STRUCTURE OF UNDER-ICE MELT PONDS IN THE CENTRAL ARCTIC AND THEIR EFFECT ON THE SEA-ICE COVER

Freezing of meltwater ponds below Arctic sea ice contributes significantly to summer ice growth. An under-ice pond, located below a surface melt puddle on a multiyear floe, was studied, comprising detailed analysis of microstructure, salinity, and deltaO-18 of ice cores. Underneath 1.6 m of solid ice, a meltwater lens 0.31 m thick (salinity, 1.5 parts per thousand) was sealed by a bottom pond ice cover (0.20 m thick), composed of intergrown ice platelets and columnar crystals, with salinity and deltaO-18 low throughout (1.0 parts per thousand and -7.8 parts per thousand). The pond was overlain by low-salinity ice (avg <0.7 parts per thousand with a linear decrease toward 0.1 parts per thousand at the bottom, paralleled by a drop in deltaO-18). Contrasting with desalination mechanisms previously described, this is shown to be a result of diffusional desalination. Analysis of Arctic multiyear ice cores suggests that under-ice ponds and diffusional desalination may be common. They modify the properties of multiyear sea ice, affect its colonization by ice biota, and may result in retainment of dissolved and particulate material within the ice cover. While increasing ice thickness and smoothing ice topography at a particular site, under-ice ponds are not likely to increase the net amount of ice grown in a particular region.