SEDIMENT EXPORT BY ICE RAFTING FROM A COASTAL POLYNYA, ARCTIC ALASKA,USA

Strong offshore winds in early 1989 produced a shore polynya that reac hed along the entire north coast of Alaska and eastward beyond the mou th of the Mackenzie River in Canada. From January through April, this open water periodically exposed the shelf to sediment entrainment by s uspension freezing. This process requires turbulence and supercooled w ater, which results in the formation of frazil and anchor ice. The res ulting granular, sediment-laden ice was observed to extend over 100 km seaward of the outer continental shelf after having been advected off shore. It was sampled to determine sediment type and to quantify the p article load. The particle size was mainly silt and clay, with local a dmixtures of as much as 27% sand and coarser clasts. Melted ice sample s contained from 31 to nearly 600 mg L-1 of sediment. Combining these data with over 400 km of shipboard and aerial observations, photograph s, and computer analysis of a summer Landsat image, we estimated the s ediment load per unit area of sea ice. Seaward of the shelf, in region s of dense pack ice, a conservatively estimated sediment load was over 289 t km-2. Using a westward summer drift rate of 3 cm s-1, the sedim ent transport through a 1-km-long north-south segment is 67,418 t duri ng 3 mo. In terms of regional sediment dynamics (littoral transport es timated at 10,000 t during the same period) and sediment budget (conti nental denudation estimated at 10 t k-m2 during the same period), this number is very significant. Benthic microfossils indicate that bottom sediment incorporated in the ice came from water depths ranging from the inner neritic seaward to 50 m. The large load of shelf-derived sed iment observed seaward of the continental shelf indicates that ice ent rainment and transport cause shelf erosion. Nothing is known about sed iment release over the Arctic Ocean Basin from these pulses of dirty i ce that are periodically introduced into the Transpolar Drift.