holocene vegetation and climate history at hurleg lake in ...ziy2/pubs/zhao2007rpp.pdfmodified...

nology 145 (2007) 275–288www.elsevier.com/locate/revpalbo

Review of Palaeobotany and Paly

Holocene vegetation and climate history at Hurleg Lakein the Qaidam Basin, northwest China

Yan Zhao a,⁎, Zicheng Yu a,b, Fahu Chen a, Emi Ito c, Cheng Zhao b

a MOE Key Laboratory of Western China's Environmental System, College of Earth and Environmental Sciences,Lanzhou University, Lanzhou 730000, China

b Department of Earth and Environmental Sciences, Lehigh University, 31 Williams Drive, Bethlehem, PA 18015, USAc Limnological Research Center, Department of Geology and Geophysics, University of Minnesota,

310 Pillsbury Drive SE, Minneapolis, MN 55455, USA

Received 10 April 2006; received in revised form 30 October 2006; accepted 3 December 2006Available online 22 December 2006

Abstract

We present fossil pollen data and discuss their climatic interpretations from a 688-cm-long sediment core from Hurleg Lake, afreshwater lake located in the Qaidam Basin on the NE Tibetan Plateau, just beyond the northern limit of the East Asian summermonsoon influence. The reconstruction of the Holocene vegetation and climate history was aided by modern surface pollenanalysis. The 14000-yr chronology of the sediment core was controlled by seven AMS 14C dates on plant macrofossils. The resultsof the surface pollen analysis showed that modern pollen spectra faithfully reflect the regional vegetation along a transect fromalpine meadow to desert steppe and desert, so fossil pollen record can be used to reconstruct Holocene vegetation change. Thepollen data showed that vegetation changed from desert before the Holocene to desert steppe dominated by Artemisia from 11.9 to9.5 ka, desert dominated by Chenopodiaceae from 9.5 to 5.5 ka, and steppe desert dominated by Artemisia and Poaceae after5.5 ka. This vegetation sequence indicates that climate was relatively wet before 9.5 ka, dry and variable from 9.5 to 5.5 ka, andrelatively wet and stable after 5.5 ka. The climate pattern reconstructed from pollen data appears to be opposite to the paleoclimatepattern inferred at Qinghai Lake, 300 km east of our study site. That site shows a moist early Holocene during the insolation andmonsoon maximum followed by a drying trend during the mid- and late Holocene. The contrast between the two sites suggests theimportance of the position of the subtropical monsoon, the mid-latitude westerlies and interactions between local topography andregional climate. Our finding has important implications for understanding complex regional vegetation and climate responses tolarge-scale forcings in arid central Asia.© 2006 Elsevier B.V. All rights reserved.

Keywords: Holocene; fossil pollen; climate change; Hurleg Lake; Qaidam Basin; Tibetan Plateau

1. Introduction

The northeastern Tibetan Plateau is located at a triplejunction of influences from the Southeast Asian summer

⁎ Corresponding author. Tel.: +86 931 891 2337; fax: +86 931 8912330.

E-mail address: [email protected] (Y. Zhao).

0034-6667/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.revpalbo.2006.12.002

monsoon, the westerlies, and the winter monsoon(Bryson, 1986). Its geographic setting suggests thatsedimentary records should be sensitive to pastvariability in regional climate, especially to shifts inthe strength or location of monsoon precipitation. Overrecent decades, many studies have been undertaken inthis region, including pollen records from lake sedi-ments and ice cores on the NE Tibetan Plateau that have

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http://dx.doi.org/10.1016/j.revpalbo.2006.12.002

276 Y. Zhao et al. / Review of Palaeobotany and Palynology 145 (2007) 275–288

been published (e.g., Du and Kong, 1983; Liu et al.,1998; Herzschuh et al., 2005; Shen et al., 2005).However, the pattern of regional vegetation change isstill poorly understood due to the limited number ofhigh-resolution records.

The Qaidam Basin is an arid and intermontane basinon the NE Tibetan Plateau (Fig. 1). Due to the uniquetopography as a closed basin on the Tibetan Plateau, therelationships among temperature, precipitation, andeffective moisture in this region are complicated. Forexample, high insolation in the basin could deliver moremonsoon precipitation by convectional activity, but itcould also enhance evaporation directly, which as aresult could reduce effective moisture. The trade-offs ineffective moisture may result in highly variable

Fig. 1. Location maps. (A) Hurleg Lake (black dot) in the Qaidam Basin, N1—Qinghai Lake; 2—Dunde ice cap. Dashed line shows the present liminorthern limit of the SW Indian monsoon. Rectangle is shown in detail in pLake to Dunde ice cap and locations of regional surface pollen samples (bboundary of meadow to the east and steppe desert/desert to the west. (C) Bacore HL05-2. The dotted line shows the approximate location of the trans

Holocene vegetation and climate changes on the NETibetan Plateau. Holocene records from the QaidamBasin are limited, especially well-dated continuousrecords. We present a Holocene pollen record from theonly freshwater lake in the arid Qaidam Basin toreconstruct the vegetation and climate history.

2. Study region and site

Hurleg Lake (lat. 37°17′N, long. 96°54′E, elevation2817 m a.s.l.) is located at the northeastern edge of theQaidam Basin on the NE Tibetan Plateau (Fig. 1), justnorth of the influence of the SE Asian summer monsoon(Winkler and Wang, 1993). The Qaidam Basin, with anarea of 120000 km2, is surrounded by the Kunlun

E Tibetan Plateau. Open circles show other nearby paleoclimate sites:t of the SE Asian summer monsoon influence, while black line theanel B. (B) Satellite image showing vegetation changes from Qinghailack triangles; S1 to S12). Dashed line approximately represents thethymetry map of Hurleg Lake. Solid dot shows the coring location ofect for collecting surface pollen samples from Hurleg Lake.

277Y. Zhao et al. / Review of Palaeobotany and Palynology 145 (2007) 275–288

Mountains to the south, the Altun Mountains to the westand the Qilian Mountains to the north and east. Thesurrounding mountains rise to an elevation of N5000 mabove sea 1evel, while the average elevation of the basinis 2800 m.

Mean annual precipitation at nearby Delinghameteorological station (at 2982 m a.s.l.) is about160 mm and is highly variable. Most moisture falls asrain during the summer months, and precipitationincreases at about 45 mm per 100 m increase inelevation on the basis of seven stations in the region.Using this relationship of precipitation and elevation,the actual precipitation at Hurleg Lake is likelyb100 mm. The mean annual temperature is 4 °C, andthe potential evaporation is about 2000 mm.

The region is characterized by desert vegetation (Fig.1B; Zhou et al., 1990), dominated by Chenopodiaceae(including Salsola abrotanoides, Kalidium gracile,Ceratoides latens, Haloxylon ammodendron, and Sym-pegma regelii), Ephedra, Nitraria, and Compositae(including Artemisia, Ajania fruticulosa, and Aster-othamnus centrali-asiaticus). Some species that aretolerant of salty soils are also found, including Glauxmaritima and Triglochin maritimum.

Distinct vegetation types characterize a transectfrom Hurleg Lake north to the mountains at differ-ent elevations (Fig. 2). The lake is surrounded by adense marsh vegetation dominated by common reeds(Phragmites communis), Hippuris spp., and Scirpusspp., whereas the lake bottom is mostly covered byChara spp., Potamogeton spp., Ceratophyllum demer-sum, and Ruppia spp. The vegetation further away fromlake shore is dominated by a mixture of Nitraria and P.communis (Phragmites plants are much shorter andsmaller than the ones in near-shore marsh). Withincreasing distance from the lake, Nitraria is replacedby S. abrotanoides, K. gracile, C. latens, H. ammoden-dron, S. regelii, and some Artemisia, while A. centrali-asiaticus and Ephedra are dominant on south-facingmountain slopes. In the mountains at higher elevation,Qilian junipers (Sabina przewalskii) are abundant.

Hurleg Lake is fed by two rivers from the surroundingmountains to the north and discharges through a smalloutlet stream to terminal Toson Lake downstream (Fig.1C). One of the inflowing rivers, Bayin River, is a largepermanent river, while the smaller river to the west of thelake is now dry due to the construction of an upstreamdam. The entire catchment area of Hurleg Lake is about12600 km2, and the lake area is about 56.7 km2. The lakewater has amean residence time of 6months, based on itstotal river inflow of 3.406×108 m3 and lake volume of1.67×108 m3 (Wang and Dou, 1998). Hurleg Lake has a

maximum water depth of 9.6 m, and it has a salinity of0.9 g/L in total dissolved solids.

3. Materials and methods

3.1. Collection of surface pollen samples and sedimentcore

Twelve surface pollen samples were collected along aregional transect from the mountains, west of QinghaiLake in the east, across the study site Hurleg Lake, andwest to Yazi Lake (Fig. 1B). This transect spanned threevegetation zones: alpine meadow, steppe desert, anddesert (Table 1). The samples were taken from mosscushions, surface sediments in permanent lakes andponds, small temporary ponds and puddles, and dry soil.Vegetation observed around each sampling site was alsodescribed in the field (Table 1). Five pollen sampleswere also taken from surface sediments at differentwater depths at Hurleg Lake (Fig. 1C).

The 688-cm-long sediment core (HL05-2) was takenat Hurleg Lake in January 2005 from ice at 2.7 m waterdepth (Fig. 1C) using a custom-designed piston corer(6 cm in diameter). The sediment cores were transportedback to the laboratory in PVC pipes and described andsubsampled shortly thereafter.

3.2. Loss-on-ignition and dating analysis

Loss-on-ignition analysis (LOI) at 500 °C was usedto estimate organic matter content in the sediments andat 1000 °C to estimate the carbonate content (Dean,1974). Seven samples of plant macrofossils were pickedand radiocarbon dated using accelerator mass spectrom-etry (AMS) at Beta Analytic, Inc. (Miami, Florida) andat Keck AMS Dating Lab at University of California-Irvine (Table 2). All dates were calibrated to calendaryears before present (BP=1950 AD) with the programCALIB Rev. 5.0.1 using IntCal04 calibration data set(Reimer et al., 2004). The age–depth model wasestablished based on the 3rd polynomial curve after6000 cal yr BP and linear interpolation of paired agesbefore 6000 cal yr BP (Fig. 3A). Calibrated radiocarbonages are used throughout the paper.

3.3. Pollen analysis method

Pollen subsamples of 2 cm3 were taken at mostly 8-cm intervals. The subsamples were treated with amodified acetolysis procedure (Fægri and Iversen,1989), including HCl, NaOH, HF, and acetolysistreatments, and fine sieving to remove clay-sized

Fig. 2. Photos of representative vegetation types around Hurleg Lake and in the study region from Qinghai Lake to Yazi Lake. (A) Ruppia-dominatedsubmerged aquatic vegetation in Hurleg Lake; (B) Phragmites-dominated marsh around Hurleg Lake; (C) Phragmites (low growth form) andNitraria north of Hurleg Lake; (D) Nitraria on saline soil near Hurleg Lake; (E) Chenopodiaceae further north of Hurleg Lake, near surfacepollen site S10; (F) Sabina on the mountain slopes, north of Hurleg Lake; (G) Alpine meadow in Xiangpi Mountain near surface pollen sample S1;(H) Steppe desert dominated by Achnatherum near surface pollen site S4; (I) Chenopodiaceae-dominated desert vegetation near S5; (J) Nitraria-dominated sand dunes near Gahai Lake; (K) Sparse Gobi desert dominated by Nitraria near Toson Lake; and (L) Kalidium (Chenopodiaceae)-dominated vegetation near Yazi lake. All photos taken in July 2005 by Z.C. Yu.


particles. The concentrate was mounted in glycerol gel.A known number of Lycopodium clavatum spores(batch # 938934) was initially added to each samplefor calculation of pollen concentration (Maher, 1981).Each pollen sample was counted under a lightmicroscope at ×400 magnification in regularly spaced

traverses; ×1000 magnification was used for criticalidentifications. Pollen sums were usually N300 terres-trial pollen grains, including unknown pollen types(well-preserved, identifiable pollen grains but outsidereference type collections). Identifications followedWang et al. (1995) aided by modern reference

Table 1Location and surrounding vegetation types of regional surface pollen samples from the NE Tibetan Plateau

Samplecode

Latitude(°N)

Longitude(°E)

Elevation(m)

Sampletype

Vegetation Dominant species Note

S1 36.45 99.36 3813 Moss Alpinemeadow

Stipa, Kobresia, Cyperaceae, Ranunculaceae,Leguminosae, Artemisia

XiangpiMountain

S2 36.46 99.05 3121 Mud Steppedesert

Kalidium, Poaceae, Cruciferae, Nitraria, Compositae Near ChakaLake

S3 37.01 98.50 3314 Mud Steppedesert

Artemisia, Poaceae, Ephedra, Nitraria, Compositae Pond withaquatic plants

S4 37.00 98.38 3184 Top soil Steppedesert

Achnatherum, Compositae, Sabina (on the mountainslopes)

Road side

S5 37.00 98.23 3172 Top soil Desert Achnatherum, Kalidium, Chenopodiaceae, Artemisia Road sideS6 37.08 97.31 2848 Lake

sedimentDesert Poaceae, Kalidum, Nitraria, Phragmites, Compositae Gahai Lake

S7 37.21 97.30 3114 Mud Desert Achnatherum, Stipa, Ephedra, Nitraria, Chenopodiaceae,Artemisia

Pond

S8 37.08 97.00 2813 Lakesediment

Desert Phragmites, Nitraria, Artemisia Toson Lake

S9 37.18 96.55 2850 Lakesediment

Desert Phragmites, Nitraria, Chenopodiaceae Hurleg Lake

S10 37.21 96.54 2850 Top soil Desert Kalidium Near HurlegLake

S11 37.33 96.06 3524 Lakesediment

Desert Chenopodiaceae, Poaceae, Phragmites, Achnatherum,Artemisia, Leguminosae, Nitraria

Yazi Lake

S12 37.34 96.05 3533 Lakesediment

Desert Chenopodiaceae, Poaceae, Phragmites, Achnatherum,Artemisia, Leguminosae, Nitraria

Yazi Lake


collections. Pollen percentages were calculated based onthe total pollen sum, but percentages of Pediastrumcolonies were calculated based on the pollen sum plusPediastrum counts. Pollen diagrams were plottedusing TGView 2.0 (E. Grimm of Illinois State Museum,Springfield, Illinois, USA).

3.4. Multivariate statistical analysis

Twelve pollen taxa with percentages N2% in anysample were used for principal component analysis(PCA) using the program CANOCO (Ter Braak, 1988).Because pollen data are ‘closed’ compositional data (aspercentages) and they have constant sum problem, a log-transformation was applied to the data (Aitchison,1986). Log-contrast PCA is an alternative form ofPCA using log-transformation and centering both by

Table 2AMS radiocarbon dates from Hurleg Lake (core HL05-2), Qinghai, China

Lab number Depth (cm) Material dated δ13C (‰ VPDB) 1

UCIAMS-15168 318 plant macrofossils −8.0 3UCIAMS-15169 386 plant macrofossils −9.2 3UCIAMS-15170 410 plant macrofossils −8.4 4Beta-201550 446 plant macrofossils −11.7 4UCIAMS-15171 531 plant macrofossils −9.6 5UCIAMS-15172 617 plant macrofossils −5.8 8Beta-201549 649 plant macrofossils −14.8 9

samples and by pollen types (double centering); it betterrepresents the real ‘ecological distance’ betweensamples that considers the abundances of all speciessimultaneously and compares differences in speciescomposition among samples.

4. Results

4.1. Lithology and chronology

Lithology of HurlegLake coreHL05-2 is dominated bycarbonate and silicates, with low organic matter (4–10%)(Fig. 3B–D). The core shows large-magnitude oscillationsbetween clay- and carbonate-dominated sediments. Per-centage carbonate oscillates between b20% and N70%,and low carbonate corresponds with high “soil-textured”clay content and abundant plant root remains.

4C date (yr BP) Error (±yr) Calibrated age (cal yr BP–2σ range)

430 20 3632–3724670 20 3959–4084615 20 5302–5446780 50 5447–5603555 25 6299–6398495 30 9473–9534270 50 10277–10576

Fig. 3. Age model and sediment lithology. (A) Age–depth model of Core HL05-2 at Hurleg Lake, Qinghai, China; (B) organic matter (%);(C) carbonate (%); (D) silicate (%).


As shown in Table 1 and Fig. 3A, the chronologyindicates that the analyzed core spanned the last14000 yr; however, ages older than 10.5 ka (1 ka=1000 cal yr BP) were extrapolated from the age–depthmodel and have greater uncertainty. The sedimentaccumulation rates based on the age–depth model were0.28 mm/yr before 6 ka, 0.79 mm/yr between 6 ka and4 ka, and 0.13 mm/yr after 4 ka.

4.2. Modern pollen spectra

4.2.1. Surface pollen assemblages along a regionaltransect in Qinghai

The sample S1 was from alpine meadow at XiangpiMountain, where surrounding vegetation is dominatedby Stipa, Kobresia, Cyperaceae, Ranunculaceae, Legu-minosae, and Artemisia (Table 1). S1 contains abundantPoaceae (41%) and Cyperaceae (20%) pollen, withsome Artemisia (14%) (Fig. 4A). Samples S2, S3 andS4 were from steppe desert consisted of Kalidium,Poaceae, Nitraria, and Compositae. At S3 and S4, thepollen assemblages are characterized by Poaceae(N23%), Artemisia, and Chenopodiaceae. S3 was froma temporary pond, with slightly more Cyperaceaepollen. Sample S2 from dry surface soil shows higherChenopodiaceae (49%) and Nitraria (10%), and lowerPoaceae (10%), compared to samples S3 and S4.

Samples S5 to S12 were collected in the desertvegetation zone dominated by Chenopodiaceae, Ni-traria, and Artemisia. These samples are represented by

high Chenopodiaceae pollen (mostly N35%) and Arte-misia (∼20%), with usually b10% Poaceae pollen.Sample S5 from dry cracked mud in a temporary pond isoverwhelmingly dominated by Chenopodiaceae pollen(N80%). High Poaceae pollen (25%) at Hurleg Lake(S9) is likely due to abundant Phragmites in the lake-shore marsh. S8 (from Toson Lake) and S9 show highBetula (N10%), which probably originated from plantedBetula trees in nearby farms, but Toson Lake has lowerPoaceae percentages than samples from Hurleg Lake.S10 from desert soil is overwhelmingly dominated byChenopodiaceae (N90%), probably caused by differen-tial pollen representation or preservation due to dryenvironments. Two Yazi samples (S11 and S12) showsimilar pollen spectra as at other desert sites, but withmore Leguminosae.

The surface pollen spectra contain low abundances ofEphedra and Nitraria, even though the sampling sitesfrom deserts are dominated by these plants. Theirabsence suggests that these species are under-repre-sented in the pollen rain. For example, at Hurleg Lake(S9) Nitraria pollen is b1%, but the surroundingvegetation is dominated by Nitraria (Fig. 2C–D).

4.2.2. Hurleg Lake transect pollen spectraPollen spectra from different water depths at Hurleg

Lake show relatively constant abundance for mostpollen types, including Poaceae (∼20%), Artemisia(20%) and Chenopodiaceae (25–50%), except at T3,which has a low pollen count (Fig. 4B). Betula pollen is

Fig. 4. Surface pollen results. (A) Percentage diagram of surface pollen samples from the NE Tibetan Plateau in Qinghai, China. See Table 1 forlocations of pollen samples. (B) Surface pollen results along a water-depth transect (shown on the right side) at Hurleg Lake.


more abundant in shallow sites, showing an increasingtrend from deep water to shallow water samples(N20%). Betula pollen is likely to originate from treesplanted by the lake-side fishery and nearby farms.Artemisia/Chenopodiaceae pollen ratios (A/C ratios)are constant around 0.7. Low pollen counts due to poorpreservation or very low concentration makes the pol-len assemblages of T3 differ from the other 4 samples,and Chenopodiaceae (50%) is obviously over-repre-sented in this sample. Another interesting pattern is thatPediastrum colonies increase with water depth fromalmost none at 0.8 m to about 90% at 8.6 m, sug-gesting that Pediastrum may be used as a good indi-cator of water depth, at least at this lake.

4.3. Fossil pollen spectra

4.3.1. Pollen assemblagesThe percentage pollen diagram was divided into 4

pollen assemblage zones, with subzones when neces-sary, based on stratigraphically constrained clusteranalysis (CONISS) (Fig. 5).

Zone HL-1 (14–11.9 ka; 782–694 cm): The pollenassemblages were dominated by Artemisia (20–30%),Chenopodiaceae (∼40%), Ephedra (up to 18%), andPoaceae (∼10%). Ephedra had the highest abundancein the core, and Pediastrum was absent in this zone.This zone had a total pollen concentration of about50000 grains/cm3. Two subzones were separated at

Fig. 5. Percentage pollen diagram of core HL05-2 at Hurleg Lake, Qinghai, China. Selected taxa shown.

Fig. 6. Biplot of PCA results from fossil pollen data of core HL05-2 atHurleg Lake, Qinghai, China.


13.5 ka (750 cm), mostly based on relative changes inabundance of Artemisia, Chenopodiaceae, and Ephedrapollen types and a decrease in A/C ratio.

Zone HL-2 (11.9–9.5 ka; 694–619 cm): Pollenassemblages were characterized by Artemisia (∼30%),Chenopodiaceae (∼35%), Poaceae (∼15%), and highCyperaceae. A/C ratios increased and were generally high(up to 1.3). Pediastrum first appeared at the top of thiszone. Pollen concentration was around 30000 grains/cm3.

Zone HL-3 (9.5–5.5 ka; 619–453 cm): Chenopodia-ceae reached a maximum value of 81%, with generallylow (0.05) and fluctuating A/C ratios. Artemisiadecreased in abundance during this period (∼15%).Poaceae and Pediastrum started to increase in this zone.Pollen concentrations reached the highest values in thiszone (up to 100000 grains/cm3). Subzones HL-3a (9.5–7.4 ka; 619–560 cm) and HL-3b (7.4–5.5 ka; 560–453 cm) were divided based mostly on the increase inPoaceae and decrease in Ephedra.

Zone HL-4 (5.5–0 ka; 453–270 cm): Pollen assem-blages were characterized by consistently high Poaceae(30–40%), high Nitraria (up to 6.7%), and lowChenopodiaceae (∼35%). Pediastrum increased to thehighest values (up to 76%) in this zone. A/C ratiosincreased and were constantly higher than 0.6. Totalpollen concentration was at its lowest value of the recordat b20000 grains/cm3.

4.3.2. Multivariate analysisPCA results of pollen types and samples of Hurleg

Lake data reflect the characteristics of the pollendiagram and summarize the vegetation shifts (Fig. 6).The first two axes show changing dominance of pollentypes. In the ordination of pollen samples, the first andsecond axes account for 52.3% and 14.1% of the totalvariance in pollen data, respectively, and variation in


sample scores represents shifts in vegetation. Fourclusters can be separated by PCA scores on the first twoaxes, corresponding with pollen zones. For example, theclosely clustered zone HL-1 samples were characterizedby Ephedra, whereas zone HL-3 was characterized byChenopodiaceae and had the most spread samplecluster. Zone HL-4 samples were clustered togetherand characterized by Poaceae and Nitraria. PCA axis 1mostly separates zone HL-4 from other three zones,while PCA axis 2 further separates zones HL-1 and HL-2 from HL-3 and HL-4.

5. Discussion

5.1. Pollen indication of vegetation and climatechanges

5.1.1. Poaceae pollen as regional or local signalsIn arid and semi-arid regions, fossil Poaceae pollen

can be used as an index of vegetation and climate. Anincrease of Poaceae can indicate an expansion of steppeover desert, suggesting relatively moist environments.Surface pollen spectra from Inner Mongolia, Xinjiang,and the Tibetan Plateau (e.g., Li, 1998; Cour et al., 1999;Liu et al., 1999; Yu et al., 2001; Herzschuh et al., 2003;Ma et al., 2004; Li et al., 2005; Shen et al., 2006)indicate that Poaceae percentages in arid and semi-aridChina are normally b10% and under-represented incomparison with Artemisia and Chenopodiaceae. How-ever, Poaceae pollen abundance is high in wetlands withPhragmites and agricultural areas. Our surface pollentransect showed that desert was represented by highChenopodiaceae and Artemisia percentages, with b10%Poaceae pollen. At Hurleg Lake (S9), Poaceae pollenaccounts for 25%, due to the local contribution fromPhragmites. Three different sizes and surface sculptureof Poaceae pollen grains were noted in downcoresamples: small (b30 μm, scabrate); medium (30–50 μm,fine reticulate, and exine thickness ∼1 μm), and large(N50 μm, scabrate). The medium size pollen usuallyaccounted for two-thirds or more of the total Poaceaepollen, which probably came from P. communis on thebasis of our observations of pollen morphology ofmodern Phragmites samples that we collected at HurlegLake. The transect of modern pollen samples at differentdistances from the surrounding marsh Phragmites plantsshowed similar Poaceae values of around 20% (Fig.4B), suggesting that the downcore changes in Poaceaepollen cannot simply be interpreted as representing oflocal plants. Therefore, Poaceae can still be useful as aregional signal in this study, even though more than halfof its pollen likely comes from local Phragmites.

5.1.2. Artemisia/Chenopodiaceae ratio as effectivemoisture indicator

The Artemisia-to-Chenopodiaceae (A/C) ratio wasfirst introduced by El-Moslimany (1990) in the MiddleEast, as an indicator of change in effective moisture insemi-arid and arid regions. The A/C ratio has been usedin many studies in arid and semi-arid China (e.g., Li andYan, 1990; Yan, 1991; Van Campo and Gasse, 1993;Van Campo et al., 1996; Cour et al., 1999; Liu et al.,1999; Yu et al., 2001) to determine steppe (high Arte-misia pollen) from desert vegetation (high Chenopodia-ceae pollen). In the Xinjiang region, A/C ratios wereb0.5 in desert, 0.5–1.2 in steppe desert, and N1 intypical steppe (Li and Yan, 1990; Yan, 1991). Cour et al.(1999) indicated that the saline desert (western Takli-makan), montane desert (Kunlun), and montane steppeor sub-desert (Karakorum) were respectively repre-sented by A/C ratio of b1, 1–2 and N2. Li et al. (2005)reported that in typical desert communities, A/C ratiosare b0.5, while in steppe-desert A/C ratios are 0.5–2.

A/C ratios in our regional surface pollen sampleswere generally b1, except S1 from alpine meadow,which had a ratio of 4.8, and S4 from alpine steppe desertwith a ratio N1.0. S2, S5 and S10 had very low A/Cratios, consistent with the fact that they were collected indesert. A/C ratios at downcore HL05-2 correspond wellwith carbonate percentages, which are used as an indexof lake-level change (Yu et al., submitted for publication;Fig. 7C). High carbonate percentage deposited underrelatively deep lake water conditions, whereas lowcarbonate deposited in shallow water and wetlandenvironments (Yu et al., submitted for publication).This lake-level interpretation of lithology was confirmedby LOI data from surface sediment samples along atransect of water depth at Hurleg Lake, which show thatpercentage carbonate increases from20.1% at 1.1mwaterto 45.4% at 8.6 m water. Percent carbonate indicate highlake level at 11.5–9.5 ka, corresponding with high A/Cratio; low and fluctuating lake levels at 9.5–4.2 ka, withlow and variable A/C ratio; and high and stable lake levelssince 4.2 ka, with high A/C ratio. This relation betweencarbonate and A/C ratio supports the idea that carbonateand A/C ratios both indicate effective moisture.

5.1.3. Ephedra and Nitraria as dry climate indicatorsNitraria and Ephedra have been used to distinguish

between steppe and desert pollen spectra (Herzschuhet al., 2004). However, Nitraria also grows abundantlyaround wet marshes (Du and Kong, 1983), thus itspresence may alternatively indicate a wet climate withincreased groundwater level, rather than an expansion ofdesert vegetation. Cour et al. (1999) found that Nitraria

Fig. 7. Scatter plots of different parameters from core HL05-2 at Hurleg Lake. (A) Chenopodiaceae vs. total pollen concentration; (B) Pediastrum vs.Poaceae; (C) carbonate vs. A/C ratio; (D) carbonate vs. total pollen concentration; (E) carbonate vs. pollen PCA 1 scores; and (F) pollen concentrationvs. pollen accumulation rate.


pollen was higher in the piedmont of Kunlun Moun-tains, possibly related to a high water table associatedwith Yarkant River. Around Hurleg Lake, Nitrariaplants become sparse in the surrounding vegetation withincreasing distance from the lake (Fig. 2). Increase inNitraria pollen may represent a high water table,under a moister climate in the past.

Ephedra is a typical desert species; however, lowvalues of Ephedra pollen may not be a good vegetationindicator because it produces large amounts of pollenthat are transported long distances. For example,Ephedra pollen has been documented in midwestern

North America, even though its parent plants arethousands of kilometers away in the deserts of thesouthwestern USA (Maher, 1964). Li et al. (2005) alsoreport that in the eastern Alashan Plateau, easternQaidam Basin, and part of Gansu Corridor, Ephedrapollen appears in most surface samples, even though noEphedra plants are recorded in the surroundingcommunities. Therefore, Ephedra pollen in low abun-dance has limited climate information (Sun et al., 1994).Ephedra pollen at Hurleg Lake shows a decreasing trendduring the last 14 ka. The high abundance in zone HL-1of Hurleg Lake was also found in the pollen diagrams


from Dunde icecap (up to ca. 40%) (Liu et al., 1998) andSumxi Lake (up to ca. 18%) (Van Campo and Gasse,1993) on the NW Tibetan Plateau before or at thebeginning of the Holocene. We hypothesize that a strongseasonality of precipitation in response to winter andsummer insolation contrast might contribute to the highabundance of Ephedra during this period at these sites.However, there is no direct paleoclimate evidence tosupport this speculation and further study is needed.

5.1.4. Pollen percentages vs. pollen concentration forclimate reconstructions

Many pollen studies in arid and semi-arid regionshave used changes in total pollen concentration to inferclimate change, based on the assumption that highpollen concentrations indicate dense vegetation coverunder a moist climate (e.g., Liu et al., 1998; Shen et al.,2005; Chen et al., 2006). However, this practice may notbe justified in desert regions dominated by Chenopo-diaceae. On the basis of modern surface pollen analysisfrom western Inner Mongolia, Herzschuh et al. (2003)showed that Chenopodiaceae is greatly over-representedin pollen rain, compared to most pollen types, includingArtemisia. High pollen concentration may simply re-present high abundance of local desert plants, includingplants from the family Chenopodiaceae, rather thandense vegetation cover under a moist climate. Variationsin sedimentation rates may also influence pollen con-centration (Van Campo and Gasse, 1993). Sedimentcores or sections with pronounced changes in lithologylikely indicate changes in depositional environment andsediment-accumulation rates that affect pollen concen-tration. Unfortunately, very few studies on the relation-ship between pollen concentration and vegetation havebeen conducted in dry land regions (Sun et al., 1994).

Minckley and Whitlock (2000) compared modernpollen spectra from lake sediments, soil, and mosspolsters in the western US and suggested that differencein pollen spectra from different types of sites existed,though the general patterns were similar. Wilmshurstand McGlone (2005) conducted a similar study ofmodern pollen spectra in New Zealand and indicatedthat the soil pollen spectra are misleading both on a localand regional scale in representing vegetation. Ourregional surface samples from Qinghai show thatsamples from soils (S2, S5 and S10) have higherChenopodiaceae pollen percentages and higher pollenconcentrations compared to lake/pond sediment samples(Fig. 4A), suggesting that Chenopodiaceae is over-represented in pollen assemblages from soil samples,which have slow accumulation rates. Obvious over-representation of Chenopodiaceae highlights the risks of

using palynomorph analysis of soil samples to recon-struct vegetation histories.

In Hurleg fossil samples, high pollen concentrationmay be due to high production and effective dispersal ofpollen from desert plants, mainly from Chenopodiaceae,as there is a positive correlation between total pol-len concentration and Chenopodiaceae percentages(Fig. 7A). Also, the increase in total pollen concentrationcorresponds with the decrease in carbonate content(Fig. 7D), suggesting a lowered lake level and dry cli-mate during high pollen concentration. Our results sug-gest that pollen concentration cannot be used as a reliableindex of vegetation cover and effective moisture.

5.1.5. Pediastrum as paleohydrological indicator inarid regions

Pediastrum occur frequently in freshwater environ-ments, being part of phytoplanktonic communities (Telland del Zamaloa, 2004). They have been largely used asan indicator of nutrient status in temperate freshwaterlakes, where the occurrence of planktonic Pediastrumindicates eutrophic and highly productive environments(del Zamaloa and Tell, 2005). Its increase has also beenrelated to a rising water level (Xu et al., 2004; Jianget al., 2006; Sarmaja-Korjonen et al., 2006). However,some researchers demonstrate that Pediastrum indicatesa low water level due to eutrophication or broad ecologyof some Pediastrum species (e.g., Prat and Daroca,1983; Chepstow-Lusty et al., 2005). The genus containsmany species of diverse ecological affiliations, so itsabundance in sediments may reflect a variety of aquaticconditions. Although the species of Pediastrum atHurleg Lake were not identified, our surface pollensamples at different depths show that Pediastrumabundance increases with water depth from almostnone at 0.8 m to about 90% at 8.6 m (Fig. 4B), sug-gesting that water depth is an important factor in-fluencing the abundance of Pediastrum at Hurleg Lake.

5.2. Holocene vegetation and climate changes

The summary pollen diagram (Fig. 5) showsvegetation changes over the last 14 ka. Before 11.9 ka,vegetation was sparse desert dominated mainly byChenopodiaceae and Artemisia, with the highestpercentages of Ephedra. Between 11.9 and 9.5 ka,steppe desert vegetation dominated by Artemisia andPoaceae occurred around Hurleg Lake, with decreasedcontributions from Chenopodiaceae and Ephedra andan increase in A/C ratio. At 9.5–5.5 ka, the vegetationwas desert, dominated by Chenopodiaceae, and withvery little Artemisia. After 5.5 ka, a wetter steppe desert


vegetation was dominated by Poaceae, Chenopodiaceae,and Artemisia. Nitraria reached the highest values (upto 7%) of the Holocene, suggesting locally high watertables after 5.5 ka. Pediastrum proliferated during thisperiod, indicating high water level in a relatively wetclimate. PCA-1 sample scores show an increase during9.5–5.5 ka and a decrease after 5.5 ka. They appear tocorrelate with A/C ratios and carbonate content, withhigh scores representing low A/C ratios and lowcarbonate content (Fig. 7E), suggesting that high scoresrepresent a dry climate. Vegetation change and PCA-1 atHurleg Lake suggest that after a dry climate interval at13–11.9 ka and a relatively wet climate in the earliestHolocene from 11.9 to 9.5 ka, a dry and variable climateoccurred during 9.5–5.5 ka, followed by a stable andless dry climate after 5.5 ka.

Other proxies from this core support the climateinterpretation inferred from fossil pollen data (Yu et al.,submitted for publication). Percent carbonate dataindicate that the climate was relatively wet at 11.5–9.5 ka, dry and variable climate at 9.5–4.2 ka, andrelatively wet and stable since 4.2 ka (see above). Oxygenisotopes of both precipitated carbonates and ostracodeshells show the highest values in the early and mid-Holocene at 9.5–4.2 ka and a decrease from the mid-tolate Holocene. A similar decrease in Mg/Ca ratios in thelow-Mg calcite shells of ostracodes occurred during themid-Holocene. Both oxygen isotope and Mg/Ca ratiostogether indicate dry climate (elevated evaporative

Fig. 8. Insolation and regional climate correlation. (A) Summer insolation atHurleg Lake; (C) total tree pollen percentage at Qinghai Lake (Shen et al., 200

enrichment) in the early and mid-Holocene and a wetclimate (low evaporation) in the mid- to late Holocene.

5.3. Complex regional responses to insolation-drivenmonsoon intensity

The climate reconstruction from Hurleg Lake con-trasts with that from Qinghai Lake (∼300 km to theeast) (Fig. 8C and D). At Qinghai Lake, evidence offorest expansion (Fig. 8C; Shen et al., 2005) and a wetclimate prior to 6.5 ka is attributed to a stronger-than-present summer monsoon during the early Holocenesummer insolation maximum (Wang et al., 2005;Fig. 8A). Oxygen isotopes of lacustrine carbonatefrom Qinghai Lake also suggest a stronger monsoonin the early Holocene (Lister et al., 1991; Fig. 8D). Since6.5 ka, tree pollen percentages gradually decreased toa minimum around 2 ka. However, PCA axis 1 (dry-ness index) at Hurleg Lake showed a general trendof increasing effective moisture in the late Holocene(Fig. 8B). In a pollen diagram from an ice core at Dundeice cap at 5325 m a.s.l., about 150 km northwest ofHurleg Lake at the northern edge of the Qaidam Basin(Fig. 1A), high A/C ratios and pollen concentrationsbetween 10 ka and 4.8 ka suggested a wet climate andstronger summer monsoon (Liu et al., 1998). QinghaiLake and Dunde ice cap records are similar, showing adecreasing Holocene moisture trend during a period ofdecreasing summer insolation.

40° N latitude (Berger and Loutre, 1991); (B) pollen PCA 1 scores at6); (D) δ18O from ostracode shells at Qinghai Lake (Lister et al., 1991).


The difference in climate interpretation suggestscomplexity of Holocene climate change near thenorthwestern limit of the summer monsoon influences.In a recent review of fossil pollen records in arid and semi-arid regions of China, Zhao et al. (2007) found thatclimate change pattern may not simply follow theinsolation trend and monsoon intensity in NW Chinaduring the Holocene. Also, on the northern TibetanPlateau, local topography may be important in modifyingthe climate patterns. Both Qinghai Lake, situated in themain part of the Tibetan Plateau, and Dunde Ice cap,located in high-elevation mountains, showed that thesummer monsoon signals followed insolation intensityhistory during the Holocene, whereas Hurleg Lake in theQaidam Basin lies an arid low-lying basin on the NWTibetan Plateau. This difference in topography not onlyallows eastward penetration of the dry westerlies into theQaidam Basin but also induces uplifting and subsidingdynamic mechanisms. The heating and upward motion ofair over the plateau causes strong air subsidence to thebasin and therefore leads to dry climate in the basin(Broccoli and Manabe, 1992; Yu et al., submitted forpublication). Our pollen results indicate that the climatehistory in the Qaidam Basin could be different from otherregions around theTibetan Plateau. The contrasting patternofmoisture histories suggests the importance of interactionbetween subtropical (monsoon), mid-latitude (westerly)atmospheric circulation systems, and local topography indetermining regional climate. Our results also suggest theneed for a denser network of sites to understand thespatially complex pattern of climate change in this region.

Acknowledgments

We thank J.W. Zhang, J.X. Chao, J.J. Chen, Z.Zheng and X.J. Liu for field and laboratory assis-tance; and Hilary Birks, Viv Jones, Hongyan Liu, CarlSayer, and Cathy Whitlock for helpful comments andsuggestions. This project was supported by the Na-tional Natural Science Foundation of China (Grants# 40301049 and # 40528001), NSFC Innovation TeamProject (# 40421101), and US National Science Foun-dation (to Yu and Ito). During the final preparation of themanuscript, the senior author was a visiting scholar at theEnvironmental Change Research Centre, UniversityCollege London (UCL), supported by a joint fellowshipfrom the China Scholarship Council and UCL.

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