Late Oligocene-early Miocene birth of the Taklimakan Desert

Late Oligocene–early Miocene birth of the Taklimakan Desert Hongbo Zhenga,b,1, Xiaochun Weic,1, Ryuji Tadad, Peter D. Clifte, Bin Wangf, Fred Jourdang, Ping Wanga, and Mengying Hea a School of Geography Science, Nanjing Normal University, Nanjing 210023, China; bCenter for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Sciences, Beijing, 100101, China; cSchool of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China; dDepartment of Earth and Planetary Science, University of Tokyo, Tokyo 113-0033, Japan; eDepartment of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803; fTourism and Environment College, Shaanxi Normal University, Xi’an 710119, China; and gWestern Australian Argon Isotope Facility, Department of Applied Geology and John de Laeter Centre, Curtin University, Perth, WA 6845, Australia Edited by Zhonghe Zhou, Chinese Academy of Sciences, Beijing, China, and approved May 11, 2015 (received for review December 22, 2014) Taklimakan Desert desertification | late Oligocene | early Miocene | tectonic uplift | S urrounded by the Tian Shan to the north, the Pamir to the west, and the West Kunlun to the south, the Taklimakan Desert is the world’s second largest sand sea (Fig. 1). Located far from any major source of moisture, and shadowed by Tibetan and central Asian mountain ranges, the Taklimakan is deprived of rainfall, with mean annual precipitation not exceeding 50 mm (1). Provenance studies suggest that mineral dust from the Taklimakan Desert contributes substantially to the global aerosol system, allowing it to play a significant role in modulating global climate on various time scales (2, 3). The formation of the Taklimakan Desert therefore marked a major environmental event in central Asia during the Cenozoic, with far-reaching impacts. Furthermore, determining when and how the desert formed holds the key to better understanding the nature of tectonic–climatic linkage in this critical region. However, the time at which the Taklimakan Desert came into existence has been strongly debated, with estimates ranging from only a few hundreds of thousands to a few million years ago (1, 4–8). In the context of this study, we define desertification to represent not only the formation of a significant sand sea but also the generation of a dynamic eolian system that supplied voluminous mineral dust on regional and even global scales. Evidence of desertification in the geological past is preserved in sedimentary sequences within, and along the margins of, the present-day Taklimakan Desert that lies within the Tarim Basin (Fig. 1). Recent geochronological work, mostly based on the magnetostratigraphy of these sedimentary sequences, proposed ∼3.4 Ma to 7 Ma for the initiation of the Taklimakan (4–8). Precise dating of these terrestrial rocks has largely been hampered by lack of dateable material. In this study, we report a newly identified volcanic tuff from two sedimentary sections www.pnas.org/cgi/doi/10.1073/pnas.1424487112 along the southwestern margin of the Tarim Basin. Radioisotopic dating of the volcanic minerals provides a robust age for the sections, and therefore we are able to determine the age of the Taklimakan Desert more precisely. Geological Setting and Lithostratigraphy From latest Cretaceous to early Paleogene, much of the western Tarim Basin was influenced episodically by a shallow sea, which was connected to the Paratethys, an epicontinental marine seaway covering large parts of Europe and southern central Asia (9). Shallow marine strata of this age are observed extensively along the western and southwestern margins of the Tarim Basin, extending to Hotan to the east and Ulugqat to the north (Fig. 1). Five major marine incursions have been recognized in the broad region, with the fourth being the final one at Aertashi, where it is represented by the Wulagen Formation (Fig. 2 A and B and SI Text). Recent biostratigraphic work at Aertashi has suggested that the Paratethys Sea finally retreated from this locality at ∼41 Ma (9, 10), a process that has been attributed to global eustasy coupled with tectonism associated with the Indo–Asia collision. Together, these have amplified the aridification of the Asian interior (9, 11). Syntectonic foreland basin deposition followed the marine regression immediately and led to accumulation of thick continental red beds (Fig. 2 A and B). The Bushiblake Formation is characterized by red, fine-gained mudstone and fine sandstone, with common evaporative gypsum, typical of low-energy, meandering river, lacustrine, and playa deposits (SI Text). The overlying Wuqia Group (SI Text) consists of thickly bedded sandstones with Significance The formation of the Taklimakan Desert marked a major geological event in central Asia during the Cenozoic, with farreaching impacts. Deposition of both eolian sand dunes in the basin center and the genetically equivalent loessite along the basin margins provide evidence for the birth of the Taklimakan Desert. This paper resolves a long-standing debate concerning the age of the Taklimakan Desert, specifically whether it dates to ∼3.4–7 Ma, currently the dominant view. Our result shows that the desert came into existence during late Oligocene–early Miocene, between ∼26.7 Ma and 22.6 Ma, as a result of widespread regional aridification and increased erosion in the surrounding mountain fronts, both of which are closely linked to the tectonic uplift of the Tibetan–Pamir Plateau and Tian Shan. Author contributions: H.Z. designed research; H.Z., X.W., R.T., P.D.C., B.W., P.W., and M.H. performed research; X.W. and F.J. analyzed data; and H.Z., X.W., and F.J. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission. 1 To whom correspondence may be addressed. Email: [email protected] or xcwnju@ gmail.com. This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. 1073/pnas.1424487112/-/DCSupplemental. PNAS Early Edition | 1 of 6 EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES As the world’s second largest sand sea and one of the most important dust sources to the global aerosol system, the formation of the Taklimakan Desert marks a major environmental event in central Asia during the Cenozoic. Determining when and how the desert formed holds the key to better understanding the tectonic– climatic linkage in this critical region. However, the age of the Taklimakan remains controversial, with the dominant view being from ∼3.4 Ma to ∼7 Ma based on magnetostratigraphy of sedimentary sequences within and along the margins of the desert. In this study, we applied radioisotopic methods to precisely date a volcanic tuff preserved in the stratigraphy. We constrained the initial desertification to be late Oligocene to early Miocene, between ∼26.7 Ma and 22.6 Ma. We suggest that the Taklimakan Desert was formed as a response to a combination of widespread regional aridification and increased erosion in the surrounding mountain fronts, both of which are closely linked to the tectonic uplift of the Tibetan–Pamir Plateau and Tian Shan, which had reached a climatically sensitive threshold at this time. Tian Shan lp Ka in TB Pamir Tibetan Plateau ra ko West K un ru 36 Thrust fault Section Nomal fault Town m ult lun Sha Mazar 76 Karak 78 ax Fa Riv Ker iya Hotan 8524m 5000m 3000m 1000m 0m n ult Tibetan Pleteau Stike-slip fault 74 Fa Ka rak ax Kekeya er r Yecheng Tashkorgan Ka Hotan River Taklimakan Desert Aertashi ve st Ri ru ax Th Mustagh Pamir Mazatagh Tarim Basin er ir gk m un Pa Kashgar er Riv ain er Riv Loess Gobi & Desert Riv su M 38 CLP QA India zle and Kir Muji B JB Tian Shan Ulugqat Yar k 40 N Yu r A 0km 100km 200km 80 Fig. 1. Location map. (A) Topographic map showing the western portion of the Taklimakan Desert (Tarim Basin) and the surrounding mountain ranges with major active faults. The location of the studied Cenozoic sedimentary sections at Aertashi, Kekeya, and Mazatagh are shown by stars. (B) Map showing the location of the Tarim Basin (TB), Junggar Basin (JB), and Chinese Loess Plateau (CLP). Qin’an loess section (QA) is marked by a red star. minor mudstone. The overall content of sand increases upward, as does the grain size. Lithofacies change to distal alluvial deposits, consisting of conglomerate, sandstone, and siltstone crossing the boundary from the Wuqia Group to the Artux Formation (SI Text). The conglomerate layers in the Artux Formation are thin-bedded debris flow deposits, containing medium-sized, angular to subrounded pebbles, of which more than half are sedimentary rocks. The overlying Xiyu Formation (SI Text) is up to 3 km thick and consists of massive boulder to cobble-grade conglomerate with increasing volumes of igneous and high-grade metamorphic clasts. The Xiyu Formation is typical of proximal diluvial fan deposits derived from unroofed mountain belts. Red beds passing upward into upwardcoarsening conglomerate and debris flow deposits recorded the change in paleoslope and sediment supply related to uplift of the northern margin of the Tibetan Plateau and Pamir. Massive siltstone lenses intercalated in the Artux and Xiyu Formations at Kekeya and many other localities are particularly noteworthy (Fig. 2 C and D). Previous sedimentological studies, including facies investigations and grain size (Fig. S1 A and B) and geochemical analyses, suggest that these siltstone lenses are eolian loessite, having been sourced from the desert, deposited, and preserved on an intermittent diluvial fan system (4), a process resembling that in the Taklimakan Desert today. The modern Taklimakan Desert is surrounded by a series of giant diluvial fans that link the uplifting mountain chains with the Tarim Basin. Sediments shed off the mountain fronts have been eroded, mostly but not exclusively, by landsliding and glaciation. They are then delivered down the slope to the basin through the fan systems, and sorted by eolian and fluvial processes into silt and sand fractions, which become the constituents of loess and desert, respectively. These processes of dust and sand production must have been in operation since the deposition of the Artux and Xiyu formations. We therefore conclude that widespread accumulation of loessite in the Artux and Xiyu formations along the margins of the Taklimakan Desert strongly suggests that the 2 of 6 | www.pnas.org/cgi/doi/10.1073/pnas.1424487112 source area had become fully arid and desertified, and was supplying dust to the proximal region at that time. Direct evidence of desertification is found in the exposed sedimentary sequence at the Mazatagh Range, in the central Taklimakan Desert (Figs. 1 and 2E). The lithostratigraphy at this location has been well established and can be generally correlated to the basin margin sequences (Fig. 2G) (5, 6), except that the lithologies are generally finer and sedimentation rates are lower. Red cross-bedded eolian sandstone (Fig. 2H and Figs. S1 C and D and S2) is present in the red bed unit as isolated sandstone lenses, likely indicating an initial stage of a desertified environment that was composed of low-energy fluvial, playa lakes and associated lunette dunes. The most prominent sedimentary unit in the Mazatagh sequence is the well-developed, cross-bedded, yellowish sandstone (Fig. 2F). It is about 200 m thick and stratigraphically continuous and is interpreted as typical desert sand dune deposits. Deposition of both eolian sand dunes in the basin center and the genetically equivalent loessite along the basin margins provides two lines of evidence to suggest that the Taklimakan Desert came into existence around that time. We have identified volcanic ash in the Aertashi and Kekeya sections (Fig. 2C, Fig. S3, and SI Text), where it is intercalated in the Xiyu Formation, the basal age of which has previously been determined by magnetostratigraphy to be of Plio-Pleistocene age (12). In this study, 40Ar/39Ar and U–Pb dating of the volcanic ash has provided, for the first time, to our knowledge, an anchor point to better constrain the strata, and therefore the age of desertification. Petrologic investigation showed that the composition of the ash includes mainly vitroclastic, sanidine, aegirine–augite, and biotite, typical of alkaline magmatic rocks (Fig. S3). These volcanic minerals are ideal for radioisotopic dating. In addition, we also carried out field investigations together with petrologic studies and found that the volcanic activity associated with the Tashkorgan alkaline complex (13) is most likely responsible for providing the Xiyu Formation ash. Zheng et al. 7 km A Aertashi B Aertashi Kashi G. Marine Continental Gypsum Ash ~11 Ma Mudstone + sandstone km 4 Sandstone Ash ~11 Ma Yellowish eolian sandstone Conglomerate Loess 15 Ma 20 3 Conglomerate Sandstone 25 Conglomerate 2 Brown sandstone 20 Sandstone 25 Loess ~26.7 or 22.6 Ma 30 40 Sandstone 1 35 Mudstone Sandstone Gypsum Kashi G. Marine 0 Final sea retreat ~41 Ma Marl Mudstone Gypsum 40 Wuqia Group Bashibulak F. Delta + playa lake + fluvial 35 Mudstone 30 Kashi G. Marine 30 Bashibulak F. Wuqia Group Delta + playa lake + fluvial 2 35 1 Mudstones km 2 Kashi G Wuqia Group Atux Formation Marine Fluvial + playa Desert +fluvial 3 GTS2012 G Mazatagh Mudstone Atux Formation Distal diluvial fan Atux Formation Distal alluvial fan Braided +Meandering river 15 Taklimakan Desert F Mazatagh 4 15 25 Yellowish eolian sandstone Xiyu Formation Proximal diluvial fan Ma 10 E Red eolian sandstone Volcanic ash D Kekeya 5 20 Conglomerate Loess Conglomerate E Mazatagh range Conglomerate Yellowish-grey sand, not well exposed 1 0 Yellowish eolian sand Mudstone Red eolian sand ~26 Ma Mudstone Sandstone Gypsum Marl Mudstone Gypsum H Mazatagh Sandstone 0 Red eolian sandstone Mudstone Sandstone Gypsum Marl Mudstone Gypsum Mudstone Gypsum Siltstone Tuff Sandstone Shell Conglomerate Eolian loess Marl Eolian sand Fig. 2. Stratigraphy and magnetostratigraphy of Cenozoic sedimentary sequences within and along the southwestern margin of the Tarim Basin (Taklimakan Desert). The magnetostratigraphy of all sections was correlated to GTS2012 (19). (A) Stratigraphy and magnetostratigraphy of the Aertashi section based on age control from volcanic ash and paleontology (9). (B) Shallow marine deposits and continental red beds at Aertashi. (C) Volcanic ash and loessite intercalated in Xiyu conglomerate at Kekeya section. (D) Stratigraphy and revised magnetostratigraphy for the Kekeya section based on age control of the volcanic ash combined with data from ref. 12. The solid line and dotted line indicate two alternative magnetostratigraphic correlations for this section (see SI Text for more information). (E) Exposed Mazatagh section. (F ) Yellowish desert sandstone at Mazatagh section. (G) Stratigraphy and revised magnetostratigraphy based on ref. 6 in the context of regional stratigraphic correlation. (H) Red eolian sandstone within the red beds at Mazatagh. See Fig. 1 for locations. 40 Ar/39Ar Dating of Biotite from Volcanic Ash We selected three samples (YC10-19, AT10-34, and AT10-35) from two locations (Kekeya section and Aertashi section) for 40Ar/39Ar dating and separated unaltered, 200- to 800-μm-size biotite. These Zheng et al. minerals were separated using a Frantz magnetic separator, and then carefully handpicked under a binocular microscope. The selected biotite crystals of sample AT10-35 were classified according to their color, with green, red, and brown populations. PNAS Early Edition | 3 of 6 EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES 6 GTS2012 Ma 10 C Kekeya Xiyu F. Xiyu Formation Proximal alluvial fan GTS2012 (ref. 19) Marl + limestone + sandstone U–Pb Dating of Zircon from Volcanic Ash Sample YC12-13-5 was taken from Kekeya section. It was the only sample in this study that contained suitable zircon grains for U–Pb dating. Zircon grains were separated using traditional flotation method, and then carefully handpicked under a binocular microscope to obtain optically clear, colorless grains. Together with zircon standard Plésovice (337 Ma) (14), the zircon grains from sample YC12-13-5 were mounted onto a 2.4-cm-diameter epoxy disk, ground, and then polished for analysis. To reveal the internal structure, all zircons were imaged using 4 of 6 | www.pnas.org/cgi/doi/10.1073/pnas.1424487112 transmitted, reflected light, as well as cathodeluminescence. The mount was vacuum-coated with high-purity gold before analysis with secondary ion mass spectrometry (SIMS). U, Th, and Pb were measured using a Cameca IMS-1280 SIMS at the Institute of Geology and Geophysics, Chinese Academy of Sciences, in Beijing. Methods used to obtain the U–Pb analytical data were similar to those developed by Li et al. (15). The standard zircon Plésovice (14) was used to determine U–Pb ratios and absolute abundances. The sample was analyzed in a continuous analytical session with standards interspersed with every four to five unknowns. Corrections for common Pb in this paper were made using measured 204Pb and an age-appropriate Pb isotopic composition of Stacey and Kramers (16). Data processing was carried out using the Isoplot 4.15 program (17). Twenty-five spots of 25 zircon grains from sample YC12-13-5 were measured during a single analytical session, and the data are reported in Dataset S4, with individual errors given at 1σ level. The analysis results showed relatively high Th/U ratios (0.391–1.903), with U contents ranging from 248 ppm to 2619 ppm. Common Pb was low for the majority of analyses, with f206 values (the proportion of common 206Pb in total measured 206Pb) lower than 4.23%, apart from spot 15, which yielded a value of 28.89%. Since the ages of zircons are quite young, we looked for 206 Pb/238U age convergence of the youngest population as the tuff eruption age. Twenty out of 25 zircon grains yielded a homogenous age population with a weighted mean age of 11.18 ± 0.11 Ma (95% confidence interval, MSWD = 1.18, P = 0.27; Fig. S5A), while the other five, which could be xenocrystic zircons, produced 12.4 ± 0.2 Ma, 16.3 ± 0.6 Ma, 28.7 ± 0.6 Ma, 207.0 ± 3.1 Ma, 30 40 Oligocene 20 Onset of desertification of Asian interior Pli. Plt. Onset of Chinese loess 10 Miocene 0 (Ma) Detrital contribution (%) 20 40 60 0 80 100 Eocene The 40Ar/39Ar analyses were performed at the Western Australian Argon Isotope Facility at Curtin University. Due to the possibility of crystal xenocrysts in the volcanic tuff layers, each biotite crystal was fused in one step using a 110-W Spectron Laser System, with a continuous laser rastered over the sample during 1 min to ensure complete melting (see SI Text for more detail). Ar isotopic data corrected for blank, mass discrimination and radioactive decay are given in Datasets S1–S3. Individual errors in Datasets S1–S3 are given at the 1σ level. Since the crystals in a single tuff can yield vastly different ages and we did not step heat the samples, we did not apply the standard plateau age criteria. Rather, we looked for age convergence of the youngest population with a minimum of four grains yielding indistinguishable age. This procedure is similar to the approach used for individual U–Pb zircon age to calculate a magmatic age. All sources of uncertainties are included in the final age proposed for a given sample. Eighteen crystals were analyzed for sample YC10-19. Fig. S4A shows a continuum of age from ∼11 Ma to 13 Ma. The six youngest ages yielded a concordant age population with a weighted average age of 11.17 ± 0.15 Ma (mean square weighted deviation (MSWD) = 1.2, possibility (P) = 0.33), interpreted as the age of the youngest eruption bearing the crystals. Those results also indicate the occurrence of older eruptions, possibly of up to 13 Ma, whose crystals have been entrained during the youngest eruption at ∼11.2 Ma; however, since this could be due to minute excess 40Ar* incorporated in the crystal, we refrain from making assumptions about the magmatic activity for the three samples. For sample AT10-34 (from the upper layer), we analyzed 14 biotite crystals. This sample includes two distinct age groups, with the oldest group (n = 7) having ages ranging from 120 Ma to 180 Ma and whose significance is beyond the scope of this paper. The youngest group (n = 7) shows a range of age (Fig. S4B), with the four youngest grains giving a concordant age population with a weighted mean age of 10.94 ± 0.20 Ma (MSWD = 0.03, P = 0.99) (Fig. S4C). This age is interpreted as the eruption age of the ash layer from which AT10-34 is derived. The last sample, AT10-35, consists of 21 crystals, and 40Ar/39Ar results show that this sample also contains crystals from much older events (Fig. S4D). Apparent ages of the older group (all red or brown crystals) range from 114 Ma to 214 Ma. Five crystals from this group yield a homogenous age population with a weighted mean age of 189.9 ± 1.4 Ma (MSWD = 1.4, P = 0.24) likely to date a Jurassic magmatic event in the region. The youngest population (n = 13) yields a range of ages from 17 Ma to 10 Ma, suggesting episodic activity during this period. The 10 youngest grains belong mostly to the green group and yield a concordant age population with a weighted mean age of 11.49 ± 0.34 Ma (MSWD = 0.70, P = 0.71) (Fig. S4E). However, it should be noted that most of these apparent ages have large age uncertainties due to the fact that the young population consists mostly of small grains that yielded a small argon beam signal. The age of 11.49 Ma is interpreted as the age of the tuff eruption. All three tuff ages are within 0.5 Ma of each other, suggesting that explosive volcanism occurred in this region over a wide area at ∼11 Ma. Since the three ages are not concordant (MSWD = 4.2), this suggests that the volcanic activity lasted for a duration of at least several hundred thousands years. Detrital Eolian quartz 0 5 10 15 20 Eolian quartz (% by weight) Fig. 3. Detrital contribution (blue dots) of core from Site 1215 and aeolian quartz percentage (green dots) of core LL44-GPC3 from the North Pacific since the Eocene (22, 24). The arrows on the right side indicate the onset of eolian loess at the Chinese Loess Plateau (20, 21) and desertification of Asian interior obtained by this study. Zheng et al. modern mineral dust suggest that the Taklimakan Desert is one of the major dust source areas to the global aerosol system (2, 3), and might have been so since the birth of the desert. In this regard, it is worth noting that the onset of loess deposition at Qin’an in the Chinese Loess Plateau (20, 21) (see Fig. 1B for location), and change in the dust to the distal north Pacific Ocean (22–24) as shown by a significant increase in the detrital contribution and quartz content, occurred at about the same time (Fig. 3). Formation of a desert over tectonic time scales requires two integral prerequisite conditions: an arid climate and a sufficient supply of fine-grained sediments. During much of the Paleogene, a roughly zonal region of China between 30°N and 50°N was under an arid climate, primarily controlled by the northern westerlies (25). The Paleogene aridity of the Tarim Basin, which was part of this arid region, is registered by widespread accumulation of red beds and intercalated evaporite. However, the climatic configuration changed dramatically during the period from late Oligocene to early Miocene, after which the Asian monsoon set in, prevailing in southeastern China, and the arid region with greatest severity retreated to the northwest. Indo–Asia collision likely starting at ∼60 Ma resulted in the progressive uplift of the Tibetan Plateau and other mountain ranges in Central Asia (26–29). These may have grown to heights of more than 3 km by the late Oligocene (30, 31), even if the extent of uplift has since continued to grow. Generation of such topography forced major climatic changes during the Cenozoic (32–35). Numerous studies suggested that significant uplift of the Tibetan–Pamir Plateau and the Tian Shan occurred around the late Oligocene–early Miocene (36–39). The uplifted plateau together with the possible retreat of the Paratethys (9, 11) forced by uplifting topography could have led to a transition from planetary climate system to monsoonal climate system (35, 40, 41). At the same time, the arid zone, with much severity, retreated to the Asian interior (25). Equally, if not more, important, is the upliftinduced erosion and weathering of bedrock, which supplied great volumes of sediments to the mountain fronts in the form of alluvial and diluvial fans. These sediments would then have been sorted by fluvial and eolian processes into sand and silt fractions, with the former becoming the constituents of a dynamic desert system and the latter being deflated and transported as eolian dust (4). We suggest that this mechanism has been in operation since the late Oligocene–early Miocene time, and that the resultant formation of the Taklimakan Desert was a direct response to a combination of widespread regional aridification and increased unroofing and erosion in the surrounding mountains, both of which are closely linked to the uplift of the Tibetan–Pamir Plateau and the Tian Shan (36–39). Birth of the Taklimakan Desert: When and How? Constrained by the volcanic ash and regional lithostratigraphic correlations, we are able to reconstruct a magnetostratigraphy for the Cenozoic sequence based on previously published data from Kekeya (12) and Mazatagh (6), as well as new measurements from Aertashi (Fig. 2 A, D, and G). All paleomagnetic samples were progressively demagnetized from room temperature up to 600–695 °C in 5–100 °C steps. Remanent magnetizations were measured using a 2G 755-R Superconducting Rock Magnetometer. The characteristic remnant magnetization (ChRM) directions were assessed on an orthogonal demagnetization diagram and calculated by application of principal component analysis to determine the direction of the best leastsquares line fit (18) (Fig. S7 and SI Text). The Aertashi section comprises a relatively complete succession of Cenozoic sediments. More importantly, the section is well constrained by the volcanic ash in the upper part and biostratigraphy at the base (10), and therefore can be used as a reference for regional correlations. The pattern of magnetic polarity zones of Aertashi can be correlated to the Geologic Time Scale 2012 (GTS2012) (19) (Fig. S8, SI Text, and Dataset S5). Two depositional hiatuses occurred when lithofacies changed to massive sandstone and conglomerate, respectively. Geomagnetic correlations to GTS2012 of the Kekeya (Fig. S9, SI Text, and Dataset S6) and Mazatagh (Fig. S10 and SI Text) sections, based on stratigraphic correlations to the Aertashi section, yielded age ranges from 37.5 Ma to 10 Ma and 34 Ma to 17.5 Ma, respectively. Of greatest significance is the timing of the transition from fluvial deposit to diluvial fan debris flow with eolian silts at Kekeya, and red eolian dunes at Mazatagh, which is, on our updated magnetostratigraphy, dated to be ∼26.7–22.6 Ma (Figs. S9 and S10 and SI Text). We therefore argue that, by late Oligocene to early Miocene time, the Tarim Basin, surrounded by a rising Tibetan– Pamir Plateau and Tian Shan, had become fully arid and desertified, supplying dust to the mountain fronts, where it accumulated as loess. Desertification of the Asian interior has had far-reaching impacts on regional and even global scales. Provenance studies of ACKNOWLEDGMENTS. The authors are grateful to the reviewers for their constructive comments. H.Z. is indebted to the late Prof. C. Powell and the late Prof. X. Wu for their contributions in the field work and analysis of sedimentological data. This work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB03020300), the National Science Foundation of China (NSFC 40025207, 90211019, and 41021002), and the Priority Academic Program Development of Jiangsu Higher Education Institutions. 1. Zhu ZD, Wu Z, Liu S, Di X (1980) An Outline of Chinese Deserts (Science Press, Beijing). 2. Uno I, et al. (2009) Asian dust transported one full circuit around the globe. Nat Geosci 2(8):557–560. 3. Shao YP, et al. (2011) Dust cycle: An emerging core theme in Earth system science. Aeolian Res 2(4):181–204. 4. Zheng HB, Powell CM, Butcher K, Cao JJ (2003) Late Neogene loess deposition in southern Tarim Basin: Tectonic and palaeoenvironmental implications. 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The age of 11.18 Ma is interpreted as the eruption age of the tuff layer from which YC12-13-5 is derived. The 206Pb/238U age of 11.18 Ma of sample YC12-13-5 is very close to the 40Ar/39Ar age of biotite (11.17 Ma) of sample YC1019. The age difference could have resulted from different eruptions. However, we believe that this minor discrepancy is more likely to have been caused by the difference of closure temperatures of the two types of minerals or age uncertainties. We therefore conclude that the volcanic ash was deposited soon after the eruption that occurred at ∼11 Ma in this region. 14. Slama J, et al. (2008) Plesovice zircon - A new natural reference material for U-Pb and Hf isotopic microanalysis. Chem Geol 249(1-2):1–35. 15. Li XH, Liu Y, Li QL, Guo CH, Chamberlain KR (2009) Precise determination of Phanerozoic zircon Pb/Pb age by multicollector SIMS without external standardization. Geochem Geophys Geosyst 10(4):Q04010. 16. 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