Characteristics and significance of the transition zone in drained thaw-lake basins of the Arctic Coastal Plain, Alaska

In the three-component conceptual model of arctic soils, the transition zone is recognized as a layer intermediate between the seasonally thawed active layer above and the stable permafrost below. Although typically frozen and therefore part of the near-surface permafrost, the transition zone episodically thaws over a time period ranging from sub-decadal to multicentennial. From an analysis of 138 pedons from the Arctic Coastal Plain near Barrow, Alaska, we were able to delineate the upper boundary of the transition zone in 78% and the lower boundary in 70% of the pedons. The transition zone exhibits the effects of cryoturbation, contains abundant redistributed organic carbon, is enriched by ice in the forms of lenses, veins, and nets (reticulate vein ice) and has abundant soil moisture. The surface (upper boundary) of the transition zone is found at an average depth of 34 +/- 7 cm below the ground surface and has an average thickness of 23 +/- 8 cm. We observed no significant differences in the thickness of the transition zone or the depth of its boundaries in drained thaw-lake basins ranging in age from 300 to 5500 years BP, suggesting that the processes leading to the development of this zone occur rapidly in Arctic Alaska. Recognition of the transition zone has implications for understanding pedogenic processes in permafrost-affected soils and for determining the response of near-surface permafrost to climate warming.