Earth and Planetary Science Letters 312 (2011) 152–163

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Earth and Planetary Science Letters

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Isotopic constraints on intensified aridity in Central Asia around 12 Ma

Guangsheng Zhuang a,b,⁎, Jeremy K. Hourigan a, Paul L. Koch a, Bradley D. Ritts c, Malinda L. Kent-Corson d

a Department of Earth and Planetary Sciences, University of California Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USAb Department of Geology and Geophysics, Yale University, PO Box 208109, New Haven, CT06520-8109, USAc Chevron Asia Pacific Exploration and Production, Chevron House, 30 Raffles Place, #08-00, Singapore, 048622, Singapored Earth Observatory of Singapore, Nanyang Technological University, 50 Nanyang Drive, Singapore 639798, Singapore

⁎ Corresponding author at: Department of Earth andof California Santa Cruz, 1156 High Street, Santa Cruz,606 5339.

E-mail address: zhuanggsh@gmail.com (G. Zhuang).

0012-821X/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.epsl.2011.10.005

a b s t r a c t

a r t i c l e i n f o

Article history:Received 2 May 2011Received in revised form 31 August 2011Accepted 4 October 2011Available online 29 October 2011

Editor: P. DeMenocal

Keywords:Himalayan–Tibetan orogenclimatic changeoxygen isotopecarbon isotopeParatethys

The relationship between central Asian aridification and the evolution of the Himalayan–Tibetan orogenremains elusive. New isotopic data from pedogenic and lacustrine carbonates sampled from well-dated Neo-gene strata (15.7–1.8 Ma) in the northeastern Qaidam basin of the northern Tibetan Plateau identify a positiveshift of ~2.5‰ in δ18O values from 12Ma to 10.7 Ma. High values were maintained until ~3.3 Ma when δ18Ovalues drop by 1‰. The timing of the positive shift in δ18O values is remarkably consistent with isotopic recordsfrom a vast region along and within the northern Tibetan Plateau. Isotopic, mineralogical, petrologic, and faciesanalyses suggest that diagenetic effects and detrital contamination are minimal. The positive shift in δ18Ovalues, as well as high δ13C values from pedogenic carbonates (which are suggestive of low soil respirationrates), is interpreted to indicate intensified aridity in central Asia ca. 12 Ma. The recognition of intensified aridityis critical to understanding the climatic effects of the development of the Himalayan–Tibetan orogen. Weattribute this climatic change in the central Asia to: (1) retreat of Paratethys from central Asia, strengthening theseasonal contrast and resulting in the loss of a nearby moisture source; (2) attainment of high elevations in theHimalayas and south-central Tibet, blockingmoisture-bearing airmasses from the Indian and Pacific oceans; and(3) enhanced isolation and outward growth of the northern Tibetan Plateau. The negative shift around 3.3 Mamight be related to the onset of Northern Hemisphere glaciation, intensification of the East Asia monsoon,and/or effects of orographic rain-out.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The coupling between the development of Himalayan–Tibetanorogen (Fig. 1A) and Cenozoic climate change in Asia has been a pro-vocative topic for decades (An et al., 2001; Dettman et al., 2001, 2003;Graham et al., 2005; Guo et al., 2002; Kutzbach et al., 1989, 1993;Molnar et al., 1993; Quade et al., 1989; Ramstein et al., 1997;Raymo and Ruddiman, 1992; Ruddiman and Kutzbach, 1989).General circulation model simulations support the inference thatthe uplift of the Himalayan–Tibetan orogen was the major mecha-nism driving shifts in atmospheric circulation and inducing theAsian monsoons (Kutzbach et al., 1989, 1993; Ruddiman andKutzbach, 1989). A host of geological observations have been citedto support these links between tectonics and climate. The expansionof C4 biomass around 7 Ma in the Indian sub-continent (inferredfrom carbon isotope records) was used to argue for the initiation orstrengthening of the Asian monsoons (Quade et al., 1989, 1995), aswas an increase in the abundance of endemic upwelling species of

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marine microfossils in the Arabian Sea (Kroon et al., 1991; Nigriniand Caulet, 1992; Prell et al., 1992). On the Chinese Loess Plateau,the onset of eolian deposition, which was linked to the desertificationin central Asia in the rain shadow of the Himalayan–Tibetan orogeny,is well dated to 8–6 Ma (Ding et al., 1998; Liu, 1985, but see, Guoet al., 2002, for an exception). The coincidence in timing around8–6 Ma among changes in paleoecology, paleoceanography, andpaleoclimate is often cited evidence supporting the casual relation-ship between the rapid uplift of the Himalayan–Tibetan orogen andclimate change.

Yet recent studies have challenged this relationship. A few isotopicstudies from Nepal and the northeastern Tibetan Plateau suggest ashift to more arid conditions at 12–10 Ma (Dettman et al., 2001,2003; Hough et al., 2011). An Nd isotope study showed that the eolianinputs to the Pacific from the Northern Tibetan Plateau began ca.15 Ma with a prominent increase around 11–12 Ma (Li et al., 2011).Monsoon-driven upwelling in the Arabian Sea may have been presentby 10–12 Ma (Kroon et al., 1991; Nigrini and Caulet, 1992). Moreover,the timing of the uplift of Himalayan–Tibetan orogen, with differentestimates ranging from the Eocene to the Pliocene, remains a matterof considerable debate (Currie et al., 2005; Cyr et al., 2005; DeCelleset al., 2007; Garzione et al., 2000a, 2000b; Molnar et al., 1993; Polissaret al., 2009; Rowley and Currie, 2006; Rowley et al., 2001; Tapponnier

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Fig. 1. (A) Topography and principle active faults of Himalayan–Tibetan orogen with localities of paleoclimatic studies (white solid circles). Abbreviations as follow: PA, Pianaman;MR, Miran River; XK, Xorkol; GC, Ganchaigou; LL, Lulehe (Kent-Corson et al., 2009); HT, Huaitoutala (this study); XH, Xunhua (Hough et al., 2011); and LX, Linxia (Dettman et al.,2003). Barbed lines are thrust faults. Black lines with opposite arrows are strike-slip faults. The large filled arrow in India indicates its motion relative to Eurasia. The yellow boxdenotes the approximate range in B with the yellow solid circle denoting the studied section-Huaitoutala (HT) in the northeastern Qaidam basin. An elevation profile shown inFig. 6 for facilitating comparison of isotopic records between Tarim, Qaidam, and Linxia basins was constructed along the white line. Three stations with meteorological and isotopicdata from the International Atomic Energy Agency (IAEA) are shown with black circles. (B) Geology map of Delingha Depression (modified from Fang et al., 2007) which is boundby Zongwulong Shan and Olongbuluke thrusts to the north and south, respectively. The HT section is well-exposed on the north limb in the footwall of Olongbuluke thrust fault.

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et al., 2001;Wang et al., 2008a;Wu et al., 2008), and the impact of theretreat of Paratethys on Asian climate has seldom been considered(see Ramstein et al., 1997; Zhang et al., 2007 as exceptions). These

observations raise two independent, but interrelated questions:(1) Does the 12–10 Ma event reflect a regional climatic change? (2) Ifso, is there geological support for intensified aridity in central Asia

154 G. Zhuang et al. / Earth and Planetary Science Letters 312 (2011) 152–163

since then? Answers to these questions will impact by any study of therelationship between uplift of Himalayan–Tibetan orogen and climatechange in Asia.

In this paper, we present a 14-million-year-long isotopic record(15.7–1.8 Ma) of climatic change from theQaidambasin in thenorthernTibetan Plateau. The record is placed in a well-refined geological con-text that summarizes emerging evidence for the growth of Himalayan–Tibetan orogen, which demonstrates that the retreat of Paratethyswas completed in the middle Miocene and that a large portion ofthe Himalayan–Tibetan orogen was at high elevations (comparableto the present) before the late Miocene. Our isotopic data revealclimatic change at 12 Ma that is remarkably synchronous with isotopicrecords from the Tarim, Qaidam, Linxia and Xunhua basins, a hugeregion (over one million square kilometers) (Dettman et al., 2003;Hough et al., 2011; Kent-Corson et al., 2009). When considered inthe light of the refined geological context for the region, this12 Ma regional climate change offers a basis to evaluate the cou-pling between climate change in Asia and the development of theHimalayan–Tibetan orogen development.

2. Geologic setting and stratigraphy

The Qaidam basin is an intermontane basin on the northern TibetanPlateau, framed by left-lateral Altyn Tagh fault (ATF) to the northwest,Qilian Shan fold-thrust belt to the north, and Qiman Tagh–East Kunlunto the south (Fig. 1A). The Qaidam basin is ~3000 m above sea leveland contains thick successions of Eocene-Quaternary deposits (Zhuanget al., 2011 and references therein). Sedimentary records from theQaidam basin document a multiple-phase tectonic history, with defor-mation starting as early as the Eocene and the main phase of construc-tion of topography beginning in the middle Miocene (Bally et al., 1986;Yin et al., 2008; Zhou et al., 2006; Zhuang et al., 2011). The Eocene de-formation, interpreted as a far-field response to the initial Indo–Asiancollision, is characterized by the faulting on basin-bounding thrustfaults, synorogenic coarse clastic sediments, and moderate crustalshortening and bedrock exhumation (Clark et al., 2010; Yin et al.,2008; Zhou et al., 2006; Zhuang et al., 2011). The post-middle Miocenedeformation is well-documented by coarsening-upward stratigraphicsequences, extensive crustal shortening, expansion of Cenozoic sedi-mentation (Yin et al., 2008; Zhou et al., 2006; Zhuang et al., 2011)and rapid cooling and exhumation of basement rocks in the surround-ing mountains, as constrained by fission-track analysis and (U–Th)/Hethermochronology (George et al., 2001; Jolivet et al., 2001; Ritts et al.,2008; Wang et al., 2006a; Zheng et al., 2010). These tectonic inferencesare consistent with the sedimentary records from the Hexi Corridor(Bovet et al., 2009).

The Huaitoutala (HT) section is well exposed on the north limb ofthe Keluke anticline in the Delingha Depression (Fig. 1B), part of thenortheastern Qaidam basin that has been incorporated into the QilianShan fold-thrust belt as deformation propagated southward into thebasin. The HT section was measured along the very canyon where adetailed magnetostratigraphic study was carried out (Fang et al.,2007). Age assignments for the HT section were refined by lithologi-cal correlation between the HT section and magnetostratigraphic col-umn with help of fauna fossil assemblages (Fang et al., 2007; Wanget al., 2007).

The lithostratigraphy of HT section was divided into six distinctlithological units based on the dominant sedimentary environment,contact relationships, and grain size (Fig. 2) (see Zhuang et al.,2011). Unit 1 is characterized by a coarsening-upward sequence rangingfrom the well-developed paleosols, to fine- to coarse-grained sandstone,to pebble–cobble conglomerate. Unit 2 is dominated by meanderingriver systems that contain two sub-facies: channel and overbankdeposits. Channel deposits are composed of fine- to coarse-grainedsandstone and show a fining-upward sequence. They occur in lenticularbeds that extend laterally tens of meters and are meters to tens of

meters thick. Overbank mudstone contains nodular calcite and burrowsand is truncated by channel deposits. Unit 3 is dominated by braidedriver systems that also contain channel and overbank deposits. Over-bank mudstone is minor and usually separated by sharp contacts fromoverlying channel deposits that are mainly composed of medium- tocoarse-grained sandstone. In the upper portion of Unit 3, intraforma-tional clasts, including flakes of mudstone, pebbles of nodular calciteand reworked siderite concretions, appear and occur in lenticular bedsabove erosive or sharp surfaces. Unit 4 is interpreted as open lacustrinefacies and dominated by massive to laminated tan to brown mudstone.Terrestrial plant macrofossils are observed in laminated mudstonelayers. Unit 5 is similar to Unit 3 in sedimentary and stratigraphicfeatures but with increasing amounts of intraformational clasts. Unit6 is dominated by pebble–cobble conglomerate and coarse-grainedsandstone.

3. Method and material

We examined the oxygen and carbon isotope composition of calcitefrom pedogenic and lacustrine carbonates (Table A.1) collected fromthe HT section. The HT section was measured at the decimeter- tometer-scale; depositional environments, clast composition, paleocur-rent indicators made on imbricated clasts, and lithologic analysesindicate that sediments were sourced from the Qilian Shan (seeZhuang et al., 2011 for detailed description). In order to facilitate faciesanalysis, mudstone content was estimated based on the decimeter- tometer-scale description of the section (Fig. 2). 107 samples were col-lected at a rough density of one sample per ~50 m (equivalent to onesample/130 ka); 20 samples were analyzed in duplicate (Table A.2).20 medium-grained sandstone samples were collected throughout theHT section for petrographic study.

About 0.3 to 1 mg of rock powder was obtained using a dentaldrill; pedogenic carbonate nodules were broken to reveal internalstructures facilitating sampling of micrite rather than spar. Powderedsamples were analyzed for δ18O and δ13C values at the University ofCalifornia Santa Cruz Stable Isotope Laboratory. Samples weredigested using an automated common acid bath carbonate deviceinterfaced with a Fisons Prism III dual-inlet isotope ratio mass spec-trometer (IRMS). Samples were reacted at 90 °C in orthophosphoricacid (specific gravity=1.92 g/cm3) to generate carbon dioxide andwater. Water was cryogenically removed from CO2, and non-condensable gases were pumped away prior to introduction of thepurified CO2 into the IRMS. Repeated analysis of NBS-19 and CarreraMarble shows that laboratory precision for both C and O isotopevalues is b0.2‰. Isotopic results for both carbon and oxygen arereported using standard delta (δ) notation with respect to ViennaPee Dee Belemnite (VPDB) (Fig. 3). We assessed temporal trends inthe oxygen and carbon isotope data with a local regression methodthat uses weighted linear least squares and a 2nd degree polynomialmodel (Loader, 1999) with a span of 10% of total samples.

In order to evaluate the possible effects from evaporation, diage-netic exchange, and detrital contamination on isotopic records, min-eralogical and petrographic studies were conducted. Mineralogicalanalyses were performed on 14 typical lacustrine samples using X-ray diffraction (Goldsmith et al., 1955). Petrographic analyses wereconducted on 20 sandstone samples to calculate their intergranularvolume (IGV) and the abundance of extrabasinal carbonate grains.About 500 total points were counted for each thin section; thesepoints were classified as framework grains and interstitial material,which includes cement, pore space, and matrix. The intergranularvolume was calculated as the percentage of interstitial material interms of the total rock volume (Houseknecht, 1987; Paxton et al.,2002). The abundance of extrabasinal carbonate grains is expressedin two ways: the ratio of extrabasinal carbonate count to the cementcount and total count, respectively (Table A.3). Petrographic analyseswere also performed on typical nodular calcite and lacustrine

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Fig. 2. Stratigraphy of the Huaitoutala section with dominant sedimentary environments for the corresponding lithologic units. Gray box in the bottom of CaCO3 weight percentagehighlights the portion of the HT section characterized by the thick well-developed paleosols with high concentration of carbonates. Bold letters with double arrow lines to the left ofthe HT stratigraphy indicate fauna fossils found in this section and Delingha Depression: A, Huaitoutala fauna (~4–5 Ma); B, Shengou fauna (~8–10 Ma); C, Tuosu fauna (~10–12 Ma), and D, Olongbuluke fauna (~12–15 Ma) (Wang et al., 2007). Absolute age constraints are interpreted from Fang et al. (2007).

155G. Zhuang et al. / Earth and Planetary Science Letters 312 (2011) 152–163

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Fig. 3. δ18O and δ13C data of measured samples from the HT section, northeastern Qaidambasin. Age assignments ofmeasured samples are interpolated based onmagnetostratigraphyand biostratigraphy (Fang et al., 2007; Wang et al., 2007). Four phases of isotope record arebuilt on the basis of average δ18O and δ13C values.

156 G. Zhuang et al. / Earth and Planetary Science Letters 312 (2011) 152–163

carbonate samples to examine the textures of pristine micrite andrecrystallized and sparry calcite.

4. Results

4.1. Mudstone content

Mudstone contents were estimated as the relative percentage ofmudstone thickness with respect to the total thickness (includingmudstone, sandstone, and conglomerate) every 10 m based upondecimeter- to meter-scale measurement and description of the HTsection. Throughout the section, we noted the following features:the lacustrine portion is dominated by mudstone, the braided riversystems lack mudstone, and the meandering river systems lie inbetween these end-members. The mudstone contents are consistentwith and support our interpretations of sedimentary environments(Fig. 2).

4.2. Isotopic results

Oxygen isotope values vary from −3.8‰ to −13.7‰ (Table A.1),with a phased evolution toward more positive values (Fig. 3). Paleosolcalcite nodules and fluvial mudstone from the lower HT section(phase-I: 15.7 Ma to 12 Ma) exhibit δ18O values averaging −9.9‰(n=35, 1σ=1.5‰). Values increase to −9.0‰ between 12 Ma and10.7 Ma (phase-II) (n=11, 1σ=0.8‰). There is a more prominent pos-itive shift in δ18O values to−7.4‰ (n=47, 1σ=1.0‰) around 10.7 Maand high values are maintained until 3.3 Ma (phase-III). The total shift inδ18O values from phase-I to phase-III is +2.5‰. The δ18O values shift tomore negative values, averaging−8.4‰ (n=12, 1σ=1.3‰) for the pe-riod from 3.3 Ma to 1.8 Ma (phase-IV). The corresponding average δ13Cvalues are: −4.3‰ (1σ=1.2‰) (phase-I), −3.3‰ (1σ=1.5‰) (phase-II), −3.0‰ (1σ=1.2‰) (phase-III), and −1.9‰ (1σ=1.7‰) (phase-IV). Thus both oxygen and carbon isotope values show the positiveshift, starting around 12Ma and becoming more pronounced around10.7 Ma (Fig. 3).

4.2.1. Covariance of δ18O and δ13C and mineralogy oflacustrine carbonates

Covariance analysis of δ18O and δ13C values and mineralogicalstudy on lacustrine samples imply an open system during sample for-mation (Fig. A.1 and Fig. A.2) and that evaporative effects contributed

little to the observed positive shift in oxygen values. A high correla-tion (correlation coefficient r>0.7) between δ18O and δ13C valueswas widely used as a paleohydrological indicator for closed lake sys-tems (Fontes et al., 1996; Li and Ku, 1997; Lister et al., 1991; Talbot,1990). In closed lake systems, evaporation and/or long residencetime causes 18O-enrichment in lake waters, whereas the drop inlake levels increases biologic export production, which removes ligh-ter carbon from lake waters, and hence increases the δ13C values inlake water dissolved inorganic carbon. The δ18O and δ13C valuesfrom our lacustrine samples showed poor correlation (n=32,r=0.23), suggesting an open lake system, as has been observedin other lake systems on the Tibetan Plateau (Cyr et al., 2005; Talbot,1990). The correlation between δ18O and δ13C values for lacustrine sam-ples is even lower than those for pedogenic andmarginal lacustrine car-bonates (Fig. A.1).

XRD analysis on 14 typical lacustrine carbonates identifies a stable[104] peak (Fig. A.2), which represents the position of low-magnesium calcite (Goldsmith et al., 1955), suggesting that low-magnesium calcite is the primary carbonate precipitatemineral, similarto studies in other parts of the plateau (Cyr et al., 2005; Fan et al., 2007;Garzione et al., 2004). The dominance of low-magnesium calcite inlacustrine carbonates (rather than evaporative concentration leadingto formation of high-magnesium carbonate or dolomite) also supportsthe inference that these were open lake systems, furthermore, ourfield observations indicate HT section lacks evaporitic phases.

4.2.2. Diagenesis and detrital input analysisWe consider burial diagenetic effects at deep depth and elevated

temperature to be minor. Throughout the entire ~5000 m section,the intergranular volume (Houseknecht, 1987; Paxton et al., 2002)of Neogene sandstone at HT is around 40% (Fig. 4; Table A.3). Highintergranular volume indicates that sandstone cementation oc-curred early, at shallow burial depths; the occlusion of intergranularvolume by cement, predominantly the calcite in this study, preventsfurther mechanical compaction of the sandstone (Houseknecht, 1987;Paxton et al., 2002). Graham et al. (2005) also observed high intergra-nular volumes in sandstones in their studies of the Tarim and Qaidambasins. Moreover, examination of petrographic thin sections of typicalnodular calcite revealed spar in veins, but this material was avoidedduring the sampling. No evidence of spar or recrystallization wasobserved in the petrographic thin sections of typical lacustrine samples(Fig. A.3).

Detrital contamination is considered to beminor as well. Throughoutthe entire section, the extrabasinal carbonate content averages 0.6%,ranging from 0 to 3.2%. In contrast, the calcite cement averages around34.6%. Thus contamination from extrabasinal carbonate is low.

5. Discussion

The most conspicuous feature of our 14-million-year-long isotoperecord from the northeastern Qaidam basin is the positive shift inδ18O and δ13C values, starting around 12 Ma, with a sharp increasearound 10.7 Ma (Fig. 3). A positive shift in δ18O values was also ob-served in the Linxia and Xunhua basins in the northeastern cornerof the plateau (Dettman et al., 2003; Hough et al., 2011). In a largeisotopic dataset from the Tarim and northwestern Qaidam, a positiveδ18O shift was identified in 13 sections (Kent-Corson et al., 2009).Sections that have absolute age controls from themagnetostratigraphy(for example, the Pianaman in the western Tarim, Ganchaigou in thenorthwestern Qaidam, and Xiao Qaidam/Da Honggou in the northernQaidam) (Gao et al., 2009; Lu and Xiong, 2009; Yin et al., 2002) demon-strate a synchronous shift in δ18O values ~12 Ma. Other sections thathave more coarse age constraints demonstrate positive shifts slightlyearlier or later than 12 Ma. The striking coincidence in timing and theidentification of the same trend across a huge region, leads us to thefirst-order conclusion that this positive shift is a regional event. We

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argue in the following sections that this event is best explained byintensification of central Asia aridification.

5.1. Dry central Asia

At present, the region from the Tarim, to the Qaidam, to the Linxiaand Xunhua basins is dominated by dry central Asian air masses(Araguás-Araguás et al., 1998; Tian et al., 2001). Two meteorologicalstations, Hetian (in the Tarim basin) and Zhangye (in theHexi Corridor),from the International Atomic Energy Agency (IAEA/WMO, 2006), arecharacterized by precipitation that is 18O-enriched in the summer rel-ative to the winter, which is opposite to the pattern in the regionsdominated by the Asianmonsoons. The third station, Lanzhou, is locat-ed at the front where monsoon-dominant and dry central Asian air-dominant systems compete; the station records the 18O-enriched sum-mer pattern, but with 18O-depleted precipitation in the late summer(Fig. 5) when the East Asian summer monsoon penetrates into thisarea. The boundary between climates dominated by dry central Asianair masses versus monsoon moisture is controlled by the altitudeand area of Himalayan–Tibetan system, as confirmed by general circu-lation climate modeling and isotopic studies (Araguás-Araguás et al.,1998; Kutzbach et al., 1989, 1993; Ruddiman and Kutzbach, 1989; Tianet al., 2001). Isotopic evidence from the Tarim and Linxia basin thatthis boundary was at roughly the same position from 12 Ma to pre-sent (Fig. Fig. 6) (Dettman et al., 2003; Hough et al., 2011; Kent-Corson et al., 2009) support the inference that the region has experi-enced a similar climate regime since that time.

5.2. Primary isotope records

Multiple approaches were used to examine the potential effects ofevaporation and/or long residence time of lake water, diagenesis, anddetrital contamination on carbonate isotope values. Low covariancebetween δ18O and δ13C values (Fig. A.1), the lack of evaporite layersin the lacustrine portion of the section (Fig. 2), and the observationthat the primary lacustrine precipitate consists of low-magnesium car-bonate (Fig. A.2) all support the conclusion that calcite formed in anopen lake system thatwas not subject to exceptional evaporative enrich-ment. Petrographic studies reveal that sparry calcite is absent in typicallacustrine carbonate samples (Fig. A.3). High intergranular volumevalues, high percentages of calcite cement, and low percentages of detri-tal calcite suggest that cementation occurred at a shallow burial depth,and that detrital contamination is trivial. These observations supportour conclusion that carbonate isotope data record near surfaceconditions in soils, lakes, and near-surface aquifers with minimal

Intergranular Volume (%)

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Fig. 4. (A) Intergranular volume (IGV) of sandstone samples across the HT section. (B) RelativHT section, northeastern Qaidam basin.

alteration or detrital contamination, and they reflect the isotopic compo-sition of ancient meteoric water.

5.3. Intensified aridity in central Asia around 12 Ma

The isotopic records from a wide region across the northernTibetan Plateau exhibit a simultaneous positive shift in δ18O valuesaround 12 Ma (Fig. 6) (Dettman et al., 2003; Hough et al., 2011; Kent-Corson et al., 2009). We argue that this positive shift in δ18O values rep-resents the intensification of central Asia aridification around 12Ma.This positive shift could be caused by the strengthened aridity in centralAsia resulting from the retreat of the Paratethys from central Asia(Ramstein et al., 1997; Ritts et al., 2008; Zhang et al., 2007), the blockageof moisture-bearing air masses by high elevations in the Himalayas andsouth-central Tibet, and/or by the growth of the northern Tibetan Pla-teau and Tian Shan and Pamir. This positive shift occurs in the contextof middle Miocene global cooling, which began ~14 Ma ago (Zachoset al., 2001).

5.3.1. Isotope recordThe major features captured by the smoothed isotopic data are the

positive shift in oxygen values between 12 and 10.7 Ma and negativeshift around 3.3 Ma (Fig. 3). The positive and negative shifts do notcorrespond to lithostratigraphic boundaries or facies changes, andhence the changes in δ18O values were not caused by changes in sed-iment sources and/or sedimentary environments.

The isotopic data demonstrate long periods of stability before andafter (stages I and III) the 2.5‰ positive shift in δ18O values, suggestingrelatively stable climate before and after the positive shift, which likelycorresponds to a significant change in climate and/or in atmospheric cir-culation patterns in the region. The δ18O values of pedogenic/lacustrinecarbonates document the composition of soil water/lake water fromwhich the pedogenic/lacustrine carbonates precipitate. The δ18O valuesof soil water/lake water can be estimated by assuming the equilibriumisotope fractionation between carbonates and soil/lake water andusing the average δ18O value of carbonates and typical temperate of25±5 °C for calcite precipitation in modern lakes of the Tibetan Plateau(Fontes et al., 1996; Lister et al., 1991). The estimated soil/lake waterδ18O value was −7.6±1‰ (SMOW) prior to 12 Ma and −5.1±1‰(SMOW) after 10.7 Ma. The +2.5‰ shift in δ18O values in soil water/lake water reflects either a shift in meteoric water δ18O values due toa change in dominant air masses (i.e., from 18O-depleted monsoonalprecipitation to 18O-enriched recycled continental air masses fromcentral Asia) and/or the relative enrichment of 18O in the soilwater/lake water with respect to meteoric water (Fig. 7). Moreover, if

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)

Fig. 5. Mean precipitation and temperature, and weighted mean δ18O values of precipitation from Hetian, Zhangye, and Lanzhou obtained from the IAEA/GNIP database (IAEA/WMO, 2006). Note the trend of enrichment of 18O in summer precipitation with the exception of late summer for Lanzhou.

158 G. Zhuang et al. / Earth and Planetary Science Letters 312 (2011) 152–163

we assume that the estimated soil/lake water δ18O values representpaleo-meteoric water δ18O values, by 10.7 Ma this water value is indis-tinguishable frommodern precipitation. This similarity implies that thelacustrine calcite might have been deposited in a similar paleoclimaticsetting as today. All these arguments support the intensified aridity incentral Asia since 12Ma.

An alternative explanation for the 2.5‰ positive shift is a changein temperature. Temperature fluctuation has a combined effect of ap-proximate 0.35‰/°C on δ18O values (~−0.23‰/°C on the fraction-ation and ~+0.58‰/°C on the δ18O value of precipitation; Koch etal., 2003; O'Neil et al., 1969; Rozanski et al., 1993); 2.5‰ positiveshift would require a sharp warming of ~7 °C between 12 and10.7 Ma, with continued high temperatures persisting for millions ofyears. We consider this an unlikely scenario considering the gradualcooling in the late Cenozoic (Zachos et al., 2001).

The drop in δ18O values at 3.3 Ma could be caused by the NorthernHemisphere glaciation (NHG) (Maslin et al., 1998; Shackleton et al.,1988), an intensified East Asian monsoon (An et al., 2001), and/ororographic rainout (Roe, 2005) in the Qilian Shan. The cooler atmo-spheric temperatures in the NHGmight be a major mechanism gener-ating more negative calcite δ18O values due to decreases in the δ18Ovalue of meteoric water (Rozanski et al., 1993). Studies on eolian de-posits in the Chinese Loess Plateau suggest intensification of the EastAsian monsoon at 3.6–2.6 Ma (An et al., 2001). The intensified EastAsian summer monsoon could penetrate into the Qaidam basin,bringing 18O-depleted precipitation (Araguás-Araguás et al., 1998).Moreover, sedimentary records and structural studies support thatthe Qilian Shan grew both laterally and vertically in the Late Pliocene(Liu et al., 2010; Métivier et al., 1998; Meyer et al., 1998; Tapponnieret al., 1990; Zhuang et al., 2011); thus the orographic effects on theprecipitation crossing the Qilian Shan, where the fluvial systems inthe northeastern Qaidam basin are draining, could also contribute tothis negative shift. We note that fission-track analysis and (U–Th)/He thermochronology on basement rocks support the Miocene rapiduplift of the Qilian Shan (George et al., 2001; Zheng et al., 2010).The orographic rainout effect from the early uplift could be

overwhelmed by the intensified aridity. In addition, we note that a3.3 Ma drop in δ18O values was not identified in the Linxia and Xunhuabasins, pointing to a local rather than a regional explanation (Dettmanet al., 2003; Hough et al., 2011).

The isotopic records reveal a continuous increase in δ13C valuesfrom phase-I to phase-IV (Fig. 3). Pedogenic carbonate δ13C valuesare determined by the local proportion of C3 to C4 plants, and bysoil respiration rate (which decreases with the extent of local plantcover in dry regions) (DeCelles et al., 2007; Quade et al., 1989,2007). Based on studies of the δ13C values of fossil tooth enameland paleosol carbonates, C4 grasses were inferred to be absent priorto 2–3 Ma in the Linxia basin (Wang et al., 2006b). To the south inthe Kunlun Pass Basin, plant cover is characterized by the dominantlyC3 vegetation (Wang et al., 2008b). Therefore, C4 plants do not con-tribute to the high values of pedogenic carbonates observed in thisstudy. The high δ13C values of pedogenic carbonates, like those inNima basin in the central Tibet (DeCelles et al., 2007), likely reflectthe sparse vegetation and low rates of soil respirations, which aresuggestive of dry climates (Quade et al., 1989, 2007).

5.3.2. Water deficitBased on studies of the δ18O values of tooth enamel from East

African mammals, a water deficit index (WD, the difference betweenthe potential evapotranspiration and mean annual precipitation)was built, which shows a linear relationship between WD and 18O-enrichment (εES–MW) between the tooth enamel of evaporation-sensitive (ES) mammals and meteoric water (MW) with a slope of5.01×10−3 (Levin et al., 2006). Surprisingly, this slope is similar tothat between WD and the 18O-enrichment between the leaf water(LW) and meteoric water (8.08×10−3, Levin et al., 2006). We will usethis latter relationship to offer very coarse estimates of the differencein water deficit associated with a shift in soil or lake water δ18O valuesof +2.5‰.

If we assume for the moment that the δ18O values of meteoricwater did not vary through time,we can assess the plausibility of dryingas an explanation for the+2.5‰ shift. As noted above, the positive shift

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Comparison between isotopic records from Tarim, Qaidam, and Linxia basinsOxygen values of

bethic foraminiferaCoupling between tectonics

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uaetalP natebiT nrehtron fo htworg drawtuo aM 8-01segnahc etamilc aM 6-8

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δ O (‰)-PA18

Fig. 6. (Upper panel) Elevation profile across the Tarim, Qaidam, and Linxia basins and surrounding mountains with the projection of paleoclimatic study localities. Refer to Fig. 1 forlocalities. Note that the Delingha Depression, northeastern Qaidam basin, is separated by the Olongbuluke Shan from the remaining Qaidam basin. Jishi Shan is an interveningmountain range between the Xunhua and Linxia basins on the northeastern corner of Tibetan Plateau. (Lower panel) Oxygen isotope records for benthic foraminifera, major tectonicand climatic events, and comparison of isotopic records from the Tarim, Qaidam, and Linxia basins. See text for detailed discussions.

159G. Zhuang et al. / Earth and Planetary Science Letters 312 (2011) 152–163

occurs in pedogenic carbonates and ismaintained through the lacustrinecarbonates, and hence cannot be explained by differences in evaporationassociated with the facies change. If the 2.5‰ shift reflects the differencein enrichment between the soil water/lake water and meteoric water(Fig. 7B), then using the slope of 8.08×10−3 between the water def-icit and 18O-enrichment between the leaf water and meteoric waterfrom Levin et al. (2006), the increase in water deficit would be~300 mm. Across the range of δ18O increases, from 1.5‰ in the Linxiabasin, to 2.0‰ in the Tarim Basin, to 2.5‰ in the Qaidam basin, theWD change would range from +200 to +300 mm. We acknowledgethat the analogy between the leaf water and the lake and soil watersrecorded by carbonates in our study is rough. Yet all of these water

sources should experience 18O-enrichment in the face of increasingwater deficit, hence the analogy provides some sense of the scaleof possible changes in this climatic parameter. Water deficit indicesappropriate for relatively open lake systems and soil carbonates wouldcertainly improve these estimates, but they are currently unavailableand beyond the scope of this study.

A 200–300 mm increase in water deficit for the region is plausible.A palynological study in the western Qaidam basin records a lateMiocene transition from a humid, temperate climate, characterizedby Podocarpus, Tsuga, Cedrus, Picea, and Pinus, to an arid and semi-aridclimate, characterized by high proportions of Chenopodiaceae andArtemisia in herbs or shrubs (Miao et al., 2010). At present, the

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mw

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sw

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mw

Fig. 7. Two alternate, end-member models explaining the observed +2.5‰ shift inδ18O values of fluvial carbonates. (A) The observed +2.5‰ shift in δ18O values of ped-ogenic carbonates (Δ1) reflects the shift in δ18O values of meteoric water (Δ2). (B) Theobserved+2.5‰ shift in δ18O values of pedogenic carbonates (Δ3) reflects the increasing18O-enrichment between meteoric water and soil water (Δ4) from which the pedogeniccarbonates precipitate. Dashed lines with arrows are equal in length and indicate thesame fractionation between pedogenic carbonates and soil water, which is dependenton the temperature. δ18OCC, δ18OSW, and δ18OMWrefer to δ18Ovalues of pedogenic carbonates,soil water, and meteoric water, respectively.

160 G. Zhuang et al. / Earth and Planetary Science Letters 312 (2011) 152–163

dominance by Chenopodiaceae and Artemisia in central Asia resultsfrom a climate with mean annual precipitation less than 200–480 mm(Zheng et al., 2008).

5.4. Controls on aridity

Given evidence for a large increase in water deficit and consideringthe critical tectonic and hydrologic changes in the region, we interpretthe 12 Ma event as evidence for intensification of central Asia aridifica-tion, which can be attributed to: (1) the retreat of the Paratethys fromthe central Asia, (2) attainment of high elevations in the Himalayasand south-central Tibet, and (3) the growth of northern Tibetan Plateauand Tian Shan and Pamir (Fig. 8).

5.4.1. Retreat of the Paratethys and active tectonism in Pamir andTian Shan

Atmospheric circulation model simulations demonstrate that thepresence of the Paratethys, which covered a large area of centralAsia during most of the Oligocene, crucially influences climate bydamping the seasonal thermal contrast with its high heat capacityand by providing an immediate source of water vapor (Ramsteinet al., 1997 and references therein). Foraminiferal assemblages dem-onstrate that the Miran River locality in the southeast Tarim basin(MR in Figs. 6 and Fig. 8) was at sea level 15 Ma ago (Ritts et al.,2008). Together with the occurrence of older Cenozoic marine stratain the southwest Tarim basin, a shallow to marginal marine connec-tion, restricted to a flexural trough along the West Kunlun MountainFront, was proposed (Ritts et al., 2008; Wang, 1985). The retreat ofthe Paratethys from the Tarim basin would result in the absence of

nearby water sources and hence causemore arid continental conditionsand strengthen the seasonal contrast, as observed in general circulationmodel simulations (Ramstein et al., 1997; Zhang et al., 2007).

The active tectonism, characterized by the extensive crustal short-ening and rapid surface uplift from the Pamir to the Tian Shan rangesduring the period of the early to late Miocene (Fig. 8) (Amidon andHynek, 2010; Bullen et al., 2001, 2003; Charreau et al., 2005, 2006;Sobel et al., 2011), has two implications for climate change in centralAsia. It accelerates the retreat of the Paratethys from central Asia andit blocks and/or splits moisture-bearing air masses from the Atlanticand Arctic oceans.

5.4.2. High Himalayan and Tibet before the late MioceneGeologic studies indicate that prior to late Miocene, a large portion

of Himalayan–Tibetan system had attained high elevations comparableto today (Fig. 8), forming an orographic barrier preventing moisturefrom the Indian and Pacific oceans from penetrating into central Asiaand hence strengthening aridity in that region. Oxygen-based paleoalti-metry studies were successfully applied to the Himalaya and southernTibet andmost give the minimum limit on the timing of the attainmentof high elevations. Paleometeoric water δ18O values reconstructed frompaleosol carbonates, lacustrinemicrites, and fossilmollusks suggest thatthe High Himalayas (Thakkhola graben and Gyirong basin, Nepal) werehigh since ~11–10 Ma (Garzione et al., 2000a, 2000b; Rowley et al.,2001). An isotopic study from the Zhada basin, southwest Tibet,provides the minimum limit of 9.2 Ma on the timing when thatportion of the plateau attained elevations comparable to or even higherthan the present (Saylor et al., 2009). A paleoaltimetry study based onleaf physiognomy in the Namling, southern Tibet, indicates high eleva-tions since 15 Ma (Spicer et al., 2003), which was confirmed later by anisotopic study (Currie et al., 2005). Isotopic studies from the Nima andLunpola basins, central Tibet, argue for high elevations since 26 and35Ma, respectively (DeCelles et al., 2007; Rowley and Currie, 2006).Moreover, though the paleoaltimetry of the Hoh Xil basin in thenorth-central Tibet has been a controversial topic (Cyr et al., 2005;DeCelles et al., 2007; Polissar et al., 2009; Quade et al., 2011), these stud-ies indicate that north-central Tibet was high no later than 15 Ma. Highnorth-central Tibet in the Miocene was supported by geologic studiesthat demonstrate highly deformed Paleogene strata coveredwith slightlydipping and extensively distributedNeogene lacustrine sediments, whichindicates substantial pre-Neogene contractile and hence deformationattainment of high elevations, prior to deposition of Neogene strata(Wang et al., 2008a; Wu et al., 2008).

5.4.3. Accelerated growth of Northern Tibetan Plateau since themiddle Miocene

In contrast to the Himalayas and south-central Tibet, which attainedhigh elevations before the late Miocene, the northern Tibetan Plateau(north of the Kunlun Shan) grew greatly both laterally and verticallysince the middle Miocene (Bovet et al., 2009; George et al., 2001;Lease et al., 2007, 2011; Métivier et al., 1998; Meyer et al., 1998; Rittset al., 2008; Zheng et al., 2006, 2010; Zhuang et al., 2011). While abun-dant geologic evidence also supports early Cenozoic deformation in theregion within 10 million years of the initial Indo–Asian collision (Clarket al., 2010; Duvall et al., 2011; Zhuang et al., 2011), sedimentary andthermochronologic evidence indicates that the preponderance ofshortening, exhumation and syn-orogenic sedimentation since themiddle Miocene. This accelerated surface uplift and exhumation sincethe Miocene is consistent with the Nd isotopic study indicating anincrease in eolian input from thenorthern TibetanPlateau into the Pacificstarting around 15 Ma and intensifying at 12–10 Ma (Li et al., 2011).Together with the rapid uplift of Tian Shan and Pamir since the lateMiocene, the outward growth of northern Tibetan Plateau necessarilycontributed to the intensified aridity in central Asia by blocking or split-tingmoisture from Atlantic and Arctic oceans by the continuous surfaceuplift and outward growth.

70˚ 08E ˚ 09E ˚ 001E ˚E 110˚E

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in the Hoh Xil basin (Wang et al., 2008a)

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8 Ma

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2CS1CS

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TK: Thakkhola (Garzione et al., 2000a; Rowley et al., 2001)GR: Gyirong (Rowley et al., 2001)ZD: Zhada (Saylor et al., 2009)NQ: Namling & Qiyug (Spicer et al., 2003; Currie et al., 2005)NM: Nima (DeCelles et al., 2007)LP: Lunpola ( Rowley and Currie, 2006)HX: Hoh Xil (Cyr et al., 2005; Polissar et al., 2009)MR: Miran River (Ritts et al., 2008)

C1: Kuitun He (Charreau et al., 2005)C2: Yaha (Charreau et al., 2006)B: Northern Tian Shan (Bullen et al., 2001; 2003)S: Pamir (Sobel et al., 2011)A: North-central Pamir (Amidon et al., 2010)Z1: North Qilian (Zheng et al., 2010)Z2: Liupan Shan (Zheng et al., 2006)

PA: Pianaman (Kent-Corson et al., 2009)HT: Huaitoutala (this study)XH: Xunhua (Hough et al., 2011)LX: Linxia (Dettman et al., 2003)

L: enhanced eolian input into the Pacific

Fig. 8. Compilation of studies on paleoclimatology, paleoaltimetry, and geology of the Himalayan–Tibetan orogen. White circles indicate the paleoclimate studies. Hexagons arepaleoaltimetry studies. The plaid gray area represents the proposed shallow to marginal marine connection in the middle Miocene based on the recovery of foraminiferaassemblages at the MR section and older Cenozoic marine strata along the South Tarim. Stars denote the geologic studies supporting the extensive crustal shortening andrapid exhumation/uplift in the northern Tibetan Plateau and Tian Shan and Pamir. The dashed linewith arrowhighlights the input of eolian deposits into the Pacific Ocean from the north-ern Tibetan Plateau since the middle Miocene.

161G. Zhuang et al. / Earth and Planetary Science Letters 312 (2011) 152–163

6. Conclusion

A 14 million-year-long isotopic record recovered from pedogenicand lacustrine carbonates in the Qaidam basin identifies a +2.5‰shift in δ18O values beginning at 12 Ma, in remarkable consistencewith isotopic records from the Tarim basin to the west and Linxiaand Xunhua basins to the east in spite of the huge lateral distance(~3000 km), high relief contrast, andwide area (on the order of millionsquare kilometer). We argue that this simultaneous positive shift inδ18O values signals a regional climatic change in the central Asia around12 Ma, i.e., he intensification of central Asia aridity. Tentative calcula-tions suggest that the increase in water deficit corresponding tothe shift in δ18O values might be on the order of 200–300 mm.With published geologic studies and numerical model results, the12 Ma intensification of aridity in the central Asia can be attributedto the combined effects of: (1) retreat of the Paratethys from central

Asia; (2) high elevations attained in the Himalayas and south-centralTibet before the late Miocene; and (3) outward growth of the northernTibetan Plateau. The negative shift around 3.3 Ma was likely to becaused by the onset of the Northern Hemisphere Glaciation, intensifiedAsian monsoon, and/or the effects of orographic rainout in the QilianShan.

Supplementary materials related to this article can be found onlineat doi:10.1016/j.epsl.2011.10.005.

Acknowledgment

We would like to thank the editor Peter DeMenocal and CarmalaGarzione and an anonymous reviewer for their thorough andthoughtful reviews of the manuscript. We thank our driver GuangduiXing and field assistants Chao Huang and Weimin Xia. Discussionswith Stephan Graham (Stanford), Mark Brandon (Yale), and Katie

162 G. Zhuang et al. / Earth and Planetary Science Letters 312 (2011) 152–163

Snell (UCSC) greatly improved the manuscript. We thank Elise Knit-tle, Scott Oliver, and Honghan Fei (UCSC) for assistance with the X-ray diffraction measurements, and Dyke Anderson and Jennifer Leh-man (UCSC) with stable isotope analysis. The financial supportscome from the National Science Foundation grants NSF-EAR TECTON-ICS 0834200 to Bradley Ritts (UCSC) and NSF-0409939 INDEPTH-IVto Simon Klemperer (Stanford), and the American Association of Pe-troleumGeologists Grants-in-Aid, Geological Society of America grad-uate grant, and Chevron Scholarship to Guangsheng Zhuang.

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