Geology Reference
In-Depth Information
climate fl uctuations within this time period are also recognized. For example, Figure 11.3
also shows the generally accepted European stratigraphic record for The Netherlands,
Central Germany, and the UK (East Anglia), together with inferred July air temperatures
compiled from palynological data for the last 120 000 years. To add complexity, however,
the North American chronostratigraphy is equally detailed. For example, Figure 11.4
attempts to correlate the Pliocene and Pleistocene cold-climate events in the western
Cordillera with those of the continental glaciations of North America.
During the last glacial stage, environmental conditions fl uctuated considerably, both
in time and space, in tandem with the growth, advance, and retreat of the ice sheets.
Smaller-scale oscillations of more temperate, or interstadial, climatic conditions can be
recognized. Thus, Late-Pleistocene periglacial conditions varied greatly in both intensity
and duration. In northwest Europe, for example, fl uctuations during the last 60 000-70 000
years (the Weichselian glacial period) resulted in arctic and sub-arctic conditions for only
about 25 000-30 000 years. There is evidence to believe that broadly similar climatic fl uc-
tuations occurred during the Wisconsinan in North America (see Figure 11.4).
It seems reasonable to assume that periglacial conditions developed around the ice
margins at the height of the cold stages, and that interstadials saw fl uctuations between
periglacial and non-periglacial conditions. The latest transition into the Holocene illus-
trates the oscillating nature of cold-climate conditions. For example, in Europe, the late
and post-glacial stages can be divided into pollen zones, indicating the dominance of dif-
ferent vegetation sequences (Table 11.1). Fluctuations in the arboreal and non-arboreal
pollen ratio indicate that during deglaciation, two periods of relatively warmer conditions
occurred, the Bolling and Allerod interstadials, at which time tundra vegetation was
replaced temporarily by shrub-tundra (dwarf birch) and birch forest. Then, in postglacial
times, there was a peak of warmth and dryness between 5000 and 3000 years BP. In
Eastern North America, the vegetation sequence is more general (Table 11.1) and suggests
that the tundra belt was narrower and the climate more humid than in Europe. It is highly
likely that the climatic amelioration in eastern North America was accentuated by large
proglacial water bodies, such as those of the proto-Great Lakes and the Champlain Sea.
As a result, the tundra belt was quickly replaced by spruce-fi r, and then by pine.
More detailed descriptions of the Quaternary climatic fl uctuations are beyond the
scope of this topic. An abundant literature is available, and the reader is referred to recent
texts by Bennett and Glasser (1996), Bell and Walker (1992), Ehlers (1996), and Bradley
(1999).
11.3. GLOBAL (EUSTATIC) CONSIDERATIONS
Any discussion of Pleistocene cold-climate conditions must be placed within the broader
context of the other global changes that occurred during the Quaternary. Two are of
special relevance to periglacial environments.
11.3.1. Sea-Level Changes
Today, if the Antarctic ice sheets were to melt completely, global sea level would rise by
∼
60 meters. Thus, fl uctuations undoubtedly occurred during the Quaternary in response
to the growth and decay of the continental ice sheets. Thus, many terrestrial lowlands that
were subject to Late Pleistocene cold-climate conditions are now being either submerged
or eroded by the latest (Holocene) transgression (see Chapter 10). Based on borehole data
from the North Sea, global sea level was more than 50 m below present level during the
