We have explored the temporal variability of the Lake Bayanchagan
record located in a sensitive region at the northern
edge of the EAM. The use of a multiproxy approach coupled
with robust statistics have enlightened the complexity of the
climatic signal. A key problem in this respect is the timing
of the monsoon enhancement. Monsoon increase is translated
in terms of increased precipitation. Then the period of
maximum EAM occurred between 10.5 and 8 ka BP. A too
rapid interpretation of the tree pollen curve should put this
maximum between 8 and 5 ka BP. It is clear that precipitation
was higher than at present time across the two periods. But,
extension of forest depends as well of temperature than precipitation,
and our quantitative evaluation of several proxies
show a more complex behaviour than Dongge and Shanbao
cave series. This may also be due to spatial differences, the
caves being located at much more lower latitudes than the
lake (Fig. 1). This is confirmed by the post-5 ka BP decrease
in the lake records where precipitation returns to the Late-
Glacial level, while in the cave record, 18O remains at an
intermediate level. This might be explained by a rapid northward
advance of the northern limit of the summer monsoon at
11.5 ka BP (beyond 41 N) followed by a slow retreat, falling
back south of Lake Bayanchangan by 5 ka, while the caves,
being further south, remain under the monsoon influence.
This illustrates well that Lake Bayanchagan, at the northern
edge of the EAM zone, is a sensitive record of the monsoon
signal.
A second implication concerns the physical mechanisms.
EAM enhancement is related to summer radiation which is
maximum at 9 ka BP and rapidly decreases to be at 6 ka BP
on the same level than at 12 ka BP (Berger, 1978; An, 2000).
When a large number of climate model simulations are compared
(Braconnot et al., 2002), a robust feature is that the
extension of the monsoon is related to the Eurasian continent
warming. This might explain why the 8–5 ka BP period
is characterised by a slight decrease of EAM accompanyed
by a decrease of temperature more marked in this
northern lake than in lower latitudes. Maximum temperature
of the warmest month falls by 5C at 8 ka BP (but keep a
level above the present one), which shows a mitigation of the
impact on vegetation of the monsoon weakening by a sharp
reduction of the evapotranspiration.
The analysis of the spatial variability of the Chinese climate
at 6 ka BP – even if the 6 ka period is not the period
of maximum monsoon enhancement – permits to replace the
timing found for Lake Bayanchagan in a larger context. Figure
3 shows that some sites in the region of this lake have
already a reduced precipitation, while , which represents
the water availability for vegetation, is still higher than at
present. This is still a period favourable to maintain forest,
even with a precipitation reduction. Annual temperature distribution
shows higher values than at present time in northern
China, but lower in southern and central China where monsoon
had still a higher influence. This illustrates well the fact
that northern China was more at that time under the influence
of the Eurasian continent while the rest of China was under
still the influence of the ocean through the Pacific Subtropical
High.
A last point is the use of a new methodology of climate
reconstruction based on vegetation model inversion. As already
mentionned, this mechanistic model offers the possibility
to escape from too constraining modern conditions as
high atmospheric CO2 concentrations or a climate seasonnality
different from modern one (in relation with insolation).
The climatic maps obtained for 6 ka BP confirmed previous
results based on modern analogues, likely because CO2 concentration
is sufficiently high. Wu et al. (2007) have shown
that, for the Last Glacial Maximum conditions, biases are introduced
by the fact that CO2 is sufficiently low to have limited
vegetation productivity in a comparable amplitude than
climate change.
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