Geology Reference
In-Depth Information
5.8.1. The Transient Layer
The transient layer is the near-surface ice-rich layer marking the long-term position of the
contact between the active layer, as traditionally defi ned, and the upper part of permafrost
(Shur, 1988a, b; Shur et al., 2005). Figure 5.12 shows a three-layer system of the active
layer that incorporates the concept of a transient layer.
The utility of the transient-layer concept follows upon an appreciation of the impor-
tance of the ice-rich layer that typically characterizes the base of the active layer and the
upper part of permafrost. In years of exceptional summer warmth, the active layer deepens
beyond its normal (average) thickness and part of this ice-rich zone thaws. This promotes
slope instability, rapid mass movements (active-layer-detachment failures), and enhanced
rates of solifl uction activity. All these processes are described in Chapter 9. Continued
global climate warming will also mean that this ice-rich layer will continue to thaw as the
active layer progressively thickens in years to come (see Chapter 15).
The ice-rich layer exists because of temperature gradients that are set up within the
active layer and the upper part of permafrost that allow moisture to migrate upwards
during winter and downwards during summer. As explained earlier in Chapter 4, it is
known that, in certain frost-susceptible soils, as much as 40% of the water content may
Depth (m)
0
Surface
Active
layer
Seasonal freezing
and thawing
(1)
1·0
Freezing and thawing on
decadal/century scales
Transient
layer
(2)
1·5
Permafrost
(3)
Long-term freezing and
thawing (permafrost) on
century/millennial scales
3·0
Ice lensing
Ice wedge in permafrost (long-term)
with upward growth of wedge
in years of no thaw (decadal/century
scales)
Figure 5.12. The three-layer model of the active layer-permafrost interface. (1)-Active layer (sea-
sonal freezing and thawing); (2)-transient layer (high ice content and freezing/thawing at decadal
to century scales; (3)-long-term permafrost (freezing and thawing at century to millennial scales).
Modifi ed from Nelson (2004) and Shur et al. (2005).
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