Buoyancy-driven lacustrine calving, Glaciar Nef, Chilean Patagonia


Glaciar Nef, a 164 km2 eastern outlet of Hielo Patago¨nico Norte (the northern Patagonia icefield), terminates in aproglacial lake that has formed in conjunction with 20th-century glacier retreat.The terminus is inferred tobe 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» ice- bergs. 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» ice- bergs 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 2326106 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 temper- ate glaciers calving in fresh water, but is nevertheless almost an order of magnitude less than calving rates at both grounded and floating tidewater glaciers.

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