Geoscience Reference
In-Depth Information
18
O proxy data support the view that the Alpine Steppe persisted
during the LGM when temperatures at these elevations were probably about 3-4
both pollen and
C
lower than today. Although some decrease in LGM precipitation seems possible,
conditions of extreme LGM aridity on the Tibetan Plateau are ruled out by the inferred
persistence of the Alpine Steppes in this area during the late Pleistocene (Miehe et al.,
2011
).
°
13.9 The glacial record from the Americas
The Andes in South America and the Rockies in North America are unlike any other
major mountain ranges on earth in that they run in a more or less direct line from
north to south, parallel to the meridian. (The Eastern Highlands of Australia also run
from north to south, but with a maximum elevation of 2,228 m, they scarcely qualify
as a major mountain range.) The Andes extend for some 7,000 km from the zone of
westerly precipitation in the far south to the zone of easterly precipitation controlled
by the seasonal migration of the ITCZ in the centre and north (see
Chapter 21
). As
a result, the rain-shadow effect enhances aridity in the Patagonian Desert, located
east of the Andes in the south, and in the Atacama and Peruvian coastal deserts,
located west of the Andes in the centre and north of the continent. We might therefore
expect that glacial activity in the Andes will reflect summer easterly precipitation in
the tropical north and winter westerly precipitation in the far south. In addition, the
location of the rain-shadow zones will vary over time in accordance with changes in
the location and persistence of the southward movement of the ITCZ in summer and
the northward movement of westerly air masses in winter. Such changes were not
necessarily synchronous.
The Rockies extend for about 5,000 km and are located further from the equator
than the equatorial northern Andes. They also display a slightly different pattern of air
mass movement, with a much stronger influence from the westerlies, bringing winter
precipitation and summer aridity to California but year round rain to Oregon and
Washington. (The Cascades and the Sierra Nevada are two smaller, separate ranges
that are aligned roughly parallel to the main Rockies.) Another important difference
between North and South America concerns the depth and extent of ice caps during
times of maximum glaciation. During the Last Glacial Maximum, much of North
America was hidden beneath a vast layer of ice up to 3 km thick - the Laurentide
ice sheet. This ice sheet (and, no doubt, its predecessors) was sufficiently high that
it seems to have split and diverted the high-level jet streams from their customary
interglacial (and present-day) mean positions over the continent, thereby altering the
circulation patterns at lower levels in the atmosphere (Kutzbach and Wright,
1985
).
However, whether this led to a significant southward displacement of the westerlies
during the LGM now seems very unlikely (Lyle et al.,
2012
). Both the Andes and the
