Ecologic shift and aridification in the northern Tibetan Plateau revealed by leaf wax n-alkane δ2H and δ13C records

Accepted Manuscript Ecologic shift and aridification in the northern Tibetan Plateau revealed by leaf wax n-alkane δ2H and δ13C records

Minghao Wu, Guangsheng Zhuang, Mingqiu Hou, Yunfa Miao PII: DOI: Reference:

S0031-0182(18)30885-X https://doi.org/10.1016/j.palaeo.2018.11.005 PALAEO 8980

To appear in:

Palaeogeography, Palaeoclimatology, Palaeoecology

Received date: Accepted date:

25 October 2018 1 November 2018

Please cite this article as: Minghao Wu, Guangsheng Zhuang, Mingqiu Hou, Yunfa Miao , Ecologic shift and aridification in the northern Tibetan Plateau revealed by leaf wax nalkane δ2H and δ13C records. Palaeo (2018), https://doi.org/10.1016/j.palaeo.2018.11.005

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ACCEPTED MANUSCRIPT Ecologic shift and aridification in the northern Tibetan Plateau revealed by leaf wax n-alkane δ2H and δ13C records Minghao Wu a, Guangsheng Zhuang a,*, Mingqiu Hou a, Yunfa Miao b Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70810, USA

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Key Laboratory of Desert and Desertification, Cold and Arid Regions Environmental and Engineering

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* Corresponding author.

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Institute, Chinese Academy of Sciences, Lanzhou 730000, China

E-mail address: [email protected] (G. Zhuang)

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Abstract

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Two competing factors, the global cooling and the uplift of Tibetan Plateau, have been proposed to drive the central Asian aridification, but their relative role has seldom been discriminated in paleoclimate and

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paleoenvironment records. Here, we reconstruct a 14-million-year-long record of paleohydrology and

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paleoecology in the western Qaidam Basin by applying the compound-specific hydrogen (δ2H) and carbon (δ13C) isotope analyses to terrestrial leaf wax long-chain n-alkanes. The δ2H values are low during

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the interval of 14.6 to 13.0 Ma. Then the δ2H increases from 13.0 to 12.2 Ma and maintains high values from 12.2 to 3.2 Ma with a peak high value of ‒156.1 ‰ at 8.0 Ma. After 3.2 Ma, the δ2H values are low

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and vary larger than 30 ‰. The δ13C values decrease from 14.6 to 13.0 Ma and are low from 13.0 to 3.2 Ma except a high value at 3.8 Ma. Then they decrease slightly after 3.2 Ma. Low δ2H values indicate

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relatively wet climate between 14.6 and 13.0 Ma. The decreasing δ13C values during the same time period support the ecologic shift with the decline of warm component of conifers after the Mid-Miocene Climatic Optimum. High δ2H values since 13.0 Ma are synchronous with the uplift of northern Tibetan Plateau, implying tectonics-driven aridity. Large-amplitude variation in δ2H values since ca. 3.2 Ma seen in East and West Qaidam and lower δ13C values reveal the climatic cyclic responses to the Northern Hemisphere Glaciation.

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ACCEPTED MANUSCRIPT Keywords: Qaidam Basin; paleoclimate; paleoecology; stable isotope; tectonic uplift; global

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cooling

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ACCEPTED MANUSCRIPT 1. Introduction The Tibetan Plateau offers a superb example for studying the interactions between tectonics and climate change. Numerical modeling studies show that the uplift of the Tibetan Plateau drives the regional climate change by physically perturbing the atmospheric circulations or

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impacting the carbon cycle via chemical weathering (An et al., 2001; Boos and Kuang, 2010; Li

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and Elderfield, 2013; Li et al., 2009; Li et al., 2017; Molnar et al., 2010; Wu et al., 2012; Zhang

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et al., 2011; Zhang et al., 2018a; Zhang et al., 2017; Zhang et al., 2015). The central Asian

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interior had experienced multiple phases of climatic changes during the Cenozoic (An et al., 2001; Bosboom et al., 2014; Chang et al., 2008; Dupont-Nivet et al., 2007; Jia et al., 2012; Li et

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al., 2014; Miao et al., 2011; Song et al., 2017; Song et al., 2014; Sun et al., 2015; Sun et al., 2017; Sun and Windley, 2015; Sun et al., 2010; Zhuang et al., 2014; Zhuang et al., 2011a). Among

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these climatic events, the Middle-Late Miocene drying and subsequent variations in climate draw

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the particular interests due to the coincidence in timing between the uplift of Tibetan Plateau and the global climate change (Li et al., 2016; Miao et al., 2012; Sun et al., 2015; Zhuang et al.,

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effect remain open.

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2014). Despite intense research, the questions regarding the driving mechanisms and the ecologic

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Zhuang et al. (2011a) argued that a vast region in the central Asia, running from the Tarim Basin, across the Qaidam Basin, to the Linxia Basin on the northeastern corner became drier since about 12 Ma. They argued for the combined effect of the uplift of the Himalaya-Tibetan plateau orogen, enhanced basin isolation, and the retreat of Para-Tethys. The northern Tibetan Plateau had grown rapidly during the Middle-Late Miocene. Coarse clastics were widely distributed across the Qaidam Basin and Hexi Corridor, indicating the proximal sedimentation in response to the basin-bounding thrust fault activation (Bovet et al., 2009; Zhuang et al., 2011b). 3

ACCEPTED MANUSCRIPT Balanced cross-section calculations show that more than half of the Cenozoic crustal shortening was completed since the Middle Miocene (Zhou et al., 2006). Low-temperature thermochronology studies from the basement rocks support a phase of fast erosion in the northern Tibetan Plateau since ca. 15-8 Ma (Clark et al., 2010; Duvall et al., 2013; Lease et al.,

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2011; Ritts et al., 2008; Wang et al., 2016b; Zheng et al., 2006; Zhuang et al., 2018). The high

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topography, built following the rapid uplift, was considered to be a key factor in driving the

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aridity via blocking moistures from oceans (Dettman et al., 2001; Graham, 2005; Kent-Corson et

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al., 2009; Zhuang et al., 2011a).

On the global scale, the Earth was transitioned from greenhouse conditions to icehouse

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conditions as the ice sheet on the East Antarctica expanded after the Mid-Miocene Climate

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Optimun (MMCO) (Flower and Kennett, 1994), which reflects on the increasing δ18O values of benthonic foraminifera (Zachos et al., 2001). During the same interval, the ecology had

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experienced a turnover from C3-forest-dominated to C4-grassland-dominated world. However,

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the ecologic history in the northern Tibetan Plateau is unclear regarding to the Middle-Late Miocene drying. The carbon isotopic data (δ13C) of terrestrial leaf wax n-alkane in the North

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Pacific Ocean does not show evidence of C3-C4 transition in the source region of northern

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Tibetan Plateau (Jia et al., 2012). Instead, an increase of xerophytic taxa and a decrease of thermophilic taxa and humid conifers after the Mid-Miocene Climate Optimun (MMCO) is suggested in the Qaidam Basin (Miao et al., 2011). Reviewing the above studies, three prominent questions stand out regarding the relationships between the central Asian aridification, the ecologic change, the uplift of northern Tibetan Plateau and the global cooling. First, the question regarding the driver of aridification (the uplift of Tibetan Plateau or the global cooling) has not been resolved. Second, there is discrepancy 4

ACCEPTED MANUSCRIPT regarding how the ecology evolved throughout the late Neogene in the northern Tibetan Plateau. Third, how ecology and climate respond to the uplift and global cooling have not been fully understood. To illuminate on the above questions, we conduct carbon and hydrogen isotope analyses on

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leaf wax n-alkanes extracted from sedimentary rocks collected at two well-dated sections in the western Qaidam Basin. The leaf wax n-alkanes hydrogen and carbon isotope analyses have been

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widely used to reconstruct the paleohydrology and paleoecology (Garcin et al., 2014; Hren et al.,

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2010; Huang et al., 2018a; Huang et al., 2018b; Jaramillo et al., 2010; Kohn, 2010; Krishnan et al., 2015; Super et al., 2018; Tipple and Pagani, 2010; Vogts et al., 2012; Zhuang et al., 2014).

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We will compare our newly collected data with the records from the eastern Qadidam Basin

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(Zhuang et al., 2014).

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2. Geological setting

The Qaidam Basin is located in the northern Tibetan Plateau which covers an area of 120,000

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km2, with a mean elevation of 2800 m (Chen and Bowler, 1986). The Qaidam Basin is bounded

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by the left-lateral strike slip Altyn Tagh fault to the west, which runs > 1500 km. To the north and south, it is bounded by the Qilian Shan fold and thrust belt and the Qimen Tagh-Eastern

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Kunlun thrust belt (Fig. 1A). The Cenozoic Qaidam Basin is a terrestrial basin. The maximum sedimentary thickness of Cenozoic sediments reaches 10,000 m (Fang et al., 2007). Cenozoic stratigraphy has been divided into the following seven lithostratigraphic units: the Early Eocene Lulehe (LLH) Formation, the Middle-Late Eocene Xiaganchaigou (XGCG) Formation, the Oligocene Shangganchaigou (SGCG) Formation, the Early-Middle Miocene Xiayoushashan (XYSS) Formation, the Middle-Late Miocene Shangyoushashan Formation (SYSS), the Late

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ACCEPTED MANUSCRIPT Miocene to Pliocene Shizigou Formation (SZG), and the Pliocene Qigequan (QGQ) Formation

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from bottom to top (Chang et al., 2015; Fang et al., 2007; Sun et al., 2005).

Fig. 1. A: Shade-relief map with superimposed color showing topographic and geological features in the northern Tibetan Plateau. The abbreviations of section names and the studies of sections are as follows: QGQ, Qigequan section (Zhang et al, 2013a; this study); HGZ, Honggouzi section (Song et al., 2014;

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ACCEPTED MANUSCRIPT Zhang et al, 2018b; this study); HTG, Huatugou section (Li et al., 2016); YH, Yahu section (Wu et al., 2011); HT, Huaitoutala section (Zhuang et al, 2011a; Zhuang et al., 2014). B: Shaded-relief map of Tibetan Plateau and surrounding areas. C: Geological map of the study area. Legend: E 3, Xiaganchaigou Formation (Middle-Late Eocene); N1, Shangganchaigou Formation (Oligocene); N21-2, Youshashan

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Formation (Miocene); N23, Shizigou Formation (Late Miocene-Pliocene); Q1-2, Qigequan Formation

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(Pleistocene); Q3-4, Late Pleistocene and Holocene.

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We studied two sections: the Honggouzi (HGZ) and the Qigequan (QGQ), which are in the western Qaidam Basin (Fig. 1A). The thickness of the HGZ and QGQ is 1050 m and 800 m,

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respectively. These two sections were dated via magnetostratigraphy. The HGZ section ranges

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from ca. 17 to 5 Ma (Song et al., 2014; Zhang et al., 2018b) and the QGQ section is from ca. 7 to 0.4 Ma (Zhang et al., 2013a). The lower part of the HGZ section is the Shangyoushashan

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Formation, which is dominated by lacustrine facies with fan delta facies on the top. The Shizigou

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Formation lays above the Shangyoushashan Formation. An angular unconformity separates these two formations apart. The Shizigou Formation has fan delta facies on its top and bottom and has

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lacustrine facies in the middle. On the top, the Qigequan Formation is separated from the

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Shizigou Formation by another unconformity. The Qigequan Formation is dominated by alluvial fan facies (Fig. 2A). The Qigequan section is composed by the Shizigou Formation and the

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Qigequan Formation from bottom to top. The Shizigou Formation is dominated by lacustrine facies which is interrupted by fan delta. The Qigequan Formation is dominated by fan delta facies (Fig. 2B). Nowadays the climate of the Qaidam Basin is mainly under the control of the westlies (Caves et al., 2015; Curio et al., 2015), while only the very eastern part is affected by the East Asian Monson (EAM) (Bothe et al., 2011; Chen et al., 2010; Chen et al., 2008; Maussion et al., 2014).

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ACCEPTED MANUSCRIPT The boundary between the westlies and the East Asian Monson locates at the east edge of the basin (Chen et al., 2010; Chen et al., 2008) (Fig. 1A). The basin receives most rainfall in summer from June to August (Tian et al., 2001). The mean annual precipitation (MAP) is less than 100 mm in the most parts of the basin and is only 25 mm in the center (Chen and Bowler, 1986),

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the moisture brought by the East Asian Monsoon (Du and Sun, 1990).

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while the mean annual precipitation is 150-200 mm at the southeast edge of the basin because of

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ACCEPTED MANUSCRIPT Fig. 2. The lithology and sedimentary facies of the HGZ and QGQ section with sample positions. Age constrains are interpreted form (Song et al, 2014; Zhang et al, 2018b). The abbreviations of grain size are as follows: C, clay; S, silt; Fs, fine sand; Ms, medium sand; Cs, coarse sand; Cong, conglomerate.

3. Method

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3.1 Compound specific isotope of carbon and hydrogen

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We combined C and H isotopes of n-alkanes for paleoclimate reconstruction aiming to reveal both ecologic (carbon isotope) and hydrologic (hydrogen isotope) information and to gain more

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solid conclusions than single isotopic analysis through more constraints on data explanations. First, the C and H isotopic compositions will reveal different aspects of environmental

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information. The δ13C values of leaf wax (δ13Cn-alk) δ will be associated with the

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C/12C

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composition of atmospheric CO2 (δ13Catm) and the isotopic discrimination (△δ13C) between leaf

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wax and atmospheric CO2 (Diefendorf and Freimuth, 2017). The isotopic discrimination will be affected by the plant taxa, especially by the differences between C3 and C4 plants. C3 plants fix

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carbon via the Calvin cycle in Mesophyll cells. C4 plants will first fix carbon by the Hatch-Slack

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cycle and then by the Calvin cycle. Because of the different carbon fixation pathways, the C4 plants are more competitive than the C3 plants in arid, warm and low atmospheric CO2

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concentration environments. The bulk δ13C values of C3 and C4 plants ranges from ‒35 ‰ to ‒20 ‰ and from ‒14 ‰ to ‒10 ‰, respectively (Tipple and Pagani, 2007). Due to the apparent fractionation factor between n-alkane and bulk tissue, the average modern n-alkane δ13C values of C3 and C4 plants are ‒32 ‰ and ‒20 ‰ (Collister et al., 1994). In addition to the large differences of △δ13C between C3 and C4 plants, smaller differences exist among C3 plants. For example, the research of conifer leaf wax △δ13C indicates a phylogenetic signal (Diefendorf et al., 2015). Also, environmental factors, such as aridity, altitude, and atmospheric CO2 9

ACCEPTED MANUSCRIPT concentration, will impact △δ13C of C3 plants by affecting the ratio of CO2 partial pressure inside the plant (pi) relative to that outside the plant (pa) (Farquhar et al., 1989; Schubert and Jahren, 2012). The δ2H values will record the H isotopic composition of leaf water which is used for synthesizing leaf wax (Tipple et al., 2015). Similar with the isotopic composition of carbon, the

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hydrogen isotopic composition will also be affected by plant types through different isotopic

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fractionation factors between leaf wax and. Leaf water ultimately comes from precipitation. The

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δ2H values of mean annual precipitation show a good positive relationship with the n-alkane δ2H values of various plants and surface sediments (Sachse et al., 2012). So, the environmental

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factors that affect precipitation δ2H values, such as elevation, temperature, and aridity, are

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reflected by leaf wax δ2H values. In addition, the synchronous changes of δ2H and δ13C values will make solid identification for some factors which affect both isotopic values. For example,

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aridity will cause the increase in the value of both δ2H and δ13C, and it can be distinguished from

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increasing elevation, which will cause the decrease of δ2H value and the increase of δ13C value.

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3.2 Sample collection

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We measured the two sections, the Honggouzi (HGZ) and Qigequan (QGQ) sections (Fig. 2). The measured HGZ section is 830 m, consisting of ca. 390 m Shangyoushashan Formation in the

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lower part, ca. 420 m Shizigou Formation in the upper part and ca. 20 m Qigequan Formation on the top. The measured QGQ section is 750 m including ca. 500 m Shizigou Formation in the lower part and ca. 250 m Qigequan Formation in the upper part. Thirty-six fine-grained samples (i.e., green and red mudstone) are taken from these two sections. The ages of the two sections are based on previous paleomagnetism chronology research (Song et al., 2014; Zhang et al., 2013a). We used the ages for major magnetostratigraphic chrons and determined the age for each sample through the linear interpolation between the major chrons. 10

ACCEPTED MANUSCRIPT 3.3 Organic matter extraction In the field, we removed the loose, weathered sediments and collected the fresh rocks. In the lab, the surface layer was further removed by the Dremel 3000 rotary tool and then rinsed with dichloromethane (DCM) to avoid cross-contamination. After that, samples were crashed into

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granule size and then freeze-dried for over 48 hours.

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Total lipids were extracted by applying the Soxhlet extractors with DCM / MeOH (2 / 1, v / v) for over 48 hours. Total lipid extracts (TLE) were then concentrated using Biotage TurboVap

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Classic II at 40 ℃ in water bath and under a stream of purified nitrogen. Total lipid extracts were then separated into apolar, intermediate and polar fractions by applying the column

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chromatography which is sequentially eluting 4 ml hexane, 4 ml DCM, and 4 ml methanol

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through the pipettes filled with ~4.0 g activated silica gel. The apolar fraction contains the n-

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alkanes and was further purified by urea adduction.

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3.4 Isotope analysis

We performed hydrogen isotope analysis on a Thermo Trace 1310 Gas Chromatography (GC)

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coupled to a Delta V Advantage Isotope Ratio Mass Spectrometer (IRMS). The n-alkanes with

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different chain length were separated in GC with a Thermo Scientific TG-5MS column (30 m long, 0.25 mm i.d., 0.25 um film thickness) and the carbon preference index (CPI) was calculated by the following equation: CPI = 0.5 × Σ A(23-33) / Σ A(24-34) + 0.5 × Σ A(25-35) / Σ A(24-34) where A is abundance.

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ACCEPTED MANUSCRIPT The GC carrier gas was helium and the flow rate was set up at 2 ml/min. The GC temperature was programmed from 60 ℃ (held for 2 min) to 170 ℃ at 14 ℃/min, to 300 ℃ at 3 ℃/min, and then to 320 ℃ at 14 ℃/min with an isothermal holding for 5 minutes. The n-alkanes were converted to hydrogen gas (H2) in a High-Temperature Convertor (HTC) reactor at the

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temperature of 1400 ℃. Sample H2 was regulated with reference H2 gas through the Isolink

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interface and analyzed by the IRMS. The H3+ factor was daily measured at the beginning of each

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analysis sequence. The average value of the H3+ factor was 7.47 ( ± 0.21, n = 21) during the hydrogen isotopic analysis period. δ2H values were corrected against the reference material (Mix

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A6, A. Schimmelmann, Indiana University Bloomington) and reported against Vienna Standard

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Mean Ocean Water (VSMOW). The δ2H values for nC16-nC30 of Mix A6 are ‒9.1 ‰, ‒117.8 ‰, ‒52.0 ‰, ‒56.3 ‰, ‒89.7 ‰, ‒177.8 ‰, ‒81.3 ‰, ‒67.2 ‰, ‒29.7 ‰, ‒263.0 ‰, ‒45.9 ‰, ‒172.8 ‰, ‒

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36.8 ‰, ‒177.8 ‰, and ‒213.6 ‰. Mix A6 was measured every six analyses. Most samples were

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δ2H = Rsample / Rstandard ‒ 1

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analyzed with at least one replicate. The δ2H values were reported by the following equation:

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where R = 2H / 1H. δ2H values are expressed in per mil (‰). Carbon isotope was analyzed with the same GC-Isolink-IRMS assemblage. The carrier gas

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was helium and the flow rate was set up at 2 ml/min. The GC oven temperature was programmed as same as that in δ2H analysis. The n-alkanes were converted to carbon dioxide gas (CO2) in the Combustion oven at the temperature of 1000 ℃. Sample CO2 was regulated with reference CO2 gas through the Isolink interface and analyzed by the IRMS. Carbon isotopic values were corrected against the reference material (Mix A6, A. Schimmelmann, Indiana University Bloomington) and reported against Vienna Pee Dee Belemnite (VPDB). The δ13C values for nC16-nC30 of Mix A6 are ‒26.15 ‰, ‒31.88 ‰, ‒32.70 ‰, ‒31.99 ‰, ‒33.97 ‰, ‒28.83 ‰, ‒ 12

ACCEPTED MANUSCRIPT 33.77 ‰, ‒33.37 ‰, ‒32.13 ‰, ‒28.46 ‰, ‒32.94 ‰, ‒30.49 ‰, ‒33.20 ‰, ‒29.10 ‰, and ‒29.84 ‰.

Mix A6 was measured every six analyses. All samples are analyzed with at least one replicate. The δ13C values are reported by the following equation: δ13C = Rsample / Rstandard ‒ 1

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where R = 13C / 12C. δ13C values are expressed in per mil (‰).

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Table 1

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δ2HnC29

Error (1σ)

δ13CnC29

Error (1σ)

‒168.9 ‒172.8 ‒184.8 ‒172.8 ‒173.0 ‒177.3 ‒169.9 ‒174.5 ‒170.8 ‒191.2 ‒165.9 ‒181.3 ‒201.4 ‒183.1 ‒192.9 ‒192.7 ‒188.1 ‒189.0 ‒194.0 ‒189.9 ‒183.4 ‒182.6 ‒173.9 ‒172.8 ‒172.1 ‒169.2 ‒167.2 ‒171.7 ‒173.2

‒30.5 ‒30.4 ‒30.1 ‒30.7 ‒28.6 ‒29.9 ‒30.7 ‒31.2 ‒30.9 ‒31.9 ‒30.5 ‒30.8 ‒30.9 ‒31.2 ‒28.2 ‒28.6 ‒29.0 ‒30.0 ‒29.6 ‒31.3 ‒31.5 ‒30.5 ‒31.1 ‒30.7 ‒30.5 ‒30.6 ‒31.6 ‒31.7 ‒31.5

0.0 0.0 0.1 0.1 0.1 0.5 0.3 0.0 0.0 0.1 0.3 0.0 0.1 0.3 0.3 0.1 0.2 0.0 0.1 0.1 0.3 0.1 0.3 0.5 0.1 0.4 0.3 0.0 0.2

0.4 0.3 1.7 2.1 0.7 4.2 5.3 3.8 1.8 5.6 0.8 1.0 0.2 3.5 0.1 1.0 n/a 0.4 2.2 0.8 n/a 3.0 2.9 1.5 5.1 3.8 2.9 4.5 3.5

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QGQ_03 QGQ_04 QGQ_05 QGQ_06 QGQ_07 QGQ_08 QGQ_09 QGQ_10 QGQ_11 QGQ_12 QGQ_14 QGQ_15 QGQ_16 QGQ_17 HGZ_03 HGZ_04 HGZ_05 HGZ_06 HGZ_07 HGZ_08 HGZ_09 HGZ_10 HGZ_11 HGZ_12 HGZ_13 HGZ_14 HGZ_15 HGZ_16 HGZ_17

Age(Ma）a 5.8 5.1 4.4 4.2 3.8 3.5 3.4 3.3 1.9 1.7 3.2 3.1 3.0 2.9 14.6 14.3 14.1 13.6 13.4 13.0 12.7 12.4 12.2 11.5 10.8 10.6 10.3 10.1 9.1

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Sample#

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δ2H and δ13C values of nC29.

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ACCEPTED MANUSCRIPT 0.3 0.5 0.1 0.4 0.3 0.2 0.4 0.1

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Absolute age is based on published magnetostratigraphic studies (Song et al., 2014; Zhang et al., 2013).

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HGZ_18 8.9 ‒163.2 0.5 ‒30.7 HGZ_19 8.0 ‒156.1 2.3 ‒30.5 HGZ_20 6.8 ‒173.2 0.4 ‒31.2 HGZ_21 6.4 ‒169.7 3.0 ‒30.5 HGZ_22 6.1 ‒174.8 3.8 ‒30.9 HGZ_23 5.8 ‒180.4 8.9 ‒30.4 HGZ_24 5.2 ‒172.6 1.6 ‒30.6 average 1.6 n/a: No standard error due to not enough sample amount for repeated analysis.

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ACCEPTED MANUSCRIPT Fig. 3. Compiled data of leaf wax nC29 δ2H (A) and δ13C (B) (this study), pollen (C) (Miao et al., 2011), leaf wax nC29 δ13C from the North Pacific (D) (Jia et al.,2012), benthic foram δ18O (E) (Zachoss et al., 2001), and atmosphere CO2 δ13C (F) (Tipple and Pagani, 2010). In A and B, the green dots are data points of the HGZ section, and the orange dots are data points of the QGQ section. The error bars in A and B

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represent ± 1σ standard error. The blue and yellow bars indicate periods of quick global cooling and the

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initial of aridification, respectively.

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4. Results

The composite records of hydrogen and carbon isotopes of nC29 comprise of the Honggouzi

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and Qigequan sections (Table 1; Fig. 2A and B). At the overlapping intervals, the δ2H and δ13C

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share similar values. To assist data interpretation, we present the composite data sets in three stages by considering the trends of δ2H and δ13C values, i.e., 14.6-13.0 Ma, 13.0-3.2 Ma, and

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since 3.2 Ma.

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From 14.6 to 13.0 Ma, the δ2H values are low, varying from ‒188.1 ‰ to ‒194.0 ‰. The δ 13C

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values decrease by 3.1 ‰ from ‒28.2 ‰ to ‒31.3 ‰. Between 13.0 and 3.2 Ma, the δ2H values are high and begin with a fast increase from ‒189.9 ‰ to ‒173.9 ‰ during 13.0 to 12.2 Ma.

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Then the δ2H maintains high values between ‒172.8 ‰ and ‒163.2 ‰ until 3.2 Ma with a peak

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high value of ‒156.1 ‰ at 8.0 Ma. The δ13C values vary from ‒31.7 ‰ to ‒29.9 ‰ with a single value of ‒28.6 ‰ at 3.8 Ma. Since 3.2 Ma, the δ2H values vary larger than 30 ‰ between ‒201.4 ‰ and ‒165.9 ‰. δ13C values are slightly lower than the previous interval, ranging from ‒31.9 ‰ to ‒30.5 ‰. The carbon preference index (CPI) values are generally between 1.0 and 2.9.

5. Discussion 5.1 Global cooling-driven ecological change after the Mid-Miocene Climatic Optimum

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ACCEPTED MANUSCRIPT During the period from 14.6 to 13.0 Ma, the δ13C values of leaf wax n-alkanes decrease by 3.1 ‰, whereas the δ2H values of leaf wax n-alkanes are generally low (Fig. 3A and B). There are several factors that impact the δ13C values of leaf wax n-alkanes, including the atmospheric CO2 isotopic composition, CO2 concentrations, and ecology. The atmospheric CO2 δ13C (δ13Catm)

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decreases by about 0.5 ‰ from 14.6 to 13.0 Ma (Tipple et al., 2010). By assuming that the

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isotopic discrimination (△δ13C) between leaf wax n-alkane (δ13Cn-alk) and atmospheric CO2

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(δ13Catm) is constant, the variation in atmospheric δ13C accounts for 0.5 ‰ for the total amount of

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3.1 ‰ change in leaf wax n-alkanes. This indicates that the varying δ13Catm is not the main factor. The isotopic fractionation of leaf wax n-alkanes against the atmospheric carbon isotopes

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(△δ13C) will decrease with reduced atmospheric CO2 concentration (Schubert and Jahren, 2012).

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Under low CO2 conditions since the Mid-Miocene Climatic Optimum (Pagani et al., 2005; Zhang et al., 2013b), the carbon isotopes in leaf wax n-alkanes will be fractionated against the

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atmospheric CO2 to the less extent, which would lead to high leaf wax δ13C values. However,

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this scenario contradicts to the observed decreasing trend of δ13C values.

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The published spore-pollen data from the same area suggests a decrease of the conifer component in the local vegetation. We argue that this change may have a dominant effect on the

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δ13C value. The palynology study shows a decrease in the ratio of arboreal pollen (AP) / nonarboreal pollen (NAP) since ca. 15 Ma. And the arboreal taxa in this region is mainly composed of conifers, most of which are favorable in humid and warm climate (Miao et al., 2011). The mean δ13C values of leaf wax n-alkanes in conifers is 2-4 ‰ heavier than the forb, graminoid and shrub in angiosperm, and slightly heavier than angiosperm trees ( < 1 ‰) (Diefendorf and Freimuth, 2017). In addition, the δ13C value of Pinaceae, a subgroup of conifer, is 2 ‰ heavier than the average conifer δ13C value (Diefendorf et al., 2015). The consistency of the leaf wax n17

ACCEPTED MANUSCRIPT alkane δ13C (δ13Cn-alk) values in this study with the spore-pollen composition supports the local ecologic shift with substantial decline in conifers, although the δ13Cn-alk values in the North Pacific Ocean does not show appreciable change during that time period (Fig. 3D) (Jia et al., 2012).

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The timing of ecologic change detected by the carbon isotope negative shift and the change in spore-pollen assemblage is synchronous with the climatic transition from greenhouse to icehouse

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conditions after the Mid-Miocene Climatic Optimum, suggesting a connection between the

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ecologic change and the global cooling. Despite the prominent change in ecology, our hydrogen isotope data of leaf wax n-alkanes reveal that the environment was wet in the Qaidam Basin from

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14.6 to 13.0 Ma. This wet climate is supported by the low values of fluvio-lacustrine carbonates

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δ18O in the Huatugou section (Li et al., 2016) and Huaitoutala section (Zhuang et al., 2011a) in

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the Qaidam Basin.

5.2 Aridification in central Asia since the late Middle Miocene

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The leaf wax n-alkane δ2H values increase from ‒189.9 ‰ to ‒173.9 ‰ during the interval

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13.0 to 12.2 Ma, then high values keep until ca. 3.2 Ma. The increase in hydrogen isotope is synchronous with the ca. 2 ‰ increase in fluvio-lacustrine carbonates δ18O values during the

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same time period identified at the Huatugou section in the same region of the western Qaidam Basin (Fig. 1A and C) (Li et al., 2016). In addition, the increasing trend seen in this study is consistent with the increasing trends of δ18O values of fluvio-lacustrine carbonates and δ2H values of leaf wax n-alkanes at the Huaitoutala section in the eastern Qaidam Basin since ca. 12 to 10 Ma (Zhuang et al., 2014; Zhuang et al., 2011a).

18

ACCEPTED MANUSCRIPT According to the greenhouse experiment (Tipple et al., 2015) and modeling results (Flanagan and Ehleringer, 1991), the δ2H value of leaf water will be determined by the isotopic composition of source water, e.g., the soil water. Though the uppermost soil waters in the soil profile may be subject to strong evaporative enrichment in arid and hyper-arid areas, studies also

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show that plants in arid areas may have been adapted to the arid climate by developing deep root

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systems to uptake water from great depth in soil horizons which are less affected by surficial

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evaporation processes and reflect annual mean signals of precipitation (Dawson, 1993; Dawson

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and Pate, 1996; West et al., 2007).

Despite the uncertainty regarding the impact of surficial evaporation, high δ2H values of leaf

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wax n-alkanes can be attributed to the enhanced aridity, as the source water is ultimately derived

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from precipitation (Sachse et al., 2012). First, the aridity has been argued to cause the enrichment of heavy hydrogen isotope in precipitation via the sub-cloud evaporation (Araguas-Araguas et al.,

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1998; Chen et al., 2015; Kong and Pang, 2016; Li and Garzione, 2017; Salamalikis et al., 2016).

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Second, in the central Asia, the recycling of moisture from enriched lake and river waters with high isotopic values substantially contributes to the local moisture (Araguas-Araguas et al., 1998;

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Curio et al., 2015; Kurita and Yamada, 2008). Third, heavy hydrogen isotope will be enriched in

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leaf water under low humidity via leaf transpiration effect (Smith and Freeman, 2006). Our inference of enhanced aridity is consistent with regional studies (Miao et al., 2011; Song et al., 2014; Zhuang et al., 2014; Zhuang et al., 2011a). The cooling temperature is an unlikely factor. The temperature in the northern Tibetan Plateau will become colder in response to the global cooling since ca.15 Ma (Zachos et al., 2001). Studies of low-temperature thermochronology, basin analysis, and structural geology suggest rapid uplift since the Middle Miocene in this region (Clark et al., 2010; Ritts et al., 2008; Wang 19

ACCEPTED MANUSCRIPT et al., 2016a; Yin et al., 2008; Zheng et al., 2006; Zhuang et al., 2011b; Zhuang et al., 2018), which will also result in colder temperatures in response to the obtainment of higher elevations. With a positive relationship between precipitation δ2H value and temperature (Dansgaard, 1964), cooling temperature will lead to the decreasing δ2H value, which is opposite to the observed

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increasing δ2H values.

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From leaf water to leaf wax, the δ2H may be impacted by the plant taxa during leaf wax

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biosynthesis processes (Sachse et al., 2012). The C3 trees and C3 grass are dominant in local

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vegetation during this time period (Wang and Deng, 2005). The predominant C3 flora is also supported by our leaf wax n-alkane δ13C values which vary between ‒31.5 ‰ and ‒30.6 ‰,

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typical values for C3 flora (Collister et al., 1994). Experiments in natural field reveal that C3 trees

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have the higher δ2H values than C3 grass grown in the same places (Chikaraishi and Naraoka, 2003; McInerney et al., 2011; Sachse et al., 2012). A decrease of C3 trees in this region since 14

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Ma is suggested by the decreasing ratio of arboreal pollen (AP) / non-arboreal pollen (NAP)

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with the high δ2H values.

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(Miao et al., 2011). In this case, the n-alkane δ2H values would decrease, which is inconsistent

5.3 The role of accelerated uplift in the northern Tibetan Plateau

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Generally, negative δ18O or δ2H values of precipitation are expected in a leeward side, contrasting to more positive δ18O or δ2H value of precipitation in a windward side due to the discrimination of heavy isotopes on the route of precipitation (Kleinert and Strecker, 2001; Stern and Blisniuk, 2002; Takeuchi and Larson, 2005). However, in extreme arid areas like the Qaidam Basin, the strong sub-cloud evaporation (Li and Garzione, 2017) and moisture recycling (Kurita and Yamada, 2008) greatly enriches the heavy isotopes in precipitation, which will reverse the negative trend caused by rain shadow isotope effect and result in more positive δ18O 20

ACCEPTED MANUSCRIPT and δ2H values. This is supported by the modern precipitation δ18O values from Zhangye and Delingha, which are located in the foreland basin (Hexi Corridor) and the intermountain Qaidam Basin (Fig. 1A). Delingha’s precipitation has a much higher δ18O value than Zhangye’s precipitation (Araguas-Araguas et al., 1998; Tian et al., 2003), though the altitude of Delingha is

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more than 1000 m higher than that of Zhangye. This reversed rain shadow effect was also

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high δ18O values in lacustrine carbonates (Rech et al., 2010).

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observed in the Miocene Calama Basin in the leeward side of the Andes Mountain which shows

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Since ca. 15 to 8 Ma, the northern Tibetan Plateau has experienced the accelerated uplift. Low-temperature thermochronology studies reveal fast erosions in the Qilian Shan range (Zheng

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et al., 2010; Zhuang et al., 2018). The sedimentary and stratigraphic studies in the Qaidam Basin

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and Hexi Corridor reveal a change in sedimentation from fine-grained, fluvio-lacustrine facies to the coarse-grained alluvial fan-braided river facies, in response to the basin-bounding thrust

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faulting (Bovet et al., 2009; Zhuang et al., 2011b). Foraminifera assemblages from ca. 15 Ma

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strata reveal a near sea-level sedimentation in shallow marine environment at the Miran He section in the Tarim Basin which is now located at 1,400 m (Ritts et al., 2008). Structural

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geology and balanced calculations of cross-section record a phase of accelerating crustal

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shortening since the Middle Miocene (Zhou et al., 2006; Zuza et al., 2016). We argue that the enhanced aridity since 13.0 Ma can be ascribed to the uplift of mountainous ranges (Altun Shan and Qilian Shan ranges) at the northern margin of Tibetan Plateau. The peak δ2H value at 8.0 Ma indicates a severe aridity in the central Asia. The aridity event is reported in the fluvio-lacustrine carbonates δ18O record (Dettman et al., 2003; Fan et al., 2007) and the geochemical proxy studies of chemical weathering and pedogenesis (Yang et al., 2016)

21

ACCEPTED MANUSCRIPT in the Linxia Basin. During the same time period, a peak dust flux appeared in the North Pacific Ocean, supporting the severe aridity in the source area in the central Asia (Rea et al., 1998). 5.4 The impact of global cooling since the Late Pliocene Since ca. 3.2 Ma, the δ2H values vary larger than 30 ‰ (Fig. 3A). For example, the lowest

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δ2H value of ‒201.4 ‰ appears at 3.0 Ma. It is noticeable that since ca. 3.0 Ma, the Northern

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Hemisphere Glaciation began and the global temperatures cooled down and varied in large amplitude (Bartoli et al., 2005; Raymo, 1994). The relationship of 0.58 ‰/ ℃ between δ18O

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value and temperature (Araguas-Araguas et al., 1996) can be translated to ca. 5.0 ‰/℃ for

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hydrogen isotopes by applying the local meteoric water line:

δ2H = (8.55 ± 0.55) × δ18O + (16.53 ± 0.67) (Li and Garzione, 2017).

(4)

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Given the fact that the globe cooled by 2-6 ℃ since about 3.0 Ma (Lear et al., 2015; Zhang et al.,

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2014), the low δ2H values of leaf wax n-alkanes since ca. 3.2 Ma may reflect low global

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temperature.

Decreasing δ13C values of leaf wax since ca. 3.2 Ma follow the pattern of the δ13C values of

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atmospheric CO2 and may also be associated with the further decrease of conifer, as suggested by

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the palynology research at the Yahu section in the central Qaidam Basin (Wu et al., 2011). 5.5 East-west comparison in the Qaidam Basin We compared two data sets of leaf wax n-alkane δ2H values, one from this study in the western Qaidam Bain and the other from the eastern Qaidam Basin (Fig. 4) (Zhuang et al., 2014). The comparison reveals similries in δ2H trend as well as disparities, which bears implications to understanding regional tectonics and climatic evolution. First, the co-incidence of high hydrogen

22

ACCEPTED MANUSCRIPT isotopes since ca. 10 Ma indicates the widespread aridity across the region from the Tarim Basin further to the west and to the Linxia Basin on the northeastern corner of the Tibetan Plateau (Zhuang et al., 2011a). The two records show divergence from around 15 to 10 Ma. The 15-10 Ma interval in the eastern Qaidam Basin is dominated by the paleosols, which was interpreted by

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Zhuang et al. (2014) to represent the elevation change in the basin level in response to the uplift,

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as seen in other geological studies of seimentology & stratigraphy, low-temperature

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thermochronology, balanced cross-section calculations, etc (section 5.2.1); while the same time interval in the western Qaidam Basin is dominated by the lacustrine facies and we interpret it to

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represent the integrated signal in the whole drainage basin. If we examine the two records, the

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short lacustrine interval at ca. 12 Ma in the eastern Qaidam Basin shows high δ2H values as well, which is consistent with the changing trend seen in the western Qaidm Basin record. The interval

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7-4 Ma with low hydrogen isotopes in the eastern Qaidam Basin was interpreted to be a wet

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phase associated with strong East Asian Monsoon (Zhuang et al., 2014); while the corresponding time period in the western Qaidam Basin is in the high-value interval. This supports the climatic

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difference that the eastern Qaidam Basin is prone to the influence of the East Asian Monsoon

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while the western Qaidam Basin is under the control of the westerlies. Both records demonstrate great-amplitude variations since ca. 3 Ma, co-eval with the onset of the North Hemisphere

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Glaciation, indicating the impact of glacial-interglacial cycles on the regional moisture budget and hence the climate.

23

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ACCEPTED MANUSCRIPT

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Fig. 4. The nC29 δ2H values of the HGZ and QGQ section in the western Qaidam basin and the

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Huaitoutala (HT) section in the east. The orange and green dots are data of QGQ and HGZ, respectively. Blue dots are from HT. The yellow bar indicates the period of widespread aridity in both eastern and

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western Qaidam Basin.

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6. Conclusions

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The integrated carbon and hydrogen isotope analyses on leaf wax n-alkanes reveal the impact of global cooling and the uplift of northern Tibetan Plateau on regional climate and ecology. A change in ecology from 14.6 to 13.0 Ma is inferred from the 3.1 ‰ decrease in δ13C

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

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values, consistent with the palynology records that show the decrease of conifers in response to the global cooling after the Mid-Miocene Climatic Optimum. The regional climate remained relatively wet at that time. 

The regional aridification is recorded by the increase in leaf wax n-alkane δ2H value since ca. 13.0 Ma. We argue that the main factor is the isolation of Qaidam Basin in response to the uplift of surrounding mountain ranges.

24

ACCEPTED MANUSCRIPT 

Global cooling since ca. 3 Ma imprints the leaf wax n-alkane record with large-amplitude variations and low values.



The east-west comparison shows similarities and disparities, supporting the present-day observations that the western and eastern Qaidam Basin are under the control of different

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climatic regimes.

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Acknowledgments

We would like to thank the editor and reviewers for their comments and suggestions. We

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appreciate the laboratory assistance from Yongbo Peng. M. Wu thanks Yang Zhang for

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constructive discussions.

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ACCEPTED MANUSCRIPT Highlights 

Decreasing δ13C since 14.6 Ma reflects the ecologic shift after the MMCO.



High δ2H values since ca. 13.0 Ma indicates aridity in central Asia.



The diachronous response of ecology and climate to MMCO indicate different driving

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Low δ2H and δ13C since ca. 3.2 Ma is synchronous with the Northern Hemisphere

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Glaciation.

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factors.

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