Geoscience Reference
In-Depth Information
The Pleistocene record of lake fluctuations in arid northern China and central Asia
is less complete than the Holocene record. Some of the Badain Jaran Desert lakes
were high around 34 ka, dry after 20 ka, and high once more by around 13 ka (Yang,
1991
; Pachur et al.,
1995
; Yang,
2001b
). In western Mongolia, the late Pleistocene
lakes were higher than their Holocene counterparts, which were high at around 8.5 ka
and around 1.5 ka (Lehmkuhl and Lang,
2001
).
The Thar Desert of India has far fewer lakes than the Badain Jaran Desert of China.
Singh et al. (
1972
;
1974
;
1990
) dated four lakes extending across a rainfall gradient
from semi-arid east to arid west and analysed the pollen within the lake sediments.
They found that after a long arid phase in the late Pleistocene, these presently saline
lakes were fresh during the early to mid-Holocene and dried out soon after 4.5 ka, with
the onset of desiccation starting a few centuries earlier in the arid western lakes than
it did in the east. Singh (
1971
) suggested that the demise of the Indus Valley Culture
was caused by this phase of desiccation, which also affected Mesopotamia, Persia
and Afghanistan (Cullen et al.,
2000
; Weiss,
2000
). Other workers have investigated
several of the lakes first studied by Singh and his colleagues in greater detail. Wasson
et al. (
1984
) carried out geochemical analyses of the sediments from Lake Didwana in
the east of the Thar Desert, and Enzel et al. (
1999
) conducted detailed analyses of the
sediments at Lake Lunkaransor in the arid west. Both studies showed that the early
Holocene was wetter with a stronger summer monsoon. Overall, the early Holocene
was a time of enhanced summer monsoon in the deserts of both India and China,
while the Last Glacial Maximum was somewhat drier than today.
19.5.6 Glaciations
The use of
10
Be surface exposure dating of moraine boulders to date glacial advances
in mountainous desert regions has revolutionised our knowledge of the glacial history
of northern Mongolia, the Pamir and the Tian Shan (Zech et al.,
2005
; Gillespie
et al.,
2008
; Sanhueza-Pino et al.,
2011
; Zech,
2012
). We noted earlier that the Tian
Shan forms the northern boundary of the Taklamakan Desert. These mountains are
1,500 km from west to east and rise to above 7,000 m. Westerly air masses from
the Atlantic and Mediterranean bring more than 1,000 mm of precipitation to the
northern and western slopes of the Tian Shan during spring and summer, but in winter
the Siberian High blocks the flow of moist westerly air. The southern and interior
slopes of the Tian Shan are arid, receiving less than 300 mm of rain a year, mainly
from convectional storms in summer. The accumulation of snow and ice in the Tian
Shan is therefore limited by temperature in the north and west and by precipitation
in the interior and south. The Pamir Mountains lie south of the Tian Shan and form
the western boundary of the Taklamakan Desert. They receive some monsoonal rain
from the south during summer. As a result of the different precipitation sources in
