THE ENIGMATIC OCCURRENCE OF GLACIAL LACUNAS

Enigmatic elliptical depressions known as lacunas, attaining lengths in excess of 100 meters and tens of meters in relief, are generally uncommon as supraglacial features, yet they always appear in clusters, as on the Lacuna Glacier in the Alaska Range. Probing their significance motivates this investigation on the surging Bering Glacier, Alaska, where lacunas are confined within a broad, 300 – 400 meter wide band of ice that parallels the eastern ice front, within a kilometer of its piedmont lobe terminus. Although this lacuna band was displaced 1.0 – 1.5 km during the 1993-95 surge, it began to reform within different ice, but at the same location as before within 4 to 6 years of downwasting and retreat. Testing for potential formative parameters, we mapped ice structures associated with the lacunas, including fractures (ablated crevasses), englacial foliation, and thrust faults, none of which are interrupted by passing through a lacuna. Lacunas appear aligned semi-parallel to the strike of fractures, and generally orthogonal to foliation and thrusts. Drainageways formed late in the surge are aligned with lacunas, some flowing through supraglacial ponds. However, the affinity of re-formed lacunas in the same location as before the surge suggests an origin unrelated to inherent ice properties. A measured downwasting rate of 10 m annually would place ice at the modern surface in a near-basal position, thus susceptible to physical alteration by passing englacial meltwater. This and other multiple working hypotheses are receiving further attention.