived in locations right at the glacier front to the time of iceberg calving. The observations included a number of significant calving events, and as a consequence the glacier retreated ~1.5 km. The data provide real‐time, high‐frequency observations in unprecedented proximity to the calving front. The glacier calved by a process of buoyancy‐force‐induced crevassing in which the ice downglacier of flexion zones rotates upward because it is out of buoyant equilibrium. Calving then occurs back to the flexion zone. This calving process provides a compelling and complete explanation for the data. Tracking of oblique camera images allows identification and characterisation of the flexion zones and their propagation downglacier. Interpretation of the GPS data and camera data in combination allows us to place constraints on the height of the basal cavity that forms beneath the rotating ice downglacier of the flexion zone before calving. The flexion zones are probably formed by the exploitation of basal crevasses, and theoretical considerations suggest that their propagation is strongly enhanced when the glacier base is deeper than buoyant equilibrium. Thus, this calving mechanism is likely to dominate whenever such geometry occurs and is of increasing importance in Greenland.

Glacier Calving in Greenland

“…Collisions between capsizing bergs and the glacier terminus are the likely source of seismic signals associated with large calving events [65][66][67]. Detailed observations by Murray et al [68] show that seismic energy release is associated with distinctive cycles of elastic frontal compression and re-extension of the remaining glacier front in response to the release and capsize of large bergs.…”

“…[30,59,61,68,81,85]). Exciting new opportunities have been created by the advent of remotely controlled or autonomous vehicles [86,87].…”

Calving Behavior at Rink Isbræ, West Greenland, from Time-Lapse Photos

“…For buoyant glacier tongues, the ice may be out of hydrostatic equilibrium at the grounding line, resulting in upward-di- rected torque forces (Warren et al, 2001;Boyce et al, 2007;Murray et al, 2015). Reduction in the ice surface elevation or advance of the terminus into deeper water can result in an increase of "superbuoyancy," which may trigger fracture propagation and calving (Benn et al, 2007b;Nick et al, 2009).…”

“…Several processes can drive the terminus out of buoyant equilibrium, including rapid thinning due to surface melt (Warren et al, 2001), rapid water-level rise (Boyce et al, 2007), enhanced ice flow forcing the terminus to advance into deeper water, and dynamic thinning (Murray et al, 2015). The first two processes can be ruled out for Rink Isbrae as they apply to warm environments with high surface melt rates, and lacustrine systems where small short-term perturbations in lake level lead to rapid increases in water depth.…”

“…Variations in buoyancy conditions at the terminus further alter the force balance and can push the ice margin to flotation, promoting calving through the propagation of basal crevasses. Glacier termini can be forced out of buoyant equilibrium by changes in ice thickness and water depth, which are sensitive to a number of processes such as rapid surface melt or water-level rise, rapid ice motion, and dynamic thinning (Benn et al, 2007b;Nick et al, 2010;Murray et al, 2015).…”