Stepwise expansion of desert environment across northern China in the past 3.5 Ma and implications for monsoon evolution Z.L. Ding a, * , E. Derbyshire b , S.L. Yang a , J.M. Sun a , T.S. Liu a a Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China b Department of Geography, Royal Holloway (University of London), Egham, Surrey TW20 0EX, UK Received 20 January 2005; received in revised form 26 May 2005; accepted 8 June 2005 Available online 19 July 2005 Editor: E. Boyle Abstract A systematic study of the last glacial cycle along three transects across the Chinese Loess Plateau shows that sand-sized particle content within loess decreases rapidly from north to south, and that markedly high sand particle contents in loess horizons occur only in the northern part of the Plateau. This suggests that variation in the sand-sized particle fraction within loess near the desert margin is closely linked to migration of the southern desert border in northern China where sand grains move mainly in saltation or modified saltation mode near the ground surface. As desert margin shift is essentially controlled by the amount of monsoon precipitation, the sand-sized particle content within loess near desert margin is regarded as a new and readily applied proxy for variations in the strength of the East-Asian summer monsoon. A continuous record of sand content in loess along the loess–desert transitional zone shows that the Mu Us Desert migrated southward at 2.6, 1.2, 0.7 and 0.2 Ma, suggesting a stepwise weakening of the East-Asian summer monsoon during the past 3.5 Ma. This evolutionary pattern is significantly different from that previously inferred from loess magnetic susceptibility records, a widely used monsoon proxy. Our results further suggest that changes in global ice volume may have been an essential factor in controlling Plio–Pleistocene monsoon evolution, and that the anticipated future melting of polar ice cover may lead to a northward migration of the monsoon rainfall belt in northern China. D 2005 Elsevier B.V. All rights reserved. Keywords: East-Asian summer monsoon; loess–red clay sequence; Mu Us desert; sand content of loess; Plio–Pleistocene 1. Introduction Formation and evolution of deserts can be regarded as a result of interactions between the atmosphere, hydrosphere, biosphere and lithosphere [1–4]. Changes in desert environment can, in turn, exert a significant 0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.06.036 * Corresponding author. Tel.: +86 10 62008111; fax: +86 10 62010846. E-mail address: zlding@mail.igcas.ac.cn (Z.L. Ding). Earth and Planetary Science Letters 237 (2005) 45 – 55 www.elsevier.com/locate/epsl