Geoscience Reference
In-Depth Information
both mountain ranges, the glacial history of the Pamir differs from that of the Tian
Shan. Zech et al. (
2005
) compared
10
Be surface exposure ages for moraines from
the Tian Shan with those from the Pamir. The Tian Shan moraines had ages of 15,
21 and
>
56 ka (MIS 3), while the Pamir showed extensive moraines during the last
interglacial (MIS 5) (Zech,
2012
). The MIS 3 glaciers in the Tian Shan may indicate
an increase in westerly precipitation at that time, with the more southerly Pamir
receiving increased monsoonal precipitation during MIS 5. Ice was more extensive in
both the Pamirs and the Tian Shan during MIS 4 than during MIS 2, suggesting an
increase in aridity in Central Asia during the last glacial cycle (Zech et al.,
2005
; Zech,
2012
).
Sanhueza-Pino et al. (
2011
) obtained
10
Be exposure ages of 67-63, 15-11 and
8-6 ka for boulders from very large landslides in three previously glaciated valleys
in the Kyrgyz Tian Shan. These ages provide a minimum age for glacier advances
in their respective valleys and also mark the maximum extent of the ice within those
valleys up to the time of the landslides. The MIS 4 age for one of the glacial advances
shows that the glacial history of these mountains cannot be widely extrapolated
and also revealed that the minimum elevations of the equilibrium snow-lines (see
Chapter 13
) in the northern Tian Shan was about 400 m higher than previously
estimated.
The two most recent glacial advances in the Darhad Basin of northern Mongolia
had ages of around 53-35 ka (MIS 3) and around 19-17 ka (MIS 2) (Gillespie et al.,
2008
). In order to establish a reliable chronology of times of maximum local ice
advance and times of ice retreat, three independent dating methods were used:
14
C,
10
Be and luminescence (IRSL) (see
Chapter 6
). The ages of these glacial advances
were the same as advances of similar extent across northern Mongolia, but they
differed from those of glacial advances in Siberia and western Central Asia. An older
and more extensive glaciation may date to MIS 6, but there was very little difference
in the equilibrium line altitude (ELA) for all three glaciations. Unlike the Tian Shan,
where greater LGM aridity confined glaciers to high elevations, glaciers advanced to
relatively low elevations during the LGM in the Darhad Basin of northern Mongolia,
indicating that it was less arid at that time (Gillespie et al.,
2008
).
The Tibetan Plateau is the largest high plateau on earth, with an area of 2.6 million
km
2
and a mean elevation of 4,600 m. Kuhle (
2001
;
2002
) has argued that most of
the plateau was covered in ice during the LGM, but the majority of workers have
concluded that glaciation was limited and spatially isolated (Owen,
2009
), not least
because both pollen and
18
O proxy data indicate that the Alpine Steppes of the
Tibetan Highlands persisted during the LGM, when temperatures were about 3-4
C
lower than they are today (Miehe et al.,
2011
). The Tibetan Plateau may have been
slightly more arid than today during the LGM, but it was not excessively dry, or else
the Alpine Steppes could not have survived.
°
