Evidence for platelet ice accretion in Arctic sea ice development

Moderate quantities of platelet ice are reported in ice cores obtained from floes in the central and western Beaufort Sea, Arctic Ocean. The crystal structure characteristics include a fabric of long, thin, ragged‐edged crystals with randomly oriented c axes. The cellular substructure that is characteristic of congelation ice is not evident in individual platelet ice crystals, but they do occur together with congelation ice in layers ranging from 0.05 to 0.6 m thick. Platelet ice crystals are observed primarily in second‐year and multiyear floes. The structural characteristics of Arctic platelet ice are similar to those of Antarctic platelet ice. The platelet ice layers have a minimum δ 18 O value of −5.3‰, and in some cases the platelet ice layers have more negative δ 18 O values than the ice layers above and below them. The salinity of platelet ice layers is not significantly different from other ice layers. A number of platelet ice growth mechanisms and sources are discussed. Processes associated with the Ellesmere ice shelves, nearshore processes such as anchor ice growth in shallow waters off the north coast of Alaska, and ice growth in freshwater impounded behind grounded ice dams might be locally important but are unlikely to contribute significant quantities of platelet ice to pack ice floes. Two processes probably account for the presence of platelet ice. The first is growth in underice melt ponds which form in inverted depressions at the base of floes when meltwater floes off the ice surface. The second is the operation of ice pumps in which ice that is melted off the deeper parts of floes and ridges is deposited as platelet ice at a higher level. In either case, initially loose accumulations of platelet crystals are entrained in the floes by the growth of congelation ice. The widespread occurrence of platelet ice in the central and western Beaufort Sea suggests that ice growth in both underice melt ponds and associated with the operation of ice pumps might be more common than was previously realized.