Dinkum Sands was mapped in 1949 as a small island, one link in a 95-km-long chain of barrier islands near Prudhoe Bay, Alaska. Questions about its status as an Arctic island and the submerged land ownership led to a Federal/State joint monitoring program using topographic surveys, tide gauges, and other approaches. On the basis of the results the Supreme Court concluded that Dinkum Sands is a shoal rather than an island. The shoal attracts the yearly formation of pressure ridges at least as high as 15 m and undergoes drastic changes in location, shape, and elevation. Pressure ridges form from 10-cm-thick new ice when shifting and compression result in crumbling and the introduction of ice slabs into the sandy gravel shoal. This leads to a seasonal increase in shoal volume and height. With summer warming and sea-ice melting, part of the ice in the crest of the shoal (∼50%) melts and its height accordingly drops to below sea level. This lowering requires neither wave action nor lateral sediment transport. The original disappearance of the island in the 1950s, however, probably was not caused by diminished sediment supply from rivers or from coastal erosion, but by diminished local sediment supply through ice pile-ups or by increased erosion from wave attack. Similar changes in sediment volume as those observed in Dinkum Sands probably also occur on fully submerged shoals ranging to water depths of 20 or more meters, which are also marked by seasonal pressure ridge formation.
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