North American deserts - Climatic Change in Deserts - page 396

Miocene and Pliocene changes in both flora and fauna on the Great Plains reflected
local and regional factors at least as much as they reflected more global climatic fluc-
tuations.

20.7 Quaternary climatic fluctuations
20.7.1 Glaciations
Progressive cooling in the Arctic during the late Pliocene resulted in rapid accumula-
tion of snow and ice and the inception of a large ice sheet over the North American
Laurentian shield by around 2.5-2.6 Ma ago. Much of the terrestrial evidence of the
older glaciations has long since been eroded and destroyed, so inferences about the
magnitude and duration of past glaciations is indirect and comes from the marine
sediment record, especially the oxygen isotopic composition of the calcareous shells
of foraminifera. The early Pleistocene ice caps were smaller than their late Pleisto-
cene counterparts, and the duration of the glacial-interglacial cycles was also not as
long. Deep-sea sediment cores show that the 23 and 19 ka precessional cycles (see
Chapter 3 ) were dominant until around 2.6 Ma. Between about 2.5 and about 0.7 Ma,
the 41 ka obliquity cycle was dominant, with the 100 ka orbital eccentricity cycle
dominant thereafter (Raymo et al., 1997 ; Williams et al., 1998 ; Lisiecki and Raymo,
2005 ; Clark et al., 2006 ; Lisiecki and Raymo, 2007 ;Luthi et al., 2008 ).
Detailed field mapping of glacial deposits and intercalated warm-climate soils and
their associated pollen spectra during the late nineteenth century led to the recognition
of four major Pleistocene glacial stages and three major interglacial stages in North
America. From oldest to youngest, these were the Nebraskan glacial, the Aftonian
interglacial, the Kansan glacial, the Yarmouth interglacial, the Illinoian glacial, the
Sangamon interglacial and the Wisconsin glacial (Flint, 1971 , p. 543, table 21-B). At
about the same time, Penck and Bruckner ( 1909 ) identified four recent glacial stages in
the piedmont zone of the European Alps, which from oldest to youngest they termed
the Gunz, Mindel, Riss and Wurm glacial stages. The corresponding interglacial
stages can be labelled quite simply as the G/M, M/R and R/W interglacial stages.
Later work showed that this fourfold division was oversimplified, because many of
the deposits associated with each glacial were in fact polygenic and represented more
than one glacial stage (see Williams et al., 1998 , for a review of this topic). Two much
older sets of glacial deposits were also recognised, and they were termed the Biber and
the Donau glacial stages. Older and highly weathered glacial deposits were likewise
identified in North America well beyond the limits of the last, orWisconsin, glaciation.
The Wisconsin glacial deposits were later subdivided on the basis of stratigraphy and
radiocarbon ages into Early ( 55,000 years BP), Middle (55,000-25,000 years BP)

and Late Wisconsin (25,000-10,000 years BP) (Flint, 1971 , p. 560).
The relative sequence of glacial and interglacial events in North America is now
quite solidly established, and where glacial deposits are lacking, use is made of the

>