Research articleChanges in the depth of Lake Qinghai since the last deglaciation and asynchrony between lake depth and precipitation over the northeastern Tibetan Plateau
Introduction
Lake Qinghai is a closed-basin salty lake on the northeastern margin of the Tibetan Plateau, located in a transitional region between arid and semi-arid areas, and the climatic junction where the Asian summer monsoon (ASM) and the Westerlies interact strongly (An et al., 2012; Yao et al., 2013). Numerous studies on Holocene climatic changes have been conducted in this region, including analyses of pollen (Liu et al., 2002; Shen et al., 2005), biomarkers (Liu et al., 2007; Wang et al., 2015), carbonate δ18O and ostracod species (Lister et al., 1991; Liu et al., 2007; Li and Liu, 2017), total organic carbon (TOC) and δ13C of TOC (An et al., 2012; Liu et al., 2013), and compound-specific leaf wax isotopes (Liu et al., 2015a; Thomas et al., 2014; Liu et al., 2017).
Unlike typical ASM climate patterns, or climatic conditions controlled by the westerly circulation, the climate of the northeastern Tibetan Plateau was relatively complicated during the Holocene as it was controlled by several climate systems (An et al., 2012; Jin et al., 2015; Chen et al., 2016). An et al. (2012) suggested that the Holocene climate in this region was dominated by the ASM but was repeatedly disrupted by North Atlantic-related climatic events. Other research has agreed that the ASM was dominant in this region during the Holocene, but propose that moisture and monsoon precipitation weakened in the early Holocene and that the maximum monsoon intensity occurred during the mid-Holocene (Chen et al., 2016; Li et al., 2017). This disparity comes from interpretations of different paleoclimate proxies, which indicate that the maximum monsoon intensity period occurred during either the early or mid-Holocene. For example, depleted isotopic values of both ostracod δ18O and n-alkane δD in lake sediments indicate a wetter climate in the early Holocene (Liu et al., 2007; An et al., 2012; Liu et al., 2017; Li and Liu, 2017). Conversely, tree-pollen studies show that the warmest and wettest climate occurred in the mid-Holocene (Shen et al., 2005; Li et al., 2017) while ASM intensity began to increase in the early Holocene and reached a maximum in the mid-Holocene (Chen et al., 2016). This is further supported by studies of paleo-shoreline records by optically-stimulated luminescence (OSL) dating (Liu et al., 2011, Liu et al., 2015b), and analyses of lake water salinity changes (Zhang et al., 1994; Li and Liu, 2014).
At present, there are different interpretations of climate patterns or environmental changes in this transitional region. Previous work has described terrestrial records (Chen et al., 1991; Lu et al., 2010; Liu et al., 2012; Liu et al., 2015b) and lacustrine records (Shen et al., 2005; An et al., 2012; Liu et al., 2015a; Thomas et al., 2014; Wang et al., 2015; Liu et al., 2017); however, reliable and quantitative climatic records are still lacking. Especially, due to the complicated limnological processes, not all lake sediment records can be directly interpreted as climatic changes. Determining the presence and proportion of aquatic and terrestrial plants in sediment cores provides valuable information relating to the changes in lakes (e.g., surface area, depth) which are indicative of climatic or environmental conditions. The use of a Paq index (an aquatic macrophyte and terrestrial plant ratio) has been proposed for distinguishing terrestrial and aquatic sources of plant material, as middle-chain n-alkanes are usually more abundant in aquatic plants than in terrestrial plants (Ficken et al., 2000). For modern terrestrial and aquatic plants on the Tibetan Plateau, Paq records indicate narrow distribution ranges, and considerable distinctions from each other (Wang and Liu, 2012; Liu et al., 2017). A Paq value of 0.35 was used as the boundary value in determining the significance of the contribution of aquatic plants in sediments (Wang et al., 2018).
Using the organic geochemistry index in lacustrine and terrestrial records, we aimed to better understand the characteristics of Holocene climatic changes in this region by verifying: 1) the history of Lake Qinghai by interpretation of biomarkers in sediment cores, 2) the changes in humidity in the region, and 3) the possible causes of asynchrony among climatic records for the northeastern Tibetan Plateau.
Section snippets
Regional setting
Lake Qinghai (36°32’to 37°15′N, 99°36′ to 100°47′E) is the largest closed basin saline lake in Northwest China with an altitude of 3193 m (Fig. 1). The lake covers about 4400 km2, with a catchment area of 29,660 km2. The modern lake water has an average salinity of 15.5 g/L and pH of 9.1. The maximum water depth is ca. 27 m and the average water depth 21 m. To the northeast and southeast of Lake Qinghai, a few small lakes have been formed because the level of Lake Qinghai has decreased in the
Paq values as indicators of lake depth changes
In lacustrine environment, the abundance of aquatic plants depended on a number of factors, including inter-annual changes in temperature and length of growing season (Rooney and Kalff, 2000), ice cover (Suren and Ormerod, 1998), latitude and altitude (Nõges et al., 2003; Jones et al., 2003), water flow and turbidity (Mackay et al., 2003), salinity and nutrients (Deegan et al., 2005). Light and water depth were considered as the critical factors in limiting the distribution of aquatic plants (
Conclusions
Reconstruction of historical changes in the depth of Lake Qinghai since the last deglaciation show that on a millennial scale, five water balance states occurred since the last deglaciation. The lake depth increased with a fluctuating upward trend from the shallowest level at 15.6 ka to the maximum depth at 5.9 ka, followed by a slow decrease in depth. The relatively humid late Holocene climate in Lake Qinghai was related to the weakened evaporation trend after the early Holocene.
We have shown
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
Authors would like to thank Prof. Xiangjun Liu for help in field work. This work was supported by National Natural Science Foundation of China (Grant No. 41773010; 41473022).
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