Geology Reference
In-Depth Information
material. This is due to the thermal properties of peat (see Table 5.1). Dry sphagnum has
thermal conductivity values approximately one order of magnitude lower than the lowest
value for mineral soil. During summer, therefore, when surface peaty layers are dry,
warming of underlying soil is limited. As a result, the depth of seasonal thaw in peat is
considerably less than in other earth materials. During autumn and early winter, however,
peat becomes saturated with moisture as rain and melting snow percolate downwards.
When peat freezes, therefore, interstices become fi lled with ice and thermal conductivity
increases dramatically. This leads to lower mean annual ground temperatures under peat
than in adjacent areas without peat.
Permafrost-related landforms that relate to these unusual thermal properties are palsas
and peat plateaus. Lithalsas are also discussed here because, although not associated with
peaty materials, the principles involved are the same.
Palsas, lithalsas, and peat plateaus occur frequently in the sub-arctic regions of north-
ern Canada, Scandinavia, and Russia. At the extreme southern limits of permafrost, they
often form “islands” of permafrost (Zoltai, 1971) and many show signs of either past or
present collapse (Delisle et al., 2003; Thie, 1974; Westin and Zuidhoff, 2001; Zuidhoff
and Kolstrup, 2000). This refl ects either natural causes, such as vegetation succession or
forest fi res, or may be an early sign of global climatic warming.
6.3.1. Palsas
A palsa is a peaty permafrost mound containing a core of alternating layers of segregated
ice and peat or mineral soil material (ACGR, 1988). Typically, a palsa may vary between
1.0 m and 7.0 m in height and is less than 100 m in diameter (Figure 6.8). Usually, palsas
occur in bogs and wetlands, where they protrude as low hills or knolls. The term is of
Fennoscandian origin (Auer, 1927, pp. 33-41), originally meaning a “hummock rising out
of a bog with a core of ice” (Seppälä, 1972b). Implicit in this defi nition are (i) their con-
structional nature, (ii) their origin in wetlands (fens or peat bogs), and (iii) that ice segre-
gation in mineral soil beneath peat is the primary process responsible for growth. Once
Figure 6.8. Abraded mature palsas on Luovdijeaggi palsa mire, western Utsjoki, Finland. In the
foreground are several small vigetation-covered mounds (pounus) and a thermokarst pond. The
photo is supplied courtesy of Professor M. Seppälä.
Search WWH ::




Custom Search