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This study shows temperature generally cooler than present one in southern China, …


Biology Articles » Bioclimatology » East Asian Monsoon and paleoclimatic data analysis:a vegetation point of view » A multi-proxy technique to reconstruct climate timevariability in Inner Mongolia

A multi-proxy technique to reconstruct climate timevariability in Inner Mongolia
- East Asian Monsoon and paleoclimatic data analysis:a vegetation point of view

The syntheses presented above are based either on pollen data or lake lavels data. A multi-proxy approach is now presented to reinforce and precise these results (Jiang et al., 2008). It is based on a record taken from Lake Bayanchagan (115.21 E, 41.65 N, 1355 m a.s.l, Fig. 1) in Inner Mongolia, which is today almost completely dry due to anthropogenic water use, with only small patches of shallow water main- tained by summer rain. It is situated at the current north- ern edge of the summer monsoon. The mean annual tem- perature in the area is about 3 C, and total annual precipita- tion is 300­400 mm. About 70% of the precipitation occurs during the summer. The data used are pollen taxa counted for 90 pollen assemblages and 2066 surface samples. The taxa are grouped into 17 plant functional types (e.g. boreal evergreen conifers, steppics, grass, temperate summergreen trees, etc.) to reduce the number of variables and also to consider together taxa which respond in the same way to climatic variations. These plant functional types (PFT) are used to reconstruct climate by the modern analogue method (PFT-MAT) proposed by Davis et al. (2003) and Jiang et al. (2006). The climatic variables considered are the tempera- ture of the coldest month (MTCO), the temperature of the warmest month (MTWA), the annual precipitation (MAP), the ratio actual evapotranspiration over potential evapotran- spiration (). These variables are calculated by linear inter- polation from meteorological stations (Jiang et al., 2006) and is obtained from monthly temperature, precipitation and sunshine variables using the Priestley-Taylor equation (Pren- tice et al., 1992).

To these proxies, are added total pollen concentrations, Pediastrum (a green algae that indicate shallow lake water) concentrations and 18 O of authigenic carbonate, i.e. on the < 40 µm fraction (Jiang et al., 2008 1 ). These three proxies show a similar general pattern during the Holocene (Fig. 2a). Before 11 000 cal yr BP, there is no Pediastrum in the lake. Pollen concentrations are lower than 2×10 5 grains/ml. All 18 O values of authigenic carbonate are between -3 and - 1 VPDB. Similar values are found after 5 ka BP and in between, there is high concentrations of pollen and Pedi- astrum and low 18 O values. As Jiang et al. (2008) 1 have shown that these three variables are controlled by balance be- tween precipitation and evaporation, they can be synthetized a common signal, given here by their first principal compo- nent (Fig. 2c). Jiang et al. (2008) 1 have used PFT-MAT constrained by the first principal component PC1 (Fig. 2c) as an indicator of , a variable directly related to the water stress. This con- strained analysis has already been proposed with different proxies by Seret et al. (1992); Guiot et al. (1993); Cheddadi et al. (1996); Magny et al. (2001). For each fossil pollen spectrum, analogues were selected from the modern pollen spectra dataset subject to a broad consistency requirement according to values. If we note the difference between of the analogue and the modern o at the lake (56%), only the analogues i with a i compatible with PC1 at time t, denoted C t , were retained. This compatibility is defined as follows:

eastm01.JPG

Figure 2 show the results obtained for the Lake Bayan- chagan core: the reconstructed climatic variables are com- pared with the constraint PC1 and the scores of the arbo- real pollen taxa (Fig. 2b).

This enables one to question the direct relationship often proposed between an increase in tree components of pollen assemblages and a warmer and wetter climate (Shi et al., 1993; Liu et al., 2002; Xiao et al., 2004). So, the highest tree scores of trees during the Holocene in Lake Bayanchagan occurred between 8000 and 5500 cal yr BP (Fig. 2b). However, the peak period of trees was not in phase with the warmest and wettest climate re- constructed between 11 000 and 8000 cal yr BP (Fig. 2d­f), suggesting that a single climatic variable is not the trigger- ing factor. In contrast, variations in tree components and were consistent (Figs. 2b and g). is an integrated measure of annual amount of growth-limiting drought stress on plants related to both temperature and precipitation, and is one of primary factors influencing vegetation distributions (Prentice et al., 1992). The similarity in tree components and varia- tions inferred from our study indicates that it is also the main controlling factor for growth of trees over the Holocene in In- ner Mongolia. does not reach its maximum before 8 ka BP even if MAP is maximum because evaporation is too strong. The water stress is minimum only when temperature has de- creased by a few degrees. The most favourable period for forest development is then between 8 ka and 5 ka BP. The MAP record during the Holocene at Lake Bayancha- gan is similar to 18 O records of stalagmite calcite from Dongge Cave and Shanbao Cave (Fig. 1) in EAM regions (Dykoski et al., 2005; Shao et al., 2006). Shifts in 18 O val- ues of the stalagmite from the cave largely reflect changes in 18 O values of meteoric precipitation at the site, which in turn relates to changes in the amount of precipitation. The 18 O results show that monsoon precipitation increased dra- matically at the start of the Holocene (11 500 cal yr BP) and remained high for 6000 cal yr BP (Dykoski et al., 2005). This timing is consistent with other paleoclimatic records in EAM regions (Zhou et al., 2004, 2005). Both the Lake Bayanchagan data and stalagmite 18 O records from Dongge Cave and Shanbao Cave show the termination of mon- soon precipitation maximum was abrupt between 6000 and 4400 cal yr BP. The second warm and humid period at Lake Bayanchagan centered at 6000 cal yr BP. This event was characterized by increased MTCO, decreased MTWA and high precipitation (Fig. 2d­f). These results agree with a marked increase in winter temperatures across eastern China at 6000 cal yr BP estimated from pollen data (Yu et al., 1998) and simulated by climatic model (Yu et al., 2003). The short-term cold event between 8500 and 8300 cal yr BP was characterized by de- creases in both winter and summer temperature (Fig. 2d­f). Even if such event has been recorded in several places and in particular in the GRIP and GISP2 records (Alley et al., 1997; Rohling and Palike, 2005), it cannot be considered as significant in our reconstruction, as several such peaks are reconstructed during the Holocene.


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