Geoscience Reference
In-Depth Information
during the early Pleistocene. As the lake(s) shrank progressively (or dried out and
refilled), dunes developed along their sandy shorelines no doubt from sands ferried in
by rivers from the adjacent high mountains. Over time, much of the evidence of these
former vast lakes has either been buried or eroded (Yang et al.,
2011b
). It thus appears
that the present sand sea in the Taklamakan is a relatively youthful feature, associated
with a progressively more arid climate, although we cannot as yet provide any reliable
suggestions as to when it first formed. During the late Pleistocene and mid-Holocene,
rivers flowed through the dunes, and small lakes came into being, some as recently
as three centuries ago (Yang,
2001a
). However, a combination of climatic desiccation
and human deforestation has hastened the processes of desertification in this region
(Yang et al.,
2011b
).
The Badain Jaran Desert is the second largest desert in China, with an area of
49,200 km
2
, and has been studied in some detail by Professor Yang Xiaoping and
his colleagues (Yang,
1991
; Yang and Williams,
2003
;Yangetal.,
2010
;Yangetal.,
2011a
;Yangetal.,
2012
). This desert is bounded to the south by the glaciated Qilian
Shan and adjoins the Tengger Desert (42,700 km
2
) to the east, with the north-south
aligned Helan Shan forming the rugged eastern boundary and a convenient zone
separating arid areas to the west and semi-arid areas to the east. The southern Badain
Jaran Desert is renowned for the height of its giant dunes, which are mostly 200 to
300 m high but attain a maximum height of 460 m, making them not only the highest
dunes on this planet, but also higher than the dunes on Mars (Yang et al.,
2011a
). A
combination of factors seems to be responsible for the great height of these dunes,
including a hilly subsurface bedrock topography, an abundance of fluvial sands, a
complex wind regime and periodic stabilisation of the dune surface by calcareous
soils during wetter climatic intervals.
This desert is also remarkable for the very large number of lakes - well over a
hundred - that occupy the swales between the dunes and provide water and fodder
for the Mongolian herders and their flocks of two-humped Bactrian camels during
the summer months (Yang,
1991
; Yang and Williams,
2003
;Yangetal.,
2010
). We
discuss the hydrological and climatic significance of these lakes in
Chapter 11
.
A much-debated point is whether the different sand seas and sandy lands have
been in contact with each other during past phases of active sand movement. Recent
studies (summarised by Yang et al.,
2012
) of the particle-size distribution, heavy
mineral content and quartz grain isotopic geochemistry in the various deserts and
sandy lands have demonstrated reasonably convincingly that each desert operated as
a self-contained unit, receiving its sand supply from river systems flowing from the
mountains adjoining the particular deserts.
Where lake sediments either underlie dune sands or are banked against dunes,
a relative chronology of wetter and drier phases can be attempted, buttressed by
OSL and radiocarbon dating,. but variable and quite large
14
C reservoir effects (see
Chapter 6
), sometimes up to several thousand years (Hofmann and Geyh,
1998
), have
