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of Pleistocene ice bodies and how permafrost bodies might refl ect different climatic
fl uctuations.
There is abundant evidence to indicate that, in high latitudes, ancient permafrost sur-
vived the impact of warmer-than-modern periods (interglacials). For example, J. R.
Mackay et al. (1972) were some of the fi rst in North America to show that ground-ice
bodies predated the last glaciation in the Western Arctic because the ice was beyond the
limit of radiocarbon dating. In more recent years, many bodies of massive ice and icy
sediment, both in the western North American Arctic and in western Siberia, have been
interpreted as deformed buried glacier ice (see Chapter 7). Thaw unconformities and
other ancient ice bodies provide additional evidence (Mackay and Matthews, 1983; Froese
et al., 2000). For example, in central Yukon, relict ice wedges, at two sites separated by
100 km, are overlain by a thaw unconformity which includes tephra beds dated at 740 000
+
60 000 years (Froese et al., 2006). Permafrost is thought to have persisted in central
Yukon through at least six interglacials, including oxygen-isotope stages 11 and 5e, both
considered to be longer and warmer than the present interglacial (the Holocene).
Insight into the actual paleo-permafrost environments can be inferred from various
sources. These include speleotherms in caves, beetles (coleoptera) and snails (non-marine
mollusca), pollen, and plant and faunal remains. There is a wide-ranging literature
(Hopkins et al., 1982; Péwé et al., 1997; Zazula et al., 2003). Rodent middens, from ice-
rich loess deposits in central Yukon, provide especially useful information (Zazula et al.,
2005). They contain a diverse assemblage of graminoids, forbs, and mosses that suggests
a range of fl oristic and animal habitats in Beringia, including steppe-like tundra on well-
drained soils, wet tundra meadows on lowlands, and hydric habitats in valley bottoms.
Inevitably, these northern ice-free areas acted as refugia for Pleistocene plants and animals
at the height of the glacial periods. Some of the most explicit evidence is provided by the
frozen carcasses of woolly mammoths and other mammals that are found preserved in the
permafrost of northern Siberia, Alaska, and Canada (Figure 11.8). The fact that these
large mammals were able to survive in relative abundance testifi es to the productivity of
the Pleistocene tundra ecosystem. Rodent middens are also excellent indicators of the
cold-climate environments in which the animals lived. For example, the remains of arctic
ground squirrel burrows and nesting sites in west-central Yukon Territory were dated at
/
25 000 years BP (Zazula et al., 2005). If one assumes that the animals were unable to
burrow into underlying permafrost, the maximum depth of the burrow refl ects the active-
layer depth. These are higher than those currently recorded, suggesting a more continental
climate (higher summer temperatures) than today.
11.4.3. Syngenetic Permafrost Growth
Long-continued cold-climate sedimentation leads to syngenetic permafrost growth. Sedi-
ment aggradation occurs for a variety of reasons; these include eolian deposition on
upland surfaces and in lee-slope positions, mass-wasting and sediment redeposition on
lower slopes, and fl uvial deposition on fl ood plains and alluvial surfaces in valley
bottoms.
Pleistocene-age syngenetic permafrost is typically formed within fi ne-grained uncon-
solidated sediments. It is often ice-rich and associated with layered and lenticular cry-
ostructures (see Chapter 7). These are regarded as typical of primary (unmodifi ed)
syngenetic permafrost (Bray et al., 2006; Shur et al., 2004) and refl ect the upward growth
of permafrost in response to an aggrading ground surface. Syngenetic permafrost also
commonly contains locally-modifi ed, or secondary, deposits possessing cryostructures
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