Highly negative oxygen isotopes in precipitation in southwest China and their significance in paleoclimatic studies
Introduction
As two important subsystems of the Asian monsoon (Huang and Tao, 1986), the East Asian summer monsoon (EASM) composed of a tropical monsoon and a subtropical monsoon mainly dominated southeast China, the middle and lower reaches of the Yangtze River, and north China. The major dominating regions of the Indian summer monsoon (ISM) as a tropical monsoon in China are the southwest corner (typically, Yunnan Province) and the south Tibetan Plateau (Wang and Lin, 2002). Meanwhile, Guizhou and Guangxi provinces in south China are considered as the region commonly influenced by both the EASM and ISM (Wei and Lin, 1994, Wei and Gasse, 1999). Modern meteorological studies have demonstrated that these two subsystems are different, and their relationship is complicated (Huang and Tao, 1986, Wang and Lin, 2002).
The paleoclimatic histories of the ISM and EASM regions and their possible interactions have been widely studied and intensely debated (e.g., Wang et al., 2003, Hong et al., 2005, Zhou et al., 2008, Han et al., 2010, Wang et al., 2010, Li et al., 2014, Allu et al., 2015). The typical and influential viewpoints are as follows. (i) Based on the lake level, pollen, eolian deposit evidence and similar data, in combination with modeling results, the Holocene optimum, as defined by the peak summer monsoon precipitation, was proposed that asynchronous occurred in China, with a withdraw trend from north to southeast China (EASM region), but occurred earliest in southwest China (ISM region; Wu et al., 1994, An, 2000). (ii) Based on a comparison between the peat α-cellulose δ13C data from the Hani site in northeast China (which are believed to indicate a record EASM intensity) and from the Hongyuan site in southwest China (which are believed to indicate a record ISM intensity), inverse phase oscillations between the EASM and ISM have been suggested on the interannual-to-orbital time scales during the past 12,000 years (Hong et al., 2005, Xu et al., 2006). (iii) Based on synthetic analyses of modeling results and stalagmite paleoclimatic records, it has been proposed that the relationship between the EASM and ISM regions is more complicated than in-phase or anti-phase, and it might vary with different time scales (Li et al., 2014).
High-quality and precisely dated stalagmite oxygen isotopic records (δ18Os) have been widely reported for both ISM and EASM regions in the last 15 years (e.g., Wang et al., 2001, Wang et al., 2005, Wang et al., 2008, Fleitmann et al., 2003, Dykoski et al., 2005, Hu et al., 2008, Cheng et al., 2009, Dong et al., 2010, Kotlia et al., 2012, Muangsong et al., 2014, Cai et al., 2015, Tan et al., 2015, Han et al., 2016), and these records were soon used in discussions on the relationship between ISM and EASM. Using a numerical climate model with an embedded oxygen isotope model, the paleoclimatic conditions and oxygen isotopic composition of precipitation (δ18Op) in both ISM and EASM regions during the last glacial maximum interval and the Heinrich 1 event have been simulated and compared with the relevant δ18Os records, and the results have demonstrated that the δ18Os records from southeast China (EASM region) reflected the ISM intensity rather than the EASM precipitation (Pausata et al., 2011). Recently, the δ18Os records from both the ISM and EASM regions, and the paleoclimatic records from the EASM region, particularly those from north China, have been comprehensively compared. The results demonstrated spatially coherent variabilities and statistically good correlations on different time scales in the δ18Os records; however, the most negative δ18Os stage during the early Holocene in southeast China is apparently different from the most humid stage during the mid-Holocene in north China (EASM region). Therefore, it has been concluded that the δ18Os in the EASM region was mainly controlled by the rainfall variability in the ISM region via the remaining moisture carried by the ISM that flowed out to the EASM region (Yang et al., 2014, Liu et al., 2015).
The δ18Os records from both the ISM and EASM regions show spatially coherent variabilities and statistically good correlations, particularly the high resolution Holocene δ18Os records (e.g., Fleitmann et al., 2003, Dykoski et al., 2005, Hu et al., 2008, Dong et al., 2010), which may not be adopted as convincing evidence for the assumption that the δ18Op signal in the EASM region comes from the ISM region. Unlike the Holocene, the most recently reported δ18Os records from the central China showed an overall opposite trend to that from India for the last 98 years (Tan et al., 2015), and to that from southern India for the period of ca. 108–99 ka BP (Allu et al., 2015). That is, the moisture sources for the ISM and EASM regions still require additional studies. In this paper, based on the comparison of δ18Os records from southwest China (ISM region) and from east China (EASM region), in combination with the analyses of modern δ18Op from both ISM and EASM regions, we show the different moisture sources for the two regions.
Section snippets
Data and methodology
Traditionally, Yunnan Province in the southwest corner of China has been widely accepted as a typically dominating area of the ISM (Cai et al., 2006, Cai et al., 2015, An et al., 2011, Chen et al., 2014). Consequently, the δ18Os records from three caves located in Yunnan Province have been selected in this work as representative of the variation of the past δ18Op values in this area. These are: the Xianren (Zhang et al., 2006), Xiaobailong (Cai et al., 2006) and Fulu (Zhu et al., 2015) caves.
Comparison of the δ18Os records
Spanning the time interval of 36–54 ka BP (Fig. 2), the δ18Os records from the Xiaobailong Cave (Yunnan Province, southwest China) (ISM region) (Cai et al., 2006), the Xiangshui Cave (Guangxi Province, south China) (Cosford et al., 2008), and the Hulu Cave (Jiangsu Province, east China) (EASM region) (Wang et al., 2001) were plotted on an identical time scale for comparison (Rao et al., 2015). Along the time sequence, these δ18Os records show a roughly consistent variation. However, the
Conclusions
The δ18Os records from Yunnan province (a typical ISM region) and from the EASM region were compared. The results clearly demonstrated that the δ18Os data in Yunnan Province were much more negative, for both the last glacial (36–54 ka BP) and mid–late Holocene (2–6.5 ka BP), as well as for modern δ18Op data, indicating a general phenomenon.
Detailed analyses demonstrated that such a phenomenon cannot be explained by any of the latitude, temperature, amount, and altitude effects. Therefore, the
Acknowledgements
The work was supported financially by the National Natural Science Foundation of China (Grant Nos. 41372181 and 41171091), and the project of Chinese Ministry of Education (Grant No. 113057A).
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