Geoscience Reference
In-Depth Information
19.5.7 Speleothems
Although the loess record from central China spans the entire Quaternary and provides
the longest continental record on earth of past climatic changes in arid areas, its chro-
nology is relatively imprecise. The speleothem records from various caves in China
provide an invaluable adjunct to the more recent loess record and have the advantage
of being very precisely dated. The caves are often located well outside the arid zone,
but they still yield important information relevant to the deserts, especially in regard
to past changes in the intensity of the summer and winter monsoons, which influence
the desert margins. In addition, because they are very precisely dated using uranium-
series dating methods (see
Chapter 6
), the speleothem records can be compared with
high-resolution records in other parts of the world, including the Greenland and Ant-
arctic ice core records, north-east Brazil, the Cariaco Basin off Venezuela, peninsular
Arabia and the North Atlantic. Such comparisons help provide a more integrated view
of global climatic fluctuations and possible forcing factors.
Wang et al. (
2001
) studied the
18
O records from five stalagmites from Hulu
Cave near Nanjing covering the past 75 ka. They found that the East Asian monsoon
was more intense when Greenland temperatures were warmer and weaker during
cold intervals in Greenland. Cheng et al. (
2006
) extended the Hulu Cave record
back to the penultimate glacial and deglacial phases and discovered that both of
the glacial terminations comprised two phases, with an interval of weak monsoon
followed by an abrupt increase in monsoon strength. They also identified at least
sixteen millennial scale events during the penultimate glacial period, comparable to
the Dansgaard-Oeschger cycles of the last glacial period (see
Chapter 3
). Wang et al.
(
2008
) later obtained a 224 ka
18
O record based on 127
230
Th ages obtained from
twelve stalagmites in Sanbao Cave in central China. They deduced that changes in
the strength of the East Asian monsoon reflect orbitally controlled variations in high
northern latitude insolation, with a 23 ka periodicity punctuated by millennial scale
events. Yuan et al. (
2004
) analysed the
18
O record of precipitation changes over the
past 160 ka from two stalagmites in Dongge Cave, 1,200 km west-south-west of Hulu
Cave. These were consistent with those inferred from Hulu Cave and showed that the
Last Interglacial Monsoon began quite abruptly at 129.3
±
0.9 ka and ended equally
abruptly at 119.6
0.6 ka, consistent with the changes in temperature in the North
Atlantic region that are evident in Greenland ice cores.
Speleothems have also been used to resolve the question of whether the East Asian
summer monsoon (EASM) and the Indian summer monsoon (ISM) were in or out
of phase. Winds blowing from the south-east flowing across the western Pacific and
the South China Sea into eastern China and central China control the EASM. Winds
blowing from the south-west across the Indian Ocean and Bay of Bengal into southern
China control the ISM. Cai et al. (
2006
) obtained a high-resolution
±
18
O record from
a stalagmite dated between 53 and 36 ka from Xiaobailong Cave in south-east China
