Surface depressions (Lacunas) on Bering Glacier, Alaska: a product of downwasting through differential ablation

Abstract. Bering Glacier lacunas are steep-sided, flat-floored hollows ranging in size from 40 to 60 m wide, 80 to 120 m long, and 35 to 50 m in deep. They are confined within a band of clean ice (1.5 km wide, 5 km long) paralleling the eastern margin of the Bering piedmont lobe. The 1993–1995 surge displaced the lacuna band several kilometers onto the foreland. Specifically significant is the formation of a new band of lacunas 5–6 years later in the same location occupied by the displaced band prior to the surge.

Conditions essential to lacuna formation were initiated during the surge as overriding ice was thrust into position across the trend of a subglacial trough, leading to stagnation of ice within the trough. Stagnation combined with saturation at depth altered ice texture and density. Exposure of this ice through normal ablation led to areas of differential ablation and the formation of lacunas.