Geoscience Reference
In-Depth Information
to allow any robust inferences about its climatic impact to be drawn. Recent work in
Greenland and Antarctica shows no evidence of sustained global cooling following
the 74 ka Toba eruption (Svensson et al.,
2013
). However, on a more regional scale,
the impact of the sudden deposition of a 10-15 cm layer of ash across peninsular India
would have seriously disrupted photosynthesis and damaged the existing plant cover,
leading to accelerated run-off and erosion, which is evident in the thick, channel-fill
deposits of Toba ash in many parts of India. Whether or not it also caused human
extinction in this region is for future work to determine.
19.5.5 Lake fluctuations
The great inland lakes of central Asia, such as the Caspian Sea, the Aral Sea and
Lake Balkhash, all show evidence of having been much larger at some time during
the Quaternary, and attracted the attention of scientific explorers more than 150
years ago (see Chapters
5
and
12
). Detailed analysis of the sediments, microfossils
(pollen, ostracods, diatoms) and stable isotopes associated with five lakes in northern
Xinjiang and the arid Qinghai-Tibetan Plateau region show a sudden increase in
summer rainfall at 12.5-11 ka that persisted until 8.7 ka, with the lowest levels
in all lakes occurring between 4.5 and 3.5 ka (Fan et al.,
1996
; Gasse et al.,
1996
;Van
Campo et al.,
1996
; Wei and Gasse,
1999
). In the arid Badain Jaran Desert of Inner
Mongolia in northern China, there are more than 100 permanent lakes in hollows
between the dunes, often with well-preserved higher shorelines and fossil shells of
freshwater mollusca. These shorelines have early to mid-Holocene radiocarbon and
luminescence ages, and were previously much less saline than they are today, at a time
when the local mean annual precipitation probably amounted to 200 mm, as compared
to only 100 mm today (Yang and Williams,
2003
). The lakes began to dry out after
about 4 ka. Further to the north-east in the Ulan Bui Desert, a former lake reached
its maximum extent between 7.8 ka and 7.1 ka (Zhao et al.,
2012
). The lake began to
dry out after 6.5 ka. Until about 8.3 ka, this area was a sand desert; it now consists of
a small salt lake (which supports a major chemical works) surrounded by active sand
dunes.
The time of maximumHolocene rainfall varied across China, with several phases of
strong summer rainfall during the early to mid-Holocene, followed by greater aridity
after 5.5 ka, and particularly after 4 ka (An et al.,
2000
; Yang and Williams,
2003
).
The lack of synchronicity in Holocene lake fluctuations in China is partly due to the
summer monsoon in China actually comprising three independent monsoon systems,
as explained in
Section 19.2
. These are the East Asian monsoon coming from the
Pacific, the Indian monsoon coming from the Indian Ocean and the Plateau monsoon
coming from the Tibetan Plateau. In addition, certain of the deserts, such as the Badain
Jaran, are in the path of the westerlies but are also close to the northern limit of the
Asian summer monsoon along their southern margins.
