Evolution and variability of the East Asian summer monsoon during the Pliocene: Evidence from clay mineral records of the South China Sea Shiming Wan a,b, ⁎, Jun Tian c , Stephan Steinke d , Anchun Li a , Tiegang Li a a Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China b Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China c State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China d MARUM – Zentrum für Marine Umweltwissenschaften/Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany abstract article info Article history: Received 4 January 2010 Received in revised form 13 May 2010 Accepted 23 May 2010 Available online 1 June 2010 Keywords: East Asian monsoon Clay minerals Pliocene South China Sea ODP Leg 184 The Late Pliocene is thought to be characterized by the simultaneous intensification of both the East Asian winter monsoon (EAWM) and East Asian summer monsoon (EASM). However, the evolution of the EASM during the Pliocene remains still controversial and only little is known about the dynamics of the EASM during the Pliocene on orbital time scales. Here we use clay mineral assemblages in sediments from Ocean Drilling Program (ODP) Site 1143 in the southern South China Sea (SCS) to obtain proxy records of past changes in the EASM climate during the Pliocene. Provenance analysis suggests that illite, chlorite and kaolinite originated mainly from the Mekong River drainage area. Smectite was derived mainly from the Indonesian islands. The kaolinite/illite ratio and the chemical index of alteration (CIA) of siliciclastic sediments allowed us to reconstruct the history of chemical weathering and physical erosion of the Mekong River drainage area and thus, the evolution of the EASM during the Pliocene. Our clay minerals proxy data suggests a stronger EASM during the Early Pliocene than during the Late Pliocene. We propose that the long-term evolution of the EASM has been driven by global cooling rather than the uplift of the Tibetan Plateau. Spectral analysis of kaolinite/ illite ratio displays a set of strong periodicities at 100 ka, 30 ka, 28 ka, 25 ka, and 22 ka, with no clear obliquity- related signal. Our study suggests that the Pliocene EASM intensity on orbital time scales is not only controlled by the Northern Hemisphere summer insolation, but also strongly influenced by equatorial Pacific ENSO-like ocean–atmosphere dynamics. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The Pliocene experienced significantly warmer temperatures than today (Ravelo et al., 2004). It represents an accessible example of a world that is similar in many respects to what model simulations estimate to be the Earth of the late 21st century (Haywood et al., 2009). Compared to the present, the Pliocene warm period (5–3 Ma) was characterized by a similar orbital configuration, 3 °C higher global surface temperatures, a 10–20 m higher sea level, slightly reduced Antarctic ice sheets, increasing but still relatively small Northern Hemisphere ice coverage, and ∼ 30% higher atmospheric CO 2 concen- trations (Ravelo et al., 2004). The Earth's climate underwent a fundamental change during the Northern Hemisphere Glaciation (NHG) between 3.3 and 2.5 Ma (e.g., Shackleton et al., 1995; Tian et al., 2002). To understand what has driven this distinct climate transition, a number of hypotheses have been proposed that focus on the climatic influence of the high latitudes (e.g., Haug et al., 2005; Molnar, 2008). A recent study suggests that the intensification of the low-latitude East Asian summer monsoon (EASM) in the Late Pliocene may have triggered the onset of NHG through enhanced rock weathering and/or organic carbon burial (Zhang et al., 2009). Furthermore, the intensification of the EASM has been ascribed to the uplift of the Tibetan Plateau (An et al., 2001). However, the notion of a stronger EASM during the Late Pliocene relative to the Early Pliocene that is based on magnetic susceptibility analysis of the Red Clay formation in China (e.g., An et al., 2001) and diffuse reflectance spectroscopy (DRS) analysis derived iron oxide minerals ratio of sediments in the SCS (Zhang et al., 2009), is in contrast to a stronger EASM during the Early Pliocene implicated by δ 13 C, faunal, floral and lithostratigraphic data of the Red Clay formation (e.g., Ding et al., 1999; Ma et al., 2005; Wu et al., 2006; Passey et al., 2009). For that reason, new and more reliable proxy indicators are needed in order to better understand the long-term evolution of the EASM during the Pliocene. Previous studies about the EASM development on orbital time scales using terrestrial and marine sediments concentrated mainly on the Pleistocene period (e.g., Wang et al., 1999; Jian and Huang, 2001; Beaufort et al., 2003; Liu et al., 2003; Sun et al., 2003; Tamburini et al., Palaeogeography, Palaeoclimatology, Palaeoecology 293 (2010) 237–247 ⁎ Corresponding author. Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China. Tel.: +86 532 8289 8535; fax: +86 532 8289 8526. E-mail address: wanshiming@ms.qdio.ac.cn (S. Wan). 0031-0182/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2010.05.025 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo