WHAT LACUNAS TELL US ABOUT DEAD-ICE DOWNWASTING

Lacunas are bowl-shaped hollows, typically 40 to 120 m in diameter and 35 to 50 m deep found on passive and stagnant ice at the terminus of most temperate glaciers where debris mantled ice is saturated at depth. Contrary to this, lacunas are also found confined within a band of clean ice (1.5 km wide, 5 km long) paralleling the eastern margin of the Bering Piedmont Glacier, Alaska. Of specific significance here is that after being displaced several kilometers by a 1993-95 surge event, the lacunas re-appeared 5-6 years later in exactly the same original location. During this interval normal downwasting removes 40-50 m of surge-thickened ice, thus exposing ice previously at depth.

Common to both occurrences is the englacial saturation of passive and stagnant ice below the glacier water table. It is proposed that saturation weakens the ice fabric, an occurrence prelude by active intracrystalline flow elsewhere. It is within pockets of this compromised ice that lacunas ultimately form through accelerated ablation, irrespective of whether surface debris is present or absent.

Bering lacunas demonstrate the significance of englacial saturation in causing accelerated, selective, differential ablation resulting in lacuna formation and the superfluous aspect of surface debris in the development of lacunas.