Geoscience Reference
In-Depth Information
Plateau and Mongolia. Both the Aral Sea and the Caspian were considerably larger at
intervals during the Quaternary, but these events remain poorly dated. Although not
as thoroughly analysed and dated as the desert lakes of Africa, the late Quaternary
record of desert lake fluctuations in China is becoming an important paleoclimatic
archive.
The deserts of north-western China cover 1.3 million km
2
and contain sporadic
but useful evidence of former hydrologic and climatic changes within this region. For
example, in the presently hyper-arid Badain Jaran Desert of north-west China, there
are more than 100 permanent lakes nestled among the tallest dunes on earth. This
desert is located in the latitude of the westerlies and its southern margin straddles
the northern limit of the Asian summer monsoon. Flanking many of these lakes are
strandlines denoting former high lake levels. Freshwater mollusc shells are commonly
associated with the beach deposits of the lakes, which range in age from early to mid-
Holocene on the basis of
14
C and TL dates (Yang and Williams,
2003
). Climatic
desiccation during the past 4,000 years has led to an increase in salinity within the
existing lakes, and water balance models point to a decrease in rainfall from around
200 mm during the early Holocene to around 100 mm today.
The summer monsoon in China actually consists of three independent monsoon
systems: the East Asian monsoon coming from the Pacific Ocean, the Indian monsoon
coming from the Indian Ocean and the Plateau monsoon coming from the Tibetan
Plateau. Consequently, the so-called Holocene climatic optimum, defined as the inter-
val of maximum wetness, was not synchronous throughout this region (An et al.,
2000
; Yang and Williams,
2003
). In fact, there were several episodes of peak summer
rains during the early to mid-Holocene, followed by increasing aridity after 5.5 ka
and especially after about 4 ka. Five lakes in the Qinghai-Tibetan Plateau region
and northern Xinjiang reveal in their oxygen isotope records evidence of an abrupt
increase in summer rainfall at 12.5-11 ka that lasted until 8-7 ka, with maximum
aridity at all five lake sites from 4.5 to 3.5 ka (Wei and Gasse,
1999
). The isotope
record was in accord with earlier studies in this region based on sediment, pollen,
ostracod and diatom analysis (Fan et al.,
1996
; Gasse et al.,
1996
;VanCampoetal.,
1996
).
Limited evidence shows that certain of the Badain Jaran desert lakes were high
around 34 ka, dry after 20 ka and high again by around 13 ka, fluctuating between
dry and less dry after that (Yang,
1991
; Pachur et al.,
1995
; Yang,
2001b
). In western
Mongolia's Valley of the Gobi, the late Pleistocene lakes reached the highest levels,
while their Holocene successors were high at around 8.5 ka and were briefly flooded
again around 1.5 ka (Lehmkuhl and Lang,
2001
). If we allow for minor differences in
the time of advance and retreat of the three separate monsoon fronts, the lake histories
in this arid region are all reasonably consistent.
Recent work in the Ulan Bui Desert has confirmed these earlier results (Zhao
et al.,
2012
). This desert is located in the arid Alashan region of Inner Mongolia in
