Erosion in northwest Tibet from in-situ-produced cosmogenic 10 Be and 26 Al in bedrock

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1 Earth Surface Processes and Landforms 116 Earth Surf. Process. Landforms 32, (2007) Ping Kong et al. Published online 17 May 2006 in Wiley InterScience ( Erosion in northwest Tibet from in-situ-produced cosmogenic 10 Be and 26 Al in bedrock Ping Kong, 1,2, * Chunguang Na, 1 David Fink, 3 Lin Ding 2 and Feixin Huang 2 1 State Key Laboratory of Lithosphere Tectonic Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, PO Box 9825, Beijing , China 2 Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing , China 3 ANSTO-Environment, Australian Nuclear Science and Technology Organisation, Menai, NSW 2234, Australia *Correspondence to: Ping Kong, State Key Laboratory of Lithosphere Tectonic Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, PO Box 9825, Beijing , China pingkong@mail.igcas.ac.cn Received 5 September 2005; Revised 25 January 2006; Accepted 13 March 2006 Abstract Concentrations of in-situ-produced cosmogenic nuclides 10 Be and 26 Al in quartz were measured by accelerator mass spectrometry for bedrock basalts and sandstones located in northwest Tibet. The effective exposure ages range between 23 and 134 ka ( 10 Be) and erosion rates between 4 0 and 24 mm ka 1. The erosion rates are significantly higher than those in similarly arid Antarctica and Australia, ranging between 0 1 and 1 mm ka 1, suggesting that precipitation is not the major control of erosion of landforms. Comparison of erosion rates in arid regions with contrasting tectonic activities suggests that tectonic activity plays a more important role in controlling long-term erosion rates. The obtained erosion rates are, however, significantly lower than the denudation rate of mm ka 1 beginning at c. 5-3 Ma in the nearby Godwin Austen (K2) determined by apatite fission-track thermochronology. It appears that the difference in erosion rates within different time intervals is indicative of increased tectonic activity at c. 5 3 Ma in northwest Tibet. We explain the low erosion rates determined in this study as reflecting reduced tectonic activity in the last million years. A model of localized thinning of the mantle beneath northwest Tibet may account for the sudden increased tectonic activity at c. 5 3 Ma and the later decrease. Copyright 2006 John Wiley & Sons, Ltd. Keywords: erosion rate; cosmogenic nuclide; northwest Tibet; uplift; tectonics Introduction The Himalayas and Tibetan plateau form the largest mountain mass on Earth, stretching over 2500 km east west from Burma to Afghanistan and over 1500 km north south from the deserts of central China to the Indo-Gangetic plain, and with an average elevation of 5000 m. The uplift of this vast area of high ground exerts an important influence on regional and global climate (Molnar and England, 1990; Harrison et al., 1998; An et al., 2001). Several lines of observations have been made to infer the uplift history of the Tibetan plateau. The occurrences of north south trending rifts in south and middle Tibet and volcanism in northern Tibet were believed to indicate that the Tibetan plateau reached its present altitude 8 Ma ago or earlier (Harrison et al., 1995; Turner et al., 1993). However, as north south rifts and late Cenozoic volcanism of similar ages are widespread in Asia, Yin and Harrison (2000) suggested that their occurrences may not only be related to the evolution of the Tibetan plateau. The abrupt increase of erosion and sedimentation rates and in grain sizes of sediments in northwest Tibet (Foster et al., 1994; Zheng et al., 2000), normally considered to be the results of mountain uplift, were argued to have resulted from Quaternary climate changes (Zhang et al., 2001; Wang et al., 2003). Distinguishing between tectonic and climatic modification of erosion rates has proved difficult. A more complicated model suggested that the rise of the Tibetan plateau probably occurred in three main steps, by successive growth and uplift of 300- to 500-km-wide crustal thrust-wedges (Tapponnier et al., 2001). The accelerator mass spectrometry (AMS)-based cosmogenic nuclide technique is an increasingly utilized method that can measure long-term (> years) average erosion rates (Lal, 1991). Determination of in-situ 10 Be and 26 Al

2 Erosion in northwest Tibet 117 in bedrock surfaces on summit flats constrains erosion rate to a narrow range of c mm ka 1 for mountain ranges within various climatic environments (Small et al., 1997), except for Antarctica and Australia where erosion rates are much lower, at m Ma 1 (Nishiizumi et al., 1991; Bierman and Turner, 1995; Bierman and Caffee, 2002). The very low erosion rates in Antarctica and Australia have been attributed to the extreme aridity of these regions. It is likely that studies of erosion rates in different geomorphic settings and across various temporal and spatial scales may help towards a better understanding of the climatic and tectonic effects on erosion of mountain ranges, and further distinguish between tectonic and climatic modification of erosion rates. The interior of the Tibetan plateau is another semi-arid to arid region (Wei and Gasse, 1999). Studies of 10 Be in central Tibet, however, give erosion rates in the range 3 29 mm ka 1 (Lal et al., 2003), much higher than those in Antarctica and Australia. Northwest Tibet is the most arid region in the Tibetan plateau, whereas exhumation rates constrained by the apatite-fission track technique are extremely high for the period 3 5 Ma ago (Foster et al., 1994). To compare erosion rates with central Tibet and similarly arid Antarctica and Australia, we have collected samples and measured in-situ 10 Be and 26 Al in bedrock surfaces in northwest Tibet. We hope to better understand the effects of climate and tectonics on erosion rate and especially to provide hints on the cause of the abrupt increase of erosion and sedimentation rates in northwest Tibet in the middle to late Pliocene. Geological Setting The Tibetan plateau shows a marked gradient in precipitation due to the decreasing influence of the Indian monsoon from east to west (Wei and Gasse, 1999). The studied area is the coldest and driest part of the Tibetan Qinghai plateau, located in the rain shadow of two high mountain ranges the Karakoram and the Kunlun (Figure 1). Annual precipitation of this area is 50 mm and evaporation/precipitation ratios range between 20 and 50 (Van Campo and Gasse, 1993). Because of the aridity, snowlines in this area are at about 6000 m. All samples we collected are from the bare bedrock surface of summit flats. No vegetation covers these flats. Among published studies of bedrock erosion rates using cosmogenic nuclides, there are very few data related to the rate of basalt erosion. This study focuses especially on the study of erosion of basalts in order to enhance comparison of erosion rates between various rock types. Samples 02KA01, 02KB01 and 02KB02 are taken from the summits of three basic volcanic cones containing quartz xenocrysts, dated Ma (Figure 2a,b). Samples 02KE08 02KE12 are from the surface of a lava mesa, 220 km 2 in size (Figure 2c). The K-Ar ages we obtained for the lava mesa range between 4 9 and 5 5 Ma, consistent with those reported in the literature (Arnaud et al., 1992). Quartz exists as xenocrysts in the lava. Although samples 02KE08 12 were taken from outcrop, we could see detritus nearby covering the mesa. Samples 02KE26 and 02KE27 are from quartz veins in Tertiary sandstone (Figure 2d). All samples are located above 5000 m over sea level. Studies of Quaternary glacial histories around this area have given controversial results: some believe there was limited ice cover (Schäfer et al., 2002; Shi, 2002), whereas others suggest an ice sheet covering the entire plateau and its flanks during the global Late Glacial Maximum (LGM) (Kuhle, 1998). Cosmogenic Nuclide Dating Theory Any material exposed to cosmic rays will inevitably contain small amounts of radioactive and stable nuclides produced by nuclear interactions between high energy cosmic ray particles and the target nuclei in the material. The amounts of cosmogenic radionuclides (for example, 10 Be, 26 Al) are a function of exposure time and erosion rate (Lal, 1991), and can be expressed as: P ( λ+ ρε/ Λ) T λt N = [ 1 e ] + N0 e (1) ( λ + ρε/ Λ) where N is the concentration of cosmogenic nuclides (atoms g 1 ), P is production rate (atoms g 1 a 1 ), T is exposure age (years), ε is erosion rate (cm a 1 ), λ is the decay constant of the nuclide, ρ is the rock density (g cm 3 ), and Λ is the exponential absorption depth dependence of cosmogenic nuclide production in rocks (g m 2 ). N 0 represents any initial concentration and is zero for a sample that has no inherited concentration. The equation is typically used to calculate either exposure age or erosion rate. If a surface has not been eroded since exposure (ε = 0) and does not contain inherited cosmogenic radionuclides (N 0 = 0), Equation 1 becomes:

3 118 Ping Kong et al. Figure 1. Map of studied area. The area is the coldest and driest part of the Tibetan Qinghai plateau, located in the rain shadow of two high mountain ranges, the Karakoram and the Kunlun. N = P(1 e λt )/λ (2) from which we can calculate the exposure age T. For a surface on which steady erosion has persisted for long enough (T = ) that the surface cosmogenic nuclide concentration has reached the steady-state value, Equation 1 can be written as: N = P/(λ + ρε/λ) or ε =Λ(P/N λ)/ρ (3) The erosion rate ε is thus calculated from measured radionuclide concentrations. To make Equation 2 or 3 valid, two other conditions must be satisfied: (i) the surface production rate is constant through time; and (ii) the surface is continuously exposed to the cosmic-ray flux and has not been buried after exposure. For many actual geological cases, however, it is hard to judge from field observations whether assumptions behind equations are met or not. For these cases, calculated exposure ages are minimal and erosion rates are maximal.

4 Erosion in northwest Tibet 119 Figure 2. Sampling sites. (a, b) Samples 02KA01, 02KB01 and 02KB02 are taken from the summits of three basic volcanic cones containing quartz xenocrysts. (c) Samples 02KE08 02KE12 are from the surface of a lava mesa where quartz exists as xenocrysts. (d) Samples 02KE26 and 02KE27 are from quartz veins in Tertiary sandstone. This figure is available in colour online at Sample Preparation and Results Chemical preparations were carried out in the cosmogenic nuclide laboratory in the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing. Samples were first crushed to mm size, then a magnetic separation step was applied. Meteoric 10 Be was removed by four to five ultrasonic leachings at 80 C with a mixed solution of dilute HF and HNO 3 (Kohl and Nishiizumi, 1992). Pure quartz samples were completely dissolved together with addition of c. 0 8 mg 9 Be and c. 0 5 mg 27 Al carriers. Be and Al were separated by ion chromatography, and their hydroxides were precipitated, then baked to oxide at 850 C. Total Al concentrations in aliquots of the dissolved quartz were quantified by ICP-OES, and 10 Be and 26 Al concentrations were measured by the AMS facility at the Australian Nuclear Science and Technology Organisation (ANSTO). Measured ratios of 10 Be/ 9 Be were normalized relative to the NIST standard SRM4325. As controversy persists with regards to the accuracy of the quoted NIST ratio (Middleton et al., 1993; Fink et al., 2000), we assign a nominal ratio of to SRM4325 obtained by correcting the NIST certified value ( ± 2 61 per cent) upwards by 1 127, a correction factor derived from the ratio of the accepted 10 Be half-life of 1 51 ± 0 06 Ma and the half-life of 1 34 ± 0 07 Ma as quoted by NIST. To ensure self-consistency, we converted all 10 Be concentrations to exposure ages and erosion rates using the former 10 Be half-life value. The 10 Be/ 9 Be and 26 Al/ 27 Al ratios of the chemical procedure blanks are at levels of (5 8) and , respectively. These values were used to correct the measured ratios for the samples. The measured 10 Be and 26 Al concentrations in quartz from bedrock samples are listed in Table I. To calculate minimum exposure ages and maximum erosion rates using Equations 2 and 3, we have to know production rates of 10 Be and 26 Al in sample sites. Production rates of cosmogenic nuclides at the Earth s surface vary with latitude and elevation. Minimum exposure ages are calculated using the scaling method of Stone (2000), assuming 2 5 per cent production by muons at sea level. The production rates we used for high latitude and sea level are 5 1 atoms g 1 a 1 and 31 1 atoms g 1 a 1 for 10 Be and 26 Al, respectively. Production rates calculated using a different scaling method of

5 120 Ping Kong et al. Table I. 10 Be and 26 Al data for bedrock samples at the surface in northwest Tibet, and calculated minimum exposure ages and erosion rates* 10 Altitude Latitude Longitude Be conc. 26 Al conc. 10 Be min. 26 Al min. Max. erosion Sample Rock type (m) (N) (E) (10 6 atoms g 1 ) (10 6 atoms g 1 ) 26 Al/ 10 Be exp. age (ka) exp. age (ka) rate (mm ka 1 ) 02KA01 basaltic andesite ± ± ± ± 5 33± 4 17± 3 02KB01 basaltic andesite ± ± ± ± 5 31± 3 18± 3 02KB02 basaltic andesite ± ± ± ± 7 16± 3 24± 7 02KE08 basalt ± ± ± ± 4 33± 3 14± 1 02KE09 basalt ± ± ± ± ± ± KE10 basalt ± ± ± ± 4 41± 3 11± 1 02KE11 basalt ± ± ± ± 5 33± 4 11± 1 02KE12 basalt ± ± ± ± ± ± KE26 sandstone ± ± ± ± ± ± KE27 sandstone ± ± ± ± 4 39± 3 15± 2 * The 10 Be and 26 Al exposure ages are calculated using scaling factors from Stone (2000). We use rock density of 2 7 gcm 3 and attenuate length of 150 g cm 2 in calculation of erosion rates. The errors with exposure ages also include 6 per cent from production rate, 1 per cent from Be carrier and 2 per cent from ICP-AES for Al. As erosion rates deduced from 10 Be and 26 Al concentrations are similar, here only those deduced from 10 Be data are given.

6 Erosion in northwest Tibet 121 Figure 3. Plot of 26 Al/ 10 Be versus 10 Be concentrations. Samples 02KA01, 02KB01 and 02KE27 are located within the erosion island suggesting constant exposure. The obvious offsets of 02KE10, 02KE11 and 02KE26 from the erosion island indicate complex exposure histories of these samples. Dunai (2000) are c. 17 per cent larger than those calculated from Stone (2000) at our sample sites and yield correspondingly younger exposure ages and larger erosion rates. According to Dunai (2001), geomagnetic corrections of the production rates may range between 1 02 and As erosion is occurring, it is difficult to assign an age for accurate geomagnetic correction. Thus, we did not make this correction and the erosion rates shown in Table I may be up to 10 per cent larger due to geomagnetic field effects. A two-isotope plot of 10 Be and 26 Al data shows that samples 02KA01, 02KB01 and 02KE27 are consistent with a model of constant exposure (Figure 3). Some samples (02KE10, 02KE11 and 02KE26) may have complex exposure histories, suggesting burial and re-exposure. As the samples are located at very high altitude (over 5000 m), a likely covering material is snow/ice. But we could not identify any relationship of burial with altitudes of samples. For example, samples 02KE26 and 02KE27 are from the same sandstone dome, not far (tens of metres) from each other. However, 02KE27 exhibits continuous exposure, but 02KE26 has a complex exposure history. It seems that the more likely covering material is detritus which was removed later. In such a case the erosion rates derived from samples with simple exposure histories would be more accurate. However, the erosion rates calculated from samples with simple and complex exposure histories do not show obvious differences in this study. The effect of any material cover will be to reduce the production of isotopes at the rock surface which would lead to a younger exposure age and a larger apparent rate of erosion. The erosion rates shown in Table I range between 4 0 and 24 mm ka 1, with an average of 12 3 ± 6 7 mm ka 1, very similar to the range of mm ka 1 (average 9 ± 8mmka 1 ) obtained for the interior of the Tibetan plateau (Lal et al., 2003). These erosion rates are also comparable with bedrock from other non-arid climatic regimes deduced from cosmogenic data (Small et al., 1997), but significantly higher than those of arid Antarctica and semi-arid Australia (Nishiizumi et al., 1991; Bierman and Turner, 1995; Bierman and Caffee, 2002). From Table I it is seen that bedrock erosion rates do not exhibit differences with lithology, consistent with previous results (Small et al., 1997; Lal et al., 2003). Discussion A basic condition for obtaining exposure ages and erosion rates is that the surface sample has not been buried since exposure. As all samples we collected are located above 5000 m, snow/ice cover during past ice ages is a critical concern of this work. Studies of Quaternary glacial histories around this area gave controversial results. One class of opinion believed there was limited snowline decrease, less than 300 m, due to the extreme aridity of this area (Shi, 2002), whereas the other class suggests an ice sheet covering the entire plateau and its flanks during the LGM (Kuhle,

7 122 Ping Kong et al. 1998). Recently, Owen et al. (2002) reviewed current knowledge about the timing of the late Quaternary glaciation in seven regions of the Himalayas, and concluded that glaciers had advanced only several tens of kilometres from the present ice front during the LGM. As the extent of snowline depression during the LGM is still in dispute, we first consider the possibility that our samples had been covered by glacial ice during the past glacial periods. If ice had covered the samples we collected during the ice age and reset or partly reset the cosmogenic nuclide clock, the obtained nuclide acitivities most likely reflect the time of deglaciation rather than erosion rates. The actual erosion rates would be smaller than those given in Table I. The other possibility is that ice had not covered the samples we collected during past ice ages. Since the exposure ages shown in Table I are several orders of magnitude smaller than the age of the erosional surface of the plateau, erosion should have reached the steady state. In such a case, the obtained nuclide activities are controlled by steadystate erosion, and the calculated maximum erosion rates should be close to steady-state erosion rates. The obtained average erosion rate for northwest Tibet is 12 3 ± 6 7 mm ka 1, similar to the value of 9 ± 8mmka 1 for the interior of the Tibetan plateau (Lal et al., 2003). This similarity may suggest an identical control of erosion rates for the two regions. These erosion rates are comparable to those with bedrock from non-arid climatic regimes (Small et al., 1997), but significantly higher than those of arid Antarctica and semi-arid Australia. Table II compares erosion rates obtained by the cosmogenic nuclide dating technique for extremely arid regions. While bedrock erosion rates in Antarctica, Australia and the Namib Desert are very low, at mm ka 1, the erosion rates in Yuma Wash, Arizona (Clapp et al., 2002) and Nahal Yael, Israel (Clapp et al., 2000) are much higher, c. 30 mm ka 1. Obviously the low erosion rates in Antarctica, Australia and the Namib Desert cannot be solely attributed to the lack of water or low annual precipitation as previously suggested (Nishiizumi et al., 1991; Bierman and Turner, 1995; Bierman and Caffee, 2002). By studying erosion rates in Sierra Nevada, California, with a temperate climate (annual precipitation mm a 1 ; annual temperature 4 15 C), Riebe et al. (2001a) reached the conclusion that climate shifts may not noticeably affect non-glacial erosion rates in mountainous granitic terrain. Besides climate, tectonics is another important control of erosion of landforms. Interestingly it appears that arid regions with high erosion rates are all tectonically active (Table II). This may suggest that tectonic activity plays a more important role in controlling erosion of landforms. Riebe et al. (2001b) and Bierman and Nichols (2004) noticed the significance of tectonic activity in controlling erosion rates. The erosion rates we determined are representative of the rate at which entire summit flats are lowered by erosion in a time interval of hundreds of thousands of years. No matter whether snow had covered the samples or not, the erosion rates are <30 mm ka 1, which are significantly lower than the denudation rate of mm ka 1 beginning at c. 5 3 Ma followed by a rate of mm ka 1 after c. 2 Ma in nearby Godwin Austen (K2) (see Figure 1) determined by the apatite fission-track technique (Foster et al., 1994). Fission-track thermochronology could model the denudation history of surface samples for a longer time interval to some million years. Thus the cosmogenic nuclide dating technique confines erosion rates with a shorter and more recent time interval and the fission-track technique confines erosion rates with a longer time interval. Although the basis of the two methods is different, results published in the literature show that erosion rates confined by the two methods are consistent for tectonically stable landforms (Kirchner et al., 2001; Bierman and Caffee, 2001). The erosion rate determined in this work is restricted to several outcrops of limited distance. However, we believe that it could be representative of a broader area. This is because the study area is located in a low-relief plateau and in fact our results are consistent with that obtained for the interior of the Tibetan plateau (Lal et al., 2003). Previous comparison of erosion rates for bedrock and catchment sediments shows that the differences are within a factor of two Table II. Comparison of long-term erosion rates in arid regions Annual precipitation Erosion rate Location Reference Altitude (m) (mm a 1 ) (mm ka 1 ) Tectonics Antarctica Nishiizumi et al. (1991) inactive Eyre Peninsula, Australia Bierman and Turner (1995) inactive Bierman and Caffee (2002) Namib Desert Van der Wateren and inacitve Dunai (2001) Northwest Tibet This work ± 7 active Yuma Wash, Arizona Clapp et al. (2002) ± 2 avtive Nahal Yael, Israel Clapp et al. (2000) ± 6 active

8 Erosion in northwest Tibet 123 to five (Bierman, 1994; Heimsath et al., 2001). Clearly the tremendous difference in denudation rate within different time intervals in northwest Tibet is not an artifact of dating material. The obviously different erosion rates within different time intervals in northwest Tibet demonstrate an extremely high denudation rate of mm ka 1 at c. 5 3 Ma followed by a decreasing rate to c. 10 mm ka 1 within the last million years which would require a significant impact by changes of tectonic activity or climate shift. As demonstrated above, tectonic activity plays a more important role than climate shift in affecting the erosion rate of landforms. In addition, temperature changes within the last million years are larger than those within the Pliocene, according to the marine δ 18 O record (Clark et al., 1999). Thus, we would not expect a smaller erosion rate within a more recent time interval due to climate effects. The much higher denudation rate at c. 5 3 Ma in northwest Tibet therefore requires intense tectonic activity in this region. Both tectonic uplift and extension can produce rapid exhumation of rocks. We prefer that tectonic uplift is responsible for the high denudation rate at c. 5 3 Ma in northwest Tibet because: (1) Pliocene normal faults are not found in northwest Tibet; and (2) sedimentation rates increased significantly along the northwest margin of the Tibetan plateau 2 4 Ma ago (Zheng et al., 2000). In addition, there is evidence that aridity intensified c. 3 6 Ma BP and c. 2 6 Ma BP in the desert lands in Asia, including those in north and northwest China (Guo et al., 2004), consistent with uplift of northwest Tibet in the Pliocene. Compared with the major uplift of southern Tibet that occurred 15 Ma ago as suggested by a recent study (Spicer et al., 2003), the uplift of northwest Tibet is substantially delayed. The local variations argue against simultaneous and homogeneous delamination of a portion of the mantle lithosphere beneath the whole of Tibet. The decrease of the extremely high erosion rates from of the early to mid-pliocene to the low rate determined in this study suggests reduced tectonic activities in the last million years in northwest Tibet. In general, two kinds of mechanisms have been proposed to account for the uplift of the Tibetan plateau: continental or intracontinental subduction (Tapponnier et al., 2001), and thinning of the mantle beneath the plateau (Molnar et al., 1993). The former would induce relatively constant tectonic activity and the latter an abrupt change of tectonics when thinning of the mantle occurred. It seems that the latter more reasonably explains the reduced tectonic activities inducing decreased erosion of northwest Tibet, but would need a localized scenario. We prefer the model of Nomade et al. (2004) that either slab break-off or changes of the underthrusting angle of the southward-subducted Qaidam slab initiated upwelling of the asthenosphere which induced delamination of the mantle lithosphere beneath northwest Tibet. Conclusions Both tectonics and climate affect erosion of landforms. Using in-situ-produced cosmogenic nuclide dating technique, we have determined steady-state erosion rates in hyper-arid northwest Tibet. The consistency of erosion rates derived from 10 Be and 26 Al suggests validity of the erosion model used in this study. Comparison of erosion rates in arid regions with contrasting tectonic activities suggests that tectonic activity plays a more important role than climate shift in modifying erosion rates. The obtained erosion rates, <30 mm ka 1, are significantly lower than the denudation rate of mm ka 1 beginning at c. 5 3 Ma in the nearby Godwin Austen (K2) determined by apatite fission-track thermochronology. It appears that the difference in erosion rates within different time intervals results from increased tectonic activity at c. 5 3 Ma in northwest Tibet. The lack of Pliocene normal faults and the significant increase of sedimentation rates and grain sizes along the northwest margin of the Tibetan plateau 2 4 Ma ago favour tectonic uplift as the cause of the high denudation rate at c. 5 3 Ma. We interpret the low erosion rates determined in this study to reflect reduced tectonic activity in the last million years. A model of localized thinning of the mantle beneath northwest Tibet may better account for the abrupt increase in tectonic activity at c. 5 3 Ma, and the later decrease. Acknowledgements This work was supported by the Chinese Academy of Sciences (Grants KZCX2-SW-33 and KZCX-3-SW-143) and the National Science Foundation of China (Grants and ). References An Z, Kutzbach JE, Prell WL, Porter SC Evolution of Asian monsoons and phased uplift of the Himalaya-Tibetan plateau since late Miocene times. Nature 411:

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