Geology Reference
In-Depth Information
air and ground temperatures, (b) the thickness and distribution of the permafrost and
whether discontinuous permafrost or deep seasonal frost was widespread, and (c) the
temporal duration of these conditions.
In North America, an early attempt to map the various ecozones which existed in the
mid-latitudes at the height of the last glaciation was made by D. Brunschweiller (1962).
A more recent synthesis (Figure 11.13) now permits a more defi nite reconstruction but
several considerations suggest this map overemphasizes the extent of tundra and steppe.
First, the pollen record for many parts of the United States and southern Canada does
not suggest a widespread frost-rubble tundra zone, and, instead, indicates that forest zones
intermingled with loess-steppe zones. Second, the maximum southern extent of the last
glacial ice sheet (the Late Wisconsinan) was further south than that in Europe; as a con-
sequence, the ice-marginal zone of severe periglacial conditions was probably more
restricted due to a tighter alignment of the displaced climatic zones. Third, retreat of the
Wisconsinan ice was accompanied by formation of extensive proglacial lakes, brought
about by isostatic depression and either the ponding of meltwater or the infl ux of marine
waters in the St Lawrence Lowlands These water bodies exerted modifying infl uences
upon regional climate and limited the land areas exposed to cold sub-aerial conditions.
By the time they disappeared, either hundreds or several thousands of years later, climate
had ameliorated suffi ciently to allow forest growth.
The most convincing evidence for Late-Pleistocene permafrost in the continental
United States comes from frost-fi ssure pseudomorphs found in north-central Wisconsin,
the interior and western plains (Iowa, Nebraska, Wyoming), and southern New Jersey.
In the Appalachians, large amounts of shattered bedrock (blockfi elds) suggest intense
frost action. As in Europe, one must envisage a time-transgressive shift of the perigla-
cial zone northwards as the Laurentide and Cordilleran ice sheets progressively
withdrew.
In Russia, several detailed maps of the extent of Late-Pleistocene periglacial conditions
have been produced (e.g. Kondratjeva et al., 1993; Popov, 1961; Popov et al. 1985, 1990;
Rozenbaum and Shpolyanskaya, 1998a, b; Velichko, 1975, 1982). There is general agree-
ment as to the changes in permafrost extent during the Late Quaternary. For example,
during the Zyriansk (50 000-60 000 years ago) and Sartansk (15 000-27 000 years ago)
cold stages, permafrost reached its maximum southern extent (48-49° N) on the Russian
Plain. However, much of the permafrost in Siberia and the Russian Plain is relict. In
formerly-glaciated areas, such as western Siberia, relict Early- and middle-Pleistocene-age
permafrost is separated at depth from Late-Pleistocene permafrost by an unfrozen zone.
By contrast, in unglaciated areas, the permafrost is continuous and thick (see Figure 11.7).
In general, one must envisage thick and cold permafrost in never-glaciated terrain and
thin or no permafrost beneath ice sheets.
Other mid-latitude regions of the world were also affected by Late-Pleistocene perigla-
cial conditions. In northern China, for example, the southern limit of Late-Pleistocene
permafrost has been mapped as far south as 40° N, based primarily upon ice-wedge pseu-
domorphs and sand-wedge casts (Figure 11.14.) In Mongolia, sand-wedge casts have been
recognized in weathered Pliocene sandstone and gravelly layers that date from between
∼
20 and 26 ka (Vandenberghe et al., 2004), and others have been reported from Tibet
(Porter et al., 2001). In the plains of central and northern Kazakhstan, several episodes
of Middle- and Late-Pleistocene permafrost are inferred on the basis of numerous sand-
wedge casts and cryogenic horizons (Aubekerov and Gorbunov, 1999). Finally, in Western
Transbaikalia (east of Lake Baikal, latitude 51-57° N), cryogenic soil structures thought
indicative of permafrost (Vogt et al., 1995) occur in sediments that, in age, span the
Bruhnes-Matuyama boundary.
