Elsevier

Quaternary International

Volume 212, Issue 1, 15 January 2010, Pages 2-13
Quaternary International

Stable isotope and ostracode species assemblage evidence for lake level changes of Nam Co, southern Tibet, during the past 600 years

https://doi.org/10.1016/j.quaint.2008.12.010Get rights and content

Abstract

Ostracode species assemblages and stable oxygen and carbon isotopes of shells from two 210Pb-dated short cores of 78 cm and 47 cm length taken from Lake Nam Co, southern Tibet, provide information about lake level history of the past approximately 600 years at decadal resolution. A transfer function is applied to reconstruct paleo-water depths from modern ostracodes. Ostracode assemblages and isotope data from five species allow a subdivision into phases of lake level evolution. Before ∼1600 AD relatively low oxygen isotope values and high abundances of deep water (>30 m) ostracodes (i.e. ?Leucocythere dorsotuberosa, ?Leucocythere dorsotuberosa f. postilirata) suggest a lake level ∼10 m higher than today. Between ∼1600 AD and 1800 AD, more positive oxygen isotope values and higher abundances of shallow water ostracodes (i.e. Ilyocypris cf. mongolica, Candona xizangensis) suggest a lake level approximately 40 m below modern level. Between 1800 AD and ∼1920 AD oxygen isotopes are characterized by a trend to more negative values followed by a trend to more positive ones. These trends are interrupted by several short-term fluctuations but overall, oxygen isotope values suggest a drop in lake level. A clearer lake level picture is provided by species assemblages that suggest a continuously increasing lake level since 1900 AD. Precipitation data from meteorological stations nearby show an increase in precipitation in the last 30 years. The low lake level between 1600 AD and 1800 AD is contemporary to the Little Ice Age. During that cold period, meltwater input into the lake decreased. This underlines the sensitivity of the level of Lake Nam Co to temperature changes on the Tibetan Plateau.

Introduction

The Tibetan Plateau responds very sensitively to climatic change because of its high altitude, strong solar radiation, and the complex interaction of moisture transport and heating. The Tibetan Plateau is also referred to as the ‘third pole’ and has significant effects on the South and East Asian monsoons, and on global climate. Today, the Tibetan Plateau is experiencing a temperature increase approximately 40% above the global mean (Wang et al., 2001, IPCC, 2007). Various proxy records from different archives, such as ice cores (e.g. Li and kang, 2006), tree rings (e. g. Bräuning, 2006), speleothems (e.g. Denniston et al., 2000), loess deposits (e.g. Chen et al., 2006; Wu et al., 2006), lake and marine sediments, reveal the mode of interaction between the components of monsoonal circulation (e.g. Morrill et al., 2006; Xu et al., 2007) and have improved the understanding of Pleistocene–Holocene evolution and feedback between the large scale atmospheric circulation systems and the regional response of the environmental systems of interior Asia. The resulting picture, however, is rather incoherent, and a plateau-wide compilation of the development of the monsoon system is still missing. This study presents a high-resolution record from a lake system in southern Tibet that reveals the timing and response to global climate change on a regional scale. Lake Nam Co, the world largest high-altitude saline lake system on the plateau, was chosen because of its position within the interference zone of the East Asian winter monsoon, Indian summer monsoon, the Westerlies and the subtropical jet stream. Lake level changes may provide information on changes in monsoon impact and associated shifts in precipitation amounts, precipitation–evaporation ratios, and meltwater input as a consequence of temperature changes.

Holocene climate records from the region show a cooling trend through the Holocene interrupted by several warm (Fontes et al., 1996, Wang et al., 2002, Bao et al., 2003, Herzschuh et al., 2006, Yi et al., 2006, Yu et al., 2006) and cold (Fontes et al., 1996, Bao et al., 2003, Herzschuh et al., 2006, Morrill et al., 2006, Yu et al., 2006) events. Highest lake levels, indicating warm and wet conditions, occurred during the early Holocene (Lister et al., 1991, Feng et al., 2006, Mischke and Wünnemann, 2006, Fan et al., 2007), but sites differ in the duration and onset of high lake levels. During the mid Holocene, after ∼4000 years BP, pollen records and lake sediments suggest a short-term fluctuation to drier conditions caused by reduced precipitation (Herzschuh et al., 2006, Morrill et al., 2006, Yi et al., 2006), which is not displayed in all regions of the plateau. The timing and the onset of climatic episodes, such as the Little Ice Age (LIA), hint at temporal and spatial heterogeneities for the Tibetan Plateau. Here we provide information from southern Tibet using ostracode species assemblages and the stable oxygen and carbon signatures of their shells. Ostracodes are the most abundant microfossils in Lake Nam Co and have already successfully been used as indicators of lake level evolution (Lister et al., 1991, Mischke et al., 2005), changes in precipitation (Holmes et al., 2007) and salinity (Mischke and Wünnemann, 2006, Mischke et al., 2007) as response to climatic change on the Tibetan Plateau. Ostracode species assemblages from two short cores are used to reconstruct the recent lake level dynamics of Lake Nam Co with decadal resolution for the past approximately 600 years. In addition, we apply a transfer function based on the water depth dependence of ostracode species distribution from surface sediment samples, and integrate results from species assemblages with the isotopic signature of their calcareous valves.

Section snippets

Regional setting

Lake Nam Co is located in the southern part (30° 33′ N, 90° 54′ E) of the Tibetan Plateau about 120 km north of the city of Lhasa (Fig. 1). The climate of this region is semi-arid to semi-humid continental (Feng et al., 2006). The catchment area of the surficially closed drainage basin comprises an area of about 10,000 km2. The lake surface covers an area of almost 2000 km2, and the maximum depth is at least 105 m (G. Daut, personal communication). Because Nam Co has no outflow, the water balance

Materials and methods

Two short sediment cores, Nam Co 4/2 and Nam Co 6/1, each 79 cm and 47 cm long, were taken in the northwestern part of the lake from 49.2 m and 64 m water depth, respectively, using a UWITEC gravity corer during September 2005 (Fig. 2). The chronology of the sediments was established using 210Pb and 137Cs dating methods. Measurements of 210Pb and 137Cs were carried out at the GGA, Hannover. Ages and sedimentation rates were calculated using a CRS model assuming a constant rate of supply of 210Pb to

Chronology and lithology

The chronology is based on 210Pb activities and additional measured 137 Cs. The constant decline of 210Pb activities corresponds to the natural radioactive decay and to a constant sedimentation rate for the last 200 years of sedimentation. The sediment record reaches back to approximately 1350 AD (Nam Co 4/2) and approximately 1400 AD (Nam Co 6/1) (Fig. 2). A chronology was established based on the 210Pb dating and the extrapolation of the measured 210Pb activities assuming long term equilibrium

Interpretation and discussion

Stable oxygen and carbon isotopes values and ostracode species assemblages are widely used in paleolimnology and paleoclimatology (Heaton et al., 1995, Holmes, 1996, Schwalb et al., 1999). There are only a few examples that have applied these proxies in regions of the Tibetan Plateau Tibet (Mischke et al., 2002, Holmes et al., 2007). δ18O values of ostracode shells reflect the temperature and the isotopic composition in which the ostracode formed their valves (Holmes, 1996, Schwalb, 2003,

Conclusions

A high resolution sedimentary record of two short cores of lake Nam Co covering the last ∼600 years reveal lake level evolution in response to climatic change on the southern Tibetan Plateau. An ostracode-based transfer function was used to reconstruct lake level changes. A comparison with stable isotope analysis of ostracode valves and ostracode species assemblages confirms a lake level lowstand of −10 m compared to modern lake level before ∼1600 AD. A distinct increase in δ18O values and

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