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Quaternary Research 70

Holocene environmental and climatic changes inferred from Wulungu Lakein northern Xinjiang, China

Xingqi Liu a,⁎, Ulrike Herzschuh b, Ji Shen a, Qingfen Jiang a,c, Xiayun Xiao a

a State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy Sciences (CAS),Nanjing 210008, China

b Alfred Wegener Institute for Polar and Marine Research, Research Unit Potsdam, 14473 Potsdam, Germanyc Graduate School, CAS, Beijing 100039, China

Received 12 October 2006Available online 8 August 2008

Abstract

Sedimentological, geochemical and palynological data from Wulungu Lake in northern Xinjiang, China, are used to reconstruct environmentaland climate changes since 9550 cal yr BP. High abundance of Sparganium and Poaceae, low Md (median diameter) and δ13Corganic values indicatearidity between 9550 and 6730 cal yr BP. High Md and δ13Corganic values, and the prevalence of desert-steppe and steppe vegetation between 4200and 560 cal yr BP, indicate that effective moisture increased after 6730 cal yr BP, peaking at 4200 and 560 cal yr BP. Low Md values, a negativeexcursion of δ13Corg, and the transition from steppe to desert vegetation since 560 cal yr BP reflect a decrease in effective moisture during thelatest Holocene. Late Holocene human activities were indicated by sharp increase in the abundance of Pediastrum then. Variations in carbonatecontents indicate that temperature was generally high between 9550 and 7740 cal yr BP, low between 7740 and 6730 cal yr BP, intermediatebetween 6730 and 560 cal yr BP, and low during the last 560 yr. Regional comparison indicates that the Asian monsoon did not extend toWulungu Lake and westerlies were the main factor in determining the moisture availability during the Holocene.© 2008 University of Washington. All rights reserved.

Keywords: Holocene; Westerlies; Northwest China; Pollen

Introduction

North-western China is located in an arid and semi-aridregion. Its climate is strongly affected by the Asian monsoonsystem (Indian Monsoon and South-East Asian monsoon) andthe Westerlies (Gao, 1962). Studies on climatic changes such asthe transition of aridity and humidity in North-western Chinaduring the geological times, can help us better understand theevolutionary relationship between Westerlies and Asian mon-soon. A close correspondence between Chinese aeolian climateproxy records and North Atlantic deep-sea records during aseries of different Geological times (Porter and An, 1995; Anand Porter, 1997; Zhou et al., 1999; Porter and Zhou, 2006)indicates that the westerly wind system can pass over theregions influenced by Asian monsoon. On the other hand, interms of humidity, oxygen isotope of carbonate and pollen data

⁎ Corresponding author. Fax: +86 25 57713063.E-mail addresses: xingqiliu@yahoo.com, xqliu@niglas.ac.cn (X. Liu).

0033-5894/$ - see front matter © 2008 University of Washington. All rights reservdoi:10.1016/j.yqres.2008.06.005

from Manas Lake indicated that Holocene climatic changes inXinjiang are similar to these in East China (Sun et al.1994;Rhodes et al., 1996; Lin et al., 1996; Lin and Wei, 1998; Weiand Gasse, 1999). Furthermore, water levels of most lakes innorthern and central Mongolia were high between 7500 and6000 yr ago (Harrison et al., 1996; Tarasov, 1996; Tarasov andHarrison, 1998; Grunert et al., 2000). These results have beenrelated to the moisture-bearing summer monsoon whichextended its influence westward to Xinjiang and northward tonorthern Mongolia during the early mid-Holocene (Sun etal.1994; Rhodes et al., 1996; Tarasov and Harrison, 1998; Qinand Yu, 1998; Tarasov et al., 2000; Blyakharchuk et al., 2004).However, since well-dated paleoclimatic records from Xinjiangare rather limited, there are some controversies concerning thetime, manner and driving circulation systems of climaticchanges in Xinjiang during the Holocene (Sun et al.1994;Zhong and Wang, 1994; Ye, 2000; Wünnemann et al., 2006).Thus, in order to fully understand the regional climate changeand its causes, more high-resolution climatic records from the

ed.

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Xinjiang region are necessary. Based on sedimentological,geochemical and palynological analysis of a sediment core fromWulungu Lake, we aim to reconstruct environmental andclimatic changes of northern Xinjiang, China, during the past9550 yr. Regional comparison is conducted to investigatewhether the influence of the Asian monsoon can extend as farinto this region during the Holocene.

Study area

Wulungu Lake (46°59′ ∼47°25′; 87°00′ ∼87°35′E; 478.60m a.s.l.) is located on the northern edge of the AutonomousXinjiang Province, China. The lake is situated between the AltaiMountains in the north and the Jungger Basin in the south (Fig.1). It is fed by Wulungu River at its southeastern margin and hasno outflow, resulting in a salinity of 2.14 g/l at present. Theaverage water depth is ∼ 8.0 m with a maximum depth of 12.8m (Wang and Dou, 1998). The lake area is 927 km2 with acatchment area of approximately 35,440 km2. The WulunguLake area is characterized by a strong continental climateindicated by high annual and daily temperature amplitude. Themean January temperature is −20.4°C, the mean Julytemperature is 23°C, and the mean annual temperature is

Figure 1. Location of Wulungu Lake in n

about 3.4°C (data from Fuhai climate station, ca. 30 km to thesoutheast from Wulungu Lake). The mean annual precipitation(116.5 mm) is by far less than the mean annual evaporationcapacity (1844.4 mm), resulting in a strong arid climate. Themain wind direction is from northwest and the strongest windspeed can reach 18.0–22.0 m/s. The modern vegetation in theWulungu Lake region is desert vegetation (up to 1000 m a.s.l.)dominated by Chenpodiaceae (several species of Anabasia andCeratoides; Chen and Yan, 1989; Wu, 2001). Elevated areas,which receive more precipitation, are covered by steppescharacterized by Artemisia and Poaceae (Cleistogenes, Stipa).Alpine Kobresia meadows cover the foothills of the AltaiMountains.

Materials and methods

Coring and sediment analyses

In July 2004, a 223 cm long sediment core (WLG-2004) wasrecovered from the central part of Wulungu Lake at a waterdepth of 12 m (Fig. 1) using UWITEC coring equipment.Sediment sub-samples of core WLG-2004 were taken every1cm for grain-size analysis and 2 cm for other proxy analysis,

orth-western China and coring site.

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which results in a mean temporal resolution of roughly 50–100calendar yr. Accelerator mass spectrometry (AMS) radiocarbonages from seven levels in the core WLG-2004 were determinedon bulk organic carbon at the Center for ChronologicalResearch, Nagoya University, Japan.

In addition, a short core (20 cm long) parallel to WLG-2004was taken using a gravity corer. The short core was sampled at0.5 cm intervals in the field and core slices were immediatelyplaced into plastic bags. Slices from the short core wereradiometrically dated by measuring 210Pb and 137Cs activity asa function of depth. Sedimentation rates and chronologies werecalculated using the 210Pb data and the constant rate of supply(CRS) model, which assumes a constant 210Pb input and achanging sedimentation rate (Robbins, 1978; Appleby andOldfield, 1978; 1983; Oldfield and Appleby, 1984). 137Cs wasmeasured to check the 210Pb-derived dates.

Furthermore, aeolian dust trapped near the ground surface inthe Wulungu Lake area and sand from Gurbantunggut Desert inJungger Basin, ca. 200 km south of Wulungu Lake (Fig. 1),were collected as reference material for grain-size analysis.

For grain-size determinations samples were pretreated with10–20 ml of 30% H2O2 to remove organic matter, washed with10% HCl to remove carbonate, rinsed with deionized water, andthen treated with 10 ml of 0.05 M (NaPO3)6 on an ultrasonicvibrator for 10 min to facilitate dispersion. Grain-size distribu-tions between 0.02 μm and 2000 μm were determined using aMalvem Mastersizer 2000 analyzer.

Total organic carbon (TOC) and total nitrogen (TN) weredetermined using a CE Model 440 Elemental Analyzer insamples decalcified with 1N HCl, rinsed repeatedly withdeionized water, and dried. The pretreated samples for TOCand TN measurements were also used for organic carbonisotope determinations. The δ13C value of CO2 generated bycombusting and oxidizing dried samples, was measured in aFinnigan Delta-plus Mass Spectrometer. All isotopic values arereported in the standard δ-notation in per mil relative to V-PDB.

Samples for carbonate analyses were observed undermicroscope and analyzed by X-Ray Diffraction (XRD). Thecarbonate content expressed as CaCO3(%) was calculatedby 0.1 N NaOH titration to neutrality after dissolution ofapproximately 0.2 g of dried sample in 0.1 NHCl and boiling for20 min.

Pollen analyses

Volumetric sub-samples (0.5 cm3) were prepared for pollenanalysis following standard methods (Fægri and Iversen, 1989):treating with HCl and HF to remove carbonate and silicate,boiling in KOH to remove excess organic fractions, and sievingover 10 and 120 μm sieving cloth to remove fine and coarsefractions, respectively. In order to calculate pollen concentra-tion, known quantities of Lycopodium spores were added toeach sample before chemical treatments. At least 400 grainswere counted for each sample (average 608). Pollen and sporepercentages were calculated using the total sum of all terrestrialtaxa. Obligate aquatic pollen taxa (including Typha/Sparga-nium) and non-pollen palynomorphs (such as Pediastrum) were

calculated in relation to the total terrestrial pollen sum. Thevalues thus obtained were used for further interpretation and forthe construction of the pollen diagrams. Only pollen taxa whichoccur with N0.5% in at least three samples have been includedin the statistical analysis. The definition of the local pollen zoneboundaries was based on the results of stratigraphicallyconstrained classification subroutine (CONISS) using TILIAsoftware.

Multivariate analysis has been applied in order to gaininformation to the following questions. Which pollen taxa showsimilar reactions in the pollen diagram? Which pollen zonesshow a similar pollen composition? Detrended correspondenceanalysis (DCA) and principal component analysis (PCA) wereperformed on the basis of the square-root transformed terrestrialpollen percentage data using CANOCO 4.5 for ordination andCANODRAW for Windows for plotting. A DCA gave acompositional gradient of less than 2 standard deviation units,showing that the data set has a mainly linear structure,suggesting the utility of the linear-based PCA (ter Braak,1987). Since the information on the pollen taxa compositionswas considered more important, the data were “species-centered,” and the distances between the samples in the“species-sample bi-plot” do not exactly correspond to itssimilarity. However, we checked it with a “sample-centered”approach (not shown here), which yielded only few differences.The obligate aquatic Sparganium and algae Pediastrum wereadded post-hoc to the ordination by projection (“supplementarytaxa” or “passive taxa” in CANOCO).

The qualitative interpretation of pollen spectra can bechecked with a quantitative method of pollen-based biomereconstruction (named biomisation, Prentice et al., 1996). Basedon contemporary knowledge of the biogeography and ecology ofmodern plants, the pollen taxa are assigned to plant functionaltypes and to main vegetation types (biomes). An affinity scorefor each single biome is then calculated according to Prenticeet al. (1996).The pollen taxa-biome matrix applied in the presentstudy is based on the biomisation procedure presented in Yu et al.(1998), but it was thoroughly improved by the information givenin Herzschuh et al. (2004) and Tarasov et al. (1998).

Results

Chronology

To establish a chronology for the Wulungu Lake record,information from 210Pb/137Cs-dating of the short core andradiocarbon dating of core WLG-2004 was used.

210Pb/137Cs dating yielded that significant levels of unsup-ported 210Pb were detected in the top 14 cm of the short core andthe oldest 210Pb-datable sediments in the core are located at14 cm which corresponds to an age of 1840 AD based on CRSmodel (Fig. 2a). The initial rise in 137Cs activity occurs at 6.75cm, and a single maximum value is at 2.25 cm, which can beprobably ascribed to the first pronounced increase in 137Cs in thenorthern hemisphere and the Chernobyl incident in 1986 (Fig.2a), respectively. Thus, the 210Pb-based chronology is generallyin good agreement with the 137Cs-based chronology (Fig. 2a).

Figure 2. (a) 210Pb and 137Cs activities versus depth in a short core parallel to core WLG-2004; (b) Radiocarbon ages and reservoir-corrected 14C ages (aftersubtraction of 760 yr from the 14C ages) versus depth in the core WLG-2004. The age–depth curve from 137Cs and 210Pb-dated short core joins the line from AMS-dated core WLG-2004 when 760 yr are subtracted from the 14C dates. For details see text.

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The 14C age/depth correlation for core WLG-2004 isshown in Figure 2b. The comparison of the age (1950 AD or0 14C yr BP) at 7 cm based on 210Pb and 137Cs model with theage (760 14C yr BP) at 7 cm based on linear extrapolationbetween the upper two 14C ages suggests that the radiocarbondating yields ages that are in excess of true age by 760 yr (Fig. 2).This presumably indicates a reservoir effect caused by dating ofaquatic plants obtaining carbon directly from hard-water lakes,which is a very common case in the radiocarbon dating oflacustine sediments in the arid-semiarid regions of WesternChina. These lakes in Western China have different reservoireffects varying from several hundred to thousand years (Morrill

et al., 2006; Shen et al., 2005; Liu et al., 2008; Herzschuh et al.,2006). Besides one sample (WLG-142), all dated samples havelow C/N ratio (6–8.4), indicating that organic matter is largelyderived from lacustrine algae (Table 1). However, sampleWLG-142 at depth of 142–143 cm has a somewhat higher C/N (12.1)(Table 1), suggesting that the organic matter in this sample mightbe a mixture of lacustrine algae and terrestrial plant and thedating is probably less influenced by the reservoir effect.Therefore, we subtracted 760 yr from all radiocarbon agesexcept for sample WLG-142. The reservoir-corrected 14C agesfit well with the 210Pb and 137Cs ages and the age of sampleWLG-142 also falls well into the line of reservoir-corrected 14C

Table 1AMS 14C and calendar ages in the core WLG-2004 and additional information on the dated material (depth, material, δ13C, C/N)

Sample number Lab I.D. Depth/cm Material dated δ13C/‰ C/N Radiocarbonage/yr BP

Reservoir-corrected14C age by 760 yr

Calendar age/cal yr BP(2σ)

WLG-26 NUTA2-9319 26–27 TOC −25.6 7.6 1026±28 266 282–330(306)WLG-52 NUTA2-9328 52–53 TOC −25.5 6.0 1371±29 611 548–654(601)WLG-96 NUTA2-9327 96–97 TOC −22.7 8.0 2466±29 1706 1543–1695(1619)WLG-114 NUTA2-9321 114–115 TOC −26.9 7.0 3040±30 2280 2300–2350(2325)WLG-142 NUTA2-9329 142–143 TOC −26.3 12.1 3161±30 / /WLG-178 NUTA2-9322 178–179 TOC −25.2 7.4 5310±31 4550 5053–5189(5121)WLG-202 NUTA2-9326 202–203 TOC −25.4 8.4 7432±33 6672 7481–7592(7537)

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age and depth (Fig. 2b). However, we still exclude the age ofsample WLG-142 from our age model because more robustevidence for this sample should be needed. All reservoir-corrected 14C ages are converted to calendar year ages using theCalib 5.1 program (Reimer et al., 2004). According to this agemodel, sediment accumulation of the core varies from 0.11 to0.75 mm/cal yr with an average of 0.37 mm/cal yr. The age ofeach sampling interval was established by linear interpolation orextrapolation between the two adjoining calendar year ages.

It is quite likely that the 14C reservoir correction of 760 yrwas not constant due to negative or positive hydrologic budgetover the past 9550 yr. However, it is very difficult to determinehow it may have varied. Thus, our chronology may be off by a

Figure 3. Grain-size distribution of the core

few centuries, but this does not affect the main aims of thispaper, i.e. to discuss millennial-scale climatic changes andwhether the influence of the Asian monsoon can extend as farinto the Wulungu Lake area during the Holocene. In a futurestudy we suggest that OSL dating can be used to check our 14Cchronology.

Proxy interpretations

Grain-sizeThe grain-size of Core WLG-2004 is dominated by silt with

an average content of 72.3% (Fig. 3). Grain-size analyses for thesediments in core WLG-2004 revealed three types of grain-size

WLG-2004 in Wulungu Lake, Xinjiang.

Figure 4. Grain-size distributions of different kinds of sediments: (A) , (B) and (C) sediments in the core WLG-2004; (D) aeolian dust trapped in theWulungu lake area;(E) sand from Gurbantunggut Desert. Most sediment samples of core WLG-2004 belong to type B (91%).

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distribution. Type A (Fig. 4A) and C (Fig. 4C), which justaccount for 8% and 1% of the total samples, respectively, showa trimodal grain-size distribution. They all possess the samemodal size of 0.5 μm, 10 μm, but a small amount of coarsecomponent (80–90 μm for type A and 60–70 μm for type C,respectively). Type B (Fig. 4B), comprising most of the samples(91%), has a bimodal grain-size distribution with a modal sizeof 20–40 μm and a small amount of fine material (0.5 μm).Aeolian dust trapped in the Wulungu Lake area and sand fromGurbantunggut Desert show bimodal grain-size distributions,with a similar modal size of the fine compound (5 μm), and adifferent modal size of the coarse compound (60 μm and 200μm, respectively). Only a few samples in core WLG-2004 showan identical (Fig. 4C) or partly similar (Fig. 4A) grain-sizedistribution to aeolian dust trapped in the Wulungu Lake areaand to sand from Gurbantunggut Desert, but most sediments(Fig. 4B) in core WLG-2004 have a quite different grain-sizedistribution pattern. As suggested by the comparison of grainsize distribution of lake sediments to those from regionalaeolian dust trapped samples and desert soil sediments (Fig. 4),sediments in core WLG-2004 are most likely transported byriver discharge, not by wind, and could, thus, be used todescribe changes of the sedimentary environment related tohydrologic energy variations. High precipitation rates wouldgenerally enhance the soil erosion over the lake region andincrease the transport capacity of streams and rivers, leading tocoarser clastic materials available for river transport andsubsequent deposition in the central part of the lake (Häkansonand Jansson 1983). Therefore, we interpret the grain-sizedistribution of core WLG-2004 as a proxy index for pastchanges of precipitation and lake level, i.e., more sand-sizefraction and higher Md values reflect higher river discharge,leading to a higher lake level caused by higher precipitationrates. Furthermore, during time of low discharge and low lakelevel the wetland vegetation might have occupied the rivermouth and acted like a sediment trap, which further diminishedthe portion of large-sized grains.

Geochemical Proxy (C/N, δ13Corganic and carbonate content)

Values of δ13C and C/N ratio of lacustrine organic matter canbe used to distinguish between autochthonous and allochtho-nous sources. Lacustrine algae have C/N ratios generally b10,submerged and floating aquatic macrophytes or organic matterof a mixed source have C/N ratios between 10 and 20, whileemerged and terrestrial plants have C/N ratios N20 (Stuiver,1975; Krishnamurthy et al., 1986; Meyers and Ishiwatari,1995). C/N ratios are mostly less than 10 (Fig. 5) in theWulungu Lake record, indicating that organic matter originatesmainly from lacustrine algae during the last 9550 cal yr.Phytoplankton preferentially utilizes 12C to produce organicmatter (Hodell and Schelske, 1998). However, this general-ization about carbon isotopic signatures erodes when theavailability of dissolved atmospheric CO2 (δ13C = −7‰) islimited and algae begin to use dissolved HCO3

− (δ13C = 1‰)as their source of carbon (Meyers, 2001). Situations whereHCO3

− becomes important include periods of high photosyn-thetic uptake of dissolved inorganic carbon during which theavailability of CO2 becomes diminished (Herczeg and Fair-banks, 1987; Herczeg, 1988; Hollander and McKenzie, 1991;Herzschuh et al., 2005) and in waters where the ratio of HCO3

−

to CO2 is kept elevated (Hassan et al., 1997). In such case, δ13C

values of algal organic matter can increase to reach as high as−9‰. Hence, high δ13Corganic from the Wulungu Lakesediments may on the one hand indicate increased primaryproduction within the lake and on the other hand high δ13C oftotal inorganic carbon in the lake (Neumann et al., 2002; Lückeet al., 2003; Shen et al. 2005). Under isotopic equilibrium withatmospheric CO2, which normally has δ13C values of ca − 7‰,lake water δ13C which can be derided from δ13C of authigeniccarbonate or ostracode shells would have a value between + 1‰and +3‰ (Leng and Marshall, 2004). The stable carbon isotopeanalysis of ostracode shells from core WLG-2004 yieldedvalues between −1.8‰ and −8.3‰ (Jiang et al., 2007).Compared to these values, the δ13C values of organic matter

Figure 5. Changes of TOC, TN, C/N, δ13C of Bulk organic and carbonate content in core WLG-2004.

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is low (−24.2‰ ∼−28.5‰, Fig. 5), which probably suggeststhat algae did mainly rely on CO2 and δ13Corganic is mainlydriven by productivity.

As revealed by microscopic analysis, carbonate particles incore WLG-2004 usually consist of small (10–15 μm long), wellpreserved idiomorphic crystals, and many Limnocythereinopinata shells. XRD data shows that carbonate minerals arealmost calcite with relatively high MgCO3 content and itscrystallinity is low, which indicates that carbonate wasautogenously formed through chemical precipitation andbiological activity. Authigenic carbonate precipitation isinfluenced by many factors such as temperature, salinity andprimary productivity within the lake (Kelts and Hsu, 1978;Tucker and Wright, 1990; Aravena et al., 1992). Among them,temperature is the most important factor. An increase oftemperature causes stronger carbonate precipitation by severalmechanisms: Higher temperature can lead to (1) the decrease ofcarbonate mineral solubility, (2) the increase of salinity due towater loss via evaporation, (3) an enhancement of primaryproductivity which decreases CO2 (aq), (4) a higher abundanceof picocyanobacteria which serves as nuclei for calciteprecipitation (Thompson and Ferris, 1990; Schelske and Hodell,1991; Hodell et al., 1998).

Pollen

In total 110 pollen samples have been analyzed (Fig. 6). Allfossil pollen spectra show high frequencies of Artemisia,Poaceae and Chenopodiaceae which sum up to more than 70%

in most samples. The following herb and shrub pollen and sporetaxa occur mostly with moderate high frequencies (1–10%):Juglans, Tamarix, Ephedra, Zygophyllum and ferns. Tree pollentaxa (mostly Betula, Cupressaceae, Pinus) never have highvalues and arboreal pollen sum is between 3 and 7% in mostsamples. With the exception of Sparganium, which has highfrequencies in the lower part of the core, obligate aquatic plants(Myriophyllum, Potamogeton, Alisma—not shown in the pollendiagram) occur with low pollen frequencies in only fewsamples. Pediastrum is absent in the lower part of the core,shows high frequencies in the upper two third of the core(mostlyN10%), but is absent again in its lower part. Pollenconcentration is between 2.5×103 and 23×103 grains/cm3

having minimum values in the lowermost and middle parts ofthe core.

PCA results of terrestrial pollen taxa and samples ofWulungu Lake (Fig. 7) reflect the characteristics of the pollendiagram, summarize the vegetation dynamics, and revealwhich taxa react in a similar way to environmental changes.The pollen taxa ordination separates the elements of wetlands(e.g. Poaceae including Phragmites, Juglans, Tamarix, (Spar-ganium as “passive taxon”)) on the positive end of the first axisfrom dominant terrestrial herbs and shrubs (Chenpodiaceae,Artemisia) on the negative end. On the second axis, taxa,rather typical for desert vegetation (Chenopodiaceae, Zygo-phyllum), show positive scores, while taxa characteristic forsteppe show negative scores (Artemisia, Lamiaceae).Treepollen taxa are scattered all over the diagram indicating strongdifferences in the reaction of single taxa. Therefore, since the

Figure 6. Pollen percentage diagram of Wulungu Lake. Only taxa which occur with N2% in at least 3 samples were included in the figure.

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arboreal pollen frequencies are generally very low, tree taxa arenot useful for the reconstruction of vegetation and environ-mental conditions.

The first two principal components together capture 61% ofvariance in the pollen data (axis 1: 52%, axis 2: 9%). Samplegroupings in the ordination space as represented by the first twoaxes are associated with local pollen zones. While most samplesof pz 1 are clustered tightly, samples of the other zones arerather loosely scattered in the single areas of the bi-plot. Thefirst axis separates samples dominated by taxa of wetlands on itspositive end (pz I) from samples with high values forChenopodiaceae and Artemisia (all other pollen zones).Samples with high values for Chenopodiaceae and Zygophyl-lum show high positive values on the second axis, whilesamples with high values for Artemisia show negative values.

Traditional interpretation of pollen records is usually basedon the presence and abundance of indicator taxa and on variouspollen ratios. However, the use of this approach in arid regionsis often limited by the low number of pollen taxa and thus by thelack of valuable indicator taxa.

Sparganium, Poaceae, Tamarix and Julans are typicalelements of azonal vegetation along rivers and lakes and inoasis where the ground-water table is close to the surface.Relatively high percentages of these “wetland” taxa areindicative for low lake levels when the core site was situatedclose to the lake shore (Davis, 1999) and vice versa. Thus, we

used these wetland taxa as an indicator of lake depth and of thedistance between shore and the centrally-located core site. Theabundances of green algae Pediastrum, which often appears infresh and eutrophic lake water, have been interpreted as anindicator of deep water (Harrison and Digerfeldt, 1993;Jankovská and Komárek, 2000; Sarmaja-Korjonen et al.,2006), or high nutrient levels caused by increased soil erosion,human impact such as fertilization by cattle dung (Bottema,1974) and by the introduction of chemical fertilizers (Burden etal. 1986; Patterson et al. 2002). Especially the sudden increaseof Pediastrum values in lake records is often interpreted as anexpansion of human activity in the catchment area (Yasuda etal., 2000; Molloy and O’Connell, 2004; Villa-Martinez et al.,2003; Paduano et al., 2003; Pollingher, 1986).

Chenopodiaceae, Ephedra, Zygophyllum were assumed toindicate desert condition, while Artemisia is rather indicativefor steppe conditions. Artemisia and Chenopodiaceae are oftenthe dominant taxa in arid regions lacking forest vegetation.When their total percentages sum up to N50%, Artemisia/Chenopodiaceae ratios (A/C) could be used for the differentia-tion between desert and steppe conditions as suggested by El-Moslinmany (1990). According to modern pollen rain studiesfrom different areas in western China, A/C ratios b0.5, between0.5 and 1, and N1 indicate desert, desert-steppe and steppecondition, respectively (Sun et al., 1994; Cour et al., 1999;Herzschuh et al., 2004).

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Palaeoenvironmental reconstruction

Integrating the palaeo-vegetation (Figs. 6 and 8) andgeochemical record (Fig. 5) and lake-level fluctuation derivedfrom grain-size and pollen data (Fig. 8), the environmental andclimatic changes in the Wulungu Lake region can be inferredand compared to records from adjacent regions in Central Asia.

Early to middle Holocene (9550–6730 cal yr BP)

The A/C ratios show strong fluctuations indicating a highvariability in the vegetation cover between steppe and desertsteppe (Fig. 6). The terrestrial pollen record is blurred by thestrong contribution of wetland pollen taxa, therefore no climateinformation can be inferred from the terrestrial vegetation cover.The high abundance of Sparganium and Poaceae (includingPhragmites) indicates that the lake was shallow and the coringsite was situated close to the shoreline and close to the wetlandvegetation on the lake shore (Fig. 8). Low Md values indicate alow transport capacity of the incoming river (Fig. 3). Theorganic matter consists of lacustrine algae mostly indicated bylow C/N ratios around 10 suggesting that the terrestrial plant

Figure 7. Results of a principle components analysis of theWulungu Lake pollen spect

input was low (Fig. 5). δ13C values of organic matter arerelatively high at the beginning and decrease towards the end ofthis period (Fig. 5), suggesting that within lake primaryproduction by algae was high only at the beginning of thisperiod. Low Md values indicate a low transport capacity of theincoming river. High carbonate content in most samplessuggests that temperature is high mostly (Fig. 5). In conclusion,the climate was generally warm and arid climate between 9550and 6730 cal yr BP. Apparently, the rate of precipitation in theregion was not enough to compensate for the rate ofevapotransration at high temperature early Holocene.

A cold and dry phase is centered at 7440 cal yr BP whenSparganium (Fig. 6), clay-size (b4 μm) fraction reached theirhighest values (Fig. 3) while carbonate content and δ13C oforganic matter reached their lowest values (Fig. 5).

Middle holocene to early late holocene (6730–560 cal yr BP)

The vegetation in the Wulungu Lake area changed fromsteppe-desert (6730–4200 cal yr BP) to desert-steppe (4200–1850 cal yr BP) to steppe (1850–560 cal yr BP) (Fig. 8),indicating that effective moisture increased within the middle

ra. Only taxawhich occur withN2% in at least 3 samples are included in the figure.

Figure 8. PCA sample scores, biome reconstruction, and lake-level fluctuation derived from grain-size and pollen data from Wulungu Lake.

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Holocene and reached its optimum between 4200 and 560 cal yrBP.

Wetland plants like Sparganium almost disappeared fromthe pollen spectra (Fig. 6), suggesting that lake water areaexpanded and the distance from shore to the core site extended.However, the climate is still arid in the middle Holocene(between 6700 and 4820 cal yr BP), which can be inferred fromthe low river discharge indicated by low Md and sand fractionvalues (Fig. 3) and the dominance of steppe-desert vegetation(Fig. 8). High Md values, a strong sand fraction contribution(Fig. 3), and the prevalence of desert-steppe and steppevegetation between 4820 and 560 cal yr BP (Fig. 8) indicatethat Wulungu Lake level increased and more humid conditionsprevailed in the region. High δ13Corganic values suggest that theprimary production enhanced between 6730 and 560 cal yr BP.The temperature was cooler between 6730 and 560 cal yr BP,which can be inferred from relatively low carbonate contents(Fig. 5). Apparently, the rate of evaporation at coolertemperature was low and thus could not exceed the rate ofprecipitation, leading to an increase in effective moisture duringMiddle Holocene to Early Late Holocene.

Latest holocene (560 cal yr BP-present day)

The vegetation of the latest phase of the Holocene ischaracterized by a dominance of desert vegetation (Fig. 8) in theWulungu Lake area indicated by low A/C ratios and the

expansion of typical desert taxa like Zygophyllum und Ephedra(Fig. 6). The transition from steppe to desert vegetation reflectsa significant decrease in effective moisture in the area likelycaused by precipitation reduction. The decrease of the sandfraction and Md might be a further hint that river discharge andriver transport capacity decreased as a result of less precipitationin the catchment area of Wulungu Lake (Figs. 3 and 8).Carbonate content and values of δ13Corganic exhibit increasingtrends since ca. 360 cal yr BP but are generally lower thanduring previous periods (Fig. 5), showing that the primaryproduction was lower and temperature was colder than beforebut continuously increased during the last 360 yr. Theabundance of Pediastrum (Fig. 6) has raised sharply around560 cal yr BP which might indicate that the nutrient levelincreased possibly due to the extension of human activities inthe area.

Discussion

Data and model results indicate that an intensified AsianSummer Monsoon occurred between 12000 and 6000 yr agolikely caused by an enhanced thermal contrast between land andsea (COHMAP Members, 1988). Many well-dated paleocli-matic records from the monsoon areas in western China such asfrom Qinghai Lake (Lister et al., 1991; Liu et al. 2002; Shen etal. 2005), Siling Co (Kashiwaya et al., 1995), Bangong Lake(Gasse et al., 1996), the Chinese Loess Plateau (Feng et al.

422 X. Liu et al. / Quaternary Research 70 (2008) 412–425

2005), indicate that the climate was much wetter during theearly Holocene than today. Furthermore, most lakes in northernMongolia experienced higher levels during the middle Holo-cene (Harrison et al., 1996; Tarasov, 1996; Tarasov andHarrison, 1998; Grunert et al., 2000), which is attributed toexpansion of the Asian monsoon to Mongolia. However, ourdata from Wulungu Lake show that the climate in Xinjiang wasarid in the early Holocene (9550–6730 cal yr BP) and stillremained relatively arid in the middle Holocene (6730–4200 calyr BP). Although Rhodes et al. (1996) stated that the climatewas humid in the Manas Lake area, about 250 km southwest ofWulungu Lake, during the first half of the Holocene, an increasein δ18O of carbonate from Manas Lake sediments from −6.0‰at 11,000 14C yr BP to 2.0‰ at 4700 14C yr BP, probablyindicates a dry climate as well. The lake-level of Aral Sea waslower during the early Holocene and the middle Holocene thanduring the middle-late Holocene (Boomer et al., 2000). Pollendata of Eastern Juyan from the Alashan Plateau (Herzschuh etal., 2004) also suggest that dry conditions with desert vegetationand low lake levels prevailed between 10,700 and 5400 cal yrBP. Likewise, early and/or middle Holocene dry conditions arerecorded at Hoton-Nur basin in northwest Mongolia (Tarasov etal., 2000), Lake Telmen in north-central Mongolia (Peck et al.,2002; Fowell et al., 2003), and Gun Nuur Lake on the northernMongolian Plateau (Feng et al., 2005). Increased summerinsolation during the early Holocene enhanced summertemperatures and evaporation, but from our record it seemsthat Asian monsoon could not bring significantly moreprecipitation to the Wulungu Lake area, even not in the so-called “mid-Holocene Climatic Optimum." Hence high rate ofevapotranspiration exceeded precipitation, resulting in areduced effective moisture during the early to middle Holocene.

The weakening of the Asian summer monsoon causes acooler and drier climate in the Asian Monsoon region since ca. 6ka (COHMAP Members, 1988; Gasse et al. 1991; Sirocko et al.1993; Overpeck et al. 1996; Herzschuh, 2006). By contrast, theclimate in the Wulungu Lake region became generally morehumid between 4200 and 560 cal yr BP. Likewise, data fromEastern Juyan (Alashan Plateau) indicate that most favorableconditions occurred between 5400 and 1700 cal yr BP exceptfor a short-term dry episode between 3200 and 3000 cal yr BP(Mischke et al., 2002; Herzschuh et al., 2004). Many similarchanges in the moisture balance are recorded in Mongolia. Themost humid conditions seem to have occurred from 4500 to2500 (possibly to 1650) 14C yr BP on the northern MongolianPlateau (Feng et al., 2005). Beginning at 4500 cal yr BP, climateof the Telmen Lake region became increasingly humid and wasmost humid between 3000 and 1600 cal yr BP. Lake Hövsgölrose to intermediate levels at ca. 5400 14C yr BP and achieved aLate Holocene high-stand between 3650 and 1800 14C yr BP(Tarasov, 1996). High available moisture with cooler tempera-ture in Central Asia was most likely caused by increasedcyclonic westerlies from the North Atlantic (Magny et al., 2001;2003; Magnya et al., 2007). Stronger thermal gradient betweenhigh and low latitudes could cause westerlies pushing farthersouth and east and bringing high precipitation in northernMongolia and in Xinjiang. The enhanced precipitation and the

decreased evaporation with lower temperatures due to thedecreasing solar insolation could have resulted in increasedeffective moisture. This increased effective moisture may havecaused denser vegetation which may have resulted in increasedprecipitation via biogeophysical feed-back mechanisms withthe regional hydrological cycle (Claussen et al., 1999).

Human impact is indicated in Lake Wulungu record since560 cal yr BP. Historical records show that Oyrat, the generalname used for the Mongolians in Moxi spread to Altai region,Hami and Junger Basin in 1436 A.D and became the mainpeople in North Xinjiang (Ma, 2002; Anivaer, 2004). WulunguLake area is one of the important regions for stationing troops tocultivate and guard the frontier (Zhou, 1999). Since the end ofMing Dynasty and the beginning of Qing Dynasty, garrisontroops and peasants began to cultivate the areas in northernXinjiang and pasture husbandry and agricultural activity playedan important role in the local economy (The compilationcommittee of Fuhai county annals, 2003).

Conclusion

The sediment record from Wulungu Lake in North Xinjiangindicates that the climate was mostly warm and arid during theEarly Holocene 9550–6730 cal yr BP) but it is characterized bystrong fluctuations. A significant cold and dry event occurred atca. 7440 cal yr BP. The moisture began to increase with thedecrease temperature at 6730 cal yr BP but the climate was stillarid during the middle Holocene (between 6730 and 4200 cal yrBP). Beginning at 4200 cal yr BP, climate of the Wulungu Lakeregion was more humid and maximum humidity with coolertemperatures occurred between 1850 and 560 cal yr BP. Theclimate became generally colder and drier during the last 560 calyr BP. Since that time indications for human impact were foundin theWulungu Lake record. Our data are generally in agreementwith climate records from Eastern Juyan in northwestern China(Mischke et al., 2002; Herzschuh et al., 2004), and lakes indifferent parts of Mongolia such as Lake Telmen (Peck et al.,2002; Fowell et al., 2003), Gun Nuur Lake (Feng et al., 2005),Lake Hövsgöl (Tarasov, 1996), and Aral Sea (Boomer et al.,2000), but our results are quite different from records of theAsian monsoonal region. This indicates that the enhanced AsianMonsoon from early to mid-Holocene probably could not reachnorthern Xinjiang. Summer insolation in the early Holocene,changes in the cyclonic activity from the westerlies andbiogeophysical feedback mechanisms may be the main factorsthat control the climatic changes and the hydrological regime inWulungu Lake.

However, temporal and spatial variations of wet or dryphases most likely existed in the central Asian. For example,most lakes in northern and central Mongolia stood high level atca 6000 yr ago (Tarasov and Harrison, 1998), Lake Issyk-Kul inKyrgyzstan went from a relatively moist climate in the earlyHolocene to a more arid climate in the late Holocene (Ricketts etal., 2001), which is attributed to maximum expansion of theAsian monsoon (Tarasov and Harrison, 1998; Ricketts et al.,2001). These regional and climatic differences are probably dueto various local hydrological or topographic factors that may

423X. Liu et al. / Quaternary Research 70 (2008) 412–425

have different responses to two different climate systems(Westerlies and Asian monsoons). Therefore, more palaocli-matic records in northern Xinjiang of China and in central Asianremain to be studied to understand the controlling mechanismsof regional variability.

Acknowledgments

We thank Prof. Singhvi, Dr. Andrew Henderson and oneanonymous reviewer for their helpful comments. This work wassupported by the National Natural Science Foundation of China(Grant No. 40772108) and the National Basic Research Programof China (Grant Nos. 2004CB720200, and 2005CB422002)).We thank Hailiang Dong for improving the quality of writing.

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