Geology Reference
In-Depth Information
QUATERNARY PERIGLACIAL CONDITIONS
11.5.4. Problems of Paleo-Environmental Reconstruction
There are several reliable indicators of past permafrost. These are (i) sedimentary struc-
tures that indicate thermal-contraction cracking (i.e. ice-wedge pseudomorphs, sand-wedge
casts, and composite-wedge casts), (ii) pingo remnants, and (iii) thermokarst phenomena.
However, the actual air and ground temperatures required for thermal-contraction crack-
ing, and for the formation of permafrost, are known only in general terms (see Chapters
5 and 6). For example, thermal-contraction cracking can occur when mean annual air
temperatures are signifi cantly warmer than
6 °C and frost cracking has been observed in
deep seasonal frost in addition to permafrost. J. B. Murton and E. Kolstrup (2003) sum-
marize the now well-known arguments against the uncritical use of frost fi ssures as paleo-
temperature indicators. Thermokarst features (see Chapter 8) clearly indicate the previous
existence of permafrost, but the “self-destroying” nature of much thermokarst activity
limits its usefulness. Even when the former occurrence of permafrost has been established,
there is still no simple relationship between permafrost and air temperatures. Permafrost
may also be relict and inherited from an earlier cold period. If it is discontinuous, site-
specifi c lithological, moisture, snow cover, and vegetation conditions may be involved.
Without the actual presence of either seasonally- or perennially-frozen ground, there
are few reliable indicators of past frost action. Attention in the past often concentrated
upon the identifi cation of “relict” features of assumed frost-action signifi cance. For
example, H. T. U. Smith (1949) listed 13 different features as being of climatic relevance.
Unfortunately, many, such as landslides, superfi cial folds, and asymmetric valleys, also
occur under non-periglacial conditions. Others, such as blockfi elds and patterned ground,
are ambiguous because their relationship with air and ground temperature is unclear.
Several problems hinder attempts to calculate possible temperature depressions during
the Pleistocene, and hence, to provide quantitative parameters to the cold-climate condi-
tions. Usually, temperature depressions are calculated on the basis of modern lapse rates
and present snowlines in alpine regions. For example, assuming a lapse rate of 0.5 °C/100 m,
a mean annual temperature at the snowline of 0 °C, and an elevation difference between
present snowline and an inferred Pleistocene snowline of approximately 1000 m, it was cal-
culated that Europe probably experienced a temperature depression of between 4 and 6 °C
during the last glacial period (Wright, 1961, pp. 966-970). This sort of analysis masks the
considerable variability of modern lapse rates which occur in mountains. Moreover, there is
no guarantee that lapse rates were similar during the Pleistocene to those of today and, fur-
thermore, that alpine conditions are representative of the lowland zone lying between the
Alps and the continental ice sheets. In fact, in continental lowlands of mid-latitudes, strong
temperature inversions up to 1500 m in elevation were probably common . As a result, it has
been suggested that mean annual air temperatures 10-12 °C lower than today were proba-
bly more realistic for the European lowlands during the last cold stage (Shotton, 1960).
Estimates of any possible drop in elevation of the Pleistocene snowline are also diffi cult
because (1) the Pleistocene snowline is usually identifi ed upon morphological evidence,
such as cirque heights or nivation hollows, which themselves may be of considerable
amplitude or variability with respect to elevation, (2) the modern snowline is not neces-
sarily at 0 °C, as is often assumed, but generally lower, and (3) in certain areas, such as
southern Africa, a hypothetical snowline has to be assumed.
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11.5.5. Extent in the Northern Hemisphere
One of the earliest attempts at periglacial morpho-climatic reconstruction involved the
identifi cation of Late-Pleistocene ecozones in Europe (Büdel, 1951). Three major zones
