Lake level variations of Qinghai Lake in northeastern Qinghai-Tibetan Plateau since 3.7 ka based on OSL dating

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Abstract

Qinghai Lake is the largest internally drained lake in China and its unique location makes it sensitive to climate changes. Late glacial climate changes associated with variation in Qinghai Lake levels have been intensively investigated for the past 40 years, with particular attention paid to lake level fluctuation histories between the last interglacial and the Holocene. However, the details of lake level fluctuations during the Holocene are still unclear. Using both optically stimulated luminescence (OSL) dating of quartz (for 22 samples) and infra-red stimulated luminescence (IRSL) of feldspars (only for sample HYW1 whose IRSL age is 37 ± 15 years), a total of 23 samples are dated from paleoshoreline deposits, fluvial sediments and aeolian sands (a total of 13 sections) near the modern lake shore, with ages from 37 ± 15 to 3710 ± 350 years. These ages are used to reconstruct the lake level fluctuation history spanning the last 3700 years. The results indicate that: (1) the youngest IRSL age of 37 ± 15 years is in agreement with the independent age of 39–29 years, suggesting that luminescence dating is able to date decadal samples for sediments from the study area; (2) the lake experienced several oscillations imposed on an overall regressive trend during the past 3700 years; (3) the dated paleoshoreline deposits are generally related to warm and wet periods, suggesting that those shoreline deposits formed during cold and dry periods, if any, may have been modified by later transgressions; (4) lake level fluctuations during the period of 3700–240 years ago are generally consistent with the climate conditions identified in other proxies, with the highest lake level occurring about 1770 years ago; and (5) after 240 years ago the lake level dropped more rapidly, which is inconsistent with the proxy records (showing a warmer and wetter phase).

Introduction

Qinghai Lake lies in the northeastern part of the Qinghai-Tibet Plateau (QTP), and is the largest saline lake in China. Its unique geographical location near the junction of three climate systems (the East Asian monsoon, the Indian monsoon, and the Westerlies) makes it one of the most sensitive regions to climate change in the world (Fig. 1). Lake level changes spanning ∼130–20 ka have been investigated recently (Madsen et al., 2008, Liu et al., 2010, Rhode et al., 2010), and Holocene lake level fluctuations have also been investigated extensively (Yuan et al., 1990, Chen et al., 1990, Lister et al., 1991).

However, different views of lake level variation still exist (Zhang et al., 1988, Zhang et al., 1994, Chen et al., 1990, Chen et al., 1991, Yuan et al., 1990, Lister et al., 1991, Wang and Shi, 1992, Rhode et al., 2010). Zhang et al. (1988) pointed out that there was no high lake level during the Holocene, and that the lake terraces around the lake were formed in the Pleistocene. Yuan et al. (1990) reported that the highest lake level during the Holocene was 10–15 m above the lake level of 1990 AD (∼3194.2 m above sea level [asl]). Chen et al. (1990) and Lister et al. (1991) concluded that the lake level was lower than 3208 m asl during the Holocene, and that the highest Holocene lake level occurred between 7.4 ka an 6 ka. This was based, in part, on a section of aeolian sediments containing several soils at 3207 m asl. Chen et al. (1990) and Lister et al. (1991) also reported that the lake level dropped 12.9 m (from 3206.6 m to 3193.7 m asl) between1884–1990 AD. Lister et al. (1991) pointed out that a shoreline terrace at +12 m probably represents the highest Holocene lake level dating to about 7–6 ka. Chen et al. (1991) investigated loess deposits and the underlying lake deposits at the southern margin of Qinghai Lake and concluded that the lake level elevations fluctuated within a 30 m range during the Holocene, and within 20 m during the past 4 ka. Using radiocarbon dating, Wang and Shi (1992) dated organic materials or charcoal from four lake terraces at different elevations around the south margin of the lake, and concluded that (1) Qinghai Lake had very high lake levels during the Holocene, and a lake level dating ∼6 ka was about 60 m above the lake level in 1992 and about 27 m higher at ∼4.5 ka, and (2) the lake level has dropped ∼10 m during the last 1 ka. The chronologies for these studies were all established by radiocarbon dating. Zhang et al. (1994) reconstructed lake level fluctuations during the Holocene based on the linear relationship between the ratio of Sr/Ca and lake salinity. Their results indicate that the lake level dropped 17.5 m during the Holocene, and that the lake level at ∼3 ka was 13 m higher than the lake level elevation in 1994 (3193.7 m asl). However, Colman et al. (2007) pointed out that salinity reconstructions, based on the trace element geochemistry of biogenic carbonate, is extremely problematic for Qinghai Lake due to aragonite overgrowth and possible diagenetic alteration, and that the relationships between Sr/Ca and lake salinity is complicated. Yu (2005) investigated the calcareous sediments within two drill cores from Qinghai Lake (Q14B and Q16C), and concluded that the lake level varied between near desiccation and a depth of 30 m in the past 14 ka. That is, the maximum Holocene depth was only 4.5 m more than the depth of 25.5 m in 2008 AD. Henderson and Holmes (2009) reviewed the state of knowledge for the last millennium in Qinghai Lake, and concluded that a detailed picture of climate change cannot be established when using radiocarbon dates due to poor chronological constraints caused by a hard-water reservoir effect. However, estimates of lake level changes during the Holocene in virtually all these studies were either obtained indirectly from proxy records or were based on radiocarbon dating of organic samples collected from sediment layers below the paleoshoreline deposits.

As remnant paleoshoreline sediments represent past periods of stable lake levels, the age of their formation and their three-dimensional coordinates can be used to retrieve several past hydrological parameters, such as lake level elevation, lake area and lake water volume, that cannot be obtained from other proxy records. Due to the lack of suitable organic materials in most beach sediments, the direct dating of paleoshorelines by radiocarbon dating is difficult. However, OSL dating is particularly appropriate for aeolian deposits (Lai et al., 2007a, Lai et al., 2009, Long et al., in press) and shoreline deposits (Madsen et al., 2008, Rhode et al., 2010, Liu et al., 2010, Sun et al., 2010, Zhang et al., submitted for publication) that are abundant around Qinghai Lake. Madsen and Murray (2009) reviewed recent applications of the OSL dating to young sediments, and concluded that OSL is an accurate and reliable tool for determining the time of deposition of young water-laid sediments from coastal zones, and aeolian deposits from both coastal and inland environments.

Most of the easily identified low elevation paleoshorelines around Qinghai Lake were formed during the late Holocene (Li et al., 1995). Older paleoshorelines have mostly been either destroyed by later erosion or covered by later sediments and cannot be readily identified. The purpose of this paper was to date the low elevation paleoshorelines along Qinghai Lake with remaining significant landforms, but which have limited chronological controls. OSL dating was used to: (1) explore if OSL dating could be used to date young beach deposits in this arid plateau environment; and (2) determine the lake level fluctuation history during the late Holocene by directly dating paleoshoreline deposits.

Section snippets

Study area

Qinghai Lake lies on the northeastern QTP (36°32′–37°15′ N; 99°36′–100°46′ E), has a lake surface area of 4473 km2 and a water volume of 850 × 108 m3 (Fig. 1, Ma, 1998). The lake level of Qinghai Lake was 3193.4 m asl on March, 2010. Its catchment is prismatic in shape, with a west–east length of about 106 km, a north–south width of about 63 km and a perimeter of about 360 km. Observational data from the Gangcha meteorological station on the northwestern margin of the lake (Fig. 1b) indicate

OSL sample collection

Although paleoshorelines provide direct evidence of past lake level elevations, it is often difficult to obtain organic materials for 14C dating; and the particle sizes of their sediments are often too coarse for OSL dating. After extensive field investigations, an area around Haiyanwan on the northern shore of the lake was selected to collect OSL samples because of the presence of a number of well preserved and easily recognized paleoshorelines with limited vegetation cover (Fig. 1b and Fig. 2

Dating results

The values for equivalent doses, dose rates and OSL ages of all samples are presented in Table 1. The OSL age determinations for 23 samples analyzed from the 13 sites and the elevations of sampling sites are shown in Fig. 4. The OSL ages of the paleoshoreline deposits was within a range of 37 ± 15 to 2370 ± 350 years, with the error associated with individual ages ranging from 8.8% to 17.2% (except HYW1 where the error reached 40.5%). The age of sample HYW1, collected from the youngest

Discussion

Holocene climate changes reflected in Qinghai Lake proxy records have been investigated by many researchers during the late 20th and early 21st centuries (Chen et al., 1990, Lister et al., 1991, Liu et al., 2003, Liu et al., 2006, Liu et al., 2007, Shen et al., 2005, Ji et al., 2005, Ji et al., 2009, Yu, 2005, Liu et al., 2008). Some of these investigators have proposed that Qinghai Lake experienced a warm and wet climate during the early and middle Holocene, but after ∼4500 cal years BP the

Conclusion

Changes in the levels of Qinghai Lake during the late Holocene were investigated using OSL dating of 23 samples from paleoshoreline deposits, fluvial sediments and aeolian sands near the modern lake shore. Twenty-two samples were dated using quartz OSL and one sample using IRSL due to the low signal level of quartz OSL. The IRSL age of 37 ± 15 years ago for young shoreline deposits (sample HYW1) is in agreement with an independent age of 39–29 years, which was based on analyses of topographic

Acknowledgements

This study was financially supported by a One-Hundred Talent Project of CAS granted to ZPL and China NSF grants (40872119, 40761010). We thank JunFeng Ji for providing the redness data, and ZhongHui Liu for the temperature and salinity reconstruction data. Thanks are also given to Lewis Owen, Steffen Mischke and an anonymous reviewer for their constructive suggestions and language corrections.

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