Geoscience Reference
In-Depth Information
clarify past climatic changes in this region and so avoid the pitfalls of premature
speculation.
Not all glacial landforms are formed as a result of actively moving ice. In some
instances, a tongue of diffluent ice may overflow across a col, perhaps creating a glacial
breach in the watershed, only to find itself cut off from the parent source of ice. The
result is stagnation of the ice mass and progressive melting and downwasting of the
ice, resulting in an abundance of meltwater and eventually a deposit of hummocky
moraine with frequent
kettle holes
caused by the collapse of ice trapped inside the
glacial till. A good example of this process comes from a mountain valley in Anatolia
in north-eastern Turkey, in which the glacier had been advancing from before 26 ka
until the LGM at 19 ka, only to collapse after 17.7
±
0.8 ka ('Termination I') (Ak¸ar
et al.,
2008
).
13.8 The glacial record from Asia
Zech (
2012
, p. 281) commented that 'establishing reliable glacial chronologies in
arid regions has been challenging due to the lack of organic material for radiocarbon
dating'. Until the relatively recent development of surface exposure dating and optic-
ally stimulated luminescence dating (see
Chapter 6
), the lack of organic material was
a major obstacle towards obtaining reliable glacial chronologies from these regions,
but this is no longer the case. A growing number of workers are using
10
Be surface
exposure dating of moraine boulders to date glacial advances in mountainous desert
regions, such as the Pamir, Tian Shan and northern Mongolia (Zech et al.,
2005
;
Gillespie et al.,
2008
; Sanhueza-Pino et al.,
2011
; Zech,
2012
).
The Tian Shan ranges form part of the northern flank of the Tarim Basin, or Takla-
makan Desert (see
Chapter 8
), and they run for about 1,500 km in an approximately
east-west direction and rise to more than 7,000 m. The northern and western slopes
receive more than 1,000 mm of precipitation a year on average, with moisture derived
from the Atlantic and Mediterranean brought by the westerlies during spring and
autumn. In winter, the Siberian High blocks the flow of westerly air. The southern
and interior slopes of the Tian Shan are very dry and receive less than 300 mm of
rain annually, mostly from convectional summer storms. Ice accumulation is therefore
limited by precipitation in the south and interior of the ranges and by temperature in
the north and west. A comparison of
10
Be surface exposure ages from the Tian Shan
with those from the Pamir (Zech et al.,
2005
) indicated moraines dated to 15 ka, 21 ka
and
56 ka (MIS 3) in the Tian Shan and extensive last interglacial moraines (MIS 5)
in the Pamir (Zech,
2012
). The Pamir Mountains lie south of the Tian Shan and form
the western flank of the Tarim Basin. A plausible interpretation of these differences is
that the Tian Shan MIS 3 glaciers reflect an increase in westerly precipitation, while
the more southerly Pamir received increased monsoonal rainfall during MIS 5. The
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