Magnetic susceptibility variations and provenance of surface sediments in the South China Sea

Sedimentary Geology 230 (2010) 77–85

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

Magnetic susceptibility variations and provenance of surface sediments in the South China Sea Jianguo Liu ⁎, Zhong Chen, Muhong Chen, Wen Yan, Rong Xiang, Xianzan Tang Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China

a r t i c l e

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Article history: Received 16 May 2009 Received in revised form 1 July 2010 Accepted 7 July 2010 Available online 14 July 2010 Editor: G.J. Weltje Keywords: Magnetic susceptibility Surface sediments Fine-grained fraction Sediment sources South China Sea

a b s t r a c t Magnetic susceptibility (MS) and grain sizes of surface sediments are measured to characterize sediment sources and their distribution in the South China Sea (SCS). Distribution characteristics of MS of bulk samples (MSB), fine-grained fraction (MSF) and coarse-grained fraction (MSC) are examined to explore the factors affecting MS values in sediment transport and deposition processes. Affected by dilution of quartz and carbonate abundance, MSC is not suitable for tracing sediment sources in the region. Instead, MSF provides a good parameter for tracking transport and deposition of complex and coarse-grained sediments such as those from Luzon Island volcanics and from the Pearl and Mekong Rivers, which often have high MSF values showing a decreasing trend with water depth on the continental shelf (water depth b 200 m). To the west of Luzon Island, Kuroshio intrusion into the SCS is the predominant factor for sediment transport after high MSF volcanic materials from the Luzon Island are discharged into the sea. Sediments from the Pearl River are transported southwestward under the China coastal current and then deposited between the Pearl River mouth and Hainan Island. To the southwest of Taiwan Island in the northeastern SCS, where sediments are mainly derived from Taiwan Island and/or the Yangtze River, both the Kuroshio intrusion and the China coastal current are significant in determining sediment transport and deposition. In the south, most of high MSF sediments from the Mekong River are transported eastward under the influence of northeastward currents in summer after entering the sea and then deposited on the northern Sunda Shelf. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Magnetic susceptibility (MS) of sediments from lakes, soil and loess has now been widely used as a proxy of paleoclimate reconstruction (e.g. An et al., 1991; Verosub et al., 1993; Peck et al., 1994; Fang et al., 1999; Nawrocki et al., 2006; Blundell et al., 2009) and rainfall estimates (Maher and Thompson, 1995; Porter et al., 2001; Vidic et al., 2004; Maher and Hu, 2006) as well as for sediment provenance in various depositional environments. In the Argentine Basin, MS variability of core-top samples was connected with deepwater circulation and useful for tracing sediment sources (Sachs and Ellwood, 1988). In loess along the Illinois and central Mississippi River valleys, MS variations are controlled by silt-sized magnetite content and are interpreted to reflect changes in sediment provenance (Grimley et al., 1998). In the southern California, MS was a quick and useful tool in provenance investigations of siliciclastic strata (Kimbrough et al., 1997). In the North Atlantic, magnetic measurements of the surface sediments were used to identify distinct spatial patterns of sediment sources and transport pathways (Watkins and Maher, 2003). Based on the anhysteretic remanent magnetism (ARM)

⁎ Corresponding author. Tel.: + 86 20 89024537; fax: + 86 20 84451672. E-mail address: [email protected] (J. Liu). 0037-0738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2010.07.001

and MS measurements, Ellwood et al. (2006) even characterized sediment source and flow pattern distribution in the Gulf of Mexico. In the western Pacific marginal seas, magnetic properties of surface sediments in the muddy area of the East China Sea (ECS) were investigated to provide environmental magnetism evidence for possible transport mechanisms of fine-grained deposit (J. Liu et al., 2003a). Ge et al. (2003) used characteristics of MS of surface sediments in the South Yellow Sea to explore the relationship between magnetic properties and sedimentary condition or sediment source. Detailed magnetic analysis of ODP cores in the South China Sea (SCS) by Kissel et al. (2003) revealed links between changes of magnetic parameters and climatic proxies. Recently, Chen et al. (2009) investigated the possible implication of MS in surface sediment samples for sub-sea methane venting in the southern SCS. Although many mineralogic and geochemical methods are used to trace sediment sources and deposition in the SCS (Shao et al., 2001; Li et al., 2003; Z. Liu et al., 2003b, 2008; Yan et al., 2007), the use of magnetic data for sediment provenance has been limited, and basinwide magnetic variability data are lacking. In this paper, we report MS results of surface sediments from the whole SCS especially the distribution characteristics of MS of bulk samples (MSB), MS of finegrained fraction (MSF) and MS of coarse-grained fraction (MSC), and discuss factors affecting sediment transport, sediment sources and areal sediment distribution in the SCS.

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2. Regional setting As the largest marginal sea of the southeast Asia in the western Pacific Ocean, the SCS ranges from the equator to 23°N and from 99°E to 121°E. Around the SCS, there are three large rivers, the Mekong River with sediment discharges of 160 × 106 tons/year to the west, the Red River with 130 × 106 tons/year to the northwest and the Pearl River with 69 × 106 tons/year to the north (Z. Liu et al., 2003b). The Mekong River transects a major Paleozoic–Mesozoic sedimentary terrain in the upper and middle reaches, while the lower basin consists of Mesozoic sedimentary rocks (Z. Liu et al., 2004, 2005b). The upper and middle reaches of the Red River are prevailed by Paleozoic–Mesozoic sedimentary rocks, with minor igneous rocks exposed along the Red River fault zone. The Pearl River drains through South China, dominated by Paleozoic–Mesozoic carbonate rocks in the west and Mesozoic–Cenozoic granitic rocks and Paleozoic sedimentary rocks (limestone, shale and sandstone) in the east (Liu et al., 2007b). As its sediment load is predominantly derived from the western river branch (about 90%), the Pearl River sediments are dominated by carbonate (Shao et al., 2009). Fang et al. (1998) and Hu et al. (2000) show that the northern SCS circulation is mainly driven by monsoon winds and, to a lesser extent, by Kuroshio intrusion through the Luzon Strait. In the central SCS, the circulation is governed by both monsoon winds and interaction between the circulation systems in the northern and the southern SCS (Hu et al., 2000). In winter, strong monsoon blows from the northeast along the coasts of China and Vietnam, resulting in southward flow of saline surface waters from the ECS and low sea surface temperatures in the northern SCS. In summer, the warm Indian Ocean waters driven by the southwest monsoon flow eastward into the southern SCS, resulting in upwelling along the coast of Vietnam (Hu et al., 2000; Fernado et al., 2007. The SCS climate is dominated by the East Asian Monsoon System, which determines flow direction and sediment transport routes and even deposition. In the SCS, different sediment types are distinctly zonated and their boundaries are roughly parallel to the isobaths. On the continental shelf (water depths b200 m), surface sediments mainly consist of modern sediments, reworked late Pleistocene sediments, relict sediments as well as a small amount of residual sediments. In the bathyal–abyssal areas (water depths N200 m), bioclast component is an important parameter for sediment classification (Luo et al., 1994) and bottom sediment types often vary with water depths and are affected by the carbonate compensation depth (CCD). Geochemical and foraminiferal analyses indicate that the CCD lies at water depths between 3400 and 4200 m in the SCS (Chen and Chen, 1989; Calvert et al., 1993; Jian and Wang, 1997; Chen et al., 1997; Wang et al., 2007). Many factors including seafloor topography, sediment sources and water masses all influence the distribution of sediment types (Luo et al., 1994). Volcanic material is mainly distributed in the deep basin especially west of Luzon Island in water depths between 2000 and 4000 m (Chen et al., 2005). Some volcanic material controlled by faults occurs on the outer shelf of the southern Taiwan Island and the upper slope (water depths 200–2000 m) of the southwestern Dongsha Islands (Yan et al., 2006), and rare volcanic material is also distributed near the northern Nansha Islands in the southern SCS.

MS of surface samples was measured using a magnetic susceptibility bridge (KLY-1, Jelinek, 1973) in the paleomagnetism laboratory of the SCSIO. The results are reported in terms of sample mass (dried at 40 °C) than volume because the former was much easier and faster to measure with high precision than the latter. Each sample was measured three times and the mean was calculated in units of m3/kg (dry weight). To identify grain size effect on MS values (Caitcheon, 1998), samples were first sieved to separate the b63 μm fine-grained size from the N63 μm coarse-grained size fractions. We calculated respectively the MS values of bulk samples (MSB), finegrained fraction (MSF) and coarse-grained fraction (MSC) relative to the dried weight of relevant samples or fractions. Grain size of bulk sediment samples was analyzed in the SCSIO using laser methods (Malvern Mastersizer 2000). Particle size analyzers categorize grains in the 0.1 to 2000 μm range. The measurement repeatability of the instrument is 0.5%, and the reproducibility is better than 2%. The contour figures of magnetic susceptibility and grain size are created with the SURFER software. 4. Results 4.1. Grain size variations of bulk samples The fine-grained fraction in surface bulk sediments ranges from 0.3% to 99.1%, with the average of 72.8%. The northern shelf and the Sunda Shelf are the two major areas with relatively low average percentage (b30%) of the fine-grained fraction in sediments, while areas with high percent (N90%) are mostly located in the bathyal– abyssal basin (Fig. 2a). A similar trend exists for the median grain size (Md, in phi) values (Fig. 2b). Data presented in Figs. 2 and 3 indicate that grain size changes are related to water depth. The percent of fine-grained fraction and Md values vary greatly on the continental shelf (water depth b200 m). The fine-grained sediments increase on the continental slope (water depths 200–2000 m) to over 60%, and the coarse-grained fraction now mainly consists of biogenic material (Luo et al., 1994). In the abyssal basin (water depths N2000 m), sediments are mostly fine-grained with some coarse-grained biogenic material at local sites such as radiolarians or calcareous nannofossils (Wang et al., 2007; Chen et al., 2008). Therefore, grain size influence on MS measurements would be obvious when investigating sediment provenance and depositional process. 4.2. MS of bulk samples (MSB) The magnetic susceptibility of surface bulk sediments ranges from 0.5 × 10− 7 m3/kg to 2.3 × 10− 6 m3/kg with the average of 3.4 × 10− 7 m3/kg. Among the 168 samples measured, 20 samples have high MSB values (N5.0 × 10− 7 m3/kg), including 15 from west of Luzon Island (8.8 × 10− 7 m3/kg in average) and southwest of the Pearl River mouth (6.0 × 10− 7 m3/kg in average). Relatively low MSB values are recorded in 32 samples (b2.0 × 10− 7 m3/kg), mainly from the coastal area between the Taiwan Strait and the Pearl River mouth in the north (1.1 × 10−7 m3/kg in average of 6 samples) and from the Sunda Shelf in the south (1.6 × 10−7 m3/kg in average of 15 samples) (Fig. 4). 4.3. MS of fine-grained fraction (MSF)

3. Materials and methods A large number of unevenly distributed surface sediment samples (first 2 cm depth) from the SCS were collected using box or grab samplers by the South China Sea Institute of Oceanology (SCSIO), the Chinese Academy of Sciences, during the past 10 years. In this paper, 168 surface sediment samples in a semi-regular square grid were selected to investigate MS variability in the SCS (Fig. 1).

The MS values of the fine-grained fraction range from 1.7 × 10−8 m3/ kg to 3.9 × 10−6 m3/kg with the average of 4.9 × 10−7 m3/kg. High MSF values (N5.0 × 10−7 m3/kg) are found in 40 samples and low MSF values (b2.0 × 10−7 m3/kg) in 9 samples. Similar to the MSB, the MSD results also indicate two high MS areas (N5.0 × 10−7 m3/kg) (Fig. 5a), but high MS values occur in more samples from wider areas. To the west of Luzon Island, the high MSF area represented by 26 samples

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Fig. 1. Locations of surface sediment samples in the South China Sea.

expands to the north and west, and the average MSF value reaches 10.2 × 10−7 m3/kg. Another high MSF area extends to the shelf and even slope between the Pearl River mouth and east of Hainan Island, where the MSF value is 10.3 × 10−7 m3/kg in average of 13 samples. Unlike those of bulk samples, however, low MSF values do not confine to any geographic areas, as low MSF sites are randomly distributed offshore from western Kalimantan Island and along the northern Nansha Trough.

4.4. MS of coarse-grained fraction (MSC) The MS values of coarse-grained fraction range from −5.2 × 10−7 m3/ kg to 1.7 × 10−6 m3/kg with the average of 2.5× 10−7 m3/kg. There are only 7 samples with high MSC values (N5.0 × 10−7 m3/kg) but 57 samples with low MSC values (b2.0 × 10−7 m3/kg). Compared with the results from the fine-grained fraction, only 6 samples from the west of Luzon Island have high MSC values (9.3 × 10−7 m3/kg in average), in an area similar to the size represented by high MSB (Fig. 5b). In contrast, sites with low MSC

values increase to 57 sites, including 29 from the North Shelf and 20 from the Sunda Shelf. 5. Discussion 5.1. Effect of grain size compositions on MS values In Chinese loess, the dominant magnetic grain size lies just above 20–25 nm (Q. Liu et al., 2005a). For lake sediments, maximum and minimum MS values are related to fine- and coarse-grained particles, respectively (Thompson and Morton, 1979). In continental shelf sediments, MS values are relatively high in the silt and sand fractions but low in the clay and coarse sand fractions (Currie and Bornhold, 1983). Sediment types in the SCS are complex, and their compositions include clay, silt, sand, and even gravel. They also show distinct zonation, with zonal9 boundaries roughly parallel to the isobaths (Luo et al., 1994). In the northern SCS, low percentages (less than 30%) of the fine-grained fraction (b63 μm) occurs on the outer shelf between 50 m and 200 m (Figs. 2a and 3a). Contents of the fine-

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Fig. 3. Variances of (a) fine-grained fraction and (b) median grain size of surface sediments with water depth in the South China Sea.

Fig. 2. Distribution of (a) fine-grained fraction (b63 μm, unit: wt.%) and (b) median grain size (unit: phi) in surface sediments from the South China Sea.

grained fraction increase sharply on the continental slope until up to more than 90% in the abyssal area (N2000 m). In the southern SCS, low content of the fine-grained fraction (b10%) is found on the Sunda Shelf (center at 5–8°N, 106–108°E), which however is replaced down slope by a rapid increase to over 90%. Due to dilution of the coarse-grained fraction, MS values of bulk samples decrease with median grain size (Fig. 6a) and increase with

the fine-grained percent (Fig. 6b). Because magnetic minerals are mostly embodied in the fine-grained fraction (b63 μm), magnetic susceptibility is mainly related with the fine-grained percent in sediments (Ellwood et al., 2000). Compared with site numbers with low MSB values (32 sites) and high MSB values (20 sites), site numbers with low MSF values and high MSF values are 9 sites and 40 sites, respectively, while site numbers with low MSC values and high MSC values are 57 sites and 7 sites. These differences confirm that grain size composition is an important factor affecting MS values in sediments. 5.2. Influence of biogenic activities on MS distribution In marine environments not only ferromagnetic minerals (magnetite, maghemite), paramagnetic minerals (clay minerals, biotite and pyrite) but also diamagnetic minerals (calcite and quartz) can become magnetized (Ellwood et al., 2000; Balsam et al., 2005). Therefore, MS

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Fig. 4. Distribution of magnetic susceptibility of bulk sediment samples (MSB) in the South China Sea (unit: 10−7 m3/kg).

of sediments is influenced by relative composition of all these minerals in sediments (Ellwood et al., 2006). Diamagnetic minerals show weak negative susceptibility (Parasnis, 1962; Kearey and Brooks, 1984). On the continental slope of the SCS, sediments are mostly calcareous silty clay to clayey silt with 10% to N60% CaCO3 (Wang et al., 1995). In addition, negative MS values are also registered in coral reef sediments on the shelf and slope (Liu et al., 2002). Because magnetic susceptibility produced by ferromagnetic and paramagnetic minerals is far greater than that by diamagnetic minerals, the effect of carbonate productivity on magnetic susceptibility is usually relatively slight (Ellwood et al., 2000). In this study, only two samples with more coarse-grained fraction have negative MS values (Fig. 7), one from near Xisha Islands in a water depth of 2445 m and another from northeast of Hainan Island in a water depth of 30 m (Fig. 5b). In contrast, fine-grained fractions of all samples have positive MS values. These results indicate that the influence of biogenic material on MS distribution can be neglected for tracking sediment sources in the SCS. 5.3. Impact of terrigenous and volcanic materials on MS distribution In the SCS, two areas are directly connected with abundant terrigenous input from rivers. One is located at the southwest of the Pearl River mouth, where sediments derived from the Pearl River are transported southwestward under the effect of Coriolis force (Luo et al., 1994). Data presented in Figs. 4 and 5 not only support the above finding but also prove that MSF can be utilized to trace sediment transport despite that the carbonate dominated sediments from the Pearl River (Shao et al., 2009) have a relatively weak MS signal. The other area with high terrigenous input is the Sunda Shelf, which is formed by sediments mainly from the Mekong River. Clay mineral analysis reveals that the Mekong River sediments are transported mainly southeastward after entering the sea (Liu et al., 2010), with a transport route also confirmed by MSF distribution shown in Figs. 4 and 5. Although the Red River is a prominent system

Fig. 5. Distribution of (a) magnetic susceptibility of fine-grained fraction (MSF) and (b) magnetic susceptibility of coarse-grained fraction (MSC) in the surface sediments from the South China Sea (unit: 10−7 m3/kg).

in the northwest, its sediments mainly stay in the Beibu Gulf and the northwest shelf and seldom enter the abyssal basin, so the impact of the Red River sediments on the deep-water deposition is minimal. However, some fine-grained material derived from Indonesian Islands can be transported to the southern SCS under the influence of the southwesterly surface current in summer (Liu et al., 2007a).

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Fig. 6. Magnetic susceptibility of bulk samples (MSB) versus (a) median grain size and (b) weight percent of fine-grained fraction in surface sediments from the South China Sea.

In the SCS, sources of volcanic glasses include Philippine arc volcanoes, deep-sea intra-basinal submarine volcanoes, Indonesian arc volcanoes, and volcanoes of Indochina Peninsula and South China (Chen et al., 2005). Heavy mineral analysis showed that volcanogenic minerals from the abyssal plain and seamounts near the latitude of 15°N in the eastern SCS mainly came from the eroded volcanic rocks in the seabed and volcano eruptions in the vicinity of islands (Su and Wang, 1992; Yan et al., 2008). Figs. 4–5 also reveal a strong impact of volcanic material on the distribution of various MS parameters in the SCS, especially in the west of Luzon Island. 5.4. Water depth and MS correlations Fig. 7 shows correlations between water depth and the three MS parameters (MSB, MSF and MSC), indicating a low level of relationship between these MS parameters and water depth. When

Fig. 7. Various magnetic susceptibility parameters (MSB, MSF, MSC) of surface sediments versus water depth (a) within 4500 m and (b) within 200 m in the South China Sea. MSB: MS of bulk samples, MSF: MS of fine-grained fraction, MSC: MS of coarse-grained fraction.

water depth is shallower than 200 m (Fig. 7b), MSC remains to be low (b2 × 10−7 m3/kg), and MSF is better correlated than MSB to water depth. This indicates that MSF is more suitable for exploring sediment transport process in a shelf environment. However, all MS parameters change clearly when water depth is deeper than 2000 m. Previous studies have revealed that due to carbonate dissolution below the CCD, carbonate content in sediments may decrease from 60 to 70% at depths b1000 m to less than 5% at 4200 m (e.g. Miao et al., 1994). Evidence supporting this comes from high MSF and MSB values in samples from below the CCD (Fig. 7a). Our results also indicate highest MSC values in samples from the west of Luzon Island, where sediments are rich in volcanic material, in accordant with clay mineral analysis (Liu et al., 2008) and geochemical analysis of tephra in the area (Haeckel et al., 2001).

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5.5. Sediment provenance Fig. 8 shows sediment sources and their relationship with surface circulation in the SCS, primarily based on MSF results as high MS values can provide independent means to track detrital sediment dispersal patterns (Sachs and Ellwood, 1988). We use MSF to trace sediment sources and their distribution because it can diminish the dilution effect of carbonate and quartz in the coarse-grained fraction through measurement of the fine-grained fraction instead of the bulk sample (Figs. 4–5). Characterized by abundant ferromagnetic minerals (magnetite, maghemite) in sediments from the Pearl and Mekong Rivers are heavy in density, they are often transported only to a relative short distance out from the river mouths. Therefore, the MSF values of sediments from various sources are different, and those from local settings can be high enough to be distinguished from others far from the continent. Many factors, especially circulation patterns and topography (Luo et al., 1994), may affect the process of sediment transport and deposition in marine environments. The overall circulation in the SCS is seasonally cyclonic in winter and anticyclonic in summer with a few stable eddies (Fig. 8, of Fang et al., 1998) driven by monsoon winds, as well as water exchange between the SCS and the ECS through the Taiwan Strait, between the SCS and the Kuroshio through the Luzon Strait (Hu et al., 2000), and between the SCS and the Indian Ocean through the Strait of Malacca (Fig. 8). Often, the dominant current flows control sediment transport and deposition. To the west of Luzon Island, sediments with the highest MSF values (Fig. 5a) are transported northwestward predominantly under the influence of the Kuroshio intrusion, which brings warm and salty waters from the western Pacific into the SCS through the Luzon Strait (Chen and Tan, 1997; Liang et al., 2003; Wu and Chiang, 2007). The

Fig. 8. Sediment sources and sediment transport based on magnetic susceptibility of fine-grained fractions (MSF) in the South China Sea. Surface circulation during summer (dashed lines) and winter (solid lines) is modified from Fang et al. (1998). Shaded areas represent various sediment sources and major deposition areas based on MSF values of surface sediments.

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high MSF values in samples from the west of Luzon Island reflect the provision of Luzon Island volcanic material, and these sediments are subsequently deposited along the route of the Kuroshio intrusion. The Kuroshio intrusion also affects transport and deposition of sediments from Taiwan Island and/or the Yangtze River in the northeastern SCS (Fig. 8), although deposition of these sediments may also be affected by the China coastal current during the northeastern monsoon in winter (Jan et al., 2002; Su, 2004; Lin et al., 2005). The China coastal current, however, exerts a more significant role in sediment depositions between the Pearl River mouth and Hainan Island on the northern shelf (Zhou and Fan, 1989; Dong et al., 2004), as already revealed in a mineral assemblage study (Chen et al., 1986). In the southwestern SCS, sediments from the Mekong River in the southeast of Vietnam are transported eastward and then deposited on the northern Sunda Shelf (Fig. 8). Previous studies revealed that sand and sandy silt on the Sunda Shelf (sand content N70% in average) covered the seafloor along the main flow paths of the near-bottom currents in shallow-water and higher-energy environments (e.g. Szarek et al., 2006). High abundances of planktonic foraminifera were reported from upwelling areas and from regions affected by major current systems including boundary currents (Arnold and Parker, 1999). Combining all these data and our MS results, we conclude that sediments on the northern Sunda Shelf are mostly derived from the Mekong River, as a result of current transportation by the predominant summer monsoon in the southern SCS (Fig. 8).

6. Conclusions Magnetic susceptibility (MS) measurements provide a sensitive tracer of terrigenous and volcanic materials in the South China Sea (SCS). MS of the fine-grained fraction (MSF) is more effective in tracking sediment sources and sediment distribution, while MS of bulk samples (MSB) and MS of coarse-grained fraction (MSC) are often affected by quartz and carbonate in coarse-grained fractions and are therefore not considered good tracers. Relatively high MSF characterizes sediments from the Pearl River in the northern and from the Mekong River in the southeastern SCS, providing a reliable means for tracking the continued sediment transport in these regions. High values of MSF, MSC and MSB can all be used to study sediment transport in the west of Luzon Island, an area enriched in high MS volcanic material in both fine-grained and coarse-grained fractions even down to over 2000 m water depths. On the continental shelf, however, MSF shows a consistent decrease with water depth, indicating that MSF, rather than of MSB, is suitable for analyzing sediment transport when grain size changes greatly and sediment types are complex. MSF results reveal that sediment sources to the northern and southern SCS are mainly from major rivers and to the eastern SCS from the island arc, and sediment distribution is controlled by predominant currents in the SCS. High MSF sediments from the west of Luzon Island indicate that the Kuroshio intrusion affects sediment transport by carrying sediments westward from Luzon Island High MSF sediments from the Pearl River are transported southwestward under coastal currents in winter and then mostly deposited between the Pearl River mouth and Hainan Island. High MSF sediments between the Pearl River mouth and Taiwan Strait are mainly derived from Taiwan Island and/or the Yangtze River, and transportation and deposition of these sediments are controlled by both Kuroshio intrusion into the SCS and the China coastal current. Most of relative high MSF sediments from the Mekong River are transported eastward under the impact of the southwest flowing current in summer and subsequently deposited on the northern Sunda Shelf.

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Acknowledgements This work was jointly supported by the Knowledge Innovation Program of the Chinese Academy of Sciences (Nos. SQ200713 and LYQY200704), the National Natural Science Foundation of China (No. 40631007), the National Key Project for Basic Research of China (Nos. 2007CB815905 and 2009CB219502-2), the Open Fund of the Key Laboratory of Marginal Sea Geology, Chinese Academy of Sciences (No. MSGL08-16), and the Open Fund of the Key Laboratory of Marine Geology and Environment, Chinese Academy of Sciences (No. MGE2008KG08). We thank Richard Gyllencreutz and two anonymous reviewers for their helpful comments, and Qianyu Li for improving the English text. References An, Z.S., Kukla, G.J., Porter, S.C., Xiao, J.L., 1991. Magnetic susceptibility evidence of monsoon variation on the Loess Plateau of central China during the last 130,000 years. Quatern. Res. 36, 29–36. Arnold, A.J., Parker, W.C., 1999. Biogeography of planktonic foraminifera. In: Sen Gupta, B.K. 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