Asian deserts - Climatic Change in Deserts - page 371

depressions flanked by recently upliftedmountain ranges (Yang, 1991 ; Derbyshire and
Goudie, 1997 ; Yang, 2002 ; Yang and Scuderi, 2010 ;Yangetal., 2011a ;Yangetal.,
2011b ;Yangetal., 2012 ). Attempts to prevent or minimise dune encroachment on
roads, railways and arable land led to the setting up of the Desert Research Institute
in Lanzhou and to detailed mapping of dune type and rates of movement (Zhu et al.,
1989 ; Zhu and Wang, 1992 ; Zhu et al., 1992 ; Zhu and Wang, 1993 ). In contrast to
the tropical deserts of North Africa, Arabia and Australia, the Chinese deserts are
located in mid-latitudes and so are subject to the mid-latitude westerlies. They are
dry because they are located well inland. Flanked by high mountain ranges, they
lie within zones of extreme rain shadow. The deserts west of the Helan Shan range
lie within deep tectonic basins surrounded by very high mountains (see Chapter 8 ,
Figure 8.12 ), many of which are capped by permanent snow and ice. Seasonal snow-
melt feeds large rivers that flow into the deserts, where they vanish today but were
more extensive in the past, especially during times when the dunes were inactive.
Large alluvial fans at the foot of the mountains were and are a major sediment source
for the active sand dunes. As a consequence, dunes cover a larger proportion of the
more arid deserts in the west of China than is the case in North Africa, where high
mountains are less common. For example, dunes cover 80-85 per cent of the arid
Taklamakan in western China. This desert is bounded to the north by the snow-clad
Tian Shan and to the south by the snow-clad Kunlun Shan, and is the largest desert
in China, with an area of 337,600 km 2 . Active dunes up to and slightly more than
100 m in height cover 80-85 per cent of its area. Very large lakes occupied the centre
of the desert basin early in the Pleistocene, but they have since been buried or eroded,
so little evidence is now left of these once wetter times (Yang et al., 2011b ). At
intervals in the late Pleistocene and mid-Holocene, rivers flowed through some of the
dunes and fed small lakes, some of which persisted until about 300 years ago (Yang,
2001 ).
The second largest sand desert in China is the Badain Jaran (49,200 km 2 ), bounded
to the south by the ice-covered Qilian Shan and to the east by the Tengger Desert
(42,700 km 2 ) (Yang, 1991 ; Yang and Williams, 2003 ;Yangetal., 2010 ;Yangetal.,
2011a ;Yangetal., 2012 ). In the southern Badain Jaran Desert, the dunes are generally
200-300 m high. Some of the dunes are up to 460 m high and are the highest dunes on
earth (Yang et al., 2011a ). The reasons for this great height include the highly dissected
bedrock beneath them, abundant fluvial sands from the Qilian Shan, a complex wind
regime and the periodic stabilisation of the dune surface by calcareous soils during
wetter climatic intervals. Between the dunes there are more than 100 small lakes,
which are discussed in Section 19.5.5 .
Evidence from grain-size distribution, heavy mineral content and quartz grain
isotope geochemistry has shown that individual deserts in China have been self-
contained, receiving their sand supply from local rivers (Yang et al., 2012 ). As a
consequence, they provide a local climatic signal that may not be representative of