Geoscience Reference
In-Depth Information
weaker magnetic susceptibility values is regarded as evidence of a stronger and more
extensive winter monsoon and a weaker summer monsoon. Chronological control
is based on the paleomagnetic time scale, cross-correlation with the marine oxygen
isotope record and a combination of radiocarbon and thermoluminescence dates for
the more recent part of the sequence.
Further refinements to our understanding of the unrivalled Chinese loess sequence
include detailed attention to dust source areas (Ding et al.,
1999
; Pullen et al.,
2011
)
and to the topographic controls over dust deposition (Sun,
2002
), as well as a finer
resolution OSL chronology of key loess sections (Lu et al.,
2007
). There have been
promising attempts to correlate the loess record with climatic events recorded in
Greenland ice cores and North Atlantic marine cores (Porter and An,
1995
), as well
as with temperature changes inferred from the oxygen isotopic record obtained from
foraminifera in marine cores collected from the East China Sea (Liu et al.,
1985
;
Kukla,
1987
) and the North Pacific (Hovan et al.,
1989
). At all events, it seems
that times of maximum loess accumulation in China coincide with times of greater
aridity, expanded source areas, reduced plant cover and both stronger and gustier wind
regimes.
9.7 Wind-blown dust in Eurasia, Africa, America and Australia
The well-studied loess deposits of America are not always strictly desert dust deposits,
given that they are often associated with deflation from the outwash plains associated
with meltwater run-off from the great Laurentide and Cordilleran ice caps, which
reached their most recent maximum extent during the LGM. However, in presently
semi-arid Nebraska, rates of loess mass accumulation obtained from luminescence
dating were exceptionally high between 18 and 14 ka (Roberts et al.,
2003
). These
authors further suggested that the high atmospheric dust loading over that area implied
by these rates may have influenced radiative forcing sufficiently to have contributed
to the several thousands of years of colder-than-present climate over central North
America at that time, despite higher-than-present summer insolation values. Much as
in China, the loess deposits of Russia and central Asia consist of alternating loess units
and buried soils, some of which form composite and other discrete soil layers (Rutter
et al.,
2003
). The loess accumulated during colder, drier, windier episodes when the
desert source areas were greatly expanded and frost action was pronounced across
the landscape. Patagonia was a major dust source for Antarctica during glacial times,
when the source area had expanded twofold as a result of lower glacial sea levels.
Once meltwater lakes had formed on the Patagonian outwash plains at the foot of the
glaciated mountains, the supply of wind-blown dust was rapidly curtailed (Sugden
et al.,
2009
; Maher et al.,
2010
).
In Australia, the two main source areas for wind-blown dust were (and are) the
Lake Eyre Basin and the Murray-Darling Basin (McGowan et al.,
2005
; Petherick
