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Highlights: • A quantitative record of Miocene EASM from northern China was obtained. • The EASM stepwise decreased from ~17.1 Ma to 7.4 Ma and increased after ~7.4 Ma. • Middle Miocene EASM evolution is mainly regulating by temperature changes. • Palaeogeography play a key role in controlling the evolution of late Miocene EASM. Abstract The East Asian summer monsoon (EASM) precipitation is vital to hydrology, ecology and societal activities in the densely populated region of East Asia. However, its long-term evolution history and driving forces during the relatively warm Miocene remain unclear, even conflicting in some intervals. Here, we present a new, and quantitative record of EASM precipitation during Miocene using the Bayesian approach of Climate Reconstruction Software (CREST) based on pollen flora from the Tianshui Basin located on the northwestern Tibetan Plateau (TP). The results demonstrate that a strong and relatively stable EASM precipitation period occurred during the Neogene in northern China at ~17.1–13.6 Ma, which was followed by a strong and gradual decreasing period between ~13.6 and 7.4 Ma. This trend was abruptly stopped at ~7.4 Ma with the beginning of a period of large amplitude precipitation increase. The comparison analysis reveals that the gradual decrease of EASM precipitation during the period of ~17.1–7.4 Ma was primarily controlled by the global cooling, whereas the significant increase period after ~7.4 Ma was mainly related to the late Miocene uplift of the TP, supporting climate model simulations, in which both the global temperature and palaeogeography play important roles in regulating the long-term evolution of EASM precipitation.

The East Asian Summer Monsoon (EASM) is the most important and active atmospheric circulation system in Asia. However, its evolution during the Early Pleistocene, and that evolution's associated drivers, remain controversial. Here, for the first time, a high resolution pollen record was obtained from a loess-paleosol sequence located in the Lanzhou Basin, western Chinese Loess Plateau (CLP), and used to reconstruct the evolution of EASM intensity during the Early Pleistocene (2.2–1.7 Ma). The type of vegetation indicated by analysis of the pollen assemblage is comparable to the forest-steppe vegetation type currently distributed on the eastern CLP. This area's present-day mean annual temperature (MAT) and mean annual precipitation (MAP) are ~6–10 °C and ~400–550 mm, respectively, implying the existence of a stronger EASM intensity during the 2.2–1.7 Ma period. When the auxiliary function of the diversity index was applied, the Early Pleistocene EASM, as recorded by the vegetation successions identified within the loess-paleosol sequence, showed a generally stronger intensity during the 2.2–2.06 Ma period, a relatively weak intensity between 2.06 Ma and 1.97 Ma, and then a gradual intensification from 1.97 Ma until 1.7 Ma. Further comparisons demonstrated that the zonal thermal gradient in the Equatorial Pacific may have played an important role in driving the evolution of the EASM over a tectonic timescale during the Early Pleistocene. In contrast, over a sub-orbital or orbital timescale, the EASM intensity appears to have been closely correlated to global temperature variations, given that the MATs of the Early Pleistocene were higher than today's. This would imply that both low-latitude forcing and global temperature change were important drivers influencing variations in precipitation in Northern China, against a background of ongoing global warming.

Chinese loess deposits preserve abundant information on the aridification of the Asian interior and Asian monsoon evolution during the Quaternary. Although substantial progress has been made in deciphering the paleoclimatic record of the thick loess deposits in the western Chinese Loess Plateau (CLP), the environmental significance of variations in the magnetic susceptibility (χ) and the ultrafine ( 40 μm fractions) of the 416-m-long Xijin loess core from the Lanzhou Basin, northeastern Tibetan Plateau. Our results indicate that the 40 μm fraction and marine oxygen isotope record. Consequently, we conclude that the pedogenic component is a more accurate and sensitive indicator of weathering and soil formation than the χ record, and it is well suited to reconstructing high-resolution Quaternary climatic change in the western CLP. Furthermore, the proportion of the pedogenic component increased systematically over the past 2.2 million years, which may have been primarily caused by long-term global cooling since the Early Pleistocene together with the episodic uplift of the Tibetan Plateau.

A comprehensive understanding of the tectonic uplift history of the Tibetan Plateau (TP), a 2000 km wide region with a mean altitude of 4000–5000 m above sea level (asl), is crucial for interpreting global Cenozoic climate change, collisional tectonics and the evolution of the Asian climatic system. However, the timing, degree and extent of the uplift of the TP remain controversial. Here we report new palynological data from Early to Mid-Miocene lacustrine-swamp sediments in the Zeku Basin, northeastern TP. The palynoflora includes 68 palynotaxa at a family or genus level, comprising gymnosperms (16.2%), angiosperms (77.9%), pteridophytes (2.9%) and others (2.9%), and is suggestive of a mixed warm temperate forests of coniferous (e.g., Picea, Abies and Tsuga) and deciduous broadleaved trees (e.g., Ulmus/Zelkova, Betula and Quercus), with some subtropical elements (e.g., Carya and Liquidambar). Three periods of vegetation succession can be recognized from the Caergen Section, indicating a stepwise warming and drying trend from bottom to top. The local palaeoclimatic parameters obtained by applying the Coexistence Approach (CoA) to palynologic assemblages rendered mean annual temperature (MAT) values of 14.2–16.1 °C and mean annual precipitation (MAP) values of 797–1113 mm. This study departs from the traditional application of the CoA, relying on published determination of plant-climate relationships, rather than on data derived from the Paleoflora Database, which is currently not fully open-access. Based on the difference in temperatures between the Early to Mid-Miocene and the present day, and the temperature lapse rate, corrected to take account of Miocene global temperature differences, the estimated palaeoaltitude of the Zeku Basin during the Early to Mid-Miocene was 1200–1400 m asl. This palaeoaltitude estimate suggests that this basin has experienced about 2200–2500 m of uplift since the Early to Mid-Miocene. Findings therefore do not support previous hypotheses that the northeastern TP attained its present altitude before the Miocene.

The Mid-Miocene Climatic Optimum (MMCO) has been regarded as an analog for future global warming due to its scale of warming being equivalent to the warming predicted for the next century by the International Panel on Climate Change (IPCC)'s fourth report. However, little is known about how the MMCO influenced paleoprecipitation in central China. In this study, two pollen floras from the eastern Longzhong Basin, northern central China, were quantitatively analyzed to reconstruct the climatic conditions likely during the MMCO. The paleoclimatic parameters obtained by applying the Coexistence Approach (CoA) to these two palynological assemblages showed that the mean annual temperature (MAT, 16.6 °C) for the MMCO was 5.7 °C higher than the present-day MAT (10.9 °C), and that mean annual precipitation (MAP, 955.4 mm) was 413.5 mm higher than at present (541.9 mm), indicating a much warmer and wetter climate. The higher mean temperature for the coldest month (CMT) and the minimum monthly precipitation (MiMP) values also indicate a warmer and wetter coldest and driest month, respectively, and thus suggest the prevalence of a more balanced seasonal paleoclimate in northern central China. These climatic conditions are equivalent to higher global temperatures of ~5–6 °C during the MMCO. This finding would imply that precipitation in northern central China will increase in the context of ongoing global warming.

Research into the surface soil pollen found within different sediments constitutes one important part of modern palynological analysis; however, such studies have not been carried out in the arid regions of Asia. In order to determine the main characteristics of the pollen assemblages of different sediments, 26 surface pollen samples were selected from five different sedimentary environments (sand dune, alluvial fan, wetland, floodplain and riverbed) found in three transverse sections in the middle reaches of the Heihe River. The results show that the five different sediment types have significantly different pollen assemblages: (1) sand dune: Chenopodiaceae - Artemisia - Nitraria - Poaceae; (2) alluvial fan: Nitraria Chenopodiaceae; (3) wetland: Poaceae - Typhaceae - Chenopodiaceae Artemisia; (4) floodplain: Picea Cyperaceae - Chenopodiaceae - Artemisia - Asteraceae; and (5) riverbed: Picea - Cyperaceae - Polypodiaceae - Chenopodiaceae Artemisia. The auxiliary functions derived from analyses of the principal components also indicates a significant difference between pollen assemblages for the five different sediment types. This established the veracity of these particular pollen assemblages representing these five different sedimentary environments, thereby improving the reliability of our research results. Those results provide the fundamental information required for any reconstruction of the region's paleovegetation and paleoclimate based on the pollen records found in different sedimentary environments. (C) 2017 Elsevier Ltd and INQUA. All rights reserved.

As a remnant elevated peneplain, the Tangxian planation surface is widely distributed in North China and the Ordos Plateau. Further into the Tibetan Plateau (TP), the Main planation surface can also be traced across this, the highest plateau on the Earth's surface. What the relation is between these two planation surfaces and whether they represent the remnants of a unique planation surface remains open to question. As the intermediate region between the low-altitude North China and the high-altitude TP, the Maxian Mountains and such bilateral basins located in the northeastern TP margin retain a series of well-preserved geomorphic features such as planation surfaces, pediment surface, and the fluvial terraces of the Yellow River. When and how the Xiaoshuizi peneplain was formed are crucial questions to elucidate the geomorphic evolution of the northeastern TP and infer the tectonic history of the TP. Here we present two parallel red clay-loess drill cores from the uplifted Xiaoshuizi bedrock peneplain in the Maxian Mountains, which document both the upper formation age of the planation surface and the evolution of Asian aridification. Based on the integration of high-resolution magnetostratigraphy analysis and Pliocene mammal fossils, we demonstrate that the red clay-loess succession deposited on the Xiaoshuizi peneplain probably began to accumulate at similar to 6.9 Ma, implying that the Xiaoshuizi peneplain had begun to form before this. These results support the hypothesis that the Xiaoshuizi planation surface is equivalent to Tangxian planation surface of North China and Main planation surface of the TP. Furthermore, the onset of red clay sequence may indicate that the Xiaoshuizi peneplain was ultimately destroyed by the intensive tectonic uplift of the northeastern TP before 6.9 Ma. Taking into account the synchronous Late Miocene red clay deposition between the Xiaoshuizi peneplain and central Chinese Loess Plateau (CLP), we propose that the onset of interior Asian aridification occurred in the Late Miocene and was principally triggered by the rapid uplift of TP.

Two drill cores were obtained from the Jilantai sub-depression (JLT(d)) and the neighboring Dengkou sub-uplift (DK(u)), within a huge, former lake basin in northern China. From an analysis of the lithology and pollen assemblages, combined with radiocarbon dating of extracted pollen and OSL dating of extracted quartz, we concluded the following: JLT(d) was continuously occupied by lakes since 85 ka; however, DK(u), the neighboring sub-uplift, was covered by lakes during 80–74 ka, 50–44 ka, 32.5–27.5 ka and

There remains no detailed record of the Middle Miocene vegetation and climatic changes which occurred in central Asia and their possible driving mechanisms. This is because there is still a lack of high resolution records. Here, we present a sporopollen record from the Wushan Basin on the northeastern Tibetan Plateau, central Asia, spanning the period ∼16.1–13.6 Ma. The sporopollen record shows that a dense mixed forest growing in rather warm and humid climatic conditions was affected by a general drying trend during the period ∼16.1–15 Ma. It demonstrates that although the climate was generally warm and humid during the Middle Miocene Climate Optimum (MMCO), it was also a time of climatic instability. The dense forest gave way to an open forest in response to a less humid climate between ∼15 and 14.4 Ma. Dense mixed forest made a return with an increasingly humid climate during ∼14.4–13.8 Ma. This vegetation and climatic succession could be associated with global cooling, or more particularly, a higher temperature rebound set against the background of a long-term cooling trend. A more open forest appearing in response to drier and colder climatic conditions dominated the study area during the ∼13.8–13.6 Ma period. This could be compared to the rapid global cooling event Mi-3b. This significant global cooling event exerted a major impact on terrestrial vegetation, climate and biota. Our high resolution sporopollen record demonstrates that global climate changes could have been the first order driving force for the Middle Miocene vegetation and climate changes seen in the Wushan Basin in central continental Asia, with the tectonic uplift of the Tibetan Plateau probably playing a subordinate role.

There have been a number of suggested driving factors for Late Miocene expansion of C4 plant coverage, including a decline in atmospheric CO2 concentrations, enhanced aridity and seasonality, shifts in fire regimes, and tectonic uplift. In order to identify and discriminate the driving factors of vegetation changes, continuous records from the same section or basin are required. We present micro-charcoal-based fire history and C4 vegetation abundance records spanning the Miocene from the Yanwan Section in the Tianshui Basin, on the northeastern Tibetan Plateau (NE TP). Based on statistical analyses, the micro-charcoal concentrations were relatively low before ~10 Ma, and after that time, there was an exponential increase. The C4 plant abundance shows a strong increase after 8 Ma, about 2 million years after the micro-charcoal concentrations started increasing. We compare our records with published CO2, pollen, seasonality, herbivorous mammalian fossil and tectonic records from the same section and basin. We find that C4 vegetation, fire intensities, and opening landscapes increased simultaneously after 8 Ma. This indicates that a fire-grassland feedback, initially driven by aridification, was the trigger for the Late Miocene expansion of C4 plants. We speculate that herbivorous mammalian species may also have played a minor role. During the Miocene, there is a trend towards enhanced seasonality and a decline in atmospheric CO2 concentrations, which were also necessary preconditions for the expansion.

Holding a climatically and geologically key position both regionally and globally, the northeastern Tibetan Plateau provides a natural laboratory for illustrating the interactions between tectonic activity and the evolution of the Asian interior aridification. Determining when and how the late Miocene climate evolved on the northeastern Tibetan Plateau may help us better understand the relationships among tectonic uplift, global cooling and ecosystem evolution. Previous paleoenvironmental research has focused on the western Longzhong Basin. Late Miocene aridification data derived from pollen now require corroborative evidence from the eastern Longzhong Basin. Here, we present a late Miocene pollen record from the Tianshui Basin in the eastern Longzhong Basin. Our results show that a general trend toward dry climate was superimposed by stepwise aridification: a temperate forest with a rather humid climate existed in the basin between 11.4 and 10.1 Ma, followed by a temperate open forest environment with a less humid climate between 10.1 and 7.4 Ma, then giving way to an open temperate forest–steppe environment with a relatively arid climate between 7.4 and 6.4 Ma. The vegetation succession demonstrates that the aridification of the Asian interior occurred after ∼ 7–8 Ma, which is confirmed by other evidence from Asia. Furthermore, the aridification trend on the northeastern Tibetan Plateau parallels the global cooling of the late Miocene; the stepwise vegetation succession is consistent with the major uplift of the northeastern Tibetan Plateau during this time. These integrated environmental proxies indicate that the long-term global cooling and the Tibetan Plateau uplift caused the late Miocene aridification of the Asian interior.

The loess-paleosol sequences of the Chinese Loess Plateau (CLP) are major paleoclimatic archives which document the evolution of the East Asian Monsoon (EAM) and changes in the Northern Hemisphere ice sheets during the Quaternary glacial-interglacial cycles. However, the mechanisms regulating the trend of EAM variations on a tectonic scale are unclear. The loess deposits of the western CLP, which have a close relationship with tectonics and climate, are much better-suited to exploring these mechanisms than those of the central CLP. However, studies of long-term EAM evolution from the western CLP have been hindered by the lack of long, accurately-dated sequences with high sediment accumulation rates. Here, we address this problem via high resolution magnetostratigraphic, magnetic susceptibility and grain-size analyses of a 416.2 m-long drill core located at Xijin Village, near Lanzhou. Paleomagnetic dating indicates that the basal age of the Xijin loess is ∼2.2 Ma. The χ and grain-size records reveal that the East Asian Summer Monsoon (EASM) and East Asian Winter Monsoon (EAWM) strengthened synchronously at ∼1.24 Ma. Subsequently, during interglacial periods, the EASM began to penetrate, and then dominate, in the Lanzhou region. This was followed by two stepwise uptrends, commencing at ∼0.87 and ∼0.62 Ma, which resulted in an increasingly moist interglacial climate in the region. We suggest that the uplift of the Tibetan Plateau was largely responsible for these three stepwise enhancements of the EASM. Overall, however, the long-term trend of strengthening in EAWM in the area may have been primarily caused by long-term global cooling from the Late Pliocene onwards.

The Pliocene climate and its driving mechanisms have attracted substantial scientific interest because of their potential as an analog for near-future climates. The late Miocene–Pliocene red clay sequence of the main Chinese Loess Plateau (CLP) has been widely used to reconstruct the history of interior Asian aridification and the Asian monsoon. However, red clay sequences deposited on the planation surface of the Tibetan Plateau (TP) are rare. A continuous red clay sequence was recently discovered on the uplifted Xiaoshuizi (XSZ) planation surface in the Maxian Mountains, northeastern (NE) TP. In this study, we analyzed multiple climatic proxies from the XSZ red clay sequence with the aim of reconstructing the late Miocene–early Pliocene climate history of the NE TP and to assess regional climatic differences between the central and western CLP. Our results demonstrate the occurrence of minimal weathering and pedogenesis during the late Miocene, which indicates that the climate was arid. We speculate that precipitation delivered by the paleo East Asian summer monsoon (EASM) was limited during this period and that the intensification of the circulation of the westerlies resulted in arid conditions in the study region. Subsequently, enhanced weathering and pedogenesis occurred intermittently during 4.7–3.9 Ma, which attests to an increase in effective moisture. We ascribe the arid–humid climatic transition near ∼4.7 Ma to the expansion of the paleo-EASM. The warming of the high northern latitudes in response to the closure of the Panama Seaway may have been responsible for the thermodynamical enhancement of the paleo-EASM system, which permitted more moisture to be transported to the NE TP.

As an analogue for predicting the future climate, Pliocene climate and its driving mechanism attract much attention for a long time. Late Miocene-Pliocene red clay sequence on the main Chinese Loess Plateau (CLP) has been widely applied to reconstruct the history of interior aridification and Asian monsoon climate. However, the typical red clay sequences deposited on the planation surface of Tibetan Plateau are rare. Recently, continuous red clay has been found on the uplifted Xiaoshuizi peneplain in the Maxian Mountains, northeastern (NE) Tibetan Plateau (TP). To reconstruct the late Miocene-early Pliocene climate history of NE Tibetan Plateau and to assess the regional differences between the central and western CLP, multiple climatic proxies were analyzed from the Xiaoshuizi red clay sequence. Our results demonstrate the minimal weathering and pedogenesis from 6.7 to 4.8 Ma, which implicates that the climate was sustained arid. We speculate that precipitation delivered by the paleo-Asian Summer Monsoon (ASM) was limited during this period, and instead the intensification of the westerlies circulation resulted in arid condition in the study region. Subsequently, enhanced weathering and pedogenesis occurred during the interval of 4.8–3.6 Ma, which attests to increasing effective moisture. Thus, we ascribe the obvious arid-humid climate transition near 4.8 Ma to the palaeo-ASM expansion. Increasing Arctic temperatures, the vast poleward expansion of the tropical warm pool into the subtropical regions and water freshening in the subtropical Pacific in response to the closure of the Panamanian Seaway may have been responsible for the thermodynamical enhancement of the paleo-ASM system, which permitted more moisture to be carried to the NE Tibetan Plateau.

The deformation of the Tibetan Plateau is central to unraveling the process and mechanism of continental tectonics. Although most agree that crust shortening and plateau growth were protracted throughout the Cenozoic Indo-Asian collision, particular deformation histories relating to tectonic kinematics and dynamics are still incomplete due to sparseness of diagnostic geological information from plateau margin. Here we present combined investigation of stratigraphy, magnetostratigraphy, and provenance for the eastern margin of Longzhong Basin to show two reorganizations of basin formation and tectonic regime during the late Tertiary. First, the depocenter migrated from the dispersed Paleogene sequences to the Wushan-Tianshui foreland sequence during the earliest Miocene (circa 22 Ma), accompanied by shift of sedimentary provenance from double sources including the eastern Qilian block and eastern West Qinling terrain to single source within the West Qinling. It suggests reorganization of deformation from NW-SE extension to NE-SW contraction and initial uplift of the eastern West Qinling. Second, massive coarse-grained fluvial beds were revived in the Wushan Basin during the late Miocene (circa 10 Ma), associated by eastward depositional expansion and another shift of sedimentary provenance toward northeast. It reflects thrusting up of the northern edge of the West Qinling and Liupan Shan Mountains linked with relocation of crust shortening from NE-SW direction to ENE-WSW direction and accelerated deformation of northeastern Tibet. These transitions of deformation regimes imply variation of geodynamic mechanisms during the process of plateau growth.

In this paper, we describe newly discovered and previously collected fossil moschid specimens from the middle Miocene of China. Besides the previously reported Hispanomeryx andrewsi, we recognize four additional fossil species in two genera of Moschidae in the middle Miocene of China based on dental morphology: Micromeryx cf. flourensianus, Micromeryx sp., Hispanomeryx sp. 1, and Hispanomeryx sp. 2. The specimens are of Tunggurian age (Neogene Chinese Land Mammal Age), corresponding to European MN6-MN7/8. The records show a diversity of fossil moschids in the middle Miocene of China comparable with the contemporaneous diversity in western European faunas.

To investigate climate evolution during the Miocene, especially during the Middle Miocene climate transition on the northeastern Tibetan Plateau, stable oxygen and carbon isotopes of carbonates from a 288-m-thick lacustrine-fluvial sediment sequence covering the period from 17.1 to 6.1 Ma from Tianshui Basin, China, were analyzed. The relatively low stable oxygen isotope values indicate the prevalence of wet climate conditions during the period of 17.1-13.6 Ma, an interval corresponding to the well-known Middle Miocene Climate Optimum. The interval between 13.6 and 11.0 Ma (i.e., the late Middle Miocene) is marked by a progressive increase in the delta O-18 values, indicative of a decrease in precipitation, probably linked to the expansion of the East Antarctic Ice Sheet and global cooling since about 14 Ma. The climate in the study area continued to get drier as shown by the enrichment of the heavy oxygen isotope from 11 Ma. We attribute these stepwise climatic changes as revealed by our carbonate delta O-18 record from the northeastern Tibetan Plateau to the sustained global cooling that may have reduced moist transport to Central Asia, which in turn led to a permanent aridification.

Long and continuous terrestrial paleoclimate records are important in understanding forcing mechanisms of Cenozoic climate. However, such records are rare. Sediments in basins near the edges of the Tibetan Plateau are potential paleoclimate archives, but dating them is a challenge. Here we report a successful application of paleomagnetism to a set of fluvio-lacustrine sediments in the Tianshui Basin, NE Tibet, and found that this set of sediments spans ∼17.1–6.1 Ma. Preliminary biomarker studies found that this set of sediments has climatically important organic components and, thus, these sediments are perfect paleoclimatic archives. Generating high-resolution biomarker records will be able to cast light on climatic history in NE Tibet during the Miocene.

The Tianshui basin of the northeastern Tibetan Plateau is located in the intersection of the eastern monsoon area,the northwest arid area and the Qinghai-Tibet Plateau cold and arid regions,so it is very sensitive to climatic change.Based on the δ18O,δ13C and TOC data along the Xiashan section,this paper deals with the Miocene climatic evolution of the Tianshui basin.The climatic change in the region can be divided into five stages in 17.02~12.26 Ma,12.26~10.92 Ma,10.92~8.50 Ma,8.50~7.050 Ma,7.05~6.05 Ma respectively.During the period of 17.02~12.26 Ma,the climate was very warm and humid under high precipitation;in the interval from 12.26 to 10.92 Ma,the climate was relative cold and humid;from 10.92 to 8.50 Ma,with less precipitation,the climate was relative cold and dry;in the phase 8.50~7.05 Ma,the climate was relative warm and humid;from 7.05 to 6.05 Ma,the climate was relative cold and dry.Global cooling may be the reason for the relative cold and wet climate during 12.26~10.92 Ma.The average δ18O values during the 10.92~8.50 Ma period are 1.5‰ more positive than that of the period prior to 10.92 Ma,probably due to the elevation of the Qinghai-Tibet Plateau that was sufficient to change the atmospheric circulation,by blocking the moisture from the Indian Ocean or south Pacific into the Tianshui basin.Since 7.05 Ma,the climate has been relatively cold and dry,marking the initiation of aridification in the inland Asian.All of the information is important to the understanding of the history of up-lifting of the Tibetan Plateau in-depth,the aridification in Asia interior as well as the climatic evolution in this region.

The Miocene vegetation and climate history of western China remains unclear. However, widely-distributed Miocene sediments of the Tianshui Basin in the NE Tibetan Plateau provide a great potential for deciphering the Miocene vegetation and climate history of this region. This paper presents first sporopollen record from these sediments, covering the period from 17.1 to 6.1 Ma. Sporopollen data reveal that temperate, warm-temperate broad-leaved forest of Quercus, Ulmus and Betula dominated the Tianshui region between 17.1 and 14.7 Ma, which was replaced by forest or forest-steppe of Ulmus, Artemisia and Betula between 14.7 and 11.7 Ma. After a return to a broad-leaved forest of Betula and Quercus during 11.7–8.5 Ma, the forest decreased rapidly and was replaced mostly by steppe vegetation (mainly composed of Artemisia, Chenopodiaceae and Poaceae) after 8.5 Ma. We interpret the observed vegetation changes as a result of global climate change, which is characterized by global cooling, development of Arctic ice-sheets and permanent El Nino state after the Middle Miocene Climatic Optimum. The rapid development of steppe at about 8.5 Ma suggested a permanent drying of the Asian interior at this time, coinciding with the onset of the well-known Hipparion red clay deposition in North China.

Holding a climatically and geologically key position both regionally and globally, the northeastern Tibetan Plateau provides a natural laboratory for understanding the interactions between tectonic activity and the evolution of Asian aridification. Determining when and how the Late Miocene climate evolved on the northeastern Tibetan Plateau may help us understand better relations between tectonic uplift, global cooling and ecosystem evolution. Previous paleoenvironmental research has focused on the western Longzhong Basin. Late Miocene aridification data derived from sporopollen now requires corroborative evidence from the eastern Longzhong Basin. Here, we present a Late Miocene sporopollen record from the Tianshui Basin in the eastern Longzhong Basin. Our results show a two-stage stepwise aridification: a temperate forest with a more humid climate developed in the basin between 11.4 and 10.1 Ma, followed by a temperate open forest environment with a less humid climate between 10.1 and 7.4 Ma; and an open temperate forest-steppe environment with a relatively arid climate occupied the basin during 7.4 to 6.4 Ma. The vegetation succession demonstrates that Asian aridification occurred after ~ 7–8 Ma, which is confirmed by other evidence from Asia. Furthermore, this persistent aridification on the northeastern Tibetan Plateau parallels the global cooling of the Late Miocene; the stepwise vegetation succession is consistent with the major uplift of the northeastern Tibetan Plateau during this time. These integrated environmental proxies indicate that global cooling may have been a potential driving force for Asian interior aridification, most likely enhanced by stepwise uplift of the Tibetan Plateau.

[1] Fine-grained Miocene sediments from Tianshui Basin, northeastern Tibetan Plateau, have received intense attention recently because these sediments were identified as loess. The presence of early Miocene loess pushes the timing of initiation of inland Asian desertification from 8 Ma back to 22 Ma. However, mudflat/distal fan and shallow lake sediments of Miocene have also been reported in Tianshui Basin. Consequently, the origin of these fine-grained Miocene sediments in this area remains controversial. Here we investigate then-alkane biomarker characteristics of Neogene sediments from a north-south transect of exposures within Tianshui Basin and compare these molecular distributions with those published Quaternary loess to help resolve the disputed origin. We found thatn-C23 and n-C25 alkanes, sourced from either aquatic macrophytes or palustrine plants, are ubiquitous in the Miocene sediments from Tianshui Basin but are largely absent in Quaternary loess. This striking difference between n-alkane distributions in the Tianshui samples and the Quaternary loess casts doubt on an eolian origin for the Tianshui samples and challenges the hypothesis of an early Miocene onset of Asian interior desertification.