Cenozoic stepwise aridification in central Asia Aeolian sediment evidence that global cooling has driven late Geological Society, London, Special Publications

It has been a long held view that uplift of the Tibetan Plateau dominated stepwise climatic drying in central Asia during the late Cenozoic. On the other hand, global cooling may also have forced Asian drying and the subsequent formation of aeolian deposits in north China. Until now, whether the Tibetan uplift or the global cooling has been the first-order driver controlling stepwise Asian drying has remained a contentious issue. In this study, we examine the thick aeolian silt deposit, which is regarded as a good archive of palaeoclimatic changes in central Asia and north China, in order to qualitatively reconstruct the drying process in Asia during the late Cenozoic. On the basis of our long-term field surveys, laboratory analyses and previous investigations, we have obtained time sequences of Asian drying from the early Miocene to late Pleistocene; we compare this newly reconstructed time series of Asian aridification with the time series of global cooling and Tibetan uplift to identify the first-order driver of stepwise Asian aridification. A good match between the drying and global cooling might indicate that global cooling was the most likely driver of stepwise drying in interior Asia. On the other hand, controversy regarding timing and amplitude of Tibetan uplift during the late Cenozoic suggests that the prevailing conclusion that Tibetan uplift forces Asian drying should be regarded as immature. A mechanism that global cooling drove the Asian drying is tentatively suggested. There is an extensive dryland covered by the Gobi and deserts in central Asia which is an important dust source on the Earth (Fig. 1a). Dust emitted from these places is entrained and deposited in an area of regional and even hemispheric scale, influencing local and regional environments (Husar et al. 2001; Zhang et al. 2003; Jickells et al. 2005). Evolution and change of this dry environment has influenced hundreds of millions of people and even climate change beyond this region, and has attracted much investigation (e.g. Liu & Ding 1998). Most of the wind-blown silt deposits, namely the thick loess and the Red Clay (a loess-like silt deposit underlying the loess, and deposited in the Miocene and Pliocene) in north and central China, cover a time from the early Miocene to the Holocene, are deposited in the downwind area of the Gobi and deserts. These aeolian silt sequences are direct indicators of environmental changes in central Asia, and many palaeoclimatic reconstructions have been carried out using these aeolian silt sequences over the past two decades (Liu 1985; Kukla & An 1989; Zheng et al. 1992; Ding et al. 1998; Sun et al. 1998; Guo et al. 2002, 2008). However, long continuous palaeoclimatic records of these drylands during the late Cenozoic are incomplete, because the surface sediments in the Gobi and deserts are moveable and easily eroded. In addition, the causal mechanism for the development of the dry environment has been hotly debated for many years without resolution. One main view is that the stepwise uplift of the Tibetan Plateau has directly driven stepwise Asian aridification during the late Cenozoic (Manabe & Terpstra 1974; Zhang 1981; Kutzbach et al. 1989; Ruddiman & Kutzbach 1989; Manabe & Broccoli 1990; Ruddiman 1997; Li 1999; Li & Fang 1999; An et al. 2001; Zhang et al. 2006; Fang et al. 2007; Sun et al. 2009). Other researchers suggest that global cooling controlled the stepwise drying in interior Asia, and that the growth of the Tibetan Plateau played a subordinate role (Zhou 1963; Chen et al. 1990; Wang & Gao 1990; Liu & Ding 1993; Guo et al. 1998, 2004; Dupont-Nivet et al. 2007; Jiang & Ding 2008; Jiang et al. 2008). Thus, more work is needed to clarify which hypothesis (global cooling or the Tibetan uplift) is the main driver of stepwise Asian drying in late Cenozoic. For this reason, we have reconstructed a time series of Asian drying during the late Cenozoic using wind-blown silt archives (the loess and the Red Clay) in north China. This newly obtained time series of Asian drying will then be compared with that of global cooling and with the uplift of the Tibetan Plateau to assess the possible forcing mechanism of Asian drying. From: Clift, P. D., Tada, R. & Zheng, H. (eds) Monsoon Evolution and Tectonics–Climate Linkage in Asia. Geological Society, London, Special Publications, 342, 29–44. DOI: 10.1144/SP342.4 0305-8719/10/$15.00 # The Geological Society of London 2010. Fig. 1. (a) The drylands and aeolian deposits in modern Asia. The dark area indicates the dryland in interior Asia, from which the dusts are emitted; the white-empty and empty arrows indicate the wind directions that transport dust to Chinese Loess Plateau and the dust storm track in spring in north Hemisphere, respectively. The inserted figure shows our filed survey routes, which provides the data for Table 1 and Figures 2, 3 and 4. (b) The index map of locations number 1287 that are listed in Table 1, and provide data sources for Figure 2. H . 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