SEASONAL FROST MOUNDS OF ARCTIC BEACHES, TUKTOYAKTUK PENINSULA, NORTHWEST-TERRITORIES

Closed-system seasonal frost mounds were observed from 1991 to 1993 on sandy spits and a barrier island of the Tuktoyaktuk Peninsula, in the Northwest Territories. The frost mounds have a circular and convex sh ape. Their diameter ranges from 5 to 15 m and their height from 15 to 25 cm. The core of the mounds consists of a lense of intrusive ice of similar diameter than that of the mound. The uppermost surface of the ice lense is convex upward while its base is horizontal. The maximum t hickness of an ice lense is equal to the maximum elevation of the fros t mound, ie. ranging from 15 to 25 cm. Frost mounds develop in a depre ssion in the backshore zone, located behind the storm berm. At the end of summer, storm surges induce the saturation of the sandy coastal ac cumulation landforms with sea water. Freeze-up then results in increas ed hydrostatic pressures of sea water trapped in the backshore depress ion, leading to the development of closed-system ice lenses. Frost mou nds form in a single winter and disappear completely during the follow ing summer. The density of frost mounds at the surface of the coastal accumulation landforms appears to be related to the frequency of storm surges between mid-August and mid-September and to the freeze-up cond itions.