Increasing effective moisture during the Holocene in the semiarid regions of the Yili Basin, Central Asia: Evidence from loess sections
Introduction
Water resources play an important role in the sustainable development of societies in the semiarid and arid regions of Central Asia (approximately 35–50°N and 50–110°E) where several climate systems converge, including the Westerlies, the Indian summer monsoon, and the East Asian summer and winter monsoons (Fig. 1a). The current moisture regime is dominated by the influences of the North Atlantic Ocean, the Mediterranean Sea, and the Black Sea via the Westerlies (Li, 1991; Aizen et al., 2004, 2006). This regime is notably different from South and East Asia, which benefits from Asian summer monsoon precipitation (Wang, 2006; An et al., 2015). Changes in past moisture in Central Asia, particularly during the Holocene, are complicated and have been hotly debated in the last few decades (e.g., Shi et al., 1993; Tarasov et al., 2000; An et al., 2006; Chen et al., 2008, 2016; 2019; Li et al., 2011; Cheng et al., 2012; Wang and Feng, 2013; Long et al., 2014, 2017; Cai et al., 2017; Li et al., 2018; Wang et al., 2019; Xu et al., 2019).
Previous studies have proposed the following three different interpretations of Holocene moisture variations in Central Asia: (1) a humid early Holocene, a moderately humid middle Holocene, and a drier late Holocene, generally in agreement with the Asian summer monsoon-influenced regions (e.g., Tarasov et al., 2000; Rudaya et al., 2009; Li et al., 2011; Cheng et al., 2012); (2) a dry early Holocene, a humid middle Holocene, and a moderately humid late Holocene (e.g., Feng et al., 2006; Chen et al., 2008; Zhao et al., 2009; Yang and Scuderi, 2010; Long et al., 2014, 2017); and (3) a general wetting trend throughout the Holocene (e.g., Ran and Feng, 2013; Wang and Feng, 2013; Hong et al., 2014; Chen et al., 2016; Wang et al., 2019; Xu et al., 2019). In these studies, the observed moisture variations have largely been attributed to changes in precipitation caused by the Asian summer monsoon and/or the Westerlies.
Most Holocene paleoclimate reconstructions for Central Asia are chiefly based on lacustrine sedimentary deposits (Feng et al., 2006; Herzschuh, 2006; Chen et al., 2008; Wang and Feng, 2013), speleothem (Cheng et al., 2012; Cai et al., 2017), ice cores (Thompson et al., 1997), loess (Li et al., 2015; Zhao et al., 2015; Chen et al., 2016; Wang et al., 2019), sand dune deposits (Yang et al., 2006a; Yang and Scuderi, 2010; Li and Fan, 2011; Long et al., 2014, 2017) and peat (Hong et al., 2014; Xu et al., 2019). Similar to the loess deposits in Central China, loess sequences in Central Asia are also believed to record paleoclimatic changes (e.g., Dodonov and Baiguzina, 1995; Ding et al., 2002; Fang et al., 2002; Machalett et al., 2008; Feng et al., 2011; Song et al., 2014; Chen et al., 2016; Li et al., 2018; Wang et al., 2019). Loess deposits there are less influenced by human activities and pedogenesis effects, making them reliable for Holocene paleoclimate reconstruction. Recent investigations of the Central Asian loess sequences (Li et al., 2015; Zhao et al., 2015; Chen et al., 2016; Wang et al., 2019) have demonstrated their potential to reveal Holocene moisture changes. For example, based on high-resolution luminescence dating and magnetic proxy analysis of a loess section on the northern slope of the Tianshan Mountains, a persistent wetting trend during the Holocene was revealed (Chen et al., 2016). More effort is needed, however, to verify previously estimated moisture evolution patterns recorded by loess at a broader scale in Central Asia.
The loess deposits in the Yili Basin have also been widely studied and verified as an archive of paleoclimate changes (e.g., Ye, 2001; Song et al., 2010; Li et al., 2018). Two recent studies have shown the potential of loess in the eastern Yili Basin at recording Holocene moisture changes (Li et al., 2018; Wang et al., 2019). However, more effort is now needed to systematically investigate the variations in Holocene moisture throughout the entire Yili Basin represented by loess deposits, particularly with high-resolution chronology. Here, we constructed chronologies for three Holocene loess sections in the semiarid regions of the Yili Basin using quartz optically stimulated luminescence (OSL) dating. Furthermore, pedogenesis and climate proxies of magnetic susceptibility (MS) and mean grain size (MGS) were used to decipher changes in effective moisture and their possible mechanisms were discussed.
Section snippets
Study area
The Yili Basin is surrounded by the Tianshan Orogenic Belt, with gentle topography to the west. The trumpet-shaped Yili Basin opens to the west, receiving rainfall from the North Atlantic Ocean, the Mediterranean Sea, and the Black Sea, and dust from the Central Asian deserts transported by the Westerlies (Fig. 1a and b). The Yili River runs through the basin from east to west (Fig. 1b). To the west of the Yili Basin are the vast Central Asian deserts, such as the Saryesik-Atyrau Desert, the
Loess sections and sampling
According to the loess distribution in the Yili Basin (Song et al., 2014), samples from three loess sections were collected, namely the Talede (TLD; 43°24′5″N, 83°2′13″E, 1020 m a.s.l.), Xiaoerbulake (XEB; 43°25′18″N, 82°59′33″E, 888 m a.s.l.), and Zhaosu (ZS; 42°56′3″N, 80°57′22″E, 1650 m a.s.l.) sections (Fig. 1b). The TLD and nearby XEB (∼5 km to the west of the TLD section) loess sections are located in the eastern Yili Basin and are situated within the high terraces of the Yili River. The
Chronology and dust accumulation
Table 1 shows 26 fine-grained quartz OSL ages and their related parameters for the TLD, XEB, and ZS sections. The dose rate is generally homogeneous across the three sites, and the De values are smaller than ∼50 Gy, which is much lower than the typical saturation value (D0 = ∼180 Gy) of fine-grained quartz at TLD (Kang et al., 2015). The OSL ages at each site gradually increase with depth, with no reversals, and range from 12.19 ± 0.81 to 0.72 ± 0.05 ka at TLD, 11.97 ± 0.62 to 1.13 ± 0.06 ka at
Conclusions
The pedostratigraphy and climatic proxies of MS and MGS coupled with high-resolution quartz OSL chronologies at the TLD, XEB, and ZS loess sections in the semiarid regions of the Yili Basin indicate that effective moisture experienced a continuously increasing trend throughout the Holocene. This resulted in a dry early Holocene, a moderately humid middle Holocene, and a wet late Holocene. Variations in effective moisture are suggested to have been primarily caused by continuously decreasing
Author statement
S.Kang designed the study, performed the fieldwork, conducted the measurements of magnetic susceptibility, grain size, and OSL ages, analyzed the proxy and age results, and wrote the paper. H. Roberts improved the English writing. W. Liu performed the fieldwork. X. Liu performed the lightness measurement. All authors contributed to discussion, interpretation of the results, and writing of the manuscript.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We sincerely thank the three anonymous reviewers for the scientific and constructive comments and suggestions that greatly improved our manuscript. We thank Prof. Z.D. Feng, Dr. W. Wang, and Dr. M. Ran for sharing the data in Fig. 4e. This study was supported by the National Key Research and Development Program of China (2016YFA0601902), the National Natural Science Foundation of China (41772177&41430532), the Strategic Priority Research Program of Chinese Academy of Sciences (XDB40010100), the
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