Geoscience Reference
In-Depth Information
Chapter 2
Cryosols as a Three-Part System
2.1
Three-Part Conceptual Model of Cryosols
The standard conception of permafrost-affected soils employs a simple two-layer
model. At depth is permafrost, which is defined as “earth material that remains
continuously at or below 0 ﾰC for at least two consecutive years” (van Everdingen
1998
). At the surface is the “active layer,” which thaws in summer and refreezes in
winter. Recent research in North America supports an earlier Russian proposal for
the existence of a “transition” or “transient” layer in polar soils (Shur et al.
2005
).
This layer—the uppermost, or near-surface, portion of the permafrost—meets the
thermal criteria for permafrost defined above (Fig.
2.1
). However, its maximum
summer thaw depth tends to fluctuate from year to year owing to interannual varia-
tions in components of the surface energy budget. During occasional deep thaw
penetration, ice in the near-surface permafrost melts, and the affected zone tempo-
rarily becomes part of the active layer. Therefore, it is not unusual for the soil profile
to extend into the near-surface permafrost (Fig.
2.2
).
2.2
Active Layer
2.2.1
Definition
The active layer is the layer of ground that is subject to annual thawing and freezing
in areas underlain by permafrost. In the zone of continuous permafrost, the active
layer generally reaches the permafrost table, but in the zone of discontinuous per-
mafrost, it may not. Some scientists consider the active layer to include the upper-
most part of the permafrost. However, this is referred to this as the transition zone
or transitory zone in this topic (Fig.
2.1
).
