Geology Reference
In-Depth Information
Figure
4.12.
Wind-abraded quartz particle showing cracks formed as a result of freezing, probably
of saline gas-liquid inclusions at cryogenic temperature. The quartz grain is of Sartan (15-27 ka)
age, and is from loess-like sediments, Cape Chukochyi, Kolyma region, Siberia. Magnifi cation
×
400;
the grain size is
∼
20
µ
m. The photo and caption information are supplied courtesy of Professor
V. N. Konishchev.
to characterize unconsolidated surfi cial sediments taken from many parts of Siberia on
the basis of a coeffi cient of cryogenic contrast (CCC), defi ned as: CCF
=
Q
1
/F
1
/Q
2
/F
2
where Q
1
=
quartz content (%) in fraction 0.05-0.01 mm, F
1
=
feldspar content (%) in
fraction 0.05-0.01 mm, Q
2
feldspar
content (%) in fraction 0.1-0.05 mm. It was concluded that deposits affected by cryogenic
weathering have CCC values in excess of 1.5.
Even more poorly understood are the physico-chemical changes which occur under
negative temperatures. Three further examples illustrate the complexities of cold-climate
(cryogenic) weathering.
First, a specifi c problem concerns the behavior of the fi ne-grained (clay) fraction. In
early experiments, V. N. Konishchev et al. (1976) and others found that, after repeated
freeze-thaw, the size of kaolin and montmorillonite particles decreased while the crystal-
line lattice and inter-parcel distances generally increased. In later experiments, particle
disintegration dominated in the early stages of cryogenic transformation but then, after
10-100 freeze-thaw cycles, aggregation and coagulation occurred together with marked
changes in pH and the ion exchange complex (Ershov, 1984). The results of Russian
studies, summarized by E. D. Yershov (1990, pp. 124-126), indicate the complexity of
physico-chemical changes that occur in fi ne-grained sediments subject to freeze-thaw.
One set of data is presented in Table 4.8.
A second illustration is provided by evidence that secondary precipitates (iron oxide,
calcite) may originate by the freezing process. This results in coatings that display fi brous
crystalline fabrics (Vogt and Corte, 1996). Because water must both freeze and thaw to
produce these precipitates, it is concluded that precipitation must take place either in the
active layer (or zone of seasonal freezing and thawing) or during the formation of the
permafrost table (Figure 4.13). Other studies suggest transformation and neoformation of
minerals in the active layer (Vogt and Larqué, 1998). In all probability, transformations
take place not only during freeze-thaw cycles, when temperature transitions cross 0 °C,
but also within negative temperature fl uctuations.
=
quartz content (%) in fraction 0.1-0.05 mm, and F
2
=
