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- East Asian Monsoon and paleoclimatic data analysis:a vegetation point of view

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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