Thawing 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 temperature was above 0°C. About one-third of the active layer froze upwards at the West Dock, Deadhorse and Franklin Bluffs sites. Temperatures were warmer than −1°C at the maximum depth of the active layer 20 days after the freeze-up date with the temperature maximum near the base of the active layer. This behaviour was caused by unfrozen water in the active layer and near-surface permafrost. A numerical model was used to determine the most appropriate unfrozen water content curves at each site. Modelling also indicates that a layer with increased unfrozen water content existed during cooling of the active layer following freeze-up and during warming prior to thawing. At the Deadhorse 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, between 0.15 and 0.25 m at West Dock, and between 0.21 and 0.31 m at Barrow. The unfrozen water contents for these layers were relatively large at low temperatures. Analytical calculations of the apparent thermal diffusivity showed that its observed variations with temperature were due to the latent heat effects of the unfrozen water. Microbial activity and trace gas evolution during the winter may be influenced by the presence of the unfrozen water. © 1997 John Wiley & Sons, Ltd. Le gel vers le haut depuis la base de la couche active a commence a la fin d'aout ou au debut de septembre quand la temperature de la surface du sol etait superieure a 0°C. Environ un tiers de la couche active a gele depuis la base dans les sites de West Dock, Deadhorse et Franklin Bluffs. Des temperatures etaient plus elevees que −1°C a la profoundeur maximum de la couche active 20 jours apres la date du regel, et la temperature maximum se trouvait pres de la base de la couche active. Ce comportement resulte de la presence d'eau non gelee dans la couche active et dans le pergelisol proche. Un modele numerique a ete utilise pour determiner les courbes les plus appropriees du contenu en eau non gelee en chaque endroit. La modelisation indique aussi qu'une couche avec des contenus accrus en eau non gelee a existe pendant le regel de la couche active et pendant le rechauffement anterieur au degel. Au site de Deadhorse, cette couche a ete continue depuis la surface du sol jusqu'a la profondeur de 0.3 m en 1987 et 0.36 m en 1991. Au site de Franklin Bluffs en 1987, a celui de West Dock en 1988 et a celui de Barrow en 1993, l'epaisseur de cette couche etait de 0.1 m et etait localisee entre 0.2 et 0.3 m a Franklin Bluffs, entre 0.15 et 0.25 a West Dock et entre 0.21 et 0.31 a Barrow. Les contenus de ces couches en eau non gelee etaient relativement eleves aux basses temperatures. Des calculs analytiques de la diffusivite thermique apparente ont montre que les variations observees avec la temperature etaient dues a la chaleur latente de fusion de l'eau non gelee. L'activite microbiologique et le deplacement de solutions et de gaz pendant l'hiver peuvent etre influencees par la presence d'eau non gelee. © 1997 John Wiley & Sons, Ltd.