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 significa ntly to summer ice growth. An under-ice pond, located below a surface melt puddle on a multiyear floe, was studied, comprising detailed anal ysis of microstructure, salinity, and deltaO-18 of ice cores. Undernea th 1.6 m of solid ice, a meltwater lens 0.31 m thick (salinity, 1.5 pa rts per thousand) was sealed by a bottom pond ice cover (0.20 m thick) , composed of intergrown ice platelets and columnar crystals, with sal inity and deltaO-18 low throughout (1.0 parts per thousand and -7.8 pa rts 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 thousa nd at the bottom, paralleled by a drop in deltaO-18). Contrasting with desalination mechanisms previously described, this is shown to be a r esult of diffusional desalination. Analysis of Arctic multiyear ice co res suggests that under-ice ponds and diffusional desalination may be common. They modify the properties of multiyear sea ice, affect its co lonization by ice biota, and may result in retainment of dissolved and particulate material within the ice cover. While increasing ice thick ness and smoothing ice topography at a particular site, under-ice pond s are not likely to increase the net amount of ice grown in a particul ar region.