FREEZING OF THE ACTIVE LAYER ON THE COASTAL-PLAIN OF THE ALASKAN ARCTIC

Upward freezing from the bottom of the active layer started at the end of August or beginning of September when the ground surface temperatu re was above 0 degrees C. About one-third of the active layer froze up wards at the West Dock, Deadhorse and Franklin Bluffs sites. Temperatu res were warmer than -1 degrees C at the maximum depth of the active l ayer 20 days after the freeze-up date with the temperature maximum nea r the base of the active layer. This behaviour was caused by unfrozen water in the active layer and near-surface permafrost. A numerical mod el was used to determine the most appropriate unfrozen water content c urves at each site. Modelling also indicates that a layer with increas ed unfrozen water content existed during cooling of the active layer f ollowing freeze-up and during warming prior to thawing. At the Deadhor se site, this layer continued from the ground surface to a depth of 0. 3 m in 1987 and 0.36 m in 1991. For Franklin Bluffs in 1987, for West Dock in 1988, and for Barrow in 1993 the thickness of this layer was 0 .1 m and it was located between 0.2 and 0.3 m at Franklin Bluffs, betw een 0.15 and 0.25 m at West Dock, and between 0.21 and 0.31 m at Barro w. The unfrozen water contents for these lavers were relatively large at low temperatures. Analytical calculations of the apparent thermal d iffusivity showed that its observed variations with temperature were d ue to the latent heat effects of the unfrozen water. Microbial activit y and trace gas evolution during the winter may be influenced by the p resence of the unfrozen water. (C) 1997 by John Wiley & Sons, Ltd.