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which showed that millennial scale variations in the ISM revealed in this cave were
indeed synchronous (at least, at that time) with those from Hulu Cave in the path of
the EASM.
One of the few cave records to come from the drier parts of China is an especially
interesting
18
O record from Wanxiang Cave in semi-arid north-west China, situated
between the Qinghai-Tibetan Plateau to the south-west and the Chinese Loess Plateau
to the east. The record covers the last 1,810 years (Zhang et al.,
2009
). The monsoon
in this locality was strong during Europe's Medieval Warm Period and weak during
the Little Ice Age. It was also weak during the final decades of the Tang, Yuan and
Ming dynasties, when poor harvests triggered widespread unrest. Weaker monsoons
coincided with times of decreased solar intensity.
An as yet unresolved question relating to interpretation of the speleothem records in
terms of monsoon intensity concerns the precise nature of the climatic signal. Maher
and Thompson (
2012
) have suggested that the
18
O records from Chinese stalagmites
may reflect changes in the source of the moisture rather than changes in the actual
amount of summer monsoonal rainfall. However, changes in air mass precipitation
sources will, in any case, cause changes in the amount of rainfall, so this may not
prove to be a major problem.
19.5.8 Marine cores
The evidence from rivers, lakes and dunes is consistent with a drier climate during
the LGM in India and further afield. One way to test this inference is to examine
the isotopic composition of deep-sea cores. At a global level, changes in the
16
O/
18
O
ratio in ocean water reflect changes in the global ice volume (see
Chapter 3
), but at
a regional level, changes in the
16
O/
18
O ratio within calcareous marine fossils can be
used to assess past changes in sea surface salinity and temperature.
Duplessy (
1982
) used this approach to estimate changes in salinity in the Bay of
Bengal. He found an increase in salinity in the northern Bay of Bengal during the
LGM, which he interpreted as resulting from a reduction in freshwater discharge into
the Bay of Bengal because of less rainfall and more arid conditions at that time. In the
Son Valley of north-central India, the presence of carbonate-cemented alluvium dated
to 25-15 ka is also consistent with drier late glacial climates in this region (Williams
and Clarke,
1984
; Williams and Clarke,
1995
).
Zhang et al. (
2009
) used changes in the hematite to goethite ratio in a deep-sea
sediment core in the South China Sea to determine changes in monsoon intensity
during the last 5 million years. The sediments were derived from the lower Mekong
River. A high proportion of hematite indicates relatively dry conditions in the basin
withminimal chemical weathering, while a high proportion of goethite denotes intense
weathering under humid conditions. They found that during the early Pliocene, from
5.0 to 4.2 Ma, the monsoon was weak, becoming intense during the mid-Pliocene,
