Global and Planetary Change 94–95 (2012) 72–81

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Global and Planetary Change

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Pliocene–Pleistocene stepwise drying of Central Asia: Evidence from paleomagnetismand sporopollen record of the deep borehole SG-3 in the western Qaidam Basin, NETibetan Plateau

Maotang Cai a, Xiaomin Fang a,⁎, Fuli Wu a, Yunfa Miao b, Erwin Appel c

a Key Laboratory of Continental Collision and Plateau uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, Chinab Key Laboratory of Desert and Desertification, Cold and Arid Regions Environmental and Engineering Institute, Chinese Academy of Sciences, Lanzhou 730000, Chinac Fachbereich Geowissenschaften, Universität Tübingen, Hölderlinstr. 12, 72074 Tübingen, Germany

⁎ Corresponding author. Tel.: +86 10 6284 9697; faxE-mail address: fangxm@itpcas.ac.cn (X. Fang).

0921-8181/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.gloplacha.2012.07.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 January 2012Accepted 5 July 2012Available online 16 July 2012

Keywords:Qaidam BasinSporopollenAridificationGlobal coolingTibet Plateau uplift

Drying of the Asian interior has generally been linked to Tibetan Plateau uplift, retreat of the Para-Tethys Seaand global cooling. However, lack of detailed aridification records hinders elucidation of how drying is con-trolled by these factors and to what extent each factor contributes. In this study, a 600 m deep core (SG-3)of lacustrine–playa deposits was obtained from the western Qaidam Basin, NE Tibetan Plateau for pollenanalysis. Magnetostratigraphic dating of the core determines its age at ca. 3.1–0.01 Ma. The palynologic com-positions show that a steppe to desert vegetation predominates the core. Artemisia-dominated steppe repre-sentative of relative warm and wet climate before 2.6 Ma changed to Chenopodiaceae-dominated steppedesert under drier climate conditions between 2.6 Ma and 0.9 Ma, interrupted by a short moister intervalof Artemisia-dominated steppe at 1.8–1.2 Ma. From 0.9 Ma to 0.6 Ma, Chenopodiaceae–Ephedraceae desertvegetation started to develop, and since 0.6 Ma, Ephedraceae-dominated desert prevailed. This vegetationchange in the western Qaidam Basin suggests a stepwise long-term aridification of the central Asia inland be-ginning at ca. 2.6 Ma, 1.2 Ma, 0.9 Ma and 0.6 Ma since the late Pliocene, most probably as a response to bothlong-term global cooling and Tibetan Plateau uplift at those times.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

The aridification of central Asia and northwestern China hasattracted wide interest since it has been regarded as a result of theuplift of the Tibetan Plateau, the retreat of the Para-Tethys Sea, globalcooling and the evolution of the East Asian monsoon (EAM)(Ruddiman and Kutzbach, 1989; Li et al., 1995; Ramstein et al.,1997; Ding and Sun, 1998; Rea et al., 1998; Li and Fang, 1999; An etal., 2001, 2006; Guo et al., 2002; Sun and Wang, 2005; Zhang et al.,2007; Jiang and Ding, 2008). The Pliocene–Quaternary drying ofAsian inland since the retreat of the Para-Tethys Sea in the Oligocene(Dercourt et al., 1993; Ramstein et al., 1997) might be linked to globalcooling and uplift of the Tibetan Plateau, both being rapid processesduring this time. However, which of these factors (global cooling oruplift of Tibetan Plateau) was the major driving mechanism is stillunclear, as most of the existing paleoclimate records concerning thisissue do not come from the interior of central Asia but have beenobtained indirectly, mostly from the Chinese Loess Plateau (CLP)(Liu and Ding, 1983; Ding et al., 1999; An et al., 2001, 2006; Guo etal., 2002; Wu et al., 2007; Sun et al., 2008). The few existing records

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from the inland arid area are either low resolution studies from theOligocene–Pliocene period (Wang et al., 1999; Sun and Liu, 2006;Miao et al., 2011; Wu et al., 2011) or only focus on some details ofthe late Pleistocene and Holocene (Zheng et al., 2002; Zhao et al.,2007). This lack of detailed well-dated Plio–Quaternary climate re-cords from the dry Asian inland hinders our understanding of themechanism of aridification.

The huge Qaidam Basin in the northeastern Tibetan Plateau receivedvery thick and continuous lacustrine deposits and provides a greatchance to detect paleoclimate changes in central Asia and the effect ofTibetan Plateau uplift (Fig. 1a). Moreover, palynologic records obtainedfrom lacustrine sediment archives bear important information on re-gional vegetation and climate changes.With theQinghai Oilfield Compa-ny of PetroChina and the Sino-German cooperation, a 600 m deep core(SG-3) was obtained from the western Qaidam Basin. In this we presentmagnetostratigraphic results in order to date the core and use thepalynological record to reconstruct in detail the Pliocene–Quaternaryvegetation and climate history in central Asia.

2. Setting of the study site, geology and stratigraphy

The Qaidam Basin, a Mesozoic–Cenozoic sedimentary basin, is thelargest intermontane basin (covering an area of ~120,000 km2) on

Fig. 1. Digitial elevation map of the Qaidam Basin (a) and geologic map of the studied region (b) showing the location of core SG-3. A,Tarim Basin; B, Linxia Basin; C, Weihe Basin;D, Loess Plateau; E, Baikal lake; F, Garze; G, Jungar; H, Yahu; I, SG-1.Panel b: (modified from Shen et al., 1993 and Fang et al., 2007).

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the northeastern Tibetan Plateau and belongs to the eastern part ofthe dry central Asian interior (Fig. 1a). The basin is bordered by theQilian Shan (Mts.) to the northeast, the Kunlun Shan (Mts.) to thesouth, the Altun Shan (Mts.) to the northwest and the Ela Shan to theeast, with altitudes ranging from 4000 m to over 5000 m, standingover 1000–2000 m above the Qaidam Basin (ca. 2800–3100 m a.s.l.)(Fang et al., 2007). This topographic situation and its location determinea typical inland hyper-arid climate, with cold (mean annual tempera-ture 0–5 °C, January: −10.6 °C; July: 16.5 °C), windy and dry condi-tions (mean annual precipitation (MAP) from 100 mm in east to less

than 20 mm in west). Most precipitation falls as rain during the sum-mer months. Mean annual evaporation is over 20 times higher thanthe MAP. Consequently, deserts (including sand dunes) and salt lakesoccur in the basin (Du and Sun, 1990). The westerlies are the dominantatmospheric pattern over the basin throughout the year, while the EastAsian summer monsoon reaches mostly the southeastern part of thebasin (Wu et al., 1985).

The western part of the basin is characterized by arid desert envi-ronment, including expanses of gravels, moving thin sands, yardangs(wind erosion geomorphology), playa and salt lake deposits. The

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modern vegetation in the region is a typical desert vegetation on gravelly,well-drained soils on alluvial fans, mainly Ephedra przewalskii, Haloxylonammodendron, Salsola collina, Kalidium foliatum, Sympegma regelii,Ceratoides lateens, Nitraria roborowskii, N. tangutorum, Tamarix chinensisandArtemisia spp. (Zhou et al., 1990). Some residual lakes are surroundedby marsh vegetation dominated by Phragmites communis along thelittoral zone. Slopes at higher elevations on surrounding mountains arecovered by shrubs adapted to cold, windy, semi-arid conditions, whichare chiefly Berberis and Salix amnematchinesis. Flat areas in the basin cen-ter are almost bare of vegetation (Wu, 1995).

The studied region consists of several rows of NW-trending foldsprogressively formed and thus younging northeastwards by propaga-tion faults rooted from the Kunlun fault (Fang et al., 2007). The foldedareas are subject to severe wind erosion to form Yardang geomor-phology, exposing old stratigraphy in their cores. The areas betweenfolded areas are less or not affected by wind erosion, mostly receivingcontinuous sediment deposition and forming flat playas and occasionallysalt lakes (e.g. Dalangtan potash salt lake) (Fig. 1). Our drilling site(38°22′47.8″ N, 91 44′53.43″E, elevation 2,736 m) is located on a veryflat playa called Fengxi Salina Plain, roughly between two rows of anti-clines, the Xiaoliang Shan–Nanyi Shan–Dafeng Shan to the south andthe Jianding Shan–Changwei Liang–Jian Shan to the north (Fig. 1b).According to the geologic map, the playa consists mainly of latePleistocene to Holocene sediments, while the surrounding anticlinesexpose mainly late Pliocene to early Quaternary sediments of mostlysiltstone, calcareous mudstone and marl, intercalated with gypsumand salt beds (Tuo and Philp, 2003) (Fig. 1b).

3. Materials and methods

The core SG-3 was drilled to a depth of 600 m with an average re-covery rate of 90%. The core sediments show an alternating sequenceof evaporite layers and calcareous mudstone and marl layers. Saltminerals occur either in cm- to m-thick beds or in scattered displacivecrystals in calcareous mudstone layers. The evaporite beds are mostlydominated by cubic/blocky halite crystals which are almost pure,white and compact with mm- to cm-size. Blodite, polyhalite andcarnallite appear in some evaporite layers in the upper part of thecore. Calcareousmudstone layers are gray, blackish, greenish, yellowishgreen and yellowish brown and contain some single halite and gypsumcrystals. (Fig. 2a).

For the magnetostratigraphic study 317 oriented cubic samples at2×2×2 cm were taken at ~1 m intervals in calcareous mudstonelayers. To isolate a characteristic remanent magnetization (ChRM)direction, samples were mainly subjected to progressive demagneti-zation in alternating field (AF) in 15–17 steps from 2 mT to 80 mTuntil the natural remanent magnetization (NRM) intensity reached5%–10% of the initial value. Remanence measurements were madeusing a 2G Enterprises Model 760-R cryogenic magnetometer installedin a magnetically shielded room (b300 nT). Magnetic measurementswere all performed at the Paleomagnetism Laboratory, Institute ofGeology and Geophysics, Chinese Academy of Sciences (Beijing).

110 samples (average sampling interval 6 m) were taken from coreSG-3 for palynological analysis. Approximately 50 g of sediment persample was treated with 10% HCl to remove carbonate and 70% HF toremove silica. For samples with a high organic content, 10% causticsoda was added and then boiled in a waterbath after the acidpretreatment. Sporopollen was concentrated twice with heavy-liquidbetween 1.9 and 2.2 g/cm3 in subsequent steps (Erdtman et al., 1969;Ke, 1994). Then, the sporopollen was collected after acetolysis. Aknown number of Lycopodium clavatum spores were initially added toeach sample for calculation of pollen concentration (Maher, 1981).Pollen and spores mounted in glycerin were counted under Olympus-BX51 light microscope with ×400 magnification in regularly spacedtraverses for rough scanning, and at ×600 magnification for criticalidentification at the Institute of Tibetan Plateau Research (ITP), Chinese

Academyof Sciences (Beijing). The ITP currently stores the pollen slides.Palynomorph identification is based on the formally published pollenplates and extensive modern pollen slides (Xi and Ning, 1994; Wanget al., 1995). More than 300 pollen grains per sample were identifiedin order to obtain a high palynomorph content of each sample. An A/Cratio was calculated by Artemisia/Chenopodiaceae pollen counts. Thepercentages of trees and herbs and shrubs' pollen taxa were based onthe sum of the total terrestrial pollen in a sample. The percentage ofeach taxon of the aquatic pollen was based on the sum of the terrestrialpollen plus the aquatic pollen of the taxon in a sample. The totalpollen concentration was deduced from the counts of fossil pollen andLycopodium spores in a sample and expressed in pollen grains pergram of samples.

4. Results

4.1. Magnetostratigraphy and timescale

AF demagnetization diagrams demonstrate that a stable singlemagnetization component can be isolated after removal of a low coer-civity magnetization at maximum 20–25 mT, but mostly alreadybelow about 10 mT (Fig. 3). High-coercive ferro(i)minerals as hema-tite obviously do not play an important role, as complete demagneti-zation could be achieved by AF treatment for most of the samples.High-temperature thermomagnetic runs of magnetic susceptibility(k–T curves), performed on some samples, reveal magnetite by itsCurie temperature of ~580 °C (Fig. 3 h). The hump in the k–T curves,starting around 250 °C, reflects new formation of magnetite duringheating arising from transformation of paramagnetic iron-bearingminerals or from iron sulfides (Roberts, 1995). Characteristic rema-nence directions (ChRM) were obtained using principal componentanalysis. From the measured 317 samples 264(83%) gave reliable re-sults. The maximum angular deviations (MAD) were smaller than15°, with 94% of the samples having MAD values smaller than 10°. Be-cause no azimuthal orientation could be determined for the drill-coreSG-3, the final magnetostratigraphic results have to be based on incli-nation data only. The inclination values of the ChRM directions from264 samples are plotted alongside the stratigraphic column in Fig. 2.

The magnetic polarity sequence for the core SG-3 shows the normalpolarity Brunhes epoch above 269 m, the predominantly reverse polar-ity Matuyama epoch from 269 to 532 m, and the top of the predomi-nantly normal polarity Gauss epoch below 532 m. Two or moresubsequent data points showing the samepolarity are considered to de-fine a subchron, event or excursion (Fig. 2b). The Brunhes–Matuyama(B/M) boundary is fairly well located at a depth of 269 m. Sixshort events or excursion (R1–R6) are recognized in the Brunhes nor-mal chron, which might be correlated with the Blake, Jamaica, CR1,Biwa III, CR2 and Delta events (Champion et al., 1988; Langereis et al,1997; Worm, 1997; Laj and Channell, 2007), but this tentative correla-tion should be treated with caution. In the Matuyama reversedchronozone, we can identify normal polarity zones N9–N12 which arelikely the Jaramillo (319–336 m), Cobb Mountain (342–351 m),Olduvai (401–450 m) and Reunion (477–487 m) normal subchrons.One short normal-polarity zone (N8) at 289–292 m in the lateMatuyama may represent the Post-Jaramillo event or ‘Burmester’subchron (Champion et al., 1988). The longer normal polarity zoneN13 can be correlatedwith the top of theGauss epoch. The two reversedsamples at the bottom of the sequence would consequently correspondto the top of Kaena subchron (Fig. 2).

Our magnetostratigraphic age determination is supported by pre-vious results. For example, the B/M boundary in the core ZK402, car-ried out by the Ministry of Mineral Resources of China for exploringsalt resources, was detected at a depth of 300 m (Shen et al., 1993),quite close to the B/M depth in our result. The core ZK402 was drilledjust west of the location of the SG-3 core (see Fig. 1b for location). Inour former core SG-1 at Chahansilatu, just north of core SG-3, the B/M

Fig. 2. Lithology, inclination data and interpreted magnetic polarity sequence of core SG-3 (black/white bars indicate normal/reverse polarity). For correlation the geomagneticpolarity timescale (GPTS) of Cande and Kent (1995) is used. Events/excursions in the Brunhes chron are shown according to Champion et al. (1988); Worm (1997); Langereiset al. (1997) and Laj and Channell (2007).

Fig. 3. Results of alternating field-demagnetizations (a–g) of some representative samples from core SG-3. Solid/open circles represent horizontal/vertical projections. In (h) ahigh-temperature curve of magnetic susceptibility (h) of a magnetic extract is shown.

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boundary is at the depth 298 m (Zhang et al., 2012), also very close tothe result of core SG-3. This magnetic polarity correlation determinesa rapid increase of the sedimentation rate since the B/M boundary.We interpret this as a direct sedimentary response to the intense tec-tonic uplift of the surrounding anticlines (Fig. 1b). A SW–NE cross sec-tion through these anticlines based on seismostratigraphy andboreholes fromhydrocarbon exploration demonstrates the rapid devel-opment of growth strata since the B/M boundary (see Fig. 1 in Zhang etal., 2012 for the location of anticlines north of SG-3). A strong increase ofthe sedimentation rate has also been reported from other boreholes inthe western Qaidam Basin (Shen et al., 1993; Sun and Liu, 2006;Zhang et al., 2012). At the southern margin of the Qaidam Basin rapidtectonic uplift of the Kunlun Shan occurred due to fast movement ofthe Kunlun fault at ca. 0.8 Ma (Cui et al., 1996; Song et al., 2005).Furthermore, sharp angular unconformities and strong incisions at ca.0.9–0.8 Ma were frequently observed at the basin margins (Li et al.,2012) and in many other regions of the NE Tibetan Plateau (e.g., Liet al., 1996; Fang et al., 2005; Liu et al., 2010).

Based on the correlation above, the age of the core SG-3 rangesfrom 3.1 Ma to 0.01 Ma. For the estimation of the bottom age thesedimentation rate in R13 was taken the same as determined withinN13. The top age was calculated by a sedimentation rates in N1 equalto the one between R1 and R2. The present salt lake still exists justwest of our core at Dalangtan. The surface at the drilling site is veryflat and is less affected by wind erosion, suggesting that the salt lakeretreated not too long ago from the drilling site. This agrees with ourage estimation of 0.01 Ma for the top of the core SG-3. 230Th/234U datingat the top of core ZK02 in NWDalangtan yielded an age of 14.9 ka (Houet al., 2010, 2011), confirming our conclusion. For further interpretationof the palynological results the timescale between polarity boundarieswas determined by linear interpolation.

4.2. Palynological record

Out of the 110 analyzed samples 104 contain 305–607 pollen grains.A total of 48 sporopollen taxa were identified from all of the samples.These include major herbs and shrubs of Artemisia, Chenopodiaceae,Poaceae, Asteraceae, Ranunculaceae, Saxifraga, Cruciferae, Umbrelliferae,Ephedraceae, Tamarix, Rosaceae and Nitraria, major trees of Picea, Abies,Pinus, Betula, Ulmus, Quercus, Juglans and Alnus, and major aquatic taxaof Typha and Potamogetonaceae, as well as a few pteridophyte and algalspores which were excluded from the total pollen sum when we calcu-lated pollen percentages. A summary percentage pollen diagram with20 selected taxa is shown in Fig. 4. A high percentage of herbs and shrubstaxa (average 94.0%) and a low percentage of tree taxa (average 6.0%)characterize the pollen assemblage. Chenopodiaceae (average 30.6%),

Fig. 4. Percentage pollen diagrams of core SG-3 showing selected taxa.

Artemisia (average 30.4%) and Ephedraceae (average 12.2%) dominatedthe pollen assemblages through thewhole sequence.Most of tree pollensare generally b2.0% and some occur only in traces. The A/C ratio showedlarge changes from 0.2 to 2.1. The pollen concentration varied from 352to 12,663 grains/g. The logarithmic ratio ln(NAP/AP) of Non ArborealPollen (NAP) to Arboreal Pollen (AP), ranges between 1.6 and 4.2.Based on variations in the abundance of the dominant taxa, A/C andln(NAP/AP) ratios, we divided the pollen diagram into six pollen zones(Fig. 4), which are described below, from the bottom to the top.

4.2.1. Zone I (depths 600–533 m, ~3.1–2.6 Ma)In this zone, pollen assemblages of herbs and shrubs dominate the

interval, although their contents are the lowest in the whole sequence(88.1%–94.5%). Chenopodiaceae and halophytic Ephedraceae pollensoccupy 20.4%–27.9% (average 24.7%) and 2.0%–21.2% (average 7.4%) re-spectively, and Artemisia reaches 32.3%–42.5% (average 37.6%). Poaceaeand Tamarix occur in moderate-high percentages (average 2.8% and4.2%, respectively). Other mesic and arid herbs appear with low anderratic percentages, including Asteraceae, Ranunculaceae, Saxifraga,Cruciferae and Umbrelliferae. Tree pollens, mainly derived from Picea,Ulmus and Betula, account for 5.5%–11.9% and reach the highest averagepercentage (9.1%) in the whole section. The Picea fluctuates between0.5% and 2.3% (average 1.2%) and the Betula between 0 and 2.4%(average 1.3%). Other subtropical-temperate deciduous broadleavedtrees including Juglans and Castanea are present in most samples inthis zone. A/C ratios were between 1.2 and 2.0 (average 1.5), beingthe highest values in the whole section. The ln(NAP/AP) ranges be-tween 2.0 and 2.9 (average 2.3) (Figs. 4 and 5).

4.2.2. Zone II (533–403 m, 2.6–1.8 Ma)This zone is characterized by a steady increase of herbs and shrubs

taxa (89.0%–97.3%, average 93.0%) and a gradual decrease in treetypes (1.8%–10.1%, average 7.0%). The proportions of desert taxa includ-ing Chenopodiaceae (average 31.8%), Ephedraceae (average 9.8%) andTamarix (average 4.3%) pollens display a remarkable increase as com-pared with the preceding zone I, whereas Artemisia of steppe taxa pol-len declines to 21.6%–37.4%, with an average of 30.3% of the totalpollens. Other mesic and arid herbs including Poaceae and Asteraceaeincreased in this zone, the former ranging from 1.1% to 16.2% (average3.8%) and the later from 0.9% to 4.1% (average 2.6%). Tree pollens inthis zone including Picea, Betula and Ulmus sow a sharp decline toaverages of 0.9%, 1.3% and 1.3%, respectively. Other tree taxa, such asPinus, Quercus, Juglans, Castanea and Alnus are also present, but almostless than 1% in abundance. The A/C ratios decline significantly and varyfrom 0.6 to 1.4 (average 0.9), whereas the ln(NAP/AP) ratios constantlyincrease and range between 2.1 and 4.1 (average 2.7) (Figs. 4 and 5).

Open curves for taxa with low abundance are 6-times exaggerated.

Fig. 5. Depth functions of pollen proxy records of core SG-3 and their comparison with the stacked δO18 isotope record from the LR04 (Lisiecki and Raymo, 2005). A/C: Artemisia/Chenopodiaceae; C+E: Xerophytic taxa (Chenopodiaceae+Ephedraceae) (%); ln(NAP/AP): ln ratios of Non Arboreal Pollen (NAP) to Arboreal Pollen; Thermophilic taxa:Betula+Quercus+Castanea+ Juglans (%).

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4.2.3. Zone III (403–350 m, 1.8–1.2 Ma)This zone is marked by a prominent recovery of Artemisia pollen to

30.1%–45.4% (average 34.9%) of the pollen sum. Chenopodiaceaepollen decreases abruptly to 23.8%–31.1% (average 26.4%). Conse-quently, A/C ratios significantly increase and vary from 1.2 to 1.9(average 1.3) in comparison with the preceding zone II. The proportionof Ephedraceae pollen remains quite stable as before (average 9.9%) inthe zone. The other mesic and arid herbs and shrubs including Poaceae,Asteraceae, and Tamarix pollen display a gradual increase. Broadleavedtree pollens decrease further in this zone in comparison with zone II.Betula, Ulmus and Quercus are present throughout the zone, but with alower average (b1%). Picea increases to 0.6%–1.8% (average 1.0%).Other conifer pollen abundances including Pinus, Larix and Cupresskeep stable but low. The ln(NAP/AP) ratios slightly increase and rangebetween 2.3 and 3.7 (average 2.9). Aquatic Typha is slightly lowerthan in the preceding zone II and Pediastrumwas found in two samplesof this zone (Figs. 2a, 4 and 5).

4.2.4. Zone IV (350–300 m, 1.2–0.9 Ma)This zone is characterized by an abrupt increase of Ephedraceae

from 3.2% to 26.9% (average 14.7%). The proportions of other deserttaxa pollens including Chenopodiaceae (average 26.4%) and Tamarix(average 4.7%) display a slightly increase in comparison with the pre-ceding zone III, whereas steppe taxa pollen Artemisia markedly de-clines to 14.3%–37.2%, with an average 29.5% of the total pollens.Other mesic herbs including Poaceae and Asteraceae decrease inthis zone, the former ranging from 1.2% to 17.3% (average 3.2%) andthe later from 0.7% to 3.7% (average 2.2%). Tree pollens in this zoneincluding Picea, Pinus, Betula, Quercus and Ulmus show a slight declinecompared with zone III. Other tree taxa, such as Juglans, Castaneaand Alnus remain quite stable as before, but almost less than 1% inabundance. Aquatic Typha abruptly increase to an average of 7.8%.The A/C ratios decline significantly and vary from 0.6 to 1.7 (average1.1), whereas the ln(NAP/AP) ratios constantly increase and range be-tween 1.7 and 3.9 (average 3.0) (Figs. 4 and 5).

4.2.5. Zone V (300–192 m, 0.9–0.6 Ma)This transition zone records a progressive increase of dry indica-

tors, characterized with a domination of similar pollen componentsas in zone IV, but with much higher-frequency fluctuations in their

abundance. Chenopodiaceae and Ephedraceae show obvious increasesfrom23.3% to 41.6% (average 31.6%) in zone IV to 6.2% to 35.9% (average17.5%) in zone V, whereas Artemisia declines to 26.2% (ranging from14.0% to 39.8%). The abundances of Tamarix and Poaceae decreaseslightly to 4.2% and 3.9%, respectively. Other mesic herbs and shrubsincluding Ranunculaceae, Saxifraga, Cruciferae, Umbrelliferae andRosaceae also gradually decline. Tree pollens continue to decline to anaverage of 4.8% (ranging from 1.9% to 8.1%) as the result of decreasingdeciduous broadleaved trees. Juglans, Castanea, Corylus and Alnuspollens are occasionally abundant. Picea increases to 0–5.5% (average1.3%) and Typha also shows a relatively high percentage (average3.6%). The A/C ratios vary from 0.4 to 1.5 (average 0.8), showing a re-markable decrease compared with the preceding zone, whereas theln(NAP/AP) ratios gradually increase and range between 2.3 and 3.9(average 3.1) (Figs. 4 and 5).

4.2.6. Zone VI (192–0 m, 0.6–0.01 Ma)The most striking feature of this zone is a considerably high pro-

portion of Chenopodiaceae and Ephedraceae pollens, reaching theirmaxima of 53.3% and 42.3% of the pollen sum, respectively. Artemisiapollen (11.5%–36.9%) continues to decrease to its lowest average per-centage 25.9% in the section. The other mesic herbs and shrubs in-cluding Rosaceae and Poaceae still show a trend of a gradualdecrease in percentage, whereas Tamarix, Asteraceae, Saxifraga andCruciferae display a slightly increase in this zone. The abundances ofthe tree pollens decrease further, varying from 1.4% to 7.2%, reachingthe lowest average (3.4%) within the section. The abundances of coni-fer taxa decrease in comparison to zone V, fluctuating between 0.4%and 3.3% (average 1.5%). Castanea pollen disappears and Juglans justoccurs in three samples in this zone. The A/C ratios are almost allless than 1.0, reaching their lowest value (0.5) within the section. Incontrast, the ln(NAP/AP) ratios range between 2.3 and 3.9 (average3.1), reaching the highest average (3.4) in the section (Figs. 4 and 5).

5. Discussion

5.1. Ecological and climatic interpretations

By studying of modern sporopollen in the Sugan Lake, located inthe western Qaidam Basin (Fig. 1a), Zhao et al. (2009) indicated

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that pollen assemblages from samples of lake surface sediments atdifferent water depths show relatively uniform abundance of mostpollen types, including Artemisia, Chenopodiaceae, Ephedraceae andNitraria. The uniform pollen composition suggests that the effectof redeposition and regathering of the pollens by lake processes(Davis, 1968) in the arid Qaidam Basin is negligible. Moreover, thepollen from lake surface sediments has regional pollen sources, most-ly from the entire lake basin catchment (Sugita, 1994; Zhao et al.,2009). Thus, we can expect that the pollen record from the lacustrinedeposits reflects the regional vegetation evolution.

According to modern surface pollen studies, Chenopodiaceae andArtemisia pollens seem to be over-represented relative to the abun-dance of their plants in the vegetation (Ma et al., 2008). Smallamounts (b30%) of arid Artemisia and Chenopodiaceae are usuallyregarded to derive from exotic sources because of their high produc-tivity (Li and Yan, 1990). Ephedraceae is typical desert plants and areoften used as indicators of dry climate (Sun et al., 1994; Li et al.,2005). Pinus is often over-represented in pollen records, but it isvery low in our samples, suggesting that it mostly came fromremote areas. Poaceae over 3–6% indicates that they exist in thesurrounding area (Tong et al., 1996; Zhao et al., 1998). The Piceavalue over 2–5% of the total pollen sum suggests local presence ofthis taxon (Tong et al., 1996). In the arid and semi-arid regions, thevegetation evolution mainly depends on the effective moisture. Inthe modern steppe and desert of northern China, Artemisia predomi-nates over Chenopodiaceae plants in the desert steppe in comparisonwith in the steppe desert (Compilatory Commission of Vegetation ofChina, 1980). Both Artemisia and Chenopodiaceae have the need foropen environments, however, Artemisia requires more moisturethan Chenopodiaceae during the growing season (El-Moslimany,1990; Van Campo and Gasse, 1993; Liu et al., 1999; Herzschuh et al.,2003; Zhao et al., 2008). Therefore, the ratio of pollen Artemisia toChenopodiaceae (A/C) can be used as an indicator of the change inrelative dominance of steppe and desert plants and of effective mois-ture in the arid region. Higher A/C ratios indicate increases in themoisture over the region. Similar studies have been carried out inother arid and semi-arid regions of the Xinjiang region (Li and Yan,1990; Tarasov et al., 1997; Cour et al., 1999; Liu et al., 1999; Li et al.,2005). Yan (1991) indicating A/C ratios b0.5 in desert, 0.5–1.2 insteppe desert, and >1 in typical steppe. Investigations of the distribu-tion of modern vegetation in relation to climate and hydrology andhigh resolution studies of Holocene pollen records in the QaidamBasin confirmed such a significance of the A/C ratio (Zhao et al.,2007, 2008). The ln(NAP/AP) ratio is usually used to indicate therelative area which is covered by shade indicating taxa versus forestcovered area (White et al., 1997). Higher ln(NAP/AP) values indicateincreasingly arid climates. In the Qaidam Basin, the vegetation zonebelongs to temperate desert. Comparing with grasses or desert vege-tations, those warm temperate broadleaved trees taxa mainly includeQuercus, Betula, Castanea and Juglans indicating relatively warmerconditions. Thus, the sum of these percentage warm broadleavedtrees reflects generally relative temperature change. Typha andSparganium generally grew along the lakeshore, and Potamogetonplants grew in the shallow-water zone of the lake. An increase in theproportion of these aquatic pollen grains could have resulted from a de-cline of the lake level (Wen et al., 2010), indicating that the lake shorewas closer to the core site. Pollen spectra from different water depthsat Hurleg Lake in the eastern Qaidam Basin show an increase ofPediastrum with lake water depth (Zhao et al., 2007). Jiang et al.(2006) suggested that an increase of Pediastrum values reflects a re-sponse to a significant rise in the level of Lake Bayanchagan in InnerMongolia (northern China). Thus, Pediastrum can be used as a good in-dicator of water depth (Xu et al., 2004; Sarmaja-Korjonen et al., 2006).

Based on the patterns of modern vegetation distribution in thebasin, the sporopollen spectra in the basin sediments record allvegetations in the basin and surrounding mountains (Sugita, 1994;

Zhao et al., 2009). We think that xeromorphic vegetation such asChenopodiaceae, Ephedraceae, Artemisia and Tamarix grew in thebasin, Typha and Sparganium generally grew along the playa lake-shore, and Potamogeton plants grew in the shallow-water zone ofplaya lake. Deciduous trees such as Quercus, Betula, Castanea andJuglans grew on the middle slope of the surrounding mountains,while Picea, Abies and perhaps Pinus lived on higher slopes of the sur-rounding mountains. Thus, the pollen record from the basin depositsreflects the regional vegetation evolution.

5.2. Ecologic environmental change and stepwise drying of the QaidamBasin

The zonation of sporopollen diagrams and index records shows clearvegetation changes and a long term stepwise drying of the QaidamBasin since ~3.1 Ma, with phase boundary ages at ca. 2.6 Ma, 1.2 Ma,0.9 Ma and 0.6 Ma (Figs. 4 and 5).

During ~3.1–2.6 Ma, the basin was dominated by Artemisia–Chenopodiaceae steppe, and humidity was high as indicated by thehighest values of A/C ratios. The higher abundance of broadleavedtrees and the lowest ln(NAP/AP) values suggest that the climatewas already dry, but the most optimal within the period 3.1–0.01 Ma. We infer that a steppe environment and a relatively warmand humid climate existed during the Late Pliocene (Figs. 4 and 5).

During the period between ca. 2.6 and 1.8 Ma, the vegetationchanged to steppe desert in the basin. The A/C ratios and ln(NAP/AP)values imply that the climate became drier during this stage. Moreover,decreasing broadleaved trees suggest that temperature started to fall ca.2.6 Ma ago. On the other hand, an obvious increase in the proportion ofaquatic pollen indicates the onset of shrinkage of the Qaidampaleo-lake(Figs. 4 and 5). These data suggest that a rapid aridification event oc-curred at ca. 2.6 Ma.

Records from other parts of the Qaidam Basin, NW China and cen-tral Asia support our interpretations. At the Yahu anticline in the cen-tral Qaidam Basin, about 270 km east of our core, detailed pollenrecords demonstrated a disappearance of some subtropical treesand a dramatic expansion of drought-tolerant plants at ca. 2.6 Ma(Wu et al., 2011). Further to the east, at the Chinese Loess Plateau(CLP) and the Weihe Basin at its southern rim, at 2.51 Ma, abroadleaved forest-grassland changed to an open forest-steppe inthe south (Tong et al., 2000), while a Cupressaceae forest shifted toa steppe in the north (Wu et al., 2007). Dry conditions wereestablished in the Baikal region after 2.62 Ma (Demske et al., 2002).An increase of eolian dust flux in deep sea sediments of the NorthPacific Ocean at 2.6 Ma (Shackleton et al., 1995), coeval with the sed-imentary transition from red clay to the loess–paleosol sequence atthe CLP (Liu and Ding, 1983; Ding et al., 1994), also indicates anaridification event at the beginning of the Quaternary.

A short interval of relative moist conditions was found betweenca. 1.8 Ma and 1.2 Ma when the vegetation returned to steppe withclearly increased A/C ratios (Figs. 4 and 5). The ln(NAP/AP) valuesshow slight increases, but this mainly results from the decrease ofbroadleaved trees, which was likely caused by the relative cold cli-mate rather than by reduced moisture during this stage, as indicatedby an increase of conifer trees. The appearance of Pediastrum may in-dicate a relatively high lake level due to increased moisture and lowertemperature (thus lower evaporation) (Figs. 2a, 4 and 5). The occur-rence of thick blackish laminated mudstone layers in this stage(Fig. 2a), indicative of a deeper lake environment (Allen andCollinson, 1986), lends robust support to pollen inference.

This relatively humid and cold climate at ca. 1.8 Ma was found alsoin other records. Liu et al. (1998) indicated that a rapid rise of thepaleo-lake level was accompanied by a sharp decrease or terminationof salt formation at ca. 1.8 Ma in the Dalangtan sub-depression of thewestern Qaidam Basin (Fig. 1). On the Chinese Loess Plateau, at ca.1.8 Ma, large quantities of spruce and fossil fir trees appeared in its

79M. Cai et al. / Global and Planetary Change 94–95 (2012) 72–81

southwestern part (Linxia Basin) (Wang et al., 1998; Li and Fang, 1999)and a steppe vegetation shifted to a Pinus forest in its central part (Wuet al., 2007) (Fig. 1).

Although the climate experienced a short interval of relative moistconditions at ca. 1.8 Ma, the stage from 1.2 to 0.9 Ma was markedby drier conditions, indicated by the decreasing trend of A/C ratiosand an increase of ln(NAP/AP) values. In this stage, steppe desertcovered the lake basin again, as indicated by the lowest percentageof Artemisia and highest proportion of Ephedraceae and Chenopodiaceae(Figs. 4 and 5).

This drying event at ca. 1.2 Ma was also observed by other records.At ca. 1.13 Ma, loess started to accumulate on the eastern TibetanPlateau, suggesting a fast drying of the Tibetan interior at that time(Yan et al., 2001). On the Chinese Loess Plateau, the lower sandloess layer (L15), which has been regarded as a result of extremelydry climate, appeared at 1.15 Ma (Liu et al., 1985; Ding et al., 1999).

From 0.9 to 0.6 Ma, the climate turned to much drier conditions,manifested by strong reduction of trees and an increase of herbs. Adecrease of A/C ratios and increase of ln(NAP/AP) values offer furtherindication. High abundance of aquatic pollen probably reflects an ex-panse of marsh in low land due to lake level drop (Figs. 4 and 5).

On the Chinese Loess Plateau, sporopollen records suggested achange of a forest-steppe to a steppe at 0.95 Ma (Wu, et al., 2007).This drying event is coeval to a strong expansion and grain size increaseof loess sediments in the Tarim and Jungar Basins at 0.9–0.8 Ma, whichare interpreted as an indication of strong expansion of the Taklimakanand Gurbantunggut Deserts at that time (Fang et al., 2002a,b).

From 0.6 Ma, the climatemay have changed to extremely dry condi-tions, since the amount of tree pollen further reduced to its minimumwhile herbs reached their maxima, especially drought-enduring herbsChenopodiaceae and Ephedraceae. The lowermost A/C ratios and thehighest ln(NAP/AP) values in the core confirm the vegetation change(Figs. 4 and 5).

More severe low moisture conditions were observed at ca. 0.5 Maby vegetation change on the CLP (Wu, et al., 2007). A rapid increase ofgrain size of loess sediments in the Tarim and Jungar Basins at ca.0.6 Ma suggests a further strong expansion of the Taklimakan andGurbantunggut Deserts and intensification of drying of the Asianinland at that time (Fang et al., 2002a,b). Both lend support to ourpollen indication of drying event at ca. 0.6 Ma.

Lithologic changes in core SG-3 provide a strong support to thelong-term drying trend suggested by the sporopollen record above.The appearance of salts in the whole core demonstrates that thestudied region was already dry since the late Pliocene, confirmingthe presence of steppe to desert environments as indicated by oursporopollen record. The upward increase of salt amounts, especiallythe occurrence of the highly soluble potassium and magnesium saltminerals carnallite and blodite (Fig. 2a), demonstrates a clear shrink-age of the salt lake and supports the interpretation of a drying trend(Figs. 2, 4–5).

Comparing the pollen proxy records with the oxygen isotope re-cord of deep sea sediments (Lisiecki and Raymo, 2005), we find thatthe long-term trends of drying and cooling, given by A/C, C+E,ln(NAP/AP) and thermophilic taxa, match well the global coolingtrend and some rapid cooling phases at ca. 2.6 Ma and 0.9 Ma (Fig. 5).

However, intensified tectonic folding of the Qaidam Basin and up-lift of the NE Tibetan Plateau have been reported at about 2.6 Ma,1.8 Ma, 1.2 Ma, 0.9–0.8 Ma, 0.6 Ma and 0.15 Ma (e.g. Li et al., 1995;Fang et al., 2005, 2007). A sharp angular unconformity between theQigequan Fm. (dated in boreholes at ca. 2.6–0.8 Ma; this study;Shen et al., 1993; Zhang et al., 2012) and the overlaying middle Pleis-tocene strata is widely distributed along the basin margin, especiallynear the foothills of Altun Shan and South Qilian Shan and dated atca. 0.9–0.8 Ma (Li et al., 2012). This unconformity, together with ourobservation of a fast development of growth strata and a rapid in-crease of the sedimentation rate (see Fig. 2 in this study and Fig. 1

in Zhang et al., 2012), demonstrates an intensive tectonic deforma-tion and uplift of the western Qaidam Basin, providing further evi-dence for rapid uplift of the NE Tibetan Plateau at that time. Theseepisodes of tectonic deformation and uplift of the Tibetan Plateaucontribute to our understanding of the long-term and stepwisedrying of the Asian inland. The uplifted topography can block mois-ture input of monsoon from the south and can intensify the westerlyjet and the Siberian High with sinking cold-dry airs (Ruddiman andKutzbach, 1989).

Thus, our result suggests that the long term stepwise drying of thewestern Qaidam Basin at ca. 2.6 Ma, 1.2 Ma, 0.9 Ma and 0.6 Ma sincethe late Pliocene might be caused by a combination of both long-termglobal cooling and rapid Tibetan Plateau uplift at those times. Furtherdetailed long-term arid climatic records from the Asian inland areneeded to decipher which mechanism is the major forcing and howthey acted through the Plio–Quaternary.

6. Conclusions

Magnetostratigraphic dating of the 600 m core SG-3 from thewestern Qaidam Basin on the NE Tibetan Plateau determined its ageat ca. 3.1–0.01 Ma. Sporopollen analysis of the core obtained a high-quality Pliocene–Pleistocene paleo-vegetation record and revealed astepwise long-term deterioration of the vegetation and drying ofthe Asian inland since ca. 3.1 Ma. The results indicate the beginningof rapid drying phases (events) at ca. 2.6 Ma, 1.2 Ma, 0.9 Ma and0.6 Ma, with a short moister interval of Artemisia-dominated steppe at1.8–1.2 Ma. Pliocene–Pleistocene rapid global cooling and episodictectonic uplift of the NE Tibetan Plateau could be the major potentialdriving forces for the drying of central Asia.

Acknowledgments

This study was co-supported by the Knowledge Innovation Programof the Chinese Academy of Sciences (grant no. KZCX2-YW-Q09-04), the(973) National Basic Research Program of China (2011CB403000) andthe NSFC grants (40920114001, 41021001). Many thanks to ZhuRixiang for the help in paleomagnetic measurements and Zan Jinbo,Yang Yibo, Wang Yadong, Zhang Zhigao, Zhang Tao, Wang Jiuyi, ChiYunping, Chen Yi and Li Xiangyu for their laboratory and fieldassistance.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.gloplacha.2012.07.002.

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