Increased contribution of terrigenous supply from Taiwan to the northern South China Sea since 3 Ma

Increased contribution of terrigenous supply from Taiwan to the northern South China Sea since 3 Ma

Marine Geology 278 (2010) 115–121 Contents lists available at ScienceDirect Marine Geology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c...

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Marine Geology 278 (2010) 115–121

Contents lists available at ScienceDirect

Marine Geology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a r g e o

Increased contribution of terrigenous supply from Taiwan to the northern South China Sea since 3 Ma Shiming Wan a,b,⁎, Anchun Li a, Peter D. Clift c, Shiguo Wu a, Kehui Xu d, Tiegang Li a a

Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China School of Geosciences, University of Aberdeen, Meston Building, Kings College, Aberdeen, AB24 3UE, United Kingdom d Department of Marine Science, Coastal Carolina University, Conway, SC 29528, USA b c

a r t i c l e

i n f o

Article history: Received 8 June 2010 Received in revised form 6 September 2010 Accepted 10 September 2010 Available online 18 September 2010 Communicated by D.J.W. Piper Keywords: Deep-water bottom deposition Clay minerals South China Sea Taiwan Ocean Drilling Program Sediment

a b s t r a c t Seismic profiles provide evidence that there has been strong transport by deep-water bottom currents and drift deposition on the northern slope of the South China Sea. Earlier geochemical studies suggest that the drift sediments originated primarily from Taiwan. However, the transport process, history and origin of the deepwater bottom deposition in the northern South China Sea, on both glacial–interglacial and tectonic time scales, remain unclear. Here, we show new high-resolution records of clay minerals, grain size and mass accumulation rate (MAR) of terrigenous materials from Ocean Drilling Program (ODP) Site 1144, together with trace element concentrations in siliciclastic sediments from ODP Site 1146. Combined with other published data, we find that the primary source for sediments at ODP Sites 1144–1148 since 3 Ma is from Taiwan, and not from Pearl River as previously thought. Before 3 Ma, however, sediment source to ODP Sites 1146 and 1148 was mainly from the Pearl River. Increased contribution of terrigenous supply from Taiwan to the northern South China Sea since ~ 3 Ma may be related to the formation of the Taiwan orogen and strengthening of deep-water bottom current transport in the northern South China Sea. Variations in clay mineralogy and sedimentology at ODP Site 1144, located on a sediment drift, shows strong glacial–interglacial cyclicity. This suggests that bottom current deposition is highly dependent on sea-level fluctuations, which control the terrigenous supply to the deep sea. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The northern South China Sea is an area of active sedimentation with possible input of terrigenous sediment supplies from the Pearl, Red and Yangtze Rivers, as well as the islands of Taiwan and Luzon (Fig. 1) (e.g., Clift et al., 2002; Liu et al., 2003; Wan et al., 2007; Shao et al., 2009). Sediment transport dynamics involve ocean circulation, which is complex in the South China Sea. Model circulation in the South China Sea can be described in three depth ranges: surface circulation above 600 m; mid-depth circulation between 600 and 1200 m; and deep circulation below 1200 m (Chao et al., 1996). The shallow circulation in the South China Sea is controlled by the East Asian monsoon system (Shaw and Chao, 1994). Surface currents flowing towards the southwest are driven by the northeast monsoon in winter, while flow is reversed by the southwest summer monsoon. The clay minerals found at most study sites in the northern South China Sea have usually been interpreted to be transported by ⁎ 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: [email protected] (S. Wan). 0025-3227/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2010.09.008

monsoon-driven surface ocean currents. As a result downcore variations in clay mineralogy have been used to indicate changes in both the intensity of surface ocean currents and continental weathering linked with the East Asian monsoon (Liu et al., 2003; Boulay et al., 2005; Wan et al., 2007). However, seismic profiles provide evidence that there is strong NE–SW-trending transport and that deposition of sediments may be controlled by deep-water bottom currents on the northern slope of the South China Sea (Lüdmann et al., 2005; Shao et al., 2007). These bottom currents carry terrigenous materials from the northern slope of the South China Sea southwestward to the central South China Sea, where they form discontinuous drifts with high sedimentation rates (Shao et al., 2007). Sediments recovered at ODP Site 1144, which is located on a sediment drift to the southeast of the Dongsha islands (Fig. 1), show an average sedimentation rate of 46 cm/ky since 1.1 Ma (Bühring et al., 2004). Moreover, the results of trace elements and Nd isotopes analyses suggest that the rapidly accumulated sediments at ODP Site 1144 were primarily derived from the island of Taiwan (Shao et al., 2001, 2009). Some of the sediment may be derived locally from the Plio–Pleistocene erosion of the Dongsha Uplift (Lüdmann and Wong, 1999), but volumetrically the flux from Taiwan would greatly dominate and is more consistent with the clay mineralogy. Although

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Fig. 1. Location map showing ODP Leg 184 Sites 1144, 1145, 1146 and 1148 in the northern South China Sea. Also shown are the major rivers in Asia, North Pacific Deep Water (NPDW, from Lüdmann et al., 2005) (orange arrows), deep-water current (solid arrows) and sediment drifts (blue crosses, Shao et al. (2007); grey shaded area, Lüdmann et al. (2005)) in the northern South China Sea.

there has been preliminary investigation of the clay minerals and elements geochemistry at ODP Site 1144, little attention has been paid to the sediment source and its link to deep bottom water currents (Boulay et al., 2003; Wei et al., 2004). Prior to this work, few studies have concentrated on the transport of deep-water bottom sediments in the northern South China Sea over glacial–interglacial time scales. In addition, because drilling at ODP Site 1144 did not reach the lower boundary of the drift body, the history and origin of deep-water deposition in the northern South China Sea over tectonic time scales still remain unknown. Drift sediments imaged by seismic profiles extend into the areas around ODP Sites 1146 and 1148 (Shao et al., 2007), where the cored sediment sequences extend at least to the Miocene–Oligocene. A detailed clay mineral and trace element study for sediments at ODP Site 1146 might thus be expected to shed light on the long-term history of deep-water bottom sedimentation in the northern South China Sea. This present paper reports new high-resolution records of clay mineral assemblages, grain size and mass accumulation rate (MAR) of terrigenous materials at ODP Site 1144, as well as the trace element geochemistry of silicate sediments at ODP Site 1146, together with other published data, in order to (1) constrain the sediment source to the study sites and their temporal changes, (2) to test whether monsoondriven surface ocean currents or deep-water current have controlled the sediment records in the northern South China Sea, and (3) to discuss the history and origin of deep-water deposition in the northern South China Sea over both glacial–interglacial and tectonic time scales. 2. Materials and methods ODP Site 1144 (20°3.18′N, 117°25.14′E) is located at a water depth of 2037 m, on a sediment drift in the northern South China Sea (Fig. 1). Coring at ODP Site 1144 recovered a mid- to upper Pleistocene and Holocene sequence of rapidly accumulated, homogenous gray-green hemipelagic clays with a basal age of ~1.1 Ma (Wang et al., 2000). ODP Site 1146 (19°27.40′N, 116°16.37′E) is located to the southwest of ODP Site 1144, in a water depth of 2092 m, within a small rift basin on the mid-continental slope of the northern South China Sea (Fig. 1). The overall drilled sediment sequences span approximately 20 Ma. For this study, a total of 149 and 275 samples were sampled at 3.0–1.5 m intervals in 0–502.02 meters composite depth (mcd) at ODP Site 1144

and in 0–642.44 mcd at ODP Site 1146, respectively. The age model for ODP Site 1144 was based on high-resolution δ18O data from Globigerinoides ruber (Bühring et al., 2004). The chronostratigraphic framework for ODP Site 1146 was established on the basis of the magnetostratigraphy and biostratigraphy (Wang et al., 2000) and interpolated linearly between control points. Clay mineral studies were carried out on the b2 μm fraction, which was separated based on conventional Stokes’ settling velocity principle after the removal of carbonate and organic matter (e.g., Wan et al., 2007, 2010a,b). Carbonate and organic matter were removed by acetic acid (15%) and hydrogen peroxide (10%), respectively. Clay minerals were identified by X-ray Diffraction (XRD) using a D8 ADVANCE diffractometer with CuKα (alpha) radiation (40 kV and 40 mA) in the laboratory of the Institute of Oceanology, Chinese Academy of Sciences (IOCAS). Three XRD runs were performed for each sample, following air-drying, ethylene–glycol salvation at 60 °C for 12 h and heating at 490 °C for 2 h. Identification of clay minerals was made according to the position of the (001) series of basal reflections on the three XRD diagrams (Moore and Reynolds, 1997). Semi-quantitative estimates of peak areas of the basal reflection for the main clay mineral groups of smectite (including mixed-layers) (15–17 Å), illite (10 Å), and kaolinite/chlorite (7 Å) were carried out on the glycolated samples using Topas 2P software with the empirical factors of Biscaye (1965). Relative clay mineral abundances were given in percent. For consistency purposes, all the clay mineralogical results referred to in this study were calculated based on the same methodology of Biscaye (1965). The illite chemical composition was estimated using the ratio of the 5 Å and 10 Å illite peak areas ratio of ethylene-glycolated samples (Esquevin-Index; Esquevin, 1969). Ratios above 0.4 indicate Al-rich illites, which are formed under strong hydrolysis. Ratios below 0.4 represent Fe–Mg-rich illites (biotites and micas) are characteristic for physically eroded, unweathered rocks (e.g., Esquevin, 1969; Ehrmann, 1998). Furthermore, the illite crystallinity was calculated as the full width at half maximum (FWHM) of the illite 10 Å peak. Generally, high values indicate poor crystallinities (highly degraded), whereas low values indicate good crystallinities (relatively unaltered). These two parameters may be used to track source regions and transport paths (e.g., Ehrmann, 1998; Wan et al., 2008, 2010a,b; Xu et al., 2009). Geochemical analysis of trace elements concentrations was performed on bulk organic- and carbonate-free sediments. Organic

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matter, calcite, and Fe–Mn oxides were removed using 10% H2O2 and 0.5 N HCl, respectively. Finally, the sediments were rinsed with deionized water three times, dried at 80 °C, then ground to powder for analysis. This chemical pre-treatment effectively removes most of the biogenic/authigenic materials in the sediments, but does not change the composition of the detrital components (Li et al., 2003; Wei et al., 2006). About 40 mg of sample powders, together with a series of United States Geological Survey (USGS) and Chinese rock and sediment standards were fully digested with concentrated 4 ml HF, 1 ml HNO3 and 0.5 ml HClO4 in airtight Teflon vessel for four days at 180 °C. Trace elements were determined using a Perkin-Elmer ELAN DRC II Inductively Coupled Plasma Mass Spectrometer (ICP-MS), at the Institute of Oceanology, Chinese Academy of Sciences (IOCAS). More detailed explanation of our experimental procedures can be found in Wan et al. (2010a,b). The analytical precision was generally better than 3% for trace elements. 3. Results and discussion 3.1. Clay mineralogy and sediment sources The clay mineral assemblage in sediments from ODP Site 1144 mainly consists of illite (~ 58%) and smectite (~21%). Kaolinite (~5%) and chlorite (~ 15%) are present in lesser amounts. The clay mineral concentrations indicate strong glacial–interglacial cycles through the last 1.1 Ma (Fig. 2). Generally, illite, chlorite and kaolinite show a similar pattern of cyclic change, with high values during glacials when illite reaches 58–65%, chlorite 15–17% and kaolinite 7–10%. Illite crystallinity and chemical index also show similar cyclic changes. In contrast, smectite displays an opposite trend with high values (22– 28%) during interglacials and low values during glacials. In addition, the mean grain size and mass accumulation rate (MAR) of terrigenous materials at ODP Site 1144 also display remarkable glacial–interglacial

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cyclicity (Fig. 2), with high values of 12–14 μm and 50–200 g/cm2/ky during glacial, and low values of 8 μm and 25 g/cm2/ky during interglacials, respectively. Rare earth element (REE) and Nd isotopic studies of sediments from ODP Site 1144 indicate that the sediment source since 1.1 Ma is very stable and mainly from Taiwan, with only a minor flux from the Pearl River (Shao et al., 2001, 2009). Localized erosion from the Dongsha Uplift can be ruled out as being volumetrically small and of Pearl River type compositions. In contrast, preliminary investigation of clay minerals at ODP Site 1144 assumed that the major sediment source had been from the Pearl River (Boulay et al., 2003). Our clay mineral analysis indicates new results of sediment sources. As shown in Fig. 3A and B, all samples from ODP Site 1144 cluster as a group and lie close to the composition of river sediments from SW Taiwan (Wan et al., 2007; Liu et al., 2008), suggesting a dominant source from SW Taiwan to the study site. Although there are no available data of clay minerals from the river sediments of the eastern Taiwan, the distinct geological characteristics between the western and eastern Taiwan island likely suggests a different detrital composition of sediments from the two geological zones (Huang, 2002). Thus the sediments from the eastern Taiwan should have no significant contribution to the study site and would in any case be largely deposited on the East side of Taiwan. More importantly, sediments younger than 450 ka at ODP Site 1145 (Boulay et al., 2005), as well as those deposited at ODP Sites 1146 (Wan et al., 2007) and 1148 (Clift et al., 2002) since ~ 3 Ma, also overlap with the clay mineral compositions at ODP Site 1144. In contrast, clay minerals in older sediments from ODP Sites 1146 (Wan et al., 2007) and 1148 (Clift et al., 2002) dated between 3 and 20 Ma are quite different from those found in SW Taiwan, but lie close to those from the Pearl River (Fig. 3). Illite + chlorite and kaolinite are characteristic of the clay minerals from SW Taiwan and the Pearl River, respectively (Liu et al., 2003; Wan et al., 2007), so that (illite + chlorite)/kaolinite can be used to monitor the relative contribution of

Fig. 2. Variation of clay minerals assemblages, crystallinity index, mass accumulation rate and mean grain size of terrigenous materials (this study), as well as δ18O of G. ruber at ODP Site 1144 since 1.1 Ma (Bühring et al., 2004). Variation of the East Asian summer monsoon intensity as reflected by the stacked FeD/FeT ratio was obtained from the Xifeng and Changwu loess-soil sequence (Guo et al., 2000) for comparison. Higher FeD/FeT ratio indicates strengthened East Asian summer monsoon intensity. The shaded bars and numbers indicate marine isotope stages.

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Fig. 3. Ternary (A) and correlation (B) diagram showing variation in clay minerals composition of sediments from ODP Sites 1144 (this study), 1145 (Boulay et al., 2005), 1146 (Wan et al., 2007), and 1148 (Clift et al., 2002), river sediments from Red River and Luzon (Liu et al., 2003), Pearl River, SW Taiwan Rivers (Wan et al., 2007; Liu et al., 2008).

these two end members to clay minerals at the study site. The clay mineral assemblage of sediments from ODP Sites 1144–1148 since about 3 Ma are characterized by higher ratios of (illite + chlorite)/ kaolinite (~ 15), consistent with a source in SW Taiwan. In contrast, the clay mineral assemblages of sediments from ODP Sites 1146 and 1148 predating 3 Ma are similar to those from the Pearl River, with much lower (illite + chlorite)/kaolinite ratios (~ 5) (Figs. 3B and 4). This result has two important implications. First, it suggests that the primary sediment source to ODP Sites 1144–1148 since 3 Ma is Taiwan, and not from Pearl River as previously thought (e.g., Clift et al., 2002; Liu et al., 2003; Boulay et al., 2005; Wan et al., 2007). Secondly, our comparison suggests that the source of sediment to ODP Sites 1146 and 1148 before 3 Ma was primarily from the Pearl River, or other rivers with similar compositions draining southern China. The Nd and especially the Sr isotopic composition of sediments deposited at ODP Site 1145 since 0.45 Ma are consistent with their derivation from SW Taiwan, similar to ODP Site 1144, but again different from Pearl River (Boulay et al., 2005; Shao et al., 2009). In contrast, Boulay et al. (2005) interpreted the sediment source to ODP Site 1145 to be a mixture between Pearl River and the Luzon Arc, rather than a strong flux from SW Taiwan, an interpretation which is inconsistent with our data. In addition, although the Nd isotopic compositions of sediments from ODP Site 1148 indicate a relatively stable sediment source from southern China since 23 Ma (Clift et al., 2002; Li et al., 2003), a detailed reconstruction of changing sources to the site since 3 Ma is difficult because of the very low temporal resolution. Here, we show a high-resolution record of variations in immobile trace element ratios, such as La/Th and Th/Yb, which can be used to distinguish between mafic and felsic source rocks and to constrain changes in provenance at high-resolution temporal (McLennan et al., 1980; Bhatia and Crook, 1986). La/Th at ODP Site 1146 displays a stable value between 20 and 3 Ma, but shows a rapid increase after ~ 3 Ma (Fig. 4), suggesting a major change in sediment source at that time. An evident change in source can also be seen in the abrupt increase in crystallinity and the percent of illite within the clay assemblage at ODP Site 1146 after ~ 3 Ma (Fig. 4), suggesting that a Taiwanese provenance, under strong physical erosion conditions, which supplied more unaltered illite to the study site after 3 Ma.

However, our results do not exclude a minor contribution of sediment supply from the Pearl River and Luzon arc to the study sites since 3 Ma. Lüdmann et al. (2001) argued that the material from the Pearl River delivered to the South China Sea during periods of sea-level lowstand could have reached the deep ocean basin via mass wasting processes during the past 690 ky. High contents of smectite within the sediments of the northern South China Sea have been suggested to originate from the Luzon arc and/or the west Philippine Sea (Liu et al., 2003, 2008; Wan et al., 2007). In addition to source variability, the possible influence of climate change on sedimentology should be also considered. An increased sedimentation rate after ~2–4 Ma has been recognized in many continental margins and has often been ascribed to global climate instability since the initiation of the North Hemisphere Glaciation (Zhang et al., 2001). A two to fourteen-fold increase in terrigenous mass accumulation rates (MAR) since 3 Ma is also shown in ODP Sites 1146 and 1148 (Fig. 4). However, the influence of climate change alone cannot explain the major change in sediment composition and thus provenance seen in the study area. Therefore, we conclude that the terrigenous supply from Taiwan to the northern South China Sea rapidly increased after about 3 Ma, consistent with the uplift and exhumation of the island since ~6 Ma (Huang et al., 2006). This finding can be in turn used to indicate the mechanism and history of deep-water current deposition in the northern South China Sea. 3.2. Glacial–interglacial deep-water bottom deposition As discussed above, the primary source for sediments at ODP Sites 1144–1148 since 3 Ma is SW Taiwan. The average long-term terrigenous MAR at ODP Sites 1144, 1146, and 1148 since 1.1 Ma is 64, 8 and 5 g/cm2/ky, respectively. Clearly, monsoon-driven surface current transportation (e.g., Liu et al., 2003; Boulay et al., 2005; Wan et al., 2007) is not needed to explain this trend to rapidly decreasing sedimentation rate into deeper water. Seismic profiles provide evidence that there has been strong deep-water current transport and deposition in the northern South China Sea (Lüdmann et al., 2005; Shao et al., 2007). Channels formed by bottom currents occur in water depths of 1000–2700 m, extending from the northeast to southwest,

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Fig. 4. Variation of (A) terrigenous MAR at ODP Sites 1146 (Wan et al., 2007) and 1148 (Clift, 2006); (B) (illite + chlorite)/kaolinite ratio at ODP Sites 1144 (this study), 1145 (Boulay et al., 2005), 1146 (Wan et al., 2007), and 1148 (Clift et al., 2002); (C) La/Th ratio (this study) and illite crystallinity (Wan et al., 2008) at ODP Site 1146; and (D) global deep sea δ18O (Zachos et al., 2001) since 20 Ma. Vertical blue bars show the standard view of the presence and extent of full scale/permanent ice sheets (solid bars) and those thought to have been partial/ephemeral (broken bars) (Edgar et al., 2007).

and leading to the accumulation of discontinuous drifts with higher sedimentation rates on the eastern side of each channel (Fig. 1; Shao et al., 2007). Furthermore, studies of pollen- and foraminiferal δ13Cdistribution patterns indicate strong water currents have been flowing via the Bashi Strait into the South China Sea since 3 Ma (Sun et al., 1999; Wang et al., 1999). Therefore, it is most likely that North Pacific Deep Water (NPDW), intruding the South China Sea through the Bashi Strait flow along the ~2000 m isobathic line, transported sediments from the coastal northeast, including Taiwan and Luzon arc, to form a contourite deposit in the northern South China Sea. This flow probably slows on the slope southeast of the Dongsha islands (Lüdmann et al., 2005; Shao et al., 2007) and terminates near the area of ODP Sites 1146 and 1148 (Fig. 1). In addition to the southwestward deep-water bottom current transport, Zhu et al. (2010) reported on middle Miocene to present northeastward migrating slope canyons off the Pear River Mouth, which were suggested as the evidence of the northeastward intermediate-water bottom current. However, these bottom currents should have no significant influence in the sedimentation in our study area because these northeastward bottom currents are only active in water depths of 450–1500 m and also not likely transport materials from Taiwan to the ODP sites.

Previous studies rarely addressed the change of the contourite deposition on glacial–interglacial time scale (Lüdmann et al., 2005; Shao et al., 2007) or just assume that it is independent of sea-level fluctuations (Shao et al., 2009). However, our results, including clay minerals, grain size and MAR of terrigenous materials at ODP Site 1144, all show strong glacial–interglacial cyclicity (Fig. 2). A similar pattern of cyclic change of clay minerals was seen in sediments at ODP Site 1145 (Boulay et al., 2005) and at Site 1146 (Liu et al., 2003). It is noteworthy that there is an opposing variation of East Asian summer monsoon intensity (Guo et al., 2000) and terrigenous flux and grain sizes at ODP Site 1144 (Fig. 2) excludes the possibility that monsoon strength is the primary control on sediment delivery rates to the continental margin in this area. If monsoon is controlling erosion rates on onshore, this sediment must be stored onshore before being redeposited into the deep sea. Thus we can conclude that bottom current sedimentation in the northern South China Sea is strongly correlated to sea-level change. Moreover, we suggest that the contourite deposition results from the combined actions of both deep-water current and terrigenous supply. The intrusion of the NPDW into the South China Sea is likely stable over glacial– interglacial cycles because the sill depth of the Bashi Strait is up to 2500 m, much higher than the magnitude of sea-level change (100–

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120 m) (Wang and Sun, 1994). However, sea-level change should have a profound influence on the terrigenous input into the northern South China Sea because at times of low sea level, the exposure of the shelf displaced the coastline close to the shelf break, thus increasing terrigenous input to the deep sea (Wang, 1999). Thus more terrigenous materials from Taiwan will be transported by the deepwater flow to the study sites during glacial periods. 3.3. Deep-water bottom deposition and Taiwan uplift Deep sea conditions have existed in the study area of the South China Sea since its opening in the early Oligocene. The intrusion of the NPDW into the South China Sea is thought to have persisted from the Miocene to present (Zhao et al., 2009). However, it remains unclear whether the emergence of the Bashi strait and the uplift of sill depth in the late Miocene–Pliocene have reduced the flow of NPDW via the Bashi strait into the South China Sea. In any case, the declined or unchanged strength of the deep-water bottom current in the northern South China Sea cannot explain the increased bottom current transport seen in the ODP Sites since 3 Ma. Thus the rapidly increased contribution of the terrigenous supply from Taiwan to the northern South China Sea since ~3 Ma may relate to the formation of the Taiwan orogen. Taiwan is one of the youngest orogenies in the world (Sibuet and Hsu, 2004). It is generally accepted that the uplift of Taiwan results from the collision of the Luzon arc with the Eurasian margin (e.g., Teng, 1990; Sibuet and Hsu, 2004; Huang et al., 2006). However, a wide spectrum of “collision” ages ranging from 12 to 2 Ma in stratigraphic records has been proposed (Sibuet and Hsu, 2004; Huang et al., 2006 and references therein). Our study shows a sharp increase in terrigenous input from SW Taiwan to the northern South China Sea, which may be related to the collision event in Taiwan. Although Lüdmann and Wong (1999) proposed that the Dongsha island may uplift at the Miocene–Pliocene boundary, our study doesn't support that the Dongsha Rise has supplied significant terrigenous materials to the ODP Sites. Intense uplift and erosion linked to arc–continent collision in Taiwan commenced in the early Late Pliocene, as evidenced by the Paliwan Formation, a Plio–Pleistocene deep marine turbidite unit (e.g., Teng et al., 1988; Teng, 1990). This deposit shows that a large amount of orogenic sediment was fed into the forearc, foreland basins and the northern South China Sea at that time. Because there was no significant sediment supply from Taiwan to the northern South China Sea before 3 Ma, as reflected in the records at ODP Sites 1146 and 1148, our study does not favor “hard” collision in Taiwan prior to 3 Ma, at least close to the modern location of the island. However, we cannot exclude the possibility that earlier collision further along the Chinese margin to the northeast may have released sediments into its nearby basin such as the Okinawa Trough but not the northern South China Sea close to ODP Site 1144 (Clift et al., 2003). 4. Conclusions Based on the comprehensive study of clay mineral assemblages, grain size and mass accumulation rate (MAR) of terrigenous materials at ODP Site 1144 and trace elemental geochemistry of silicate sediments at ODP Site 1146, together with other published data, we draw the following conclusions: (1) Provenance analysis suggests that the primary source for sediments at ODP Sites 1144–1148 since 3 Ma is from Taiwan. However, the sediment source to ODP Sites 1146 and 1148 before 3 Ma is mainly from the Pearl River. This increased contribution of terrigenous supply from Taiwan to the northern South China Sea since ~3 Ma may be related to the formation of the Taiwan orogen and strengthening of deep-water bottom current transport in the northern South China Sea. (2) Variations in clay mineralogy and sedimentology at ODP Site 1144, located on a sediment drift, show strong glacial–interglacial cyclicity. The opposing variation of East Asian summer

monsoon intensity and terrigenous flux and gain-size at ODP Site 1144 excludes the possibility that monsoon strength is the main control on sediments delivery to the study area. Bottom current deposition is strongly dependent on sea-level fluctuations, which control the terrigenous supply to the deep South China Sea. Acknowledgments All samples for this study were supplied by the Ocean Drilling Program (ODP), which is sponsored by the U.S. National Science Foundation (NSF) and participating countries under management of Joint Oceanographic Institutions (JOI) Inc. We thank Dr. Christophe Colin and Zhengtang Guo for providing the data of clay minerals from ODP Site 1145 and FeD/FeT ratio from the loess-soil sequence, respectively. We are grateful to the Editor-in-Chief, David Piper, Thomas Lüdmann and an anonymous reviewer for their helpful reviews and suggestions, as well as Prof. Qianyu Li in Tongji University for constructive discussions. Thanks also go to Xuebo Yin for his kind help during experiments. This work was supported by the National Natural Science Foundation of China (40706025), Knowledge Innovation Program of CAS (KZCX2-YW-229), the National Natural Science Foundation of China (41076033), and the National Basic Research Program of China (2007CB411703 and 2007CB815903). Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10.1016/j.margeo.2010.09.008. References Bhatia, M.R., Crook, K.A.W., 1986. Trace element characteristics of graywackes and tectonic setting discrimination of sedimentary basins. Contributions to Mineralogy and Petrology 92, 181–193. Biscaye, P.E., 1965. Mineralogy and sedimentation of recent deep-sea clay in the Atlantic Ocean and adjacent seas and oceans. Geological Society of America Bulletin 76, 803–832. Boulay, S., Colin, C., Trentesaux, A., Pluquet, F., Bertaux, J., Blamart, D., Buehring, C., Wang, P., 2003. Mineralogy and sedimentology of Pleistocene sediment in the South China Sea (ODP Site 1144). In: Prell, W.L., Wang, P., Blum, P., Rea, D.K., Clemens, S.C. (Eds.), Proceedings of the Ocean Drilling Program, Scientific Results, 184, pp. 1–21 (online). Boulay, S., Colin, C., Trentesaux, A., Frank, N., Liu, Z., 2005. Sediment sources and East Asian monsoon intensity over the last 450 ky, Mineralogical and geochemical investigations on South China Sea sediments. Palaeogeography, Palaeoclimatology, Palaeoecology 228, 260–277. Bühring, C., Sarnthein, M., Erlenkeuser, H., 2004. Toward a high resolution stable isotope stratigraphy of the last 1.1 million years: site 1144, South China Sea. In: Prell, W.L., Wang, P., Blum, P., Rea, D.K., Clemens, S.C. (Eds.), Proceedings of Ocean Drilling Program, Scientific Results, 184, pp. 1–29 (online). Chao, S.-Y., Shaw, P.T., Wu, S.Y., 1996. Deep water ventilation in the South China Sea. Deep Sea Research, Part I 43, 445–466. Clift, P.D., 2006. Controls on the erosion of Cenozoic Asia and the flux of clastic sediment to the ocean. Earth and Planetary Science Letters 241, 571–580. Clift, P.D., Lee, J.I., Clark, M.K., Blusztajn, J., 2002. Erosional response of South China to arc rifting and monsoonal strengthening: a record from the South China Sea. Marine Geology 184, 207–226. Clift, P.D., Schouten, H., Draut, A.E., 2003. A general model of arc–continent collision and subduction polarity reversal from Taiwan and the Irish Caledonides. In: Larter, R.D., Leat, P.T. (Eds.), Intra-Oceanic Subduction Systems; Tectonic and Magmatic Processes, 219. Geological Society, London, pp. 81–98. Edgar, K.M., Wilson, P.A., Sexton, P.F., Suganuma, Y., 2007. No extreme bipolar glaciation during the main Eocene calcite compensation shift. Nature 448, 908–911. Ehrmann, W., 1998. Implications of Late Eocene to Early Miocene clay mineral assemblages in McMurdo Sound (Ross Sea, Antarctica) on paleoclimate and ice dynamics. Palaeogeography, Palaeoclimatology, Palaeoecology 139, 213–231. Esquevin, J., 1969. Influence de la composition chimique des illites sur le cristallinite. Bulletin de Centre Recherche Pau 3, 147–153. Guo, Z., Biscaye, P., Wei, L., Chen, X., Peng, S., Liu, T., 2000. Summer monsoon variations over the last 1.2 Ma from the weathering of loess-soil sequences in China. Geophysical Research Letters 27, 1751–1754. Huang, C.Y., 2002. Taiwan Tectonics (in Chinese). Geological Society China, Taibei. 1– 207. Huang, C.Y., Yuan, P.B., Tsao, S., 2006. Temporal and spatial records of active arc– continent collision in Taiwan: a synthesis. Geological Society of America Bulletin 118, 274–288.

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