Geoscience Reference
In-Depth Information
as a result of increased continental area when sea level was lower), and the vegetation
at that time was dominated by C
3
plants, in contrast to C
4
grasses during full glacial
times. Some recent research involving currently active speleothems in Australia has
focussed on comparing seasonal records from instrumentally monitored climatic data
with subannual records of trace elements and
18
O fluctuations evident in annual
speleothem growth rings (Treble et al.,
2003
;Trebleetal.,
2005
).
14.3.4 Speleothem studies from monsoonal China
The loess record fromnorth-central China extends backmore than 2.5Ma and provides
the longest and most detailed continental record of past climatic oscillations for
anywhere on earth (
Chapter 9
). However, although this record is reasonably accurately
dated, the error terms are large, and so the loess chronology is far from precise. In order
to supplement themore recent portions of the loess record, increasing attention is being
paid to the speleothem records from different parts of China. Most of the caves studied
so far are not within the drier parts of China, but they are nonetheless very important
for our understanding of past climatic changes in the deserts of Asia and elsewhere.
There are several reasons for making this assertion. First, the Chinese speleothems
are very precisely dated and provide a high-resolution record of past changes in
the intensity of the summer and winter Asian monsoon and its two variants, the
Indian Monsoon and the East Asia Monsoon, both of which affect the desert margins.
Second, the fine resolution allows detailed comparison with both the Greenland ice
core records and those from Antarctica, providing a glimpse into possible changes
in global atmospheric circulation patterns. Third, the record allows comparison with
regions as far distant as north-east Brazil, the Cariaco Basin off Venezuela, peninsular
Arabia and the North Atlantic, once again providing a more coherent view of global
climatic fluctuations in and beyond the desert margins.
The
18
O records from five stalagmites from Hulu Cave near Nanjing spanning the
past 75 ka showed that the East Asian monsoon was more intense when Greenland
temperatures were warmer and weaker during cold intervals in Greenland (Wang et al.,
2001
;Wangetal.,
2005
). The Hulu Cave record was later extended back to the pen-
ultimate glacial and deglacial phases, and analysis of three stalagmites demonstrated
that both of the glacial terminations occurred in two phases, with an interval of weak
monsoon (135.5 to 129.0 ka) followed by an abrupt increase in monsoon strength
(Cheng et al.,
2006
). The overall trend reflected insolation changes related to orbital
fluctuations, with some possible influence from changing ice sheet dynamics. Within
these broader trends, at least sixteen millennial scale events were evident during the
penultimate glacial period, comparable to the Dansgaard-Oeschger cycles of the last
glacial period (see
Chapter 3
). A 224 ka
18
O record based on 127
230
Th ages obtained
from twelve stalagmites in Sanbao Cave in central China confirmed that the record
of changes in the strength of the East Asian monsoon reflects orbitally controlled
