Cryosols as a Three-Part System - Cryopedology - page 18

Fig. 2.6 Distribution of permafrost in Antarctica (Bockheim 1995 ). The solid black represents
ice-free areas with continuous permafrost. Subglacial permafrost may be restricted to the gray
areas . Subglacial lakes are by crosses . The −1 and −8 ﾰC isotherms for mean annual air tempera-
ture may correspond with the northern and southern limit of permafrost, respectively
The “zero curtain” refers to the persistence of a nearly constant temperature close to
the freezing point during annual freezing (and occasionally during thawing) of the
active layer (Fig. 2.7 ). Despite that it is below freezing the upper part of permafrost
is subject to temperature variations in response to air temperature variation.
However, at a depth of 10-20 m, the permafrost temperature does not vary.
There is considerable variation in the ice content of permafrost. The ice content
of permafrost is dependent on the texture of the material and the amount of “excess
ice.” Excess ice exists where the volume of ice in the ground exceeds the total pore
volume of the material. The sandy dry-frozen materials in Antarctica may contain
less than 5 % gravimetric water (Campbell and Claridge 2006 ). However, in cases
where there is abundant excess ice, gravimetric water contents exceed 100 %.
The presence of segregation ice (ice in discrete lenses or veins formed by ice
segregation) yields are variety of cryostructures, including basal, basal-layered,
crust-like, layered, lens-type, massive-agglomerate, massive, massive-porous, retic-
ulate and reticulate-blocky (French and Shur 2010 ). Some of these structural char-
acteristics are depicted in permafrost cores from arctic Alaska (Fig. 2.8 ).