Geoscience Reference
In-Depth Information
valleys in the upper Arkansas River Basin illustrate how local topography can override
regional climate (Young et al.,
2011
). Maximumglacier advances in these three valleys
were far from synchronous, ranging from 22 to 16 ka, although glacier retreat began at
16-15 ka in all three valleys. The ages obtained for other Pinedale glacial moraines at
widely different sites in the western United States show that although climatic change
exerts a major influence on glacier advances and retreats, local factors such as aspect
and topography modulate this influence (Young et al.,
2011
).
One long-standing issue in glacial geology is whether the late Pleistocene glacial
advances and retreats were synchronous or out of phase in both hemispheres. In an
effort to resolve this question, Clark et al. (
2009
) analysed 5,704
14
C,
10
Be and
3
He
ages from across the globe. They found that the Northern Hemisphere ice sheets
(Greenland, North America, Europe and parts of Asia) began to retreat at roughly the
same time (20-19 ka) regardless of size, as did most Northern Hemisphere mountain
glaciers. However, the mountain glaciers of Tibet and those of the Southern Hemi-
sphere started to retreat somewhat later (18-16 ka), and the West Antarctic Ice Sheet
retreated later still (around 14.5 ka). Mountain glaciers in many areas were already
at or near their maximum extent by around 30 ka, which was when global ice sheets
began to reach their maxima. The period of minimum global sea level (26.5-19 ka)
was when the global ice sheets were in near equilibrium with climate. According
to Clark et al. (
2009
), melting of the northern ice sheets was likely related to three
main forcing factors: an increase in high northern latitude insolation, an increase in
atmospheric CO
2
concentration and a rise in tropical Pacific sea surface temperatures.
Another as yet unsolved problem in regard to Quaternary climates is why the
lead up to glaciation involved such a long interval of cooling, whereas the melting
of the ice caps was the result of a relatively short phase of warming. Denton et al.
(
2010
) considered this question, noting that the postglacial melting of the Northern
Hemisphere ice caps to present volumes 'represents one of the largest and most rapid
natural climatic changes in Earth's recent history' (op. cit., p. 1652). During this rapid
warming and melting, sea level rose by 120 m and atmospheric CO
2
increased by
100 ppmv. The influx of meltwater into the North Atlantic led to cold conditions in the
Northern Hemisphere and a change in oceanic and associated atmospheric circulation
patterns. They speculated that during each northern cold stadial event, the Southern
Hemisphere westerlies shifted to the south, resulting in warming and deglaciation in
the Antarctic and in the Southern Ocean.
20.7.2 Loess, dunes and fossil soils
The loess deposits of North America were not always recognised as eolian dust, and
Darwin's contemporary, the great geologist Charles Lyell, was convinced that the
loess along the Mississippi Valley was alluvial in origin (see
Chapter 9
). However,
by the late nineteenth century, the reality of widespread Quaternary glaciations had
