Buoyancy-driven lacustrine calving, Glaciar Nef, Chilean Patagonia

Glaciar Nef, a 164 km(2) eastern outlet of Hielo Patagonico 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 less than or equal to 6.5 in above lake level, and the ubiquitous presence since 1975 of "tabular" icebergs with surface areas < 0.3 km(2). 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 x 10(6) m(3) a(-1) or 0.2 km(3) 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.