Holocene moisture evolution in arid central Asia and its out-of-phase relationship with Asian monsoon history
Introduction
A variety of proxy records has been used to document climate change during the Holocene in many parts of the world. Understanding the spatial and temporal patterns of climate change in a given region may provide insights into the underlying climate-forcing mechanisms. In east and south Asia, Asian monsoon variations during the Holocene have been well-documented by precisely dated cave deposits (e.g., Fleitmann et al., 2003; Yuan et al., 2004; Shao et al., 2006). The Indian and East Asian summer monsoons were enhanced shortly after the Younger Dryas (YD) at the onset of the Holocene, was strongest in the early and mid-Holocene, and weakened after the mid-Holocene. This pattern closely follows changes in summer insolation at low latitudes (Kutzbach, 1981). Similar changes in the strength of the Asian monsoons have been documented by other proxy records from peats (Hong et al., 2003), lake sediments (Lister et al., 1991; Gu et al., 1993; Hodell et al., 1999; Xiao et al., 2004; Shen et al., 2005) and marine sediments (Wang et al., 1999; Gupta et al., 2003). However, Holocene climate patterns in arid central Asia (referred to as ACA in this paper) are poorly documented and understood. This lack of understanding is partly due to the complex interplay of competing forcing factors controlling regional climate; these factors include the low-latitude summer monsoonal circulation, the mid-latitude Westerlies, and orographic influences of the Tibetan Plateau. For example, the western part of ACA experienced a wet mid-Holocene and a dry early and late-Holocene, as documented by lake-level changes in the Aral Sea (Boomer et al., 2000), while the eastern part of the region near the monsoon limit showed a variable wetness during the entire Holocene, with some possibly drier intervals in the mid-Holocene (Chen et al., 2003a, Chen et al., 2003b, Chen et al., 2006; Schettler et al., 2006; Zhao et al., 2007).
For expediting the following discussion, we divide the southeastern part of the Eurasian continent into three climatically distinct regions: humid Asia mainly controlled by summer monsoonal circulation; ACA dominated by the Westerlies; and a transitional zone around modern Asian summer monsoon limit in semi-arid northwest China and the southern Mongolian Plateau. In this study we focus only on the westerly dominated ACA for several reasons. First, it is one of the driest regions in the world, and its sparse water resources and fragile ecosystems in a general dry climate would be very sensitive to abrupt changes in rainfall (Qin et al., 2005; Narisma et al., 2007). Second, many lakes in this region, including Aral Sea and Caspian Sea, experienced similar changes during the Holocene (Qin and Yu, 1998). Also, a dramatic decline in lake levels has occurred during the last century at several lakes; such changes have attracted considerable attention because of concerns about shortage of water resources and related ecological problems (Qin and Yu, 1998; Ferronskii et al., 2003). However, a recent lake-level rise and lake expansion in the last 20 years at most lakes in arid west China suggests a possible shift from a warm-dry to a warm-humid climate under global warming (Shi et al., 2007). Several syntheses have been published over the last decade, which provide valuable insights into understanding the Holocene moisture history of the region. However, most of these reviews have focused only on parts of arid central Asia, as previous studies were mostly restricted to China (Shi et al., 1993, Shi et al., 1994; An et al., 2000; Feng et al., 2006). A comprehensive review of Holocene moisture evolution for the entire region is useful to decipher and understand the complex palaeo-moisture evolution under the same atmosphere circulation, namely the Westerlies. In addition, there are potential problems with chronology and proxy interpretations in some of the early published palaeo records (cf. Qin and Yu, 1998; An et al., 2000; Feng et al., 2006). As a result, there remains confusion about the regional climate patterns, and there is an urgent need to standardize and synthesize palaeoclimatic information about ACA mainly based on recent published records. Such an updated synthesis will help not only understand the Holocene moisture history of the region, but also provide information to assess climate simulations using general circulation model (GCM) and regional climate model (RCM, e.g., Sato and Kimura, 2005).
Here we review 11 lake records from the westerlies-dominated region of ACA (Fig. 1). The objectives of this study are (1) to review recently published proxy climate records from lake sediments; (2) to derive a semi-quantitative reconstruction of moisture history during the Holocene; and (3) to understand potential controls and mechanisms of the observed temporal and spatial patterns.
Section snippets
Data sources and analyses
In ACA there are abundant climatic records from continental eolian deposits, including loess and dune deposits. However, these records tend to have low temporal resolution and be discontinuous, even in the thick loess deposits on the Chinese loess Plateau that have high accumulation rates (e.g., Lu et al., 2006; Stevens et al., 2006). Therefore, in this study we have selected records from lake sediments based on four criteria: (1) the selected sites should be from extant lakes, so that any
Temporal moisture change during the Holocene in arid central Asia
Many lakes, including Bosten Lake (5), Bayan Nuur (6), Juyan Lake (9), Gun Nuur (10) and Hulun Nuur (11), were totally dried up or were very shallow during the early Holocene before 8 ka (Fig. 3). Large deep lakes, such as the Aral Sea, Issyk-Kul and Lake Van, were still present, but at low lake levels. The Caspian Sea also reportedly had a lower water level before 8 ka than at present (Kazanci et al., 2004), though this site is not included in this synthesis because of its low sampling
Spatial patterns of Holocene moisture evolution in arid central Asia and monsoonal Asia
We divide the Holocene into four periods to investigate the regional patterns of Holocene moisture change in ACA and compared them with those in monsoonal Asia: early Holocene (ca 11–8 ka), mid-Holocene (8–5 ka), late Holocene (5–2 ka), and the last 2 ka. The Holocene moisture histories show distinct spatial patterns between ACA and monsoonal Asia (Fig. 4). Climate was clearly dry in the westerly dominated ACA during the early Holocene but it was humid during the mid-Holocene (Fig. 4). A moderately
Contrasting patterns of Holocene moisture histories between arid central Asia and monsoonal Asia
Various proxy data from speleothem, lake sediments, peat cores and marine sediments as discussed above show similar patterns in effective moisture and precipitation change during the Holocene in monsoonal Asia. In their review, An et al. (2000) proposed that the Holocene optimum of the East Asian monsoon was asynchronous among different regions. However, evidence from recently well-dated records does not appear to support this hypothesis (Feng et al., 2006). Also, our synthesis shows that the
Possible forcing mechanisms for Holocene moisture evolution in arid central Asia
The Holocene ACA moisture history appears to be similar to the pattern of the Holocene sea-surface temperatures (SSTs) in the North Atlantic (e.g., Kaplan and Wolfe, 2006) and in the Norwegian Sea (Koç et al., 1993; Birks and Koç, 2002; Figs. 6c and d), as well as the air temperatures recorded in the GRIP ice-core (Dahl-Jensen et al., 1998; Fig. 6e) and in the European pollen records (Davis et al., 2003). The North Atlantic was warmest in the mid-Holocene between ca 9–8 ka and 5 ka after the
Conclusions
On the basis of palaeoclimatic records from 11 selected lakes in westerly dominated ACA, we find an out-of-phase relationship in Holocene moisture histories between ACA and the Asian monsoon-controlled region. In contrast to monsoonal Asia where maximum moisture occurred during the early to mid-Holocene, maximum moisture (precipitation) occurred during the mid-Holocene in ACA. We suggest that the out-of-phase pattern results from different controlling factors of moisture change in both regions.
Acknowledgements
This synthesis results from discussions at the INQUA RACHAD 2006 Workshop held on 25–27 July 2006 in Lanzhou, China. We thank Zhaodong Feng (Lanzhou University, China), Hilary Birks (University of Bergen, Norway), Georg Schettler (Geoforschungszentrum Potsdam, Germany), Xunlin Yang (Lanzhou University, China) and other workshop participants for their valuable contributions and discussion. We also thank two anonymous reviewers for their constructive comments and suggestions. This research was
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