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Global and Planetary Chang

Sensitive response of desert vegetation to moisture change based on anear-annual resolution pollen record from Gahai Lake in

the Qaidam Basin, northwest China

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

a MOE Key Laboratory of Western China's Environmental System, College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, Chinab Department of Earth and Environmental Sciences, Lehigh University, 31 Williams Drive, Bethlehem, PA 18015, USA

c Limnological Research Center, Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, MN 55455, USA

Received 3 June 2007; accepted 17 December 2007Available online 3 January 2008

Abstract

We present a 50-year pollen record at near-annual resolution from Gahai Lake in the Qaidam Basin on the northeastern Tibetan Plateau.Chronology of a 22-cm short core was established by 210Pb and 137Cs analysis. The pollen results at 0.5 cm intervals show large changes inArtemisia/Chenopodiaceae (A/C) ratios from b0.2 to 0.95 in the last 50 years. High (low) A/C ratios represent increase (decrease) in steppe pollenproduction, which correspond to high (low) relative humidity observed at nearby Delingha weather station. On the basis of good correspondencewith instrumental records and carbonate content from Gahai Lake, we conclude that A/C ratio is sensitive to moisture change and can be a veryuseful index in reconstructing paleoclimate of arid regions. Comparison with pollen and snow accumulation data from Dunde ice core suggeststhat effective moisture at low and high elevations shows the opposite relationship when mountain precipitation was extremely high, possibly dueto topography-induced uplifting and subsiding air dynamics.© 2008 Elsevier B.V. All rights reserved.

Keywords: fossil pollen; A/C ratio; effective moisture; climate data; Gahai Lake; Qaidam Basin

1. Introduction

Instrumental climate records show increasing warmingduring the 20th century in the Northern Hemisphere (IPCC,2007). However, hydrologic variations tend to show complexspatial patterns at regional scales (e.g., in China; Qian et al.,2003). The mechanisms that cause hydrologic change andregional difference are still poorly understood. Several high-resolution climate records from tree ring and ice core areavailable from the northeastern Tibetan Plateau. Tree-ringrecords north of the Qaidam Basin (Shao et al., 2005) and Dulanto the east of the basin (Zhang et al., 2003) have shown thatmoisture conditions changed at regular centennial timescalesover the last 2000 years. High temporal-resolution geochemical

⁎ Corresponding author. Tel.: +86 931 891 2337; fax: +86 931 891 2330.E-mail address: yanzhao@lzu.edu.cn (Y. Zhao).

0921-8181/$ - see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.gloplacha.2007.12.003

and pollen records are available from the Dunde ice core in themountain north of the Qaidam Basin that might have recordedchanges in temperature, precipitation or monsoon intensityduring the Holocene (Liu et al., 1998; Thompson et al., 2003).However, these studies are from higher elevation sites thatreceive greater precipitation, while comparable paleoclimatedata from the Qaidam Basin itself, a lower elevation basin thattoday receives much less precipitation, are lacking. Moreover,tree ring and Dunde proxy data can only be compared withinstrumental records available from meteorological stationslocated at low elevation.

We here present pollen data at near-annual resolution fromGahai Lake sediment core in the northeastern Qaidam Basin.The pollen data can be compared directly with the instrumentaldata collected at the Delingha meteorological station located inthe basin. Our objectives were to understand the response offossil pollen and vegetation to changes in effective moisture by

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comparison with instrumental climate data, to evaluate pollenindices that can be potentially used for high temporal-resolutionpaleoclimate reconstructions, and to understand the relationshipbetween effective moisture in the basin and in the mountains.This modern process study, rarely carried out for paleoclimateinvestigations, will put on a much firmer footing the use ofpollen indices (particularly of A/C ratio=pollen abundanceratio of Artemisia/Chenopodiaceae) to the reconstruction ofwet–dry oscillations in arid regions. The verification of theutility of A/C ratios as an indicator of aridity will increase ourunderstanding of the severity and timing of past droughts andthus better project possible future droughts in interior Eurasiancontinent.

2. Study region and site

Gahai Lake (37°08′N latitude, 97°31′E longitude, at anelevation of 2848m above sea level) is located at the northeasternedge of the Qaidam Basin on the northeastern Tibetan Plateau(Fig. 1A). The surrounding mountains rise to elevations ofN5000 m above sea 1evel (Fig. 1B), while the average elevationof the basin is 2800 m. Mean annual precipitation at nearbyDelingha climate station (at 2982 m a.s.l.), ca. 25 km north of

Fig. 1. Hydrologic and climatic settings. A. Map of China showing location of study rline indicates the Tibetan Plateau. B. Satellite image of study region showing study sittemperature (°C), precipitation (mm) and relative humidity (%) from Delingha weathefrom Delingha during 1956–2004.

Gahai Lake, is about 160 mm (Fig. 1C), and most precipitationfalls as rain during the summer months (Fig. 1D). Mean annualtemperature is 4 °C (January: −10.6 °C; July: 16.5 °C) based onclimate normals 1971–2000. The potential evapotranspiration isabout 2000 mm.

The regional vegetation changes from alpine meadow (mainlydominated by Stipa, Kobresia, Cyperaceae, Ranunculaceae,Leguminosae and Artemisia), to steppe desert (consisting of Ar-temisia, Poaceae, Kalidium, Ephedra, Nitraria, Cruciferae andotherAsteraceae), and to desert along a precipitation gradient fromca. 350 mm near Qinghai Lake at 3200 m a.s.l. to 100 mm nearGahai Lake. The vegetation aroundGahai Lake is characterized bydesert vegetation (Hou, 2001), dominated by Chenopodiaceae(consisting of Salsola abrotanoides, Kalidium gracile, Cera-toides aritim, Haloxylon ammodendron and Sympegma regelii),Ephedra, Nitraria, and Asteraceae (including Artemisia, Ajaniafruticulosa and Asterothamnus centrali-asiaticus).

Gahai Lake is a topographically closed-basin lake, fedmostly by groundwater, and has no permanent inflow streamstoday. The surface area of the lake is about 35 km2. Gahai Lakehas a maximum water depth of 15 m, and total dissolved solidsof 90.6 g/L based on our hydrochemical analysis of watersamples collected in 2005 and 2006.

egion (Rectangle area as shown in detail in B) in the NE Tibetan Plateau. Dottede (Gahai Lake) and other sites: Hurleg Lake and Dunde ice core. C. Mean annualr station. D. Monthly temperature (°C), precipitation (mm) and evaporation (mm)

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3. Materials and methods

Two 22-cm long sediment cores (GL05-1 and GL05-2) weretaken at Gahai Lake in July 2005 at 6.4 m water depth using aGlew-type gravity corer (6 cm in diameter), which has a one-way valve to allow displaced water to pass from the core tube asthe core is taken and to preserve the surface/water interface asthe corer is raised. The sediment cores were subsampled atcontinuous 0.5 cm intervals shortly after core collections. BothGL05-1 and GL05-2 were used for loss-on-ignition analysis,while GL05-2 was used for 210Pb, 137Cs dating and pollenanalysis. In this paper we only present and discuss results fromcore GL05-2.

Fig. 2. Chronology and lithology of core GL05-2 at Gahai Lake, Qinghai, China. A. 2

line as estimated from 226Ra activity). B. 137Cs activities. C. Age-depth model base

Sediment subsamples of 1 cm3 were dried at 100 °C andcombusted at 500 °C and 1000 °C to estimate organic matterand carbonate based on dry weight loss, respectively. A total of15 sediment slices were analyzed for 210Pb activities and 14slices for 137Cs using a gamma spectrometer at the University ofManitoba in Winnipeg, Canada. 226Ra was also estimated byradon de-emanation after growing-in the Rn daughters in orderto estimate the supported activity of 210Pb. Although 210Pbdidn’t reach background level in our core, the supported 210Pbvalue as independently estimated from 226Ra activities help uscalculate unsupported (excess) 210Pb profile. Dates weredetermined according to the Constant Initial Concentration(CIC) Model, which assumes that 210Pb activity in the

10Pb activity (total and excess) and the calculated supported 210Pb (vertical dashd on CIC model. D. Organic matter (%). E. Carbonate content (%).

Fig. 3. Percentage pollen diagram of core GL05-2 at Gahai Lake, Qinghai, China. Open curves are 5× exaggerations. Analyst: Xiuju Liu.

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uppermost layer of sediment is constant through time (Binford,1990). This derived age model provides a chronology from1955 to 2005, also consistent with 137Cs data (Fig. 2A–C).

About 1 g of freeze-dried sediment from each of the 45subsamples was used for pollen analysis. The subsamples weretreated with a modified acetolysis procedure (Fægri and Iversen,1989), including HCl, NaOH, HF and acetolysis treatments, andfine sieving to remove clay-sized particles. A known number ofLycopodium clavatum spores were initially added to eachsample for calculation of pollen concentration (Maher, 1981).About 300 terrestrial pollen grains per sample were counted andused as pollen sum for pollen percentage calculations. A/C ratiowas calculated by Artemisia/Chenopodiaceae pollen counts.Pollen zonation was aided by the CONISS cluster analysisprogram (Grimm, 1992) using nine pollen taxa with percentagesN2% in any one sample.

4. Results

Two cores generally show similar change in organic matter(5–10%) and carbonate (10–50%). In core GL 05-2, carbonatecontent shows large magnitude fluctuations between 13% and41% (Fig. 2D–E). Based on 137Cs and 210Pb dating andchronology, near-annual pollen data fromGL05-2 were obtainedfor the last 50 years from 1955 to 2005 (Fig. 3). All fossil pollenspectra are dominated by Chenopodiaceae, with abundancemostly ranging from 42% to 76%. Artemisia is the second mostabundant pollen taxon with abundance between 12% and 43%.Nitraria and Ephedra occur throughout the sequence, mostlywith a low abundance of b2% and b10%, respectively. Theextremely high Nitraria (88.1%) in the uppermost sample isprobably due to the planting and proliferation of Nitraria forharvesting its seeds in the last few years. The Artemisia-to-Chenopodiaceae (A/C) ratio showed large changes from b0.2 to0.95. Pollen concentration varied from 12,400 to 35,000 grains/g.Five local pollen assemblage zones were recognized based onCONISS analysis (Fig. 3): GL-1 (22–17.5 cm, 1955–1966 AD);GL-2 (17.5–10.5 cm, 1966–1981AD); GL-3 (10.5–4.5 cm,1981–1995 AD); GL-4 (4.5–3 cm, 1995–1998 AD); and GL-5(3–0 cm, 1998–2005 AD).

5. Discussion

5.1. Changes in pollen production, vegetation and moistureinferred from fossil pollen record over the last 50 years

In arid and semi-arid regions, vegetation density andproductivity are limited by the water available to the plants.Therefore pollen indices that are sensitive to effective moisturewould be valuable for vegetation and climate reconstructions.Ephedra and Nitraria are typical desert plants and are oftenused as indicators of dry climate. However, Ephedra produceslarge amounts of pollen with good dispersal capability, so a lowpercentage of this pollen type is not necessarily an indication oflocal vegetation change but may simply represent long distancetransport (Li et al., 2005; Sun et al., 1994). Hence, the lowabundance of Ephedra pollen from Gahai Lake carries limited

climate information. Nitraria has been cultivated and manu-factured as a traditional medicine product in this region, so itsvariation may simply reflect human activity. In forest or stepperegions, pollen concentration is commonly used as an indicatorof vegetation cover; however, in desert regions high pollenconcentration mostly represents high production and goodpreservation of pollen grains from desert plants, as indicated bysurface pollen sample analysis (Herzschuh et al., 2003) and ourpollen results from nearby Hurleg Lake (Zhao et al., 2007). Thepollen spectra from Gahai Lake also suggest that high pollenconcentration probably relates more to production of pollenfrom Chenopodiaceae and Nitraria (Fig. 3). Both Artemisia andChenopodiaceae are characteristic of highly continental climatewith cold winter and dry summers (EI-Moslimany, 1990);however, Artemisia requires more water than Chenopodiaceaeduring the growing season (EI-Moslimany, 1990; Liu et al.,1999; Herzschuh et al., 2003). A/C ratio was successfully usedin the Middle East as an indicator of change in relativedominance of steppe and desert plants and in effective moisture(EI-Moslimany, 1990). Similar studies have been carried out inother arid and semi-arid regions of the world (Van Campo andGasse, 1993; Tarasov et al., 1997; Gunin et al., 1999; Cour et al.,1999; Cohen et al., 2000; Davis and Fall, 2001;Herzschuh, 2007).

The pollen spectra from Gahai Lake show that over the past50 years pollen assemblages were consistently dominated byChenopodiaceae and Artemisia, with some Nitraria, Ephedra,Poaceae and other Asteraceae. Because our record covers only50 years at near-annual resolution, it may be more appropriate torefer to pollen production, rather than to change in plantpopulation size or vegetation. Changes in A/C ratios (from b0.2to 0.95) demonstrate large shifts in relative abundances of Ar-temisia and Chenopodiaceae pollen. Chenopodiaceae pollencould come from plants growing on dry soils on upland(responding to precipitation) or from saline soils around the lake(responding to lake level and salts). At the present there are onlya few Chenopodiaceae plants growing on saline flats aroundGahai Lake based on our observations, so we think the pollenmore likely represent plants growing well outside the immediatevicinity of the lake. Therefore, high A/C ratio at Gahai Lakelikely results from higher production of Artemisia pollen, due tohigher water availability during the growing season. During theperiods of 1958–1962, 1966–1972, 1978–1982, 1987–1992,2000–2004, Artemisia pollen is more abundant with correspond-ingly higherA/C ratios (generallyN0.5), suggesting relativelywetclimates. While during other periods (1963–1965, 1973–1977,1983–1986, and 1993–1999), Chenopodiaceae dominates withlow A/C ratios (generally b0.5), suggesting drier climates.

5.2. Sensitive response of arid region vegetation to changes ineffective moisture

To establish the climate sensitivity of Gahai Lake pollenrecord, we correlated the pollen data with the records from thenearby Delingha weather station and with lithology data from thesame core. Instrumental records from 1956–2004 at Delinghashow that the mean annual temperature has increased ~3 °C,which has also been observed in other stations on the Tibetan

Fig. 4. Correlation of Gahai Lake records with instrumental climate data and Dunde ice core data. A. Relative humidity (RH) from Delingha 1956–2004. B. A/C(Artemisia/Chenopodiaceae) pollen ratios at near-annual resolution from Gaihai Lake. C. Carbonate content from Gahai Lake. D. Pollen records from Dunde ice coreat annual resolution from 1957–1986. Solid black lines indicate probable correlations, and dotted lines with question marks suggest possible correlations. Gray thicklines show the correlation of the lowest Delingha RH, low Gahai A/C ratio, relatively high Gahai carbonate, and the highest A/C ratio and pollen concentration atDunde in the 1970s, as discussed in the text. All curves are 3-point moving averages.

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Plateau, especially in theQaidamBasin (Tao et al., 2004), and thatannual precipitation has increased from ~100 to near 300 mmover the last 48 years (Fig. 1C). As a result of increasing temper-ature and precipitation, relative humidity, as a measure of effec-tive moisture, shows no trend but with fluctuations between 33%and 46% during the same period (Fig. 1C).

A/C ratio at Gahai Lake appears to show a good correlationwith relative humidity data from Delingha (Fig. 4A–B), withperiods having a high A/C ratio (1958–1962, 1966–1972, 1978–1982, 1987–1992, and 2000–2004) corresponding with periodsof higher relative humidity (1959–1961, 1966–1972, 1981–1983, 1987–1990 and 1998–2001). Because of the strongevapotranspiration in the Qaidam Basin, relative humidityseems to be a better measure of water availability to plants thanprecipitation. We interpret relative humidity as an approximatemeasure of water available to plants. The slight offset in agesbetween the two records could have been caused by the datinguncertainty at Gahai Lake, possibly caused by 210Pb model usedand by changes in the sediment accumulation rate over the last50 years. The good correlations strongly support that A/C ratio isextremely sensitive to change in effective moisture in the QaidamBasin, as suggested by our regional surface pollen results along aprecipitation transect from Qinghai Lake to Dunde Icecap (Zhaoet al., 2007).

Pollen assemblages dominated by desert pollen types includ-ing Chenopodiaceae and with lowA/C ratios tend to occur duringlow-carbonate periods, while high Artemisia and Poaceae and

high A/C ratios occur during high-carbonate periods (Fig. 4C).Our modern process study and multiple proxy downcore datafrom nearby Hurleg Lake show that high (low) carbonaterepresents high (low) lake levels (Zhao et al., 2007). Thediscrepancy between low A/C ratio and high carbonate in the1970s might be caused by decreasing water availability to plantsin the basin and at the same time increasing precipitation in thesource region of the groundwater from the mountains. The highmountain precipitation caused increased groundwater and Ca andHCO3 inputs to the lake, resulting in high lake level and morecarbonate precipitation. Unfortunately, there are no meteorologi-cal stations in the surroundingmountains to compare with, but seediscussion below for support of this explanation based on com-parison with the Dunde ice core.

5.3. Comparison with annual resolution records from Dundeicecap

Liu et al. (1998) reported a 30-year pollen record from 1957to 1986 at annual resolution from the annual ice layers at theDunde icecap at an elevation of 5325 m, about 200 kmnorthwest of Gahai Lake (Fig. 1B). The climate and vegetationat Dunde icecap differ greatly from around Gahai Lake. Annualsnow accumulation rate measured from the annual ice layers atDunde is on average 400–500 mm (water equivalent) over thelast 100 years (Davis et al., 2005). Steppes and alpine meadows,dominated by Artemisia and Cyperaceae, respectively, occur

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around the icecap and also to the east and southeast (Liu et al.,1998). Artemisia and Chenopodiaceae pollen are abundant atboth sites; however, at Dunde ice core Chenopodiaceae is lower(mostly b20%) and trees/shrubs (up to 25%) and Cyperaceae(up to 20%) are higher than at Gahai Lake.

A/C ratios and pollen concentrations (reflecting vegetationcover related to effective moisture) at Dunde ice core (Fig. 4D)appear to show generally opposite correlation with relativehumidity from Delingha (Fig. 4A) and with A/C ratios fromGahai Lake (Fig. 4B), based on their respective chronologies.This opposite correlation is especially striking during the late1970s, when relative humidity at Delingha was lowest in the last50 years but A/C ratios and pollen concentrations at Dundewere highest. Net annual snow accumulation rate at Dundereached up to 650 mm during the period 1974–1978, the highestin the last 80 years (Davis et al., 2005; their Fig. 7a). Thisobservation seems to support our interpretation for the mis-match between A/C ratio and carbonate content at Gahai Lakeduring the 1970s (see above). High precipitation in themountains would increase pollen production and induce highA/C ratios at Dunde, and resultant increase in stream and ground-water flows would cause high lake level and high-carbonatecontent at Gahai Lake.

What would have caused the opposite relationship betweeneffective moisture at low and high elevations, especially duringthe 1970s? As a large portion of annual precipitation at Dunde ismonsoon-derived as interpreted by Davis et al. (2005), the highsnow accumulation rate in the late 1970s suggests strongmonsoon influence. The intense heating and upward motion ofair over the Tibetan Plateau during strong monsoon causes strongair subsidence to the northwest and north of the plateau, inducinga dry climate in central Asia (He et al., 1987; Broccoli andManabe, 1992). This mechanism also appears to operate at aregional scale as shown in regional climate simulations thatuplifting air in the surrounding mountains and plateau causes theair subsidence to extend into the QaidamBasin (Sato and Kimura,2005). Therefore, the interactions between subsiding air flow andmonsoon precipitation (direct or recycled) into the Qaidam Basinmight have been responsible for the observed opposite change ineffective moisture. Stronger monsoon and increased precipitationat Dunde would correlate with stronger subsidence of warm, dryair at Gahai Lake in the basin, causing stronger evaporation andthus lower relative humidity, though the direct monsoonprecipitation in the basin is also high in the late 1970s.

Acknowledgements

We thank J.J. Chen and C. Zhao for field assistance, andP.Wilkinson and E. Slavicek for 210Pb and 137Cs dating analysis,and three anonymous reviewers and the editor for useful com-ments and suggestions. This project was supported by theNational Natural Science Foundation of China (Grants#40528001, 40421101 and 40771212), and US NSF (EAR#0518774 and #0519255). During the final preparation of themanuscript, the senior author was a Visiting Research Scientistin the Department of Earth and Environmental Sciences atLehigh University, supported by the US NSF.

References

Binford, M.W., 1990. Calculation and uncertainty analysis of 210Pb dates forPIRLA project lake sediment cores. Journal of Paleolimnology 3, 253–267.

Broccoli, A.J., Manabe, S., 1992. The effects of orography on midlatitudeNorthern Hemisphere dry climates. Journal of Climate 5, 1181–1201.

Cohen, A.S., Palacios-Fest, M.R., Negrini, R.M., Wigand, P.E., Erbes, D.B.,2000. A paleoclimate record for the past 250,000 years from Summer Lake,Oregon, USA: П. Sedimentology, paleontology and geochemistry. Journalof Paleolimnology 24, 151–182.

Cour, P., Zheng, Z., Duzer, D., Calleja, M., Yao, Z., 1999. Vegetational andclimatic significance of modern pollen rain in northwestern Tibet. Review ofPalaeobotany and Palynology 104, 183–204.

Davis, C.P., Fall, P.L., 2001. Modern pollen precipitation from an elevationaltransect in central Jordan and its relationship to vegetation. Journal ofBiogeography 28, 1195–1210.

Davis, M.E., Thompson, L.G., Yao, T., Wang, N.L., 2005. Forcing of the Asianmonsoon on the Tibetan Plateau: evidence from high-resolution ice core andtropical coral records. Journal of Geophysical Research 110, D04101.doi:10.1029/2004JD004933.

El-Moslimany, A.P., 1990. The ecological significance of common nonarborealpollen: examples from dryland of the Middle East. Review of Palaeobotanyand Palynology 64, 343–350.

Fægri, K., Iversen, J., 1989. Textbook of Pollen Analysis, (4th Edition). JohnWiley and Sons, London, UK.

Grimm, E.C., 1992. CONISS: a Fortran 77 program for stratigraphicallyconstrained cluster analysis by the method of incremental sum of squares.Computers & Geosciences 13, 13–35.

Gunin, P.D., Vostokova, E.A., Dorofeyuk, N.I., et al., 1999. VegetationDynamics ofMongolia. Kluwer Academic Publishers, Dordrecht, pp. 46–54.

He, H.Y., McGinnis, J.W., Song, Z.S., Yanai, M., 1987. Onset of the Asiansummer monsoon in 1979 and the effect of the Tibetan Plateau. MonthlyWeather Review 115, 1966–1995.

Herzschuh, U., 2007. Reliability of pollen ratios for environmental reconstruc-tions on the Tibetan Plateau. Journal of Biogeography. doi:10.1111/j.1365-2699.2006.01680.x.

Herzschuh, U., Kürschner, H., Ma, Y.Z., 2003. The surface pollen and relativepollen production of the desert vegetation of the Alashan Plateau, westernInner Mongolia. Chinese Science Bulletin 48, 1488–1493.

Hou, X., 2001. Vegetation Atlas of China. Science Press, Beijing.IPCC WGI Fourth Assessment Report, Summary for Policymakers, 2007. The

10th Session of Working Group I of the IPCC, Paris.Li, Y.C., Xu, Q.H., Zhao, Y.K., Yang, X.L., Xiao, J.L., Chen, H., Lu, X.M.,

2005. Pollen indication to source plants in the eastern desert of China.Chinese Science Bulletin 50, 1632–1641.

Liu, H.Y., Cui, H.T., Pott, R., Speier, M., 1999. The surface pollen of thewoodland-steppe ecotone in southeastern Inner Mongolia, China. Review ofPalaeobotany and Palynology 105, 237–250.

Liu, K.B., Yao, Z.J., Thompson, L.G., 1998. A pollen record of Holoceneclimatic changes from Dunde ice cap, Qinghai-Tibetan Plateau. Geology 26,135–138.

Maher, L.J., 1981. Statistics for microfossil concentration measurementsemploying samples spiked with marker grains. Review of Palaeobotanyand Palynology 32, 153–191.

Qian, W., Hu, Q., Zhu, Y., Lee, D.-K., 2003. Centennial-scale dry–wetvariations in East Asia. Climate Dynamics 21, 77–89.

Sato, T., Kimura, F., 2005. Impact of diabatic heating over the Tibetan Plateauon subsidence over northern Asian arid region. Geophysical ResearchLetters 32, L05809. doi:10.1029/2004GL022089.

Shao, X.M., Huang, L., Liu, H.B., Liang, E., Fang, X., Wang, L.L., 2005.Reconstruction of precipitation variation from tree rings in recent 1000 yearsin Delingha, Qinghai. Science in China (Series D) 48, 939–949.

Sun, X.J., Du, N.Q., Wong, C.Y., Lin, R.F., Wei, K.Q., 1994. Paleovegetation andpaleoenvironment of Manas Lake, Xinjiang, N.W. China during the last14,000 years. QuaternarySciences 3, 239–246 (inChinese,with EnglishAbstr.).

Tao, N., Chen, L.X., Zhou, Z.J., 2004. The characteristics of climate change overthe Tibetan Plateau in the last 40 years and the detection of climatic jumps.Advances in Atmospheric Sciences 21, 193–203.

114 Y. Zhao et al. / Global and Planetary Change 62 (2008) 107–114

Tarasov, P.E., Jolly, D., Kaplan, J.O., 1997. A continuous late glacial andHolocene record of vegetation changes in Kazakhstan. Palaeogeography,Palaeoclimatology, Palaeoecology 136, 281–292.

Thompson, L.G., Thompson, E.M., Davis, M.E., Lin, P.N., Henderson, K.,Mashiotta, T.A., 2003. Tropical glacier and ice core evidence of climatechange on annual to millennial time scales. Climatic Change 59, 137–155.

Van Campo, E., Gasse, F., 1993. Pollen- and diatom-inferred climatic andhydrological changes in Sumxi Co Basin (Western Tibet) since 13,000 yr B.P. Quaternary Research 39, 300–313.

Zhang, Q.B., Chen, G.D., Yao, T.D., Kang, X.C., Huang, J.G., 2003. A 2,326-year tree ring record of climate variability on the northeastern Qinghai-Tibetan Plateau. Geophysical Research Letters 30, 1939–1742.

Zhao, Y., Yu, Z.C., Chen, F.H., Ito, E., Zhao, C., 2007. Holocene vegetation andclimate history at Hurleg Lake in the Qaidam Basin, northwest China.Review of Palaeobotany and Palynology 145, 275–288.