Extreme Aridification Since the Beginning of the Pliocene in The

Palaeogeography, Palaeoclimatology, Palaeoecology 485 (2017) 189–200

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Palaeogeography, Palaeoclimatology, Palaeoecology

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

Jimin Sun a,b,c,⁎, Weiguo Liu d,ZhonghuiLiue, Tao Deng b,c,f, Brian F. Windley g, Bihong Fu h a Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China b Chinese Academy Science Center for Excellence in Tibetan Plateau Earth Sciences, China c University of Chinese Academy of Science, Beijing, China d Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, China e Department of Earth Sciences, The University of Hong Kong, HK, China f Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing 100044, China g Department of Geology, University of Leicester, Leicester LEI 7RH, UK h Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, Beijing 100094, China article info abstract

Article history: Mid-latitude Central Asia is characterized by an extreme arid landscape. Among the Central Asian deserts, the Ta- Received 31 January 2017 klimakan Desert is the largest shifting sand desert which is located in the rain shadow of the Tibetan Plateau and Received in revised form 5 June 2017 of the other central Asian high mountains. The formation of this desert is important for placing widespread Accepted 13 June 2017 aridification into a regional tectonic context at the western end of the Himalayan-Tibetan orogen. However, Available online 15 June 2017 there still exists considerable controversy regarding the timing of desert formation, thus impeding our understanding of the climatic effects of the Tibetan uplift and regional environmental changes. Here we report new bio- Keywords: Tarim Basin stratigraphic age control and multiple high-resolution climatic records from the center of the Taklimakan Desert. Western China Our results reveal dramatic environmental changes at ~5 Ma, suggesting that an extremely dry climate has Taklimakan Desert prevailed since the beginning of the Pliocene, which is consistent with findings from a high-resolution borehole Late Cenozoic record about 670 km to the east in the same basin. The two records combined demonstrate that an extremely dry Aridification 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. © 2017 Elsevier B.V. All rights reserved.

1. Introduction for understanding climate dynamics in response to regional tectonic forcing and global climatic change. The Taklimakan Desert is the largest desert in Central Asia and is lo- Different theories explain the formation of the dry lands in the Asian cated within the Tarim Basin, northwestern China. The basin is part of interior: 1) the rain shadow effect of the Tibetan Plateau (Broccoli and the largest arid zone in the northern hemisphere and it is bounded by Manabe, 1992; Kutzbach et al., 1993; Miao et al., 2012), 2) global cooling the Kunlun Mountains to the south, the Pamir Plateau to the west, and (Miao et al., 2013), and 3) the reduced westerly moisture transport from the Tian Shan Mountains to the north (Fig. 1a). Due to an extremely the Neotethys Ocean (Bosboom et al., 2014a; Caves et al., 2015; Sun and dry climate (annual precipitation is less than 50 mm) and minimal veg- Windley, 2015). etation, about 85% of the Taklimakan Desert consists of shifting sand The Cenozoic uplift of the Tibetan Plateau and the northward dunes (Zhu et al., 1980), which are important source of dust emissions indentation of the Pamir salient turned the Tarim Basin into a semi- in Central Asia (Sun et al., 2001; Laurent et al., 2006). Knowledge on enclosed basin (Fig. 1a), which lay in the rain shadows of the southwest the timing of desert formation in the Tarim Basin has major implications Indian monsoon and the westerly moisture transport from the Mediterranean/Caspian/Black Sea, the remaining relics of the Neotethys Ocean. The onset of the extremely dry environment in the Tarim Basin is ⁎ Corresponding author at: Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China. still under debate. Earlier studies suggested a young age ranging from E-mail address: [email protected] (J. Sun). 3.5 Ma (Sun et al., 2011; Zheng et al., 2002) to Pleistocene (Zhu et al.,

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 Sections as well as the Lop Nor borehole mentioned in the text; b, interpretation of the seismic cross-section indicates that the Mazartag fault is a basement-involved thrust created in response to intracontinental deformation in the northern Tibetan Plateau (modiﬁed from Liang et al. (2012)); c, an illustration based on an outcrop of relict Paleogene marine deposits and terrestrial Neogene strata, related to the northward propagation of the Mazartag thrust.

1980; Fang et al., 2002). Later studies revealed that the formation time linked to the intracontinental deformation in the Tarim Basin in re- of the Taklimakan Desert dates back to ~5 Ma (Sun and Liu, 2006; sponse to the India-Eurasia collision. The south-dipping Mazartag thrust Chang et al., 2012; Liu et al., 2014). However, one of the most recent is the frontal structure of the foreland fold-and-thrust belt of the Kunlun publications suggested a much older age dating back to late Oligocene Mountains (Fig. 1b); being a basement (Proterozoic and Paleozoic stra- time (Zheng et al., 2015). ta)-involved northward underthrusting (Fig. 1b). The Mazartag thrust The formation of the Taklimakan Desert marks an important change was a main contributor to uplift and exposure of the Cenozoic strata in the Asian landscape. The desert has contributed abundant mineral to its south (Fig. 1c). aerosols to the atmosphere, which accordingly has played a signiﬁcant The Mazartag section has Paleogene and Neogene strata exposed role in modulating global climate (Uno et al., 2009). Moreover, this de- with younger sediments to the southwest. Field investigations indicate sert lies in a rain shadow at the western end of the Tibetan-Himalaya that the Paleogene deposits consist of relict white Paleocene gypsum orogen, and thus the aridiﬁcation can potentially be linked to the with thin interbedded reddish mudstone, gypsum-mudstone and marl India-Asia tectonic collision. We therefore aim at better constraining (Fig. 1c), whereas the late Paleocene to early Eocene grey limestones the timing and cause of the extreme climate in the Tarim Basin by are rich in marine bivalve fossils demonstrating deposition during a ma- using multiple high-resolution paleoclimate records from the outcrop rine transgression when the Tarim Basin was connected to the in the center of the basin. Neotethys Ocean (Yong et al., 1983; Lan and Wei, 1995). There is a hia- tus in sedimentation between the Paleogene marine and the terrestrial 2. Geological setting and stratigraphy Neogene deposits (Fig. 1c). Generally, all Neogene strata dip moderately (about 34°) to the southwest, except where there are three minor folds In this study, we focus on the Mazartag Section at the end of the in the lower part of the Neogene strata (Fig. 1c). Mazartag range (Fig. 1a), which exposes unique Cenozoic strata in the In this paper, we focus on the well-exposed, 1071 m–thick Neogene heart of the Tarim Basin. The uplift of the Mazartag range is closely strata, which can be divided into two parts according to their color J. Sun et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 485 (2017) 189–200 191

Fig. 2. Stratigraphy and subdivision of Neogene strata of the Mazartag Section. a, Photograph shows the late Miocene (reddish) to Pliocene light-yellowish strata; b–d, photos show Pliocene, latest Miocene (just below the Pliocene/Miocene boundary), and late Miocene sediments, respectively.

characteristics (Fig. 2a): light-yellowish Pliocene strata (Fig. 2b) overlies where M is the amount of sediment material; C1 is the concentration of reddish late Miocene strata (Fig. 2c, d). the HCl solution after calibration; C2 is the concentration of the NaOH The middle-lower part of the reddish Miocene strata mainly consists solution after calibration; and V1 and V2 are the volumes of the NaOH of fine-grained lacustrine, commonly laminated, mudstones (Fig. 2d); solution used for titration of the sample and the blank. The analytical but several beds of reddish sandstones are intercalated with the lacus- error of this method is about 0.5%. trine mudstones in its upper part (Fig. 2c). The sandstones are characterized by cross-beddings dip of 30° ± 2°, being close to the angle of 3.3. Stable isotope analysis of carbonates 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 elec- One hundred and seventy six bulk samples were analyzed for their 13 18 tronic scanning microscope microstructure evidence of quartz grains δ Ccarb and δ Ocarb values of carbonates. Each sample (∼1g)was (Sun et al., 2009a). ground and sieved through a 100-mesh screen, and then analyzed The light-yellowish Pliocene strata consist of alternating sandstones with an isotope ratio mass spectrometer [MAT-252 (Finnigan)] with and siltstones, occasionally with conglomerates. Their sedimentary fa- an automated carbonate preparation device (Kiel II) at the Institute of cies is dominated by fluvial deposits and interbedded thin lacustrine Earth Environment, Chinese Academy of Sciences (IEECAS), Xi'an, siltstones and eolian dune sands (Fig. 2b). China. Oxygen and carbon isotope compositions are expressed in the delta (δ) notation relative to the V-PDB standard. The typical standard 3. Methods deviation for the repeated analyses of these laboratory standards is smaller than ±0.1‰. 3.1. Color parameter 3.4. Carbon isotopic analysis of organic matter Three hundred and forty two bulk samples were used for color mea- 13 surements. Each sample (2 g) was first dried at 40 °C for 24 h and then One hundred and ten samples were prepared for δ Corg analysis of crushed and measured using a Minolta-CM2002 spectrophotometer. their organic carbon. The samples were first screened for modern root- For all samples, colorimeters of L* (lightness), a*(redness relative to lets and then digested for at least 15 h in 1 M HCl to remove any inor- greenness), and b* (yellowness relative to blueness) were used ganic carbonate. The samples were then washed with distilled water (Commission Internationale de l'Éclairage, 1978). 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,

3.2. Carbonate content 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 Three hundred and fifteen samples were analyzed for their carbon- mass spectrometer [MAT-252 (Finnigan)] at the IEECAS. The analytical ate content applying the neutralization-titration method (Hasegawa et error is within ±0.1‰. 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 3.5. Scanning electron microscope (SEM) analysis 2500 rpm on an 80-2 low-speed bench top centrifuge, 10 mL of super- natant was titrated with NaOH solution (∼0.1 mol/L) with two drops The microstructures of authigenic carbonates from lacustrine of phenolphthalein solution as indicator. A blank experiment was also deposits of the Mazartag Section were examined using a Nova conducted in parallel. The content of carbonate can be calculated as fol- NanoSEM 450 instrument, which is a field-emission scanning lows: electron microscope (FE-SEM) with ultra-high imaging resolution. Elemental compositions were detected using the Oxford AZtec microanalysis system at the Institute of Geology and Geophysics, % ¼ ½ − ðÞ− = ; CaCO3 150 C1 15 V1 V2 C2 M Chinese Academy of Sciences in Beijing. 192 J. Sun et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 485 (2017) 189–200

4. Results and interpretations of the distal facet on the posterior aspect are all typical features of the Plesiohipparion (Qiu et al., 1987; Deng et al., 2012). 4.1. Additional age constraints for the Mazartag Section Plesiohipparion is a subgenus of the genus Hipparion (Qiuetal., 1987), but it is regarded as an independent genus by other research As chronology forms the basis for understanding paleoclimate (e.g., Bernor and Sun, 2015). Plesiohipparion first appeared in the latest history, except for the previous paleomagnetic polarity time scale Miocene beds in the Yushe Basin in Shanxi and the Zanda Basin in (Sun et al., 2009a), a better age constraint for the Mazartag Section Tibet, and became a common equid in the Pliocene of China (Qiu et al., is required. This is obtained through the discovery of additional fossil 1987; Li and Li, 1990; Deng et al., 2012; Wang et al., 2013; Bernor and mammals. Sun, 2015). Its age ranges from 6 Ma to 3.4 Ma in the Zanda Basin During two joint field expeditions in 2015 and 2016, a mammal fos- where up to 55 localities of Plesiohipparion fossils were discovered sil of the second phalanx III of the left foreleg of a three-toed horse genus (Wang et al., 2013). Hipparion was discovered in a medium-grained sandstone (Fig. 3a) in The position of the Plesiohipparion fossil is just above the boundary the lowest part of the light-yellowish Neogene strata (Fig. 3b). between the light-yellowish Pliocene and the reddish Miocene units In anterior view, the central part of the fossil is concave and rough (Fig. 3b). Although the fossil position is about 16 km west of the studied (Fig. 3a). The proximal articular facet is inclined anteriorly, and divided Mazartag Section, the color difference of the two units is distinct and into lateral and medial portions by a weak sagittal ridge. The posterior clearly visible in both high-resolution remote-sensing images (Fig. 3b) aspect of the distal facet is larger than its anterior one, with a very shal- and in photos along the Mazartag range (Fig. 3c), it is easy to define low distally directed depression on its proximal border, and the flexor the corresponding position in the Mazartag Section (Figs. 3cand4). tuberosity is weak (Fig. 3a). In lateral and medial view, the scars of the Therefore, the new age assignment of Plesiohipparion supports the ligamentum collaterale are small and shallow. The robust size, weak previous paleomagnetic correlation of the Mazartag Section (Fig. 4), ac- sagittal ridge in the proximal articular facet, small scars of the cording to which the light-yellowish strata have a Pliocene age (Sun et ligamentum collaterale, weak flexor tuberosity, and shallow depression al., 2009a)(Fig. 4).

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. J. Sun et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 485 (2017) 189–200 193

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-de- pendent. 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 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; Spielvogel et al., 2004). In this study, we used dried samples for color measurements, thus obviating the effect of moisture content. Lightness correlates negatively with 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 reduction in the content of organic matter. The fact that organic content is somewhat influenced by biomass (Jenny, 1980), suggests, to some extent, a decline in biomass driven by a drier climate since the beginning of the Pliocene. Fig. 4. Published magnetostratigraphy attributes the light-yellowish strata a Pliocene age (Sunetal.,2009a), this is consistent with the newly discovered mammal fossil of 4.3. Carbonate content Plesiohipparion from the lowest part of the light-yellowish strata which has an age range of 6–3.4 Ma. Before ~5 Ma, the CaCO3 content of sediments shows stronger fluctuations (between 4% and 30%), but after ~5 Ma the fluctuations became weaker (between 7% and 22%) (Fig. 5c). This change is linked to different environmental conditions; our analysis on the Mazartag Section in- 4.2. Color variations dicates 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 Vertical variations of the color parameters show that the redness sandstones interbedded in the lacustrine sediments (Fig. 2c), indicating (a*) and lightness (L*) exhibit abrupt changes at ~5 Ma (Fig. 5a, b), im- episodic deposition of sand dunes in a temporary dry environment. plying dramatic environmental changes immediately after the Mio- The generally higher values of CaCO3 before ~5 Ma are associated cene-Pliocene boundary. with more authigenic carbonates in a dominantly lacustrine environ- The redness of sediments is associated with the amount of iron ox- ment during the late Miocene. The SEM microstructure analysis of thin ides released during chemical weathering (Walker, 1967). Those oxides sections of the lacustrine strata indicate the presence of very small (2– are usually present in limited amounts in sediments, but play an impor- 4 μm), well-developed, authigenic euhedral calcite crystals (Fig. 6a); tant role in sediment color. Two main iron oxides are responsible of the element composition of the calcites is confirmed by energy

18 13 13 Fig. 5. Multiple climatic proxies indicate dramatic environmental changes at ~5 Ma, (a) redness, (b) lightness, (c) carbonate content, (d) δ Ocarb,(e)δ Ccarb, and (f) δ Corg. The age of ~5 Ma is based on the chronology of Fig. 4. 194 J. Sun et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 485 (2017) 189–200

al., 2014). Moreover, oxidation of sinking organic matter will return 12C to the water column, leading to depleted δ13C of dissolved inorganic carbon (Teranes and Bernasconi, 2005). After ~5 Ma, the input of more detrital carbonates account for the 13 relatively more positive δ Ccarb 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 13 (δ Corg) in the Mazartag Section show remarkable changes at ~5 Ma marked by a shift to more 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; Read et al., 1991, 1992; Ehleringer et al., 1993). Drought stress decreases stomatal conductance, increasing the

ratio of external to internal CO2 partial pressures, leading to a 1–3‰ dif- 13 ference of δ Corg as demonstrated in laboratory and ﬁeld experiments (Farquhar and Richards, 1984; Read et al., 1991, 1992). Although the av- 13 erage δ Corg value of modern C3 plants is −27‰,C3 plants can be 13 13 enriched in C under water-stressed conditions, and δ Corg values Fig. 6. Authigenic carbonates in the late Miocene lacustrine strata in the Mazartag Section. can reach as high as −20‰ in arid environments (Diefendorf et al., a, Scanning electron microscopy images of euhedral calcite crystals indicate an authigenic δ13 origin; b, element compositions of authigenic calcite measured by energy microanalysis. 2010; Kohn, 2010). In this context, the positive shift of Corg in the Tarim Basin can be explained by the effect of aridity on the enrichment

of C3 plants. 13 In addition, the change in δ Corg is also explicable by the worldwide 13 microanalysis (Fig. 6b). The decreased carbonate content after ~5 Ma expansion of C4 grasses, which generally have higher δ C values than coincides with the disappearance of lakes, and with the change to pre- C3 plants (Cerling et al., 1993). However, although modern investiga- dominant ﬂuvial deposits and eolian dune sands in the central Tarim tions suggest that there are up to 72 C4 species in the Tarim Basin (Su Basin; i.e. less authigenic carbonates were precipitated. et al., 2011), their total amount is very limited evidenced by modern vegetation studies (Zhang et al., 2003), and by the current global distri-

bution map of C4 vegetation (Still et al., 2003). The expansion of C4 13 4.4. Stable isotopes of carbonates grasses may therefore have played only a minor role in this δ Corg shift at ~5 Ma. Stable oxygen isotopes are the most widely applied proxies in the study of past climate changes due to their isotopic fractionations during geochemical processes (Leng and Marshall, 2004). High-resolution 5. Discussion 18 13 δ Ocarb and δ Ccarb curves of carbonate also show a distinct change at 18 around 5 Ma (Fig. 5d, e). From approximately 5 Ma, the δ Ocarb values 5.1. Age determinations of the Mazartag Section become more negative (Fig. 5d). 18 Generally, the δ Ocarb values of sedimentary carbonates are affected The chronology of the Mazartag Section was first established by Sun 18 by both detrital and authigenic carbonates. It is well known that δ Ocarb et al. (2009a) who used paleomagnetic polarity correlation, in combina- depends mostly on the isotopic composition of lake water controlled by tion with biostratigraphic age constraints derived from analyses on a changes in precipitation/evaporation ratios (Siegenthaler and Oeschger, mammal fossil (Fig. 7b). The results of the analyses indicate that the 1980; Edwards and Wolfe, 1996). In an inland lake system, the 16Oiso- Mazartag Section has an age range of ~9.7 to 2.6 Ma. topes are preferentially transferred into the vapor phase leaving lake Recently Zheng et al. (2015) re-interpreted the published paleomag- 18 18 water enriched in O, leading to relatively positive δ Ocarb values in netic polarity data of Sun et al. (2009a), and derived a 18 the authigenic carbonates. The higher δ Ocarb values before ~5 Ma magnetostratigraphy indicating that the Mazartag Section has an age were associated with more authigenic carbonates in a predominant la- range of ~34 to 17.5 Ma (Fig. 7c). Based on their age model, Zheng et custrine environment. al. (2015) conclude that sand dune mobilization in the Tarim Basin 13 The δ Ccarb values of carbonates also exhibit a pronounced change and hence the formation of the Taklimakan Desert started as early as 13 at ~5 Ma marked by a shift toward higher values (Fig. 5e). The δ Ccarb 26.7 Ma. values were relatively negative before 5 Ma, probably due to biological However, the re-interpreted age model of Zheng et al. (2015) is in effect in lake waters (Talbot and Kelts, 1990; Barešić et al., 2011). Gen- conflict with the biostratigraphic age constraints as presented by Sun 13 erally, the δ Ccarb in lake sediments mainly reflects the carbon isotope et al. (2009a). In addition, our newly discovered Plesiohipparion fossil evolution of dissolved inorganic carbon (DIC) during the carbon cycle has an age range of 6 to 3.4 Ma. Another mammalian fossil (Olonbulukia in the lake system (Paprocka, 2007). The dominant controls on the car- tsaidamensis) was discovered in 2008 in the lower reddish Miocene bon isotope composition of DIC are CO2 exchange between the atmo- strata, having a biostratigraphic age of 8–9Ma(Deng, 2006)(Fig. 7b). sphere and lake water, photosynthesis/respiration of aquatic The biostratigraphic ages of these mammalian fossils disagrees strongly organisms (Vreča, 2003), and lake stratification and circulation with the re-interpreted age range of ~34 to 17.5 Ma of Zheng et al. (Myrbo and Shapley, 2006). Phytoplankton and bacteria preferentially (2015), but does support the earlier paleomagnetic correlation of Sun incorporate 12C, resulting in biogenic carbonates (e.g., ostracods, mol- et al. (2009a). Therefore, we will use the age model of Sun et al. lusk shells and charophyte algae) with depleted δ13C(de Kluijver et (2009a) for further discussions. J. Sun et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 485 (2017) 189–200 195

Fig. 7. Two different age models of the Mazartag Section. a, The stratigraphic column; b, paleomagnetic time-scale of Sun et al. (2009a), note that the age assignments of the mammal fossils support the paleomagnetic correlation of Sun et al. (2009a); c, the re-interpreted magnetostratigraphy of Zheng et al. (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.

5.2. Basin-wide paleoclimatic correlation in the Tarim Basin paleoclimatic records to conﬁrm the timing and mechanisms of the formation of the Taklimakan Desert. Given the large area of the Tarim Basin and the relatively few studies In recent years, a continuous 1050.6 m-long core was drilled in the in the basin so far, it is important to have more late Cenozoic Lob Nor paleo-lake basin in the eastern Tarim Basin (see Fig. 1afor

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 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. 196 J. Sun et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 485 (2017) 189–200 location). The chronology of the Lob Nor borehole was based on high- (Fig. 9). Apatite and zircon (U/Th)-He ages record an early period of ac- resolution magnetostratigrahy that yielded an age range from 7 Ma to celerated exhumation at ∼50–40 Ma driven by the India-Asia collision in the present (Chang et al., 2012). the Pamir (Amidon and Hynek, 2010). This time is similar to the crustal The high-resolution climate record of the Lob Nor borehole provides thickening in the Pamir at ∼50 Ma (Ducea et al., 2003; Hacker et al., another unique late Cenozoic paleoclimatic archive from the Tarim 2005) and in the western Kunlun at ∼47 Ma (Yin et al., 2002) as well 18 Basin (Liu et al., 2014). The contents and δ Ocarb of carbonates of the as the late Eocene sea-water retreat in the eastern margin of Tajik Mazartag Section are correlated with those of the Lob Nor borehole Basin (Carrapa et al., 2015) and the western Tarim Basin (Bosboom et (Fig. 8). This correlation indicates the two following features. al., 2014a, 2014b; Sun et al., 2016). First, the change of carbonate content of the Mazartag Section corre- During this time interval, the southwest Indian monsoon was al- lates well with that of the Lop Nor borehole in that they both show a ready established in the Eocene (Shukla et al., 2014; Spicer et al., general decrease after ~5 Ma (Fig. 8). This is consistent with the shift 2016), but geologic and paleoaltimetric studies show that parts of the from an episodic lake facies (relatively more authigenic carbonates) to Tibetan Plateau were already elevated at this time (e.g. Kapp et al., a later drier environment in the Tarim Basin. 2005; Dupont-Nivet et al., 2008)(Fig. 9a). This blocked moisture trans- 18 Second, the fluctuating pattern of the δ Ocarb in the Mazartag Sec- port of the Indian monsoon to the Tarim Basin as early as the Eocene ev- tion also correlates with that of the borehole record in Lop Nor (Fig. idenced by δ18O records of central Asia (Caves et al., 2015). 18 8). Additionally, we note that before ~5 Ma, the δ Ocarb values in the Except for the Indian monsoon, another important moisture source Lop Nor borehole are relatively more positive and have greater changes is the water vapor from the Neotethys Ocean (Fig. 9a). Although sea- compared with those of the Mazartag Section (Fig. 8). This suggests that water retreated from the Tarim Basin at ∼40 Ma (Bosboom et al., in the late Miocene, Lop Nor lake was much shallower and probably 2014a, 2014b; Sun et al., 2016), there still existed a large Neotethys dried up temporarily, causing evaporative isotope enrichment relative Ocean to the west of the Tarim Basin (Rögl, 1999; Popov et al., 2004), to the central Tarim Basin record. from which water-vapor was easily carried to the mid-latitude Asian in- We also note that, although the earliest in situ eolian dune sands in terior by westerlies (Caves et al., 2015) through a ~ 300 km-wide water- the Mazartag Section are ~7 Ma old, the sedimentary facies at that time vapor channel between the Pamir and the Tian Shan ranges (Fig. 9a). was dominated by lacustrine deposits with only minor layers of inter- Predominant lacustrine deposits of the Suweiyi Formation were present bedded eolian sands (Fig. 2c), thus implying that there were very limit- in the northern Tarim Basin during the late Eocene (Zhang et al., 2016). ed dune fields in the interior of the basin between 7 and 5 Ma (Sun et al., A later tectonic pulse occurred at 25–18 Ma marked by accelerated 2009a). The multiple high-resolution climatic records of this study, to- northward indentation of the Pamir, considerable crustal shortening, gether with the Lob Nor borehole record, demonstrate that the en- and initiation of the right-lateral Karakoram fault (Lacassin et al., hanced aridity and the extensive desert in the Tarim Basin appeared 2004)(Fig. 9b). On the eastern flank of the Pamir, accelerated tectonic only after ~5 Ma. convergence is recorded at this time by detrital low-temperature Considering the fact that the Lob Nor borehole is about 670 km east thermochronology near the Main Pamir Thrust (MPT) (Sobel and of the Mazartag Section, the good correlation of the high-resolution re- Dumitru, 1997; Bershaw et al., 2012). In the western flank of the cords between the Mazartag Section and the Lop Nor borehole confirms Pamir, basin subsidence curves constructed for the Tajik depression a basin-wide paleoenvironmental change from approximately 5 Ma, show the onset of rapid subsidence at ∼22 Ma, which was likely in re- which shifted from a predominant lake environment to a later drier sponse to tectonic loading (Leith, 1985). Thermochronologic results desert. 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 5.3. Mechanisms accounting for the extreme aridity after ~5 Ma in the Ta- motion and prominent offset in the eastern flank of the Pamir were a re- rim Basin sponse to accelerated northward displacement of the Pamir salient. In addition, at this time the ongoing northward indentation of the The Tarim Basin lies in the rain shadow of the Tibetan Plateau and of Pamir salient (Fig. 9b), together with the climatic effect of global the other central Asian high mountains. The timing of the extremely dry cooling, resulted in the westward retreat of the Paratethys, leading to climate and the birth of the Taklimakan Desert at ~5 Ma has important the decreased moisture transport by the westerlies, which enhanced implications for the role of global climate and regional tectonics. the aridity in the mid-latitude Asian interior in the Oligocene to early Recently, Herbert et al. (2016) reported a global cooling event at Miocene (Guo et al., 2002; Sun et al., 2010; Bosboom et al., 2014a; Sun about 7–5 Ma, and suggested that this was associated with an increased and Windley, 2015; Caves et al., 2016). meridional temperature gradient, linked to increased aridity and eco- However, the presence of widespread paleosols and pedogenic system changes on land. However, in the Tarim Basin, a predominant la- carbonate nodules in the Asian interior (Guo et al., 2002; Sun and custrine environment appears to have remained through this global Windley, 2015; Caves et al., 2016) suggests that the climate was cooling event from at least ~9.7 Ma to ~5 Ma, though there were occa- mostly semi-arid to sub-humid rather than hyperarid. It is generally sionally lake regressions between 7 and 5 Ma. This would suggest that known that the occurrence of pedogenic carbonate is mostly the paleoclimatic changes in the Tarim Basin in the late Neogene may observed in soils where there is enough water for chemical leaching not be due solely to global cooling. The regional tectonic deformation (Deocampo, 2010). Thus, pedogenic carbonates are characteristic of and its consequent climatic impact were also critical influences on the soils where the moisture conditions strongly alternate (Cerling, paleoenvironmental history of the Tarim Basin. 1984; Gocke et al., 2012). At present, the Pamir Plateau forms a prominent tectonic salient Moreover, although the earliest eolian dust is around 22 Ma in the along the western end of the Himalayan-Tibetan orogen separating Chinese Loess Plateau (Guo et al., 2002), there is no evidence to demon- the Tarim and Tajik Basins (Fig. 1a), although they were once connected strate that it was derived from the Tarim Basin. On the contrary, recent (Molnar and Tapponnier, 1975). It has long been known from geological detrital-zircon U-Pb geochronology, heavy-mineral, and bulk-petrogra- surveys that during the Cenozoic era the northern Pamir had indented phy analyses rule out any direct eolian sediment transport from the northwards for ~300 km relative to stable Eurasia (Burtman and Tarim Basin to the Chinese Loess Plateau (Rittner et al., 2016). Molnar, 1993), and geophysical data confirm that the Indian Plate un- 87Sr/86Sr ratios also demonstrate that the Taklimakan Desert in the derthrust the Eurasian crust in the Pamir to a depth of 300–500 km Tarim Basin was not the provenance of the Loess Plateau (Chen et al., (Negredo et al., 2007; Burtman, 2013). 2007). The above results are similar to modern 40-year meteorological The uplift and northward indentation of the Pamir salient is an ongo- observational data, which indicate that the Taklimankan Desert is not ing process, but it underwent at least three pulses of tectonic activity the provenance of the Chinese Loess Plateau (Sun et al., 2001). J. Sun et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 485 (2017) 189–200 197

Fig. 9. Conceptual models showing the progressive tectonic evolutions of the Pamir salient. a, Initial collision at the west end of the Himalayan-Tibetan orogen (∼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 Tarim Basin transported by the westerlies; b, a second episode (~25–18 Ma) was marked by large-scale northward indentation of the Pamir salient and the initiation of the right-lateral Karakoram fault (Lacassin et al., 2004), although the Paratethys had mostly retreated westwards, the presence of the space 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 Tarim Basin; c, ~7–5 Ma, the ongoing indentation resulted in the collision of the 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.

During this period, the corridor between the Pamir and the Tian from the Paratethys to the Tarim Basin. Geological investigations indi- Shan ranges signiﬁcantly narrowed due to the substantial northward in- cate that the early Miocene Jidike Formation in the northern Tarim dentation of the Pamir salient (Fig. 9b). But, there still existed a narrow Basin comprises ﬂuvial-lacustrine deposits (Bureau of Geology and water-vapor channel between them, leading to the moisture transport Mineral Resources of Xinjiang, 1982; Zhang et al., 2016). 198 J. Sun et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 485 (2017) 189–200

A final tectonic episode at ~7–5 Ma is marked by another pulse of chemical weathering (Singer et al., 1995)) of the Kuqa Section shows outward growth of the Pamir and initiation of the Pamir Frontal Thrust a general decreasing trend and a remarkable shift to lower values after (PFT) (Thompson et al., 2015), created by the first collision of the North ~5 Ma (Fig. 10b). This latter shift is consistent with our findings in the Pamir with Tian Shan (Fig. 9c). This is evidenced by considerable Mazartag Section (Fig. 10 a, b). It is interesting that global marine ben- slowing of strike–slip motion along the Kashgar-Yecheng transfer fault thic oxygen isotopes also show a general trend toward more positive system in the Late Miocene-Pliocene (Sobel et al., 2011), the latest Mio- δ18Ovalues(Fig. 10c), which mostly reflect increased ice-volumes cene syntectonic strata in the southern foreland of Tian Shan (Zachos et al., 2001), but the transition from Miocene to Pliocene was (Hubert-Ferrari et al., 2007; Sun et al., 2009b), and the tectonic defor- not abrupt (Fig. 10c). mation in the Pamir–Tian Shan convergence zone at ~5 Ma (Fu et al., The above correlations suggest that the general trend of regional cli- 2010; Thompson et al., 2015). matic aridification in the Tarim Basin was controlled by first-order glob- This latest phase of tectonics in the Pamir had a profound effect on al cooling, whereas the abrupt extreme aridification event in the Tarim the environmental evolution in the Tarim Basin. The collision between Basin since the latest Miocene was greatly enhanced by the closure of a the North Pamir and the Tian Shan ranges resulted in regional tectonic water-vapor pathway caused by the collision of the Pamir salient with uplift in their convergence zone which gradually closed the previous in- the Tian Shan Mountains in response to the India–Eurasia collision. tervening water-vapor pathway, thus effectively blocking water-vapor transport by the westerlies from the upwind Paratethys Ocean to the 6. Conclusions Tarim Basin (Fig. 9c). Moreover, global climate cooling is also evidenced by the initiation Analyses of the Mazartag Section in the central Tarim Basin allows us of increased ice-rafted debris (IRD) in the northern Hemisphere at to reconstruct the paleoenvironmental history and to explore the inter- ~7–5Ma(Jansen and Sjøholm, 1991; St. John and Krissek, 2002). The play between regional tectonic uplift and climate change in an active enlarged global ice sheet resulted in a decline in sea-level that in turn tectonic region. Our new biostratigraphic age control and high-resolu- would lead to considerable shrinkage of the Paratethys at the end of tion multiple climatic records, together with a compilation of geological the Miocene (Popov et al., 2004) and to reduced moisture transport by evidence, support the following conclusions: westerlies to the Tarim Basin. Given the fact that both tectonics and global climate underwent substantial changes over this time period, it is useful to finally compare the (1) A basin-wide lacustrine-fluvial environment prevailed in the regional climatic record of the Tarim Basin with the global marine re- Tarim Basin until the latest Miocene. At this time, there still cord (Fig. 10). existed a water vapor passage between the northern Pamir and The regional paleoclimatic records from the Mazartag and the Kuqa the Tian Shan ranges, and the oceanic moisture transported by Sections in the Tarim Basin (see Fig. 1a for locations) all exhibit abrupt the prevailing westerlies from the Paratethys Ocean were able changes at ~5 Ma (Fig. 10a, b). Moreover, the content of free Fe (iron ex- to reach eastwards to the Tarim Basin. The fact that there were tractable by citrate–bicarbonate–dithionite, reflecting Fe released by few eolian sand dunes between 7 and 5 Ma, could be an indicator

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 5.32–13.3 Ma are from Lisiecki and Raymo (2005) and Zachos et al. (2001), respectively. The data show abrupt changes at ~5 Ma in the Mazartag and Kuqa Sections, and a minor, gradual change in the global marine record, see text for discussion. J. Sun et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 485 (2017) 189–200 199

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