Extreme aridification since the beginning of the Pliocene in the Tarim Basin, western China

Accepted Manuscript Extreme aridification since the beginning of the Pliocene in the Tarim Basin, western China

Jimin Sun, Weiguo Liu, Zhonghui Liu, Tao Deng, Brian F. Windley, Bihong Fu PII: DOI: Reference:

S0031-0182(17)30105-0 doi: 10.1016/j.palaeo.2017.06.012 PALAEO 8328

To appear in:

Palaeogeography, Palaeoclimatology, Palaeoecology

Received date: Revised date: Accepted date:

31 January 2017 5 June 2017 13 June 2017

Please cite this article as: Jimin Sun, Weiguo Liu, Zhonghui Liu, Tao Deng, Brian F. Windley, Bihong Fu , Extreme aridification since the beginning of the Pliocene in the Tarim Basin, western China, Palaeogeography, Palaeoclimatology, Palaeoecology (2017), doi: 10.1016/j.palaeo.2017.06.012

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ACCEPTED MANUSCRIPT Extreme aridification since the beginning of the Pliocene in the Tarim Basin, western China

Jimin Suna,b,c*, Weiguo Liud*, Zhonghui Liue, Tao Dengb,c,f, Brian F. Windleyg, Bihong Fuh Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and

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a

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Geophysics, Chinese Academy of Sciences, Beijing 100029, China b

c

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Chinese Academy Science Center for Excellence in Tibetan Plateau Earth Sciences

University of Chinese Academy of Science, Beijing, China

d

Department of Earth Sciences, The University of Hong Kong, HK, China

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e

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Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, China

f

Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences,

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Beijing 100044, China g

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Department of Geology, University of Leicester, Leicester LEI 7RH, UK

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Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, Beijing 100094,

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China

_______________________________________ 

Corresponding author. Tel: + 86 10 8299 8389; Fax: + 86 10 6201 0846

E-mail address: [email protected] (J. M. Sun)

1

ACCEPTED MANUSCRIPT ABSTRACT Mid-latitude Central Asia is characterized by an extreme arid landscape. Among the Central Asian deserts, the Taklimakan Desert is the largest shifting sand desert which is located in the rain shadow of the Tibetan Plateau and of the

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other central Asian high mountains. The formation of this desert is important for

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placing widespread aridification into a regional tectonic context at the western

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end of the Himalayan-Tibetan orogen. However, there still exists considerable controversy regarding the timing of desert formation, thus impeding our

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understanding of the climatic effects of the Tibetan uplift and regional

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environmental changes. Here we report new biostratigraphic age control and multiple high-resolution climatic records from the center of the Taklimakan

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Desert. Our results reveal dramatic environmental changes at 5 Ma, suggesting

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that an extremely dry climate has prevailed since the beginning of the Pliocene, which is consistent with findings from a high-resolution borehole record about

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670 km to the east in the same basin. The two records combined demonstrate

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that an extremely dry environment prevailed in the entire Tarim Basin from approximately 5 Ma. This was related to the reduced transport of water vapor by westerlies in response to the retreat of the Paratethys Ocean driven by global climatic cooling, and the closed oceanic water-vapor pathway between the Pamir and the Tian Shan ranges driven by the ongoing India-Eurasia collision. ______________________________________ Key words: Tarim Basin; Western China; Taklimakan Desert; late Cenozoic; aridification 2

ACCEPTED MANUSCRIPT 1. Introduction

The Taklimakan Desert is the largest desert in Central Asia and is located within the Tarim Basin, northwestern China. The basin is part of the largest arid zone in the

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northern hemisphere and it is bounded by the Kunlun Mountains to the south, the

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Pamir Plateau to the west, and the Tian Shan Mountains to the north (Fig. 1a). Due to

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an extremely dry climate (annual precipitation is less than 50 mm) and minimal vegetation, about 85 percent of the Taklimakan Desert consists of shifting sand dunes

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(Zhu et al., 1980), which are important source of dust emissions in Central Asia (Sun

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et al., 2001; Laurent et al., 2006). Knowledge on the timing of desert formation in the Tarim Basin has major implications for understanding climate dynamics in response

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to regional tectonic forcing and global climatic change.

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Different theories explain the formation of the dry lands in the Asian interior: 1) the rain shadow effect of the Tibetan Plateau (Broccoli and Manabe, 1992; Kutzbach

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et al., 1993; Miao et al., 2012), 2) global cooling (Miao et al., 2013), and 3) the

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reduced westerly moisture transport from the Neotethys Ocean (Bosboom et al., 2014a; Caves, et al., 2015; Sun and Windley, 2015). The Cenozoic uplift of the Tibetan Plateau and the northward indentation of the Pamir salient turned the Tarim Basin into a semi-enclosed basin (Fig. 1a), which lay in the rain shadows of the southwest Indian monsoon and the westerly moisture transport from the Mediterranean/Caspian/Black Sea, the remaining relics of the Neotethys Ocean. 3

ACCEPTED MANUSCRIPT The onset of the extremely dry environment in the Tarim Basin is still under debate. Earlier studies suggested a young age ranging from 3.5 Ma (Sun et al., 2011; Zheng et al., 2012) to Pleistocene (Zhu et al., 1980; Fang et al., 2002). Later studies revealed that the formation time of the Taklimakan Desert dates back to 5 Ma (Sun

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and Liu, 2006; Chang et al., 2012; Liu et al., 2014). However, one of the most recent

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publications suggested a much older age dating back to late Oligocene time (Zheng et

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al., 2015).

The formation of the Taklimakan Desert marks an important change in the Asian

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landscape. The desert has contributed abundant mineral aerosols to the atmosphere,

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which accordingly has played a significant role in modulating global climate (Uno et al., 2009). Moreover, this desert lies in a rain shadow at the western end of the

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Tibetan-Himalaya orogen, and thus the aridification can potentially be linked to the

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India-Asia tectonic collision. We therefore aim at better constraining the timing and cause of the extreme climate in the Tarim Basin by using multiple high-resolution

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paleoclimate records from the outcrop in the center of the basin.

2. Geological setting and stratigraphy

In this study, we focus on the Mazartag Section at the end of the Mazartag range (Fig. 1a), which exposes unique Cenozoic strata in the heart of the Tarim Basin. The uplift of the Mazartag range is closely linked to the intracontinental deformation in the Tarim Basin in response to the India-Eurasia collision. The south-dipping 4

ACCEPTED MANUSCRIPT Mazartag thrust is the frontal structure of the foreland fold-and-thrust belt of the Kunlun Mountains (Fig. 1b); being a basement (Proterozoic and Paleozoic strata)-involved northward underthrusting (Fig. 1b). The Mazartag thrust was a main contributor to uplift and exposure of the Cenozoic strata to its south (Fig. 1c).

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The Mazartag section has Paleogene and Neogene strata exposed with younger

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sediments to the southwest. Field investigations indicate that the Paleogene deposits

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consist of relict white Paleocene gypsum with thin interbedded reddish mudstone, gypsum-mudstone and marl (Fig. 1c), whereas the late Paleocene to early Eocene grey

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limestones are rich in marine bivalve fossils demonstrating deposition during a marine

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transgression when the Tarim Basin was connected to the Neotethys Ocean (Yong et al., 1983; Lan and Wei, 1995). There is a hiatus in sedimentation between the

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Paleogene marine and the terrestrial Neogene deposits (Fig. 1c). Generally, all

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Neogene strata dip moderately (about 34) to the southwest, except where there are three minor folds in the lower part of the Neogene strata (Fig. 1c).

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In this paper, we focus on the well-exposed, 1071m-thick Neogene strata, which

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can be divided into two parts according to their color characteristics (Fig. 2a): light-yellowish Pliocene strata (Fig. 2b) overlies reddish late Miocene strata (Figs. 2c, d).

The middle-lower part of the reddish Miocene strata mainly consists of fine-grained lacustrine, commonly laminated, mudstones (Fig. 2d); but several beds of reddish sandstones are intercalated with the lacustrine mudstones in its upper part (Fig. 2c). The sandstones are characterized by cross-beddings dip of 30° ± 2°, being close 5

ACCEPTED MANUSCRIPT to the angle of repose of 32 to 34° for dry eolian dune sand (Bagnold, 1941). The eolian origin of the above sandstones can be also demonstrated by the electronic scanning microscope microstructure evidence of quartz grains (Sun et al., 2009a). The light-yellowish Pliocene strata consist of alternating sandstones and siltstones,

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occasionally with conglomerates. Their sedimentary facies is dominated by fluvial

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deposits and interbedded thin lacustrine siltstones and eolian dune sands (Fig. 2b).

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3. Methods

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3.1. Color parameter

Three hundred and forty two bulk samples were used for color measurements. Each

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sample (2g) was first dried at 40C for 24 h and then crushed and measured using a

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Minolta-CM2002 spectrophotometer. For all samples, colorimeters of L* (lightness), a*(redness relative to greenness), and b* (yellowness relative to blueness) were used

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(Commission Internationale de l'Éclairage, 1978).

3.2. Carbonate content Three hundred and fifteen samples were analyzed for their carbonate content applying the neutralization-titration method (Hasegawa et al., 2009). Approximately ∼0.3 g material was decarbonated with 30 mL of HC1 solution (∼0.2 mol/L) for 24 h. After centrifugation at 2,500 rpm on an 80-2 low-speed bench top centrifuge, 10 mL of supernatant was titrated with NaOH solution (∼0.1 mol/L) with two drops of 6

ACCEPTED MANUSCRIPT phenolphthalein solution as indicator. A blank experiment was also conducted in parallel. The content of carbonate can be calculated as follows: CaCO3%= [150 ×C1 − 15 × (V1 − V2) ×C2]/M; where M is the amount of sediment material; C1 is the concentration of the HCl

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solution after calibration; C2 is the concentration of the NaOH solution after

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calibration; and V1 and V2 are the volumes of the NaOH solution used for titration of

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3.3. Stable isotope analysis of carbonates

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the sample and the blank. The analytical error of this method is about 0.5%.

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One hundred and seventy six bulk samples were analyzed for their δ13Ccarb and δ18Ocarb values of carbonates. Each sample (∼1 g) was ground and sieved through a

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100-mesh screen, and then analyzed with an isotope ratio mass spectrometer

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[MAT-252 (Finnigan)] with an automated carbonate preparation device (Kiel II) at the Institute of Earth Environment, Chinese Academy of Sciences (IEECAS), Xi'an,

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China. Oxygen and carbon isotope compositions are expressed in the delta (δ)

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notation relative to the V-PDB standard. The typical standard deviation for the repeated analyses of these laboratory standards is smaller than ±0.1‰.

3.4. Carbon isotopic analysis of organic matter One hundred and ten samples were prepared for δ13Corg analysis of their organic carbon. The samples were first screened for modern rootlets and then digested for at least 15 h in 1 M HCl to remove any inorganic carbonate. The samples were then 7

ACCEPTED MANUSCRIPT washed with distilled water and dried. The dried samples (~480 mg) were combusted for over 4 h at 900 C in evacuated/sealed quartz tubes in the presence of 0.5 g Cu, 4.5 g CuO and 0.2 g Ag foil. The released CO2 was purified and isolated by cryogenic distillation for isotopic analysis using an isotope ratio mass spectrometer [MAT-252

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3.5. Scanning Electron Microscope (SEM) analysis

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(Finnigan)] at the IEECAS. The analytical error is within ±0.1‰.

The microstructures of authigenic carbonates from lacustrine deposits of the

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Mazartag Section were examined using a Nova NanoSEM 450 instrument, which is a

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field-emission scanning electron microscope (FE-SEM) with ultra-high imaging resolution. Elemental compositions were detected using the Oxford AZtec

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of Sciences in Beijing.

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microanalysis system at the Institute of Geology and Geophysics, Chinese Academy

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4. Results and interpretations

4.1. Additional age constraints for the Mazartag Section As chronology forms the basis for understanding paleoclimate history, except for the previous paleomagnetic polarity time scale (Sun et al., 2009a), a better age constraint for the Mazartag Section is required. This is obtained through the discovery of additional fossil mammals. During two joint field expeditions in 2015 and 2016, a mammal fossil of the 8

ACCEPTED MANUSCRIPT second phalanx III of the left foreleg of a three-toed horse genus Hipparion was discovered in a medium-grained sandstone (Fig. 3a) in the lowest part of the light-yellowish Neogene strata (Fig. 3b). In anterior view, the central part of the fossil is concave and rough (Fig. 3a). The

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proximal articular facet is inclined anteriorly, and divided into lateral and medial

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portions by a weak sagittal ridge. The posterior aspect of the distal facet is larger than

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its anterior one, with a very shallow distally directed depression on its proximal border, and the flexor tuberosity is weak (Fig. 3a). In lateral and medial view, the

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scars of the ligamentum collaterale are small and shallow. The robust size, weak

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sagittal ridge in the proximal articular facet, small scars of the ligamentum collaterale, weak flexor tuberosity, and shallow depression of the distal facet on the posterior

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aspect are all typical features of the Plesiohipparion (Qiu et al., 1987; Deng et al.,

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2012).

Plesiohipparion is a subgenus of the genus Hipparion (Qiu et al., 1987), but it is

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regarded as an independent genus by other research (e.g., Bernor and Sun, 2015).

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Plesiohipparion first appeared in the latest Miocene beds in the Yushe Basin in Shanxi and the Zanda Basin in Tibet, and became a common equid in the Pliocene of China (Qiu et al., 1987; Li et al., 1990; Deng et al., 2012; Wang et al., 2013; Bernor and Sun, 2015). Its age ranges from 6 Ma to 3.4 Ma in the Zanda Basin where up to 55 localities of Plesiohipparion fossils were discovered (Wang et al., 2013). The position of the Plesiohipparion fossil is just above the boundary between the light-yellowish Pliocene and the reddish Miocene units (Fig. 3b). Although the fossil 9

ACCEPTED MANUSCRIPT position is about 16 km west of the studied Mazartag Section, the color difference of the two units is distinct and clearly visible in both high-resolution remote-sensing images (Fig. 3b) and in photos along the Mazartag range (Fig. 3c), it is easy to define the corresponding position in the Mazartag Section (Figs. 3c and 4).

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Therefore, the new age assignment of Plesiohipparion supports the previous

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paleomagnetic correlation of the Mazartag Section (Fig. 4), according to which the

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light-yellowish strata have a Pliocene age (Sun et al., 2009a) (Fig. 4).

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4.2. Color variations

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Vertical variations of the color parameters show that the redness (a*) and lightness (L*) exhibit abrupt changes at 5 Ma (Figs. 5a, b), implying dramatic environmental

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changes immediately after the Miocene-Pliocene boundary.

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The redness of sediments is associated with the amount of iron oxides released during chemical weathering (Walker, 1967). Those oxides are usually present in

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limited amounts in sediments, but play an important role in sediment color. Two main

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iron oxides are responsible of sediment color. Hematite gives sediments a typical reddish color, whereas goethite gives a more yellowish color (Torrent et al., 1983; Escadafal and Huete, 1992). Because the contents of authigenic hematite in sediments are positively related to the strength of chemical weathering (Boero et al., 1992), the redness is likely to be climate-dependent. This is demonstrated by the higher values of redness in warm-humid interglacial sediments than in cold-dry glacial deposits in the semi-arid Chinese Loess Plateau (Yang and Ding, 2003). The redness values 10

ACCEPTED MANUSCRIPT decreased dramatically after 5 Ma (Fig. 5a), suggesting weaker chemical weathering and colder/drier climates since the Pliocene. The lightness of sediments is known to be mainly affected by factors such as moisture content and organic matter concentration (e.g., Schulze et al., 1993;

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Spielvogel et al., 2004). In this study, we used dried samples for color measurements,

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thus obviating the effect of moisture content. Lightness correlates negatively with

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their organic content (Escadafal and Huete, 1992; Spielvogel et al., 2004). Therefore, the abrupt increase of lightness in the Pliocene (Fig. 5b) can be partially attributed to a

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reduction in the content of organic matter. The fact that organic content is somewhat

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influenced by biomass (Jenny, 1980), suggests, to some extent, a decline in biomass

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4.3. Carbonate content

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driven by a drier climate since the beginning of the Pliocene.

Before 5 Ma, the CaCO3 content of sediments shows stronger fluctuations

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(between 4% and 30%), but after 5 Ma the fluctuations became weaker (between 7%

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and 22%) (Fig. 5c). This change is linked to different environmental conditions; our analysis on the Mazartag Section indicates that the middle-lower part (before 5 Ma) was dominated by lacustrine sediments (Fig. 2d). Only at the top, did we find a few eolian sandstones interbedded in the lacustrine sediments (Fig. 2c), indicating episodic deposition of sand dunes in a temporary dry environment. The generally higher values of CaCO3 before 5 Ma are associated with more authigenic carbonates in a dominantly lacustrine environment during the late Miocene. 11

ACCEPTED MANUSCRIPT The SEM microstructure analysis of thin sections of the lacustrine strata indicate the presence of very small (2–4 μm), well-developed, authigenic euhedral calcite crystals (Fig. 6a); the element composition of the calcites is confirmed by energy microanalysis (Fig. 6b). The decreased carbonate content after 5 Ma coincides with

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the disappearance of lakes, and with the change to predominant fluvial deposits and

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eolian dune sands in the central Tarim Basin; i.e. less authigenic carbonates were

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precipitated.

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4.4. Stable isotopes of carbonates

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Stable oxygen isotopes are the most widely applied proxies in the study of past climate changes due to their isotopic fractionations during geochemical processes

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(Leng and Marshall, 2004). High-resolution δ18Ocarb and δ13Ccarb curves of carbonate

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also show a distinct change at around 5 Ma (Figs. 5d, e). From approximately 5 Ma, the δ18Ocarb values become more negative (Fig. 5d).

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Generally, the δ18Ocarb values of sedimentary carbonates are affected by both

the

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detrital and authigenic carbonates. It is well known that δ18Ocarb depends mostly on isotopic

composition

of

lake

water

controlled

by

changes

in

precipitation/evaporation ratios (Siegenthaler and Oeschger, 1980; Edwards and Wolfe, 1996). In an inland lake system, the 16O isotopes are preferentially transferred into the vapor phase leaving lake water enriched in 18O, leading to relatively positive δ18Ocarb values in the authigenic carbonates. The higher δ18Ocarb values before 5 Ma were associated with more authigenic carbonates in a predominant lacustrine 12

ACCEPTED MANUSCRIPT environment. The δ13Ccarb values of carbonates also exhibit a pronounced change at ~5 Ma marked by a shift toward higher values (Fig. 5e). The δ13Ccarb values were relatively negative before 5 Ma, probably due to biological effect in lake waters (Talbot and

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Kelts, 1990; Barešić et al., 2011). Generally, the δ13Ccarb in lake sediments mainly

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reflects the carbon isotope evolution of dissolved inorganic carbon (DIC) during the

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carbon cycle in the lake system (Paprocka, 2007). The dominant controls on the carbon isotope composition of DIC are CO2 exchange between the atmosphere and

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lake water, photosynthesis/respiration of aquatic organisms (Vreča, 2003), and lake

preferentially incorporate

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stratification and circulation (Myrbo and Shapley 2006). Phytoplankton and bacteria C, resulting in biogenic carbonates (e.g., ostracods,

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mollusk shells and charophyte algae) with depleted δ13C (de Kluijver, 2014).

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Moreover, oxidation of sinking organic matter will return

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C to the water column,

leading to depleted δ13C of dissolved inorganic carbon (Teranes and Bernasconi,

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2005).

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After 5 Ma, the input of more detrital carbonates account for the relatively more positive δ13Ccarb values following the shift from a predominant lacustrine environment to alternations of fluvial sediments and eolian dunes.

4.5. Organic carbon isotopes The temporal variations in the carbon isotope of organic matter (δ13Corg) in the Mazartag Section show remarkable changes at 5 Ma marked by a shift to more 13

ACCEPTED MANUSCRIPT positive values after that time (Fig. 5f). We provide the following interpretations for this event. The change can be explained by the effect of water-stress on the carbon isotopic composition of plants (Farquhar and Richards, 1984; Ehleringer and Cooper, 1988;

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Read et al., 1991, 1992; Ehleringer et al., 1993). Drought stress decreases stomatal

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conductance, increasing the ratio of external to internal CO2 partial pressures, leading

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to a 1–3‰ difference of δ13Corg as demonstrated in laboratory and field experiments (Farquhar and Richards, 1984; Read et al., 1991, 1992). Although the average δ13Corg

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value of modern C3 plants is -27‰, C3 plants can be enriched in

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C under

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water-stressed conditions, and δ13Corg values can reach as high as -20‰ in arid environments (Diefendorf et al., 2010; Kohn, 2010). In this context, the positive shift

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enrichment of C3 plants.

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of δ13Corg in the Tarim Basin can be explained by the effect of aridity on the

In addition, the change in δ13Corg is also explicable by the worldwide expansion of

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C4 grasses, which generally have higher δ13C values than C3 plants (Cerling et al.,

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1993). However, although modern investigations suggest that there are up to 72 C4 species in the Tarim Basin (Su et al., 2011), their total amount is very limited evidenced by modern vegetation studies (Zhang et al., 2003), and by the current global distribution map of C4 vegetation (Still et al., 2003). The expansion of C4 grasses may therefore have played only a minor role in this δ13Corg shift at 5 Ma.

5. Discussion 14

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5.1. Age determinations of the Mazartag Section The chronology of the Mazartag Section was first established by Sun et al. (2009a) who used paleomagnetic polarity correlation, in combination with biostratigraphic age

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constraints derived from analyses on a mammal fossil (Fig. 7b). The results of the

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analyses indicate that the Mazartag Section has an age range of 9.7 to 2.6 Ma.

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Recently Zheng et al. (2015) re-interpreted the published paleomagnetic polarity data of Sun et al. (2009a), and derived a magnetostratigraphy indicating that the

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Mazartag Section has an age range of 34 to 17.5 Ma (Fig. 7c). Based on their age

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model, Zheng et al. (2015) conclude that sand dune mobilization in the Tarim Basin and hence the formation of the Taklimakan Desert started as early as 26.7 Ma.

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However, the re-interpreted age model of Zheng et al (2015) is in conflict with the

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biostratigraphic age constraints as presented by Sun et al. (2009a). In addition, our newly discovered Plesiohipparion fossil has an age range of 6 to 3.4 Ma. Another

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mammalian fossil (Olonbulukia tsaidamensis) was discovered in 2008 in the lower

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reddish Miocene strata, having a biostratigraphic age of 89 Ma (Deng, 2006) (Fig. 7b). The biostratigraphic ages of these mammalian fossils disagrees strongly with the re-interpreted age range of 34 to 17.5 Ma of Zheng et al. (2015), but does support the earlier paleomagnetic correlation of Sun et al. (2009a). Therefore, we will use the age model of Sun et al. (2009a) for further discussions.

5.2. Basin-wide paleoclimatic correlation in the Tarim Basin 15

ACCEPTED MANUSCRIPT Given the large area of the Tarim Basin and the relatively few studies in the basin so far, it is important to have more late Cenozoic paleoclimatic records to confirm the timing and mechanisms of the formation of the Taklimakan Desert. In recent years, a continuous 1050.6 m-long core was drilled in the Lob Nor

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paleo-lake basin in the eastern Tarim Basin (see Fig. 1a for location). The chronology

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of the Lob Nor borehole was based on high-resolution magnetostratigrahy that yielded

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an age range from 7 Ma to the present (Chang et al., 2012).

The high-resolution climate record of the Lob Nor borehole provides another

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unique late Cenozoic paleoclimatic archive from the Tarim Basin (Liu et al., 2014).

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The contents and δ18Ocarb of carbonates of the Mazartag Section are correlated with those of the Lob Nor borehole (Fig. 8). This correlation indicates the two following

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features.

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First, the change of carbonate content of the Mazartag Section correlates well with that of the Lop Nor borehole in that they both show a general decrease after 5 Ma

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(Fig. 8). This is consistent with the shift from an episodic lake facies (relatively more

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authigenic carbonates) to a later drier environment in the Tarim Basin. Second, the fluctuating pattern of the δ18Ocarb in the Mazartag Section also correlates with that of the borehole record in Lop Nor (Fig. 8). Additionally, we note that before ~5 Ma, the δ18Ocarb values in the Lop Nor borehole are relatively more positive and have greater changes compared with those of the Mazartag Section (Fig. 8). This suggests that in the late Miocene, Lop Nor lake was much shallower and probably dried up temporarily, causing evaporative isotope enrichment relative to the 16

ACCEPTED MANUSCRIPT central Tarim Basin record. We also note that, although the earliest in situ eolian dune sands in the Mazartag Section are 7 Ma old, the sedimentary facies at that time was dominated by lacustrine deposits with only minor layers of interbedded eolian sands (Fig. 2c), thus

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implying that there were very limited dune fields in the interior of the basin between 7

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and 5 Ma (Sun et al., 2009a). The multiple high-resolution climatic records of this

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study, together with the Lob Nor borehole record, demonstrate that the enhanced aridity and the extensive desert in the Tarim Basin appeared only after 5 Ma.

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Considering the fact that the Lob Nor borehole is about 670 km east of the

Mazartag

Section

and

the

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Mazartag Section, the good correlation of the high-resolution records between the Lop

Nor

borehole

confirms

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basin-wide

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paleoenvironmental change from approximately 5 Ma, which shifted from a

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predominant lake environment to a later drier desert.

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5.3. Mechanisms accounting for the extreme aridity after ~5 Ma in the Tarim Basin

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The Tarim Basin lies in the rain shadow of the Tibetan Plateau and of the other central Asian high mountains. The timing of the extremely dry climate and the birth of the Taklimakan Desert at 5 Ma has important implications for the role of global climate and regional tectonics. Recently, Herbert et al. (2016) reported a global cooling event at about 7‒5 Ma, and suggested that this was associated with an increased meridional temperature gradient, linked to increased aridity and ecosystem changes on land. However, in the 17

ACCEPTED MANUSCRIPT Tarim Basin, a predominant lacustrine environment appears to have remained through this global cooling event from at least 9.7 Ma to 5 Ma, though there were occasionally lake regressions between 7 and 5 Ma. This would suggest that the paleoclimatic changes in the Tarim Basin in the late Neogene may not be due solely to

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global cooling. The regional tectonic deformation and its consequent climatic impact

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were also critical influences on the paleoenvironmental history of the Tarim Basin.

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At present, the Pamir Plateau forms a prominent tectonic salient along the western end of the Himalayan-Tibetan orogen separating the Tarim and Tajik Basins (Fig. 1a),

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although they were once connected (Molnar and Tapponnier, 1975). It has long been

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known from geological surveys that during the Cenozoic era the northern Pamir had indented northwards for 300 km relative to stable Eurasia (Burtman and Molnar,

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1993), and geophysical data confirm that the Indian Plate underthrust the Eurasian

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crust in the Pamir to a depth of 300–500 km (Negredo et al., 2007; Burtman, 2013). The uplift and northward indentation of the Pamir salient is an ongoing process,

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but it underwent at least three pulses of tectonic activity (Fig. 9). Apatite and zircon

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(U/Th)-He ages record an early period of accelerated exhumation at ∼50–40 Ma driven by the India-Asia collision in the Pamir (Amidon and Hynek, 2010). This time is similar to the crustal thickening in the Pamir at ∼50 Ma (Ducea et al., 2003; Hacker et al., 2005) and in the western Kunlun at ∼47 Ma (Yin et al., 2002) as well as the late Eocene sea-water retreat in the eastern margin of Tajik Basin (Carrapa et al., 2015) and the western Tarim Basin (Bosboom et al., 2014a, b; Sun et al., 2016). During this time interval, the southwest Indian monsoon was already established 18

ACCEPTED MANUSCRIPT in the Eocene (Shukla et al., 2014; Spicer et al., 2016), but geologic and paleoaltimetric studies show that parts of the Tibetan Plateau were already elevated at this time (e.g. Kapp et al., 2005; Dupont-Nivet et al., 2008) (Fig. 9a). This blocked moisture transport of the Indian monsoon to the Tarim Basin as early as the Eocene

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evidenced by δ18O records of central Asia (Cave et al., 2015).

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Except for the Indian monsoon, another important moisture source is the water

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vapor from the Neotethys Ocean (Fig. 9a). Although sea-water retreated from the Tarim Basin at ∼40 Ma (Bosboom et al., 2014a, b; Sun et al., 2016), there still existed

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a large Neotethys Ocean to the west of the Tarim Basin (Rögl, 1999; Popov et al.,

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2004), from which water-vapor was easily carried to the mid-latitude Asian interior by westerlies (Cave et al., 2015) through a 300 km-wide water-vapor channel between

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the Pamir and the Tian Shan ranges (Fig. 9a). Predominant lacustrine deposits of the

(Zhang et al., 2016).

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Suweiyi Formation were present in the northern Tarim Basin during the late Eocene

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A later tectonic pulse occurred at 25–18 Ma marked by accelerated northward

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indentation of the Pamir, considerable crustal shortening, and initiation of the right-lateral Karakoram fault (Lacassin et al., 2004) (Fig. 9b). On the eastern flank of the Pamir, accelerated tectonic convergence is recorded at this time by detrital low-temperature thermochronology near the Main Pamir Thrust (MPT) (Sobel and Dumitru, 1997; Bershaw et al., 2012). In the western flank of the Pamir, basin subsidence curves constructed for the Tajik depression show the onset of rapid subsidence at ∼22 Ma, which was likely in response to tectonic loading (Leith, 1985). 19

ACCEPTED MANUSCRIPT Thermochronologic results show that the initiation of the Karakoram strike-slip fault was mostly between 25 and 21 Ma (Valli et al., 2007), implying that right-lateral motion and prominent offset in the eastern flank of the Pamir were a response to accelerated northward displacement of the Pamir salient.

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In addition, at this time the ongoing northward indentation of the Pamir salient

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(Fig. 9b), together with the climatic effect of global cooling, resulted in the westward

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retreat of the Paratethys, leading to the decreased moisture transport by the westerlies, which enhanced the aridity in the mid-latitude Asian interior in the Oligocene to early

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Miocene (Guo et al., 2002; Sun et al., 2010; Bosboom et al., 2014a; Sun and Windley,

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2015; Caves et al., 2016).

However, the presence of widespread paleosols and pedogenic carbonate nodules

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in the Asian interior (Guo et al., 2002; Sun and Windley, 2015; Caves et al., 2016)

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suggests that the climate was mostly semi-arid to sub-humid rather than hyperarid. It is generally known that the occurrence of pedogenic carbonate is mostly observed in

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soils where there is enough water for chemical leaching (Deocampo, 2010). Thus,

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pedogenic carbonates are characteristic of soils where the moisture conditions strongly alternate (Cerling, 1984; Gocke et al., 2012). Moreover, although the earliest eolian dust is around 22 Ma in the Chinese Loess Plateau (Guo et al., 2002), there is no evidence to demonstrate that it was derived from the Tarim Basin. On the contrary, recent detrital-zircon U–Pb geochronology, heavy-mineral, and bulk-petrography analyses rule out any direct eolian sediment transport from the Tarim Basin to the Chinese Loess Plateau (Rittner et al., 2016). 20

ACCEPTED MANUSCRIPT 87

Sr/86Sr ratios also demonstrate that the Taklimakan Desert in the Tarim Basin was

not the provenance of the Loess Plateau (Chen et al., 2007). The above results are similar to modern 40-year meteorological observational data, which indicate that the Taklimankan Desert is not the provenance of the Chinese Loess Plateau (Sun et al.,

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2001).

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During this period, the corridor between the Pamir and the Tian Shan ranges

SC

significantly narrowed due to the substantial northward indentation of the Pamir salient (Fig. 9b). But, there still existed a narrow water-vapor channel between them,

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leading to the moisture transport from the Paratethys to the Tarim Basin. Geological

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investigations indicate that the early Miocene Jidike Formation in the northern Tarim Basin comprises fluvial-lacustrine deposits (Bureau of Geology and Mineral

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Resources of Xinjiang, 1982; Zhang et al., 2016).

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A final tectonic episode at 7‒5 Ma is marked by another pulse of outward growth of the Pamir and initiation of the Pamir Frontal Thrust (PFT) (Thompson et al., 2015),

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created by the first collision of the North Pamir with Tian Shan (Fig. 9c). This is

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evidenced by considerable slowing of strike–slip motion along the Kashgar-Yecheng transfer fault system in the Late Miocene-Pliocene (Sobel, et al., 2011), the latest Miocene syntectonic strata in the southern foreland of Tian Shan (Hubert-Ferrari et al., 2007; Sun et al., 2009b), and the tectonic deformation in the Pamir–Tian Shan convergence zone at  5 Ma (Fu et al., 2010; Thompson et al., 2015). This latest phase of tectonics in the Pamir had a profound effect on the environmental evolution in the Tarim Basin. The collision between the North Pamir 21

ACCEPTED MANUSCRIPT and the Tian Shan ranges resulted in regional tectonic uplift in their convergence zone which gradually closed the previous intervening water-vapor pathway, thus effectively blocking water-vapor transport by the westerlies from the upwind Paratethys Ocean to the Tarim Basin (Fig. 9c).

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Moreover, global climate cooling is also evidenced by the initiation of increased

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ice-rafted debris (IRD) in the northern Hemisphere at 7‒5Ma (Jansen and Sjøholm,

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1991; St. John and Krissek, 2002). The enlarged global ice sheet resulted in a decline in sea-level that in turn would lead to considerable shrinkage of the Paratethys at the

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end of the Miocene (Popov et al., 2004) and to reduced moisture transport by

MA

westerlies to the Tarim Basin.

Given the fact that both tectonics and global climate underwent substantial

D

changes over this time period, it is useful to finally compare the regional climatic

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record of the Tarim Basin with the global marine record (Fig. 10). The regional paleoclimatic records from the Mazartag and the Kuqa Sections in

b).

Moreover,

the

content

of

free

Fe

(iron

extractable

by

AC

10a,

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the Tarim Basin (see Fig. 1a for locations) all exhibit abrupt changes at 5 Ma (Figs.

citrate–bicarbonate–dithionite, reflecting Fe released by chemical weathering (Singer et al., 1995)) of the Kuqa Section shows a general decreasing trend and a remarkable shift to lower values after 5 Ma (Fig. 10b). This latter shift is consistent with our findings in the Mazartag Section (Figs. 10 a, b). It is interesting that global marine benthic oxygen isotopes also show a general trend towards more positive δ18O values (Fig. 10c), which mostly reflect increased ice-volumes (Zachos et al., 2001), but the 22

ACCEPTED MANUSCRIPT transition from Miocene to Pliocene was not abrupt (Fig. 10c). The above correlations suggest that the general trend of regional climatic aridification in the Tarim Basin was controlled by first-order global cooling, whereas the abrupt extreme aridification event in the Tarim Basin since the latest Miocene was

PT

greatly enhanced by the closure of a water-vapor pathway caused by the collision of

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the Pamir salient with the Tian Shan Mountains in response to the India–Eurasia

SC

collision.

MA

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6. Conclusions

Analyses of the Mazartag Section in the central Tarim Basin allows us to

D

reconstruct the paleoenvironmental history and to explore the interplay between

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regional tectonic uplift and climate change in an active tectonic region. Our new biostratigraphic age control and high-resolution multiple climatic records, together

CE

with a compilation of geological evidence, support the following conclusions:

AC

(1) A basin-wide lacustrine-fluvial environment prevailed in the Tarim Basin until the latest Miocene. At this time, there still existed a water vapor passage between the northern Pamir and the Tian Shan ranges, and the oceanic moisture transported by the prevailing westerlies from the Paratethys Ocean were able to reach eastwards to the Tarim Basin. The fact that there were few eolian sand dunes between 7 and 5 Ma, could be an indicator of a relatively mild drought, and the lakes were predictably shallow and vulnerable to become ephemerally dry. 23

ACCEPTED MANUSCRIPT (2) Our new records together with evidence from the Lob Nor borehole demonstrate that basin-scale extreme aridification and an extensive desert appeared after 5 Ma in the Tarim Basin. Although the general trend of regional aridification in the Tarim Basin was controlled by first-order global cooling, the aridity of the basin

PT

was greatly enhanced by closure of the water-vapor channel between the Pamir and

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CE

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SC

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the Tian Shan ranges caused by the ongoing India-Eurasia collision.

24

ACCEPTED MANUSCRIPT Acknowledgements This work was supported by the National Nature Science Foundation of China (41290251 and 41672168), the “Strategic Priority Research Program” of the Chinese Academy of Sciences (XDB03020500), and the National Basic Research Program of

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China (2013CB956400). We thank Dr. Jeremy Caves and an anonymous reviewer for

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their valuable comments and suggestions that substantially improved our manuscript.

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We also thank Drs. Shiqi Wang, Qiang Li, Yingying Jia and Zhiliang Zhang for their

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helps during the fielding expedition.

25

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ACCEPTED MANUSCRIPT Figure Captions Fig. 1. Tectonic background and section description. a, Digital elevation model (DEM) map showing the Tarim Basin and the adjoining orogens, the seismic cross-section line (X‒X'), and the locations of the Mazartag and Kuqa

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Sections as well as the Lop Nor borehole mentioned in the text; b,

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interpretation of the seismic cross-section indicates that the Mazartag fault is

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a basement-involved thrust created in response to intracontinental

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deformation in the northern Tibetan Plateau (modified from Liang et al. (2012)); c, an illustration based on an outcrop of relict Paleogene marine

of the Mazartag thrust.

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deposits and terrestrial Neogene strata, related to the northward propagation

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Fig. 2. Stratigraphy and subdivision of Neogene strata of the Mazartag Section. a, Photograph shows the late Miocene (reddish) to Pliocene light-yellowish

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strata; b-d, photos show Pliocene, latest Miocene (just below the Pliocene/Miocene boundary), and late Miocene sediments, respectively.

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Fig. 3. New mammal fossil in the Mazartag Section. a, The newly discovered mammal fossil in Pliocene sandstone (to the left), and the anterior and posterior views of Plesiohipparion fossils (to the right); b, the stratigraphic position of the mammal fossil is just above the Pliocene/Miocene boundary, and the section line (X-X') of the Mazartag Section is indicated; c, photo shows the corresponding position of the fossil in the Mazartag Section.

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Fig. 5. Multiple climatic proxies indicate dramatic environmental changes at 5 Ma,

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(a) redness, (b) lightness, (c) carbonate content, (d) δ18Ocarb, (e) δ13Ccarb, and (f)

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δ13Corg. The age of 5 Ma is based on the chronology of Figure 4.

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Fig. 6. Authigenic carbonates in the late Miocene lacustrine strata in the Mazartag Section. a, Scanning electron microscopy images of euhedral calcite crystals

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assignments of the mammal fossils support the paleomagnetic correlation of Sun et al. (2009a); c, the re-interpreted magnetostratigraphy of Zheng et al.

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(2015), which used the paleomagnetic data of Sun et al. (2009a) to indicate an age range of 3417.5 Ma, which is in conflict with the late Miocene to Pliocene age of the mammalian fossils. Fig. 8. Paleoclimatic correlation between the Mazartag Section and the Lop Nor borehole in the Tarim Basin. The contents and oxygen isotopic compositions of carbonates of the Mazartag Section correlate well with the same records 40

ACCEPTED MANUSCRIPT from the Lop Nor borehole (data from Liu et al. (2014)), suggesting a dramatic, basin-scale climatic deterioration from a dominant episodic lake environment to fluvial/eolian sedimentation since the end of the Miocene.

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Fig. 9. Conceptual models showing the progressive tectonic evolutions of the Pamir

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salient. a, Initial collision at the west end of the Himalayan-Tibetan orogen

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(∼50–40 Ma), when an epicontinental sea (referred as the Jajik-Tarim Sea) connected with the Neotethys Ocean, enabling water-vapor to easily reach the

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Tarim Basin transported by the westerlies; b, a second episode (25-18 Ma)

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channel between the Pamir and the Tian Shan ranges was still wide enough to let sufficient Paratethys moisture to be transported by the westerlies to the

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Tarim Basin; c, 7-5 Ma, the ongoing indentation resulted in the collision of the

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North Pamir with the Tian Shan Mountains, which closed the previous intervening water-vapor pathway, which in turn significantly enhanced the aridity of the Tarim Basin.

Fig. 10. Regional and global paleoclimatic correlations. a, Climatic records from the Mazartag Section; b, temporal variations of chemical weathering index of the Kuqa Section, c, global marine benthic δ18O record, data of 0–5.32 Ma and 41

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ACCEPTED MANUSCRIPT Research Highlights

New biostratigraphic age constrain on the Late Cenozoic strata in the Tarim Basin

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Multiple high-resolution records indicate extreme aridification at 5 Ma

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The extreme aridification is largely enhanced by regional tectonics

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