Buoyancy-driven lacustrine calving, Glaciar Nef, Chilean Patagonia

Abstract

Glaciar Nef, a 164 km2 eastern outlet of Hielo Patagónico Norte (the northern Patagonia icefield), terminates in a proglacial lake that has formed in conjunction with 20th-century glacier retreat. The terminus is inferred to be transiently afloat. A hinge-calving mechanism is proposed in which buoyant forces impose a torque on the glacier tongue, resulting in the release of coherent sections of the glacier tongue as “tabular” icebergs. A simple model shows how torque and tensile stress reach a maximum at the up-glacier limit of the buoyant zone, and that glacier thinning causes this point to migrate up-glacier. Empirical evidence supporting this model includes elevated thermo-erosional notches ≤6.5 m above lake level, and the ubiquitous presence since 1975 of “tabular” icebergs with surface areas ≤0.3 km2. Flow speeds of 1.2–1.3 m d−1 were measured near the terminus in February 1998. Extrapolations from these short-term data yield a calving rate of 785–835 m a−1 and a calving flux of 232 × 106 m3 a−1 or 0.2 km3 a−1. The calculated mean water depth at the terminus is 190 m. This calving rate is higher than at grounded temperate glaciers calving in fresh water, but is nevertheless almost an order of magnitude less than calving rates at both grounded and floating tidewater glaciers.

Seasonal Variations in Ice-Front Position Controlled by Frontal Ablation at Glaciar Perito Moreno, the Southern Patagonia Icefield

“…While the ice-front position and the flow speed are relatively easy to measure using satellite data, calving and melting in water are difficult to observe. An approach commonly taken in previous studies is to define frontal ablation rate,ȧ, as the sum ofċ andṁ, and compute the frontal ablation as a residual of dL/dt and u m (e.g., Warren et al, 2001;Haresign and Warren, 2005;Luckman et al, 2015;McNabb et al, 2015). Recent studies on tidewater glaciers have shown that calving (ċ) and melting under waterline (ṁ) vary in time under the influence of atmospheric and fjord water conditions (e.g., Benn et al, 2007;Bartholomaus et al, 2013;Luckman et al, 2015;Pȩtlicki et al, 2015).…”

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.…”

Hazard assessment investigations due to recent changes in Triftgletscher, Bernese Alps, Switzerland

“…First because such glaciers differ from tidewater glaciers because of lower velocities and calving speed. Second because they can sustain floating tongues for long periods prior to the onset of calving phases (Warren et al, 2001;Boyce et al, 2007). In this study, we propose to apply a flotation model to explain the retreat of the Triftgletscher terminus.…”