Geoscience Reference
In-Depth Information
of interglacial deposits with fossil pollen, snails and beetles indicative of warmer
conditions sandwiched between glacial till deposits, some of which contained erratic
rocks reflecting long-distance transport by former continental ice caps. However, the
fragmentary nature of the terrestrial glacial sedimentary record and the lack of an
absolute chronology beyond the limits of radiocarbon dating meant that stratigraphic
correlations within and between continents were always open to doubt.
The way out of this impasse came with the pioneering work of Emiliani (
1955
)and
the recognition that the foraminifera preserved in deep-sea sediment cores provided a
record of alternating cold and warm sea-surface temperatures. Later work by Shack-
leton (
1967
;
1977
;
1987
) demonstrated that the stable oxygen isotopic composition
of the calcareous shells of the foraminifera could be used as a measure of changes
in global ice volume and not simply as a measure of changes in ocean temperature
or salinity. On this basis, a sequence of glacial-interglacial cycles was identified and
numbered from the most recent backwards in time (see Chapters
3
and
6
). Thus, the
current postglacial is denoted as Marine Isotope Stage 1 (MIS 1) and the immediately
preceding glacial maximum as MIS 2. In fact, the Last Glacial Maximum has been
defined in two ways. One, used for the sake of clarity and simplicity in this volume, is
the time of the most recent minimum sea level coincident with maximum global ice
volume, as inferred from the marine isotopic record, and is considered by Mix et al.
(
2001
) to date back to 21
2 ka. The other, based on the revised time of most recent
minimum global sea level, constrains the timing of the LGM to between 26.5 and
19.0 ka (Clark et al.,
2009
). The Mix et al. (
2001
) age of 21
±
±
2 ka falls within the age
±
range of the Clark et al. (
2009
) estimate of 22.75
3.75 ka, and because the dating
of glacial events is seldom very precise, we will continue to use the 21
±
2kaage
estimated for the LGM.
In this context, it is of interest to compare the timing of maximum advances of
mountain glaciers with that of the continental ice caps. A comprehensive review by
Gillespie and Molnar (
1995
) of the timing of mountain glacier advances in widely
spaced localities in North and South America, Europe, Asia, Hawai'i, Tasmania and
New Zealand revealed that some mountain glaciers advanced much further early in the
last glacial cycle, roughly 115,000 to 30,000 years ago, than they did during the LGM,
some 20,000 years ago. These authors also concluded that a number of undated glacial
landforms, such as moraines, that had been attributed to the LGM could actually be
two to four times older. In light of these uncertainties, a reappraisal of the chronology
of desert glacial advances is also warranted.
We saw in
Chapter 3
that the astronomical factors identified by Milutin Mil-
ankovitch (
1920
;
1930
;
1941
) have acted as the pacemaker of the glacial cycles and
control the length of each cycle (Hays et al.,
1976
; Imbrie and Imbrie,
1979
; Williams
et al.,
1998
). Three astronomically controlled variables influence the amount of solar
radiation received from the sun in any given latitude, namely, the distance of the earth
from the sun, which reflects changes in the elliptical path of the earth around the sun
