Geoscience Reference
In-Depth Information
distinctive loess deposits (see
Chapter 9
) downwind of the ice caps and blown from
the glacial outwash plains. As in China, the loess deposits often consist of a lower,
relatively unweathered unit and an upper, often highly weathered unit in which a soil
had developed. However, despite the establishment of an impressive relative sequence
of glacial and interglacial stages, there was no way of knowing howmuch of the North
American continental record was missing until the deep-sea sediment records across
the oceans of the world revealed that there had been more than thirty glacial stages
in the last 2 million years. How the Marine Isotope Stages (MIS) were determined
is explained in
Chapter 6
. Each unevenly numbered one represents an interglacial
or an interstadial stage, with the present-day interglacial stage designated as MIS 1.
All counting is from the present back in time. Each evenly numbered marine isotope
stage represents a glacial or a stadial stage, with the Last Glacial Maximum (LGM)
designated as MIS 2. Further subdivision of each stage is shown by letters of the
alphabet, with, for example, the peak of the last interglacial designated as MIS 5e.
MIS 3 is an interstadial, not a full interglacial, which is confusing for the unwary.
The vast size and high elevation of the Laurentide ice sheet (coeval with the
Late Wisconsin glacial stage) and, no doubt, its earlier equivalents, may have caused
the high-level jet streams to split and deviate from their interglacial positions over
North America, thus altering the lower-level patterns of circulation over the continent
(Kutzbach andWright,
1985
). However, more recent work suggests that these changes
were unlikely to have caused any major southward displacement of the westerlies
during the LGM (Lyle et al.,
2012
).
Relative to the ice caps, mountain glaciers showed a far more variable response
to late Pleistocene climatic change. This is to be expected, given that glaciers will
respond to local differences in valley size, shape and aspect, not to mention local
influences on precipitation. In addition, until the advent of cosmogenic nuclide dating
(see
Chapter 6
) allowed glacial moraines to be directly dated for the first time,
all previous late Quaternary chronologies relied on radiocarbon dating of organic
materials sandwiched between glacial deposits and were therefore at best indirect
measures of the timing of glacial advances and retreats. Porter et al. (
1983
)have
summarised much of the earlier work on Late Wisconsin mountain glaciation in the
western United States, which was based primarily on
14
C dating. Since that time, the
widespread deployment of cosmogenic exposure dating has confirmed earlier work
while also adding significant new detail. Indeed, the late Pleistocene and Holocene
glacial history of the North American deserts is without a doubt the best-dated of
any desert region on earth (Porter,
1983
; Wright,
1984
; Ruddiman and Wright,
1987
;
Wright et al.,
1993
; Easterbrook,
2003
; Clark et al.,
2009
; Young et al.,
2011
).
Many of the mountain glaciers in the drier regions of North America were at or near
their maximum limits during the Last Glacial Maximum, but there was still significant
local variation. The
10
Be surface exposure ages from moraines, bedrock and river
terrace gravels relating to the late Pleistocene Pinedale glaciation from three adjacent
