Desert dunes - Climatic Change in Deserts - page 132

yet been studied in any real detail from a perspective of past climatic changes (Yang,
1991 ; Derbyshire and Goudie, 1997 ; Yang, 2002 ; Yang and Scuderi, 2010 ;Yangetal.,
2011a ;Yangetal., 2011b ;Yangetal., 2012 ). The Chinese deserts differ from their
tropical Saharan, Arabian and Australian counterparts in three important respects.
First, they are located in mid-latitudes rather than tropical latitudes and therefore lie
within the zone of the mid-latitude westerlies rather than being located within the
zone of dry subsiding air masses associated with the tropical anticyclones discussed
in Chapter 2 . They owe their aridity to distance inland and to pronounced rain shadow
effects. Second, the deserts west of the Helan Shan ( shan means mountain range)
occupy tectonic depressions bounded by high mountain ranges ( Figure 8.12 ). The
higher of these mountains have permanent snow and ice, meltwater from which feeds
rivers that are often substantial and flow into and, on occasions in the past, across the
present dune fields. Location within large and deep tectonic depressions means that
one important source of sediment for the evolving sand seas comes from the large
alluvial fans flanking the foot of the mountains and fromwhat Weissmann et al. ( 2010 )
termed 'distributive fluvial systems', which encompass the 'mega-fans' identified by
Leieretal.( 2005 ) that are discussed in Chapter 10 . Third, and unsurprisingly, given the
active tectonic history of western China (and central Asia more generally), the Chinese
deserts span a considerable altitudinal range. The dune fields range in elevation from
less than 1,000 m to nearly 5,000 m on the Tibetan Plateau and appear to occur at
discrete elevations (Jakel, 2002 ), although this may be coincidental.
In China, the term 'desert' ( shamo ) is restricted to active sand seas, and the term
'sandy land' denotes areas of fixed and vegetated sand dunes. According to this well-
established classification, there are seven sand sea deserts in China and five main
areas of sandy land ( Figure 8.12 ). These sandy lands are the Chinese equivalents of
the ancient erg of Hausaland in northern Nigeria described by Grove ( 1958 )andthe
vegetated dune fields of the northern Kalahari (Thomas, 1984 ). As a general rule, the
desert sand seas of northern China are confined to the most arid areas, while the sandy
lands lie within the semi-arid regions of the centre and east ( Figure 8.12 ).
Although Yang et al. ( 2011a ) concluded from their comprehensive review of Qua-
ternary environments in the desert lands of northern China that 'little is also known
about why dune type and size vary greatly across the drylands of China and the forcing
factors that cause these differences', there is still a great deal of useful environmental
information that can be gleaned from the dunes within this vast region. As with any
scientific research, the interpretation of this information remains a work in progress.
The Taklamakan is the largest desert in China, with an area of 337,600 km 2 .Active
dunes up to and slightly more than100 m in height cover 80-85 per cent of its area.
The setting of this desert within the almond-shaped Tarim Basin is spectacular, with
the snow-covered Tian Shan forming the northern boundary and the similarly snow-
covered Kunlun Shan forming the southern boundary ( Figure 8.12 ).Oneormorevast
lakes seem to have occupied the centre of the basin to an elevation of at least 1,100 m