Geoscience Reference
In-Depth Information
times of maximum dust flux were more arid than today and had a much reduced plant
cover, an expanded dust source area and strong, very gusty winds (see
Chapter 9
).
International recognition of the paleoclimatic importance of the unique Chinese loess
archive stands as an enduring legacy of the pioneering efforts of the late Professor Liu
Tungsheng.
19.5.2 Desert dunes of India and China
The Thar Desert of north-west India and eastern Pakistan (
Chapter 8
,
Figure 8.10
)
forms the eastern limit of the vast tropical deserts that extend from the Sahara across
Arabia, Iraq, Iran and Afghanistan - a distance of 8,000 km spanning 110
of lon-
gitude, making this the largest stretch of deserts anywhere on earth. The Thar Desert
itself covers an area of about 320,000 km
2
. Mean annual rainfall diminishes from
more than 500 mm in the east to less than 100 mm in the west, close to the Indus
Valley. Here the linear dunes are aligned parallel to the dominant sand-transporting
winds, which occur at the onset of the south-west summer monsoon. The desert is
criss-crossed by ephemeral river channels indicative of once wetter times, when pre-
cipitation was enough to support perennial drainage. The seasonal Luni River is the
only surviving integrated drainage system in the desert today.
In favourable circumstances, such as where the bedrock hills run at right angles to
the dominant sand-moving winds, a long record of dune accumulation is preserved.
For example, near Didwana in the north of the desert, one site shows twelve cycles
of dune sand deposition, soil development, calcrete formation, erosion and renewed
sand accumulation extending back to around 190 ka (Singhvi et al.,
2010
). Each
full cycle lasted about 19,000 years, indicating a precessional influence. The onset
of sand deposition coincided with the onset of monsoonal activity in this region, at
least during the very late Pleistocene. Analysis of the carbon isotopic composition of
organic matter within the sand profiles at this site indicated deposition of the sand
during a transition from a landscape dominated by C
3
grassland to one covered in
C
4
woodland or forest (Singhvi et al.,
2010
). This and other sites in the Thar Desert
were occupied during more humid intervals from Lower Palaeolithic times onwards
(Misra,
1983
;Dhiretal.,
1992
;Dhiretal.,
2010
). In both the Thar Desert and the
Wahiba Sands of Arabia, the period of most recent maximum dune building was
not during the cold, dry and windy Last Glacial Maximum, as in the Rub al Khali
and other nearby sand deserts (
Chapter 8
,
Figure 8.9
), but at about the time when the
south-west summer monsoon was again becoming stronger, around 15-14 ka (Wasson
et al.,
1983
;Chawlaetal.,
1992
;Dhiretal.,
1992
; Thomas et al.,
1999
; Singhvi et al.,
2010
). Dune activity in the Thar Desert and Wahiba Sands was therefore controlled
more by wind strength than aridity.
Unlike India, which has only the one big desert located in an area of relative tectonic
stability, China has twelve deserts or semi-deserts, many of them located in tectonic
°
