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2 Marine Geology 243 (2007) Geochemical compositions and provenance discrimination of the central south Yellow Sea sediments Shouye Yang a,, Jeung-Su Youn b a Department of Marine Geology, State Key Laboratory of Marine Geology, Tongji University, Shanghai , China b Department of Oceanography, College of Ocean Sciences, Cheju National University, Jeju-do , South Korea Received 28 September 2006; received in revised form 1 May 2007; accepted 12 May 2007 Abstract Geochemical compositions and sedimentation rates of surface and core sediments in the central south Yellow Sea were analyzed for the identification of sediment origins. The sediment accumulation rates measured by 210 Pb geochronology decrease significantly from the western part to the central area. Calcium carbonate, total organic carbon and most elements are more enriched in the western muddy sediments than in the sandy eastern part. Overall, geochemical compositions of the central south Yellow Sea sediments vary between those of Chinese and Korean river sediments supplied into the sea. Although CaCO 3 content exhibits regional variations in the central south Yellow Sea, the discrimination diagram of Sc/Al vs Cr/Th distinctly suggests that the muddy Holocene sediments in the central western part are derived ultimately from Chinese rivers, especially the Huanghe River, whereas the eastern sandy sediments primarily came from the Korean rivers during the postglacial transgression Elsevier B.V. All rights reserved. Keywords: sediment; provenance; Yellow Sea; geochemical composition; rivers 1. Introduction The Yellow Sea is a typical epicontinental sea, bordered by China and the Korea Peninsula. It is characterized by shallow water, gently sloping seafloor and featureless massive muddy deposits in the central and western parts, and a steeper seafloor and sandy sediments in the east, with the deepest water depth of about 100 m in the southeastern part (Fig. 1). The sedimentation in the Yellow Sea is primarily controlled Corresponding author. Tel.: ; fax: addresses: syyang@online.sh.cn (S. Yang), jsyoun@cheju.cheju.ac.kr (J.-S. Youn). by river inputs from the surrounding land including large rivers (e.g. the Huanghe, Changjiang, Yalujiang/ Aprok rivers) in China and relatively smaller rivers (Han, Keum, Yeongsan rivers) in Korea (Qin and Li, 1983; Milliman et al., 1985, 1987; Lee and Chough, 1989; Zhao et al., 1990, 1997, 2001; Alexander et al., 1991; Park and Khim, 1992; Chough et al., 2000; Park et al., 2000; Lee and Chu, 2001; Yang et al., 2003). Although at present both the Changjiang and the Huanghe do not empty directly into the Yellow Sea, they are still considered as the major sediment sources to the Yellow Sea. The Huanghe and Changjiang annually discharge about and tons of suspended sediments into their estuaries respectively, in which some escapes the estuaries and deposits in the Yellow Sea (Yang et al., 2003). A number of small rivers /$ - see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.margeo

3 230 S. Yang, J.-S. Youn / Marine Geology 243 (2007) Fig. 1. Sketch map of the Yellow Sea showing bathymetric contour in meters, the sampling locations, and the oceanic circulation pattern (modified after Yang et al., 2003). BCC: Bohai Coastal Current; LDCC: Liaodong Coastal Current; KCC: Korea Coastal Current; YSCC: Yellow Sea Coastal Current; YSWC: Yellow Sea Warm Current; CDFW: Changjiang Diluted Freshwater. Both surface grab samples and gravity cores were collected from Sites 2, 8 and 10 while only grab samples were taken from the other sites. draining the Korea Peninsula contribute small amounts of suspended sediments to the Yellow Sea annually (Schubel et al., 1984; Yang et al., 2003). In the past two decades many scientists have attempted to identify the sediment origins and reconstruct late Quaternary paleoenvironmental changes of the Yellow Sea using oceanographic, geophysical, sedimentological, and mineralogical methods (Chough and Kim, 1981; Qin and Li, 1983; Zheng and Klema, 1982; Yang and Milliman, 1983; Milliman et al., 1985; Lee and Chough, 1989; Alexander et al., 1991; Park and Khim, 1992; Chough et al., 2000; Lee and Chu, 2001; Gao, 2002; Yang et al., 2003; Yang and Liu, 2007). Although the Yellow Sea has increasingly attracted many research interests from a variety of scientific fields, more in-depth studies are urgently needed in order to better understand the problem Yellow Sea (Yang et al., 2003), and especially the sediment source-to-sink history needs to be clarified. More recently, the sediment origins of the Yellow Sea have been widely investigated using geochemical methods (Zhao et al., 1990, 2001; Cho et al., 1999; Kim et al., 1998, 1999, 2000; Yang et al., 2003, 2004), but consensus has rarely been attained. A detailed review of provenance discrimination of Yellow Sea sediments with special emphasis on geochemical approaches has been made by Yang et al. (2003). However, few case studies were included in that review. There are several muddy areas in the Yellow Sea, in which a mud patch in the central part of the south Yellow Sea (CYSM, Yang et al., 2003) has attracted most research considerations with respect to sediment provenance and depositional processes (Zhao et al., 1990, 2001; Park and Khim, 1992; Park et al., 2000; Yang et al., 2003). The CYSM has been suggested to be derived mostly from the Huanghe (Qin and Li, 1983; Milliman et al., 1987; Qin et al., 1989; Lee and Chough, 1989; Alexander et al., 1991; Park and Khim, 1992; Cho et al., 1999; Yang and Liu, 2007). However, other scientists suggested that it is a multi-sourced deposit based on mineralogical and geochemical compositions (Zhao et al., 1990, 1997), or a relict mud formed during the last glacial period (Hu, 1984). Therefore, the sediment provenance of the central south Yellow Sea remains unresolved and more lines of substantial evidence are needed to reliably identify the sediment origins from Chinese and Korean rivers. In this study, we measured grain-size patterns, major and trace element compositions, and sedimentation rates

4 S. Yang, J.-S. Youn / Marine Geology 243 (2007) of surface and core sediments in the central south Yellow Sea (CSYS). The main purposes of this study are to characterize compositional variations of the surface and core sediments, to establish provenance proxy indicators to distinguish Chinese and Korean river sediments, and then identify the sediment provenances of the CSYS. 2. Sample sources and methods A total of thirteen surface sediments (St-1 to St-13) and five gravity cores (CY96010, CY96008, CY96002, YS2 and YS3) were obtained from the CSYS in August 1996 and September 2001 (Fig. 1). Three gravity cores (CY96010, CY96008, CY96002) with lengths shorter than 50 cm were taken from the muddy CSYS together with the surface grab samples. These three short cores were only used for the measurements of sediment accumulation rate. The other two cores YS2 and YS3 are located at N, E and N, E with water depths of 78 m and 82 m; core lengths are 200 cm and 74 cm respectively. In the laboratory, grainsize patterns of surface and core sediments were measured using different methods after the samples were processed with 10% H 2 O 2 and 1 N HCl to remove organic matter and biogenic carbonate, respectively. For the surface sediments, a standard procedure was performed to measure the grain size, i.e. the sand fraction was analyzed by sieving methods and the silt and clay fractions by pipette techniques following Stokes Law. In comparison, grain-size compositions of cores YS2 and YS3 were measured by laser grain-size analyzer (Coulter LS 230) with the analytic precision of about 1%. Total organic carbon (TOC) was analyzed using a CHNS Analyzer (EA1110, Carlo-Erba, Italy) in the State Key Laboratory of Marine Geology at Tongji University. The relative deviations between the measured and certified concentrations of standards are lower than 0.3%. Calcium carbonate was determined using a Bernard Calcimeter for the surface sediments, and calculated from total inorganic carbon measured by a CHNS Analyzer for the core sediments. Fine-grained fraction(b 63 μm) was used for chemical analysis in order to minimize the grain-size effect on element concentrations (Loring and Asmund, 1996; Datta and Subramanian, 1998; Yang et al., 2002). The b63 μm fraction was separated from bulk sediments in deionized water by pipetting, dried at 50 C in a clean oven, and ground in an agate mortar. For element analysis, the powdered subsamples were digested with HF HNO 3 HClO 4 mixed solution in an airtight Teflon bomb and then leached with a diluted HNO 3 solution. The detailed sampling process follows Yang et al. (2002). Concentrations of major and trace elements were measured by Table 1 Grain size, total organic carbon (TOC), and carbonate compositions of surface and core sediments from the central Yellow Sea Samples Sand Silt Clay Sediment Mz Sorting Sk. Kurt. TOC CaCO 3 (%) (%) (%) Type (Φ) (Φ) (Φ) (Φ) (%) (%) St C St sc St sc St ms St cs St ms St sm St M St M St sm St ms St ms St S YS2-avg cs, sc YS3-avg ms YS2-std YS3-std YS2-CV YS3-CV Note: Mz: mean grain size; Sk: skewness; Kurt: kurtosis; ms: muddy sand; cs: clayey sand; sm: sandy mud; sc: sandy clay; M: mud; C: clay; avg: average; std: standard deviation; CV: coefficient of variation. The classification of sediment type is based on Folk et al. (1970).

5 232 S. Yang, J.-S. Youn / Marine Geology 243 (2007) Fig. 2. Lithology, grain size and elemental compositions of cores YS2 and YS3 sediments from the central south Yellow Sea. C: clay; Z: silt; S: sand; Mz: mean grain size; The 14 C dates after Chen et al. (2003) and Kim and Kucera (2000). For location of the cores, see Fig. 1. an Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES, JY58P-1) at the Korea Basic Science Institute. The analytic errors monitored by international geostandards are below 8%. For the estimation of sediment accumulation rates in the CSYS, three short cores (CY96010, CY96008 and CY96002) were analyzed by the radiochemical 210 Pb method. Core samples were sectioned at every 1 cm interval and each subsample was separated for 210 Pb analysis. Approximately 3.0 g of dried subsamples was passed through a 1-phi sieve to remove the coarse fraction and then was spiked with a known amount of 208 Po tracer. The sample was dissolved in an acid mixture of HNO 3, HClO 4, HCl and HF and then dried at 40 C. The Po isotope was digested in 1 N HCl and plated onto 1 cm 2 silver planchets. The 210 Po activities were determined at the Cheju Applied Radioisotope Research Institute by alpha spectrometry, and the counting error was generally less than 3%. The 226 Ra activity was also determined throughout the length of the cores by measuring the gamma ray emission of 214 Pb and 214 Bi leached samples.

6 S. Yang, J.-S. Youn / Marine Geology 243 (2007) Results and discussion 3.1. Sediment grain-size composition and depositional rates The surface sediment types in the CSYS vary from clay to sand, with mean grain size ranging from 2.7 Φ to 9.3 Φ (Table 1), with an overall coarsening trend to the east. The western part is covered by clay and silty clay with a mean grain size of 7.8 Φ and sand content lower than 10% (St-1, 9, 10). A sandy deposit exists in the northeastern part (St-11, 12, 13) and has a mean grain size of 3.6 Φ. The sandy deposit represents a transgressive basal layer formed during the sea-level rise in the early Holocene (Lee and Chough, 1989; Chough et al., 2004). The muddy and clayey sands are distributed in the eastern part of the Yellow Sea (St-4, 5, 6), and the mean grain size is about 3.4 Φ. The sandy and silty clays are dominant in the central region (St-2, 3, 7, 8) and have a mean grain size of 7.2 Φ. Overall, the surface sediments in the CSYS are poorly or very poorly sorted, fineskewed to coarse-skewed (Table 1). Core YS2 is primarily composed of dark silt in the upper 60 cm, dark-grey homogeneous mud from 60 to 140 cm, and dark-grey silt to sandy silt in the lower 60 cm. Shell fragments are occasionally present in the lower part of the core (Fig. 2). The mean grain size of core sediments ranges from 4.9 Φ to 7.5 Φ and averages 6.6 Φ (Table 1). Comparatively, Core YS3 mainly consists of grey muddy sand with sediment grain size ranging from 3.7 Φ to 5.0 Φ and an average of 4.3 Φ (Table 1, Fig. 2). Sand fractions in core sediments are all higher than 45% while clay fractions are lower than 15%. Similar to the surface sediments, core sediments are all poorly sorted for Core YS2 and very poorly sorted for Core YS3, and vary from very fine-skewed to coarse-skewed. The sediment character and lithofacies of cores YS2 and YS3 are very similar to those of nearby cores collected by Kim and Kucera (2000) and Chen et al. (2003).The 14 Cdates of these previously-studied cores suggest that sediments of cores YS2 and YS3 primarily formed during the postglacial period. The excess 210 Pb activity was used to estimate sediment accumulation rates following a simplified equation by DeMaster et al. (1985). The 210 Pb profile of station CY96010 shows a clear exponentially decreasing 210 Pb activity with depth (Fig. 3). The sedimentation rate, sediment material flux, and input of 210 Pb yield 0.68 cm/yr, g/cm 2 yr, and dpm/ cm yr, respectively. Similarly, the estimated sediment accumulation rates, sediment material flux and input of 210 Pb in sediment cores CY96008 and CY96002 range from 0.21 to 0.23 cm/yr, to g/cm 2 yr, and to dpm/cm yr, respectively. It is distinctive that the sedimentation rates are relatively lower in the central part than in the western region of the CSYS. Fig. 3. Depth profiles of 210 Pb activity from three box cores CY96010 (St-10), CY96008 (St-8) and CY96002 (St-2) in the central south Yellow Sea. For location of the cores, see Fig. 1.

7 234 S. Yang, J.-S. Youn / Marine Geology 243 (2007) Fig. 4. Correlations between mean grain size (Mz) and the elemental concentrations of the central south Yellow Sea sediments. Nevertheless, our data fall well within the data range of previous reports (Alexander et al., 1991; Li et al., 2002). Station CY96010 is located just to the east of Shandong Peninsula, thereby being directly affected by the Huanghe sediment input (Fig. 1, Qin and Li, 1983; Milliman et al., 1985, 1987; Qin et al., 1989; Zhao et al., 1990; Alexander et al., 1991; Zhao et al., 1997; Gao, 2002; Liu et al., 2004). The 210 Pb deposition rates of the mud wedge enclosing the Shandong Peninsula vary between 1.24 and 0.64 cm/yr (Liu et al., 2004). The decreasing deposition rates from the northwest to eastern part in the CSYS suggest a dispersal pattern of suspended

8 S. Yang, J.-S. Youn / Marine Geology 243 (2007) sediment from the entrance between the Shandong Peninsula and Korean Peninsula to the central zone of mud Major and trace element compositions Concentrations of TOC and CaCO 3 The contents of CaCO 3 and TOC in the surface sediments vary from 2.8% to 10.5% and from 0.3% to 1.3%, respectively (Table 1). Contents of CaCO 3 and TOC in the western muddy sediments (St-1, 9, 10) are higher than those in the sandy sediments in the eastern part of the CSYS (St-4, 5, 6, 12, 13). The decreasing trends of CaCO 3 and TOC contents from the western to the eastern part of the CSYS are in general agreement with the sediment grain-size pattern. A good correlation between TOC and mean grain size (R 2 =0.91; Fig. 4) is present in the surface sediments. Contents of TOC and CaCO 3 in Core YS2 sediments with ranges of % and % respectively are more variable than those in the surface sediments. In contrast, Core YS3 sediments have markedly lower contents and smaller variations of TOC and CaCO 3, ranging from 0.18% to 0.43% and from 0.14% to 0.85%, respectively (Fig. 2). There exists no clear correlation between TOC and mean grain size in core sediments (Fig. 4). It is well known that the majority of the Huanghe river sediment comes from the Loess Plateau in northwest China, which is characterized by a high content of calcium carbonate (Qin and Li, 1983; Qin et al., 1989; Yang et al., 2003). Therefore, high CaCO 3 content has been widely used to discriminate the Huanghe sediment from other Chinese river sediments and particularly from the Korean river sediments which have extraordinarily low CaCO 3 contents (Yang and Milliman, 1983; Milliman et al., 1985; Alexander et al., 1991; Cho et al., 1999; Yang et al., 2003, 2004). The CaCO 3 contents in the western muddy sediments of the CSYS are about two times higher than those in the eastern sandy sediments, suggesting that most of the fine-grained sediments in the western part of the CSYS were probably derived from the Huanghe. The CaCO 3 contents in the muddy sediments of Core YS2 mostly fall within compositional variations of the surface sediments, and are relatively lower than those of a core which was taken from a more western part of the CYSY (Core B10, Chen et al., 2003). The CaCO 3 contents in Core YS2 are significantly higher than those in Core YS3 which is located in the eastern sandy part of the CSYS. It has been suggested that CaCO 3 content increases with decreasing particle grain size and is relatively enriched in the clay fraction of the Huanghederived sediments (Yang et al., 2004). This further Table 2 Element concentrations in the central Yellow Sea sediments (major elements : wt.%; trace element: μg/g) Elements St-1 St-2 St-3 St-4 St-5 St-6 St-7 St-8 St-9 St-10 St-11 St-12 St-13 K Na Ca Fe Mg Al Ti Mn Ba Sr Rb Zr Ni Co V Cr Cu Zn Pb Th Nb Li Sc

9 236 S. Yang, J.-S. Youn / Marine Geology 243 (2007) suggests that the muddy sediments in the central and western parts of the CSYS including those in Core YS2 are mostly derived from the Huanghe. However, the CaCO 3 content alone cannot be used directly as a reliable provenance indicator to discriminate sediment origins of the Yellow Sea because its content in marine sediments is readily influenced by biogenic carbonate which can be difficult to distinguish from detrital components (Yang et al., 2003) Concentrations of major and trace elements Different major and trace elements show large variations in concentrations in the CSYS sediments, and overall compositional variations in the core sediments are smaller than those in the surface sediments (Table 3). In comparison, alkaline and alkaline earth elements such as K, Na, Ba, Rb, and Sr and trace elements of Zr and Pb have relatively smaller compositional variations. Most elements with the exceptions of K, Na, Ba, and Zr are relatively enriched in the sediments of Core YS2 than in Core YS3. Similar compositional variations with depth for most analyzed elements were observed in Core YS2, showing higher concentrations in the middle part ( cm) (Fig. 2). Comparatively, most elements have uniform variations in Core YS3, showing lower concentrations in the upper part (Table 2). The surface and core sediments of the CSYS have average element concentrations mostly between those of Chinese and Korean river sediment (Table 3). The surface sediments in the CSYS are characterized by higher concentrations of K, Na, and Li, and remarkably lower concentrations of Ba and Zr than in the river sediments (Table 3). Correlation plots show that concentrations of Al and most transition elements have good correlations with mean sediment grain size of the CSYS sediments, especially in the surface sediments (Fig. 4), which shows that chemical compositions reflect sediment grain-size patterns. A poor correlation between sediment grain size and chemical composition in sandy Core YS3 sediments reflects the fact that the b63 μm fraction was used for chemical measurements while bulk samples were used for grain-size analyses. Similar co-variance of sediment grain size and chemical compositions of sediments in the south Yellow Sea was reported by Kim et al. (1998), Cho et al. (1999), and Yang et al. (2003). Although the b 63 μm fine-grained fraction of Table 3 Comparisons of element compositions between the CSYS sediments and the river sediments (major elements : wt.%; trace element: μg/g) Elements YS2 YS3 Surface sediments Rivers Mean CV Mean CV Mean STD CV Changjiang Huanghe Keum K Na Ca Fe Mg Al Ti Mn Ba Sr Rb n.d. n.d. n.d. n.d Zr Ni Co V Cr Cu Zn Pb Th Nb Li Sc Note: STD = standard deviation; CV = coefficient of variation; n.d. means no determination. Note the element compositions of the river sediments are from Yang et al. (2003, 2004) and the others are unpublished data from the authors.

10 S. Yang, J.-S. Youn / Marine Geology 243 (2007) sediment has been widely considered for sedimentary geochemical study in order to minimize grain-size effect on element concentrations (Loring and Asmund, 1996; Datta and Subramanian, 1998; Yang et al., 2002), our data indicate that the b63 μm fraction cannot ultimately eliminate the grain-size effect. In view of this, it is unreliable to identify the sediment origins of the Yellow Sea by using absolute element concentrations even in the same grain-size fraction, and more reliable provenance indicators are needed Provenance discrimination of the CSYS based on geochemical composition It is well known that the Huanghe sediments are characterized by high concentrations of Ca, Na, Sr, and CaCO 3 and the Changjiang sediments have relatively higher concentrations of most transition elements, whereas the Keum river sediment is noticeably rich in Al, K, and Ba (Zhao et al., 1990, 2001; Cho et al., 1999; Kim et al., 1999; Yang et al., 2003, 2004). Accordingly, some of these geochemical indices, e.g. concentrations of alkaline and alkaline earth elements and CaCO 3 may give general distribution and dispersal patterns of the river sediments in the marginal seas. Core YS3 sediments have extraordinary low CaCO 3 contents compared to those surface and Core YS2 sediments (Table 1; Fig. 2). The CaCO 3 contents are lower than 1% in Core YS3 sediments, which is very similar to that in the Korean river sediment (Yang et al., 2003, 2004). In contrast, the muddy surface and Core YS2 sediments have more variable element concentrations, and CaCO 3 contents are closer to those of Chinese river sediments. The compositional variations of these major elements Fig. 5. Correlation plots between Al and trace elements in the surface and core sediments of the CSYS. Note that variable correlations exist between different elements and relatively good correlations occur between Al and Nb, Th, and Sc.

11 238 S. Yang, J.-S. Youn / Marine Geology 243 (2007) apparently suggest that most of the surface and Core YS2 sediments may have a provenance similar to that of the Chinese river sediments, whereas most of Core YS3 sediments are probably derived from Korean rivers. However, these geochemical indices may produce misleading results because they are not invariable under marine depositional environments. Furthermore, biogenic activity in marine environment may significantly mask these relationships, which makes it difficult to discriminate the biogenic components from the detrital sources through the river and aeolian detrital inputs (Cho et al., 1999; Yang et al., 2003, 2004). Element ratios, especially normalizing elemental concentration to Al content, are commonly used to distinguish the sources of terrigenous materials from autogenic components (Ergin et al., 1996; Cho et al., 1999; Yang et al., 2003). Correlation plots reveal that positive correlations exist between Al and some trace elements such as Sc, Cr, Nb, and Th, especially high correlations for the surface sediment samples (Fig. 5). In this study, element ratios of Sc/Al and Cr/Th are used to identify the sediment provenances of the CSYS in view of the relatively conservative behaviors of these elements during sediment formation and their high enrichments in residual fractions of marine sediments (Zhao and Yan, 1994). Moreover, these element ratios yield distinct values between the Changjiang, Huanghe and Kuem river sediments and therefore, can be treated as good provenance indicators for the identification of these river sediments. Scatter plots of Cr/Th vs Sc/Al clearly show that the CSYS sediments can be clustered Fig. 6. Discrimination diagram of Cr/Th vs Sc/Al of the sediments from the central south Yellow Sea and Chinese and Korea rivers. into different types (Fig. 6). Overall, Core YS2 sediments have geochemical ratios close to the Changjiang and Huanghe sediment averages despite their large variations, whereas Core YS3 sediments have element ratios similar to those of the Keum river sediment. The surface sediments in the CSYS also exhibit a systematic pattern in the ratios. The sandy sediments in the eastern part (St-4, 5, 6, 12, 13) plot closely to the Keum river sediment, whereas the muddy sediments in the western and central parts more closely approximate the Chinese river sediments (Fig. 6). Consequently, the discrimination diagram suggests that the muddy surface and Core YS2 sediments in the central western part of the CSYS are primarily derived from the Changjiang and Huanghe, whereas the sandy surface and Core YS3 sediments in the eastern part are supplied mostly by Korean rivers such as the Keum River. The Keum River has been suggested to supply a large amount of sediment into the coastal area of the southeastern Yellow Sea (Chough and Kim, 1981; Lee and Chu, 2001; Yang et al., 2003) Sediment distribution and dispersal patterns of the CSYS Many previous studies have revealed that suspended sediments from the modern Huanghe are transported southeastward, pass around the Shandong Peninsula and deposit in the northwest and central parts of the south Yellow Sea (Qin and Li, 1983; Yang and Milliman, 1983; Milliman et al., 1985, 1987; Qin et al., 1989; Alexander et al., 1991; Zhao et al., 1990, 2001; Gao, 2002; Liu et al., 2004; Yang and Liu, 2007). Another pathway of the Huanghe-originated sediment to the CSYS is from the abandoned Huanghe Delta in the northern Jiangsu coastal plain which formed from 1128 to 1855 when the Huanghe directly emptied into the south Yellow Sea (Yang et al., 2002, 2003). The abandoned Huanghe Delta experienced significant erosion after the shift of the Huanghe course northward to the present-day Bohai Sea. At present, the Changjiang sediments are mostly trapped in its estuary and partly escape southeastward to the Hangzhou Bay and, therefore, have a small contribution to the CSYS. Consequently, the muddy surface sediments in the central western CSYS are primarily derived from the modern and old Huanghe. The sediment accumulation rates also suggest the sediment transport pathway from the western part to the central south Yellow Sea. Our data show that the Keum River does not supply much fine-grained sediments to the central Yellow Sea, because it has a lower sediment flux than Chinese

12 S. Yang, J.-S. Youn / Marine Geology 243 (2007) rivers, and most of its sediments are trapped in the estuary and transported towards the southeastern Yellow Sea (Chough and Kim, 1981; Lee and Chough, 1989; Cho et al., 1999; Chough et al., 2000; Lee and Chu, 2001; Chough et al., 2004). Nevertheless, geochemical data of the surface and Core YS3 sediments in the east CSYS suggest that these sandy sediments predominantly come from the Keum River and/or local coastal sources. The lower sediments of Core YS3 are considered to have formed at 12 ka B. P. according to the radiogenic 14 C dating of a neighboring core (Fig. 2; Kim and Kucera, 2000). Geophysical and sedimentological studies revealed that these sandy sediments formed during the postglacial transgression with rapid retreat of the shoreface, and are presently exposed on the sea floor due to meager sediment supply from the neighboring Korea Peninsula (Lee and Chough, 1989; Chough et al., 2004). The sediment source and formation process of Core YS2, however, differ significantly from those of Core YS3. Core YS2 sediments were derived from different sources with the Changjiang-originated material dominant in the middle and lower parts. Core YS2 is located in the central part of the south Yellow Sea where the Yellow Sea Cold Water (YSCW, Yang et al., 2003) occurs, forming a low-energy, cold-eddy depositional environment with the intrusion of the Yellow Sea Warm Current (YSWC, Hu and Li, 1993; Yang et al., 2003). The YSWC and the present-day ocean circulation pattern in the Yellow Sea were suggested to have been formed during the middle Holocene of 8 6 kab.p.(liu et al., 1999; Kim and Kucera, 2000). Therefore, we deduce that the lower part of Core YS2 ( cm in depth) might have formed during the deglacial period with the strong influence of river input. At that time, the Huanghe entered the Bohai Sea predominantly in the north and exerted a minor control on the sedimentation of the CSYS (Yang et al., 2002). In contrast, the Changjiang might have supplied a large amount of sediments to the south Yellow Sea before the formation of its modern delta. In the middle Holocene, with the establishment of the YSWC and YSCW over the CSYS, the oceanic circulation as well as sedimentation pattern changed significantly. Consequently, the middle part of Core YS2 ( cm in depth) primarily consists of Changjiangderived homogeneous clayey sediments which formed during eddy-dominated depositional setting. During the late Holocene, the general ocean circulation and sedimentation patterns of the south Yellow Sea remained stable (Kim and Kucera, 2000; Chough et al., 2004). The provenance of the CSYS muddy sediments, however, might have changed because of the frequent shift of the Huanghe river course and the rapid development of the Changjiang and Huanghe deltas. The upper part of Core YS2 (0 60 cm in depth) seems to be of Huanghe origin in view of the higher Ca content and relatively lower Cr/ Th and Sc/Al ratios (Fig. 2). During the late Holocene the Huanghe frequently changed its course to the northern Jiangsu coastal plain, and directly emptied its huge sediment supply into the southwest Yellow Sea (Milliman et al., 1987; Yang et al., 2002). In addition, recent geophysical study suggested that the modern Huanghederived sediments can be resuspended in the Bohai Sea and transported into central south Yellow Sea by coastal alongshore currents (Yang and Liu, 2007). At that time, the majority of the Changjiang sediments were trapped within the estuary and developed its large delta, and little escaped southeastward to the Hangzhou Bay. Therefore, the Changjiang-originated material made a small contribution to the CSYS sedimentation. Consequently, our preliminary study suggests that the sediment provenance of the south Yellow Sea is complex and varies significantly in different areas and at different times because of variable river inputs during the Holocene. Detailed sedimentation processes, sediment dispersal patterns and related paleoenvironmental changes in the south Yellow Sea need more oceanographic, geophysical, and geological work. 4. Conclusions A total of thirteen surface sediments and five gravity cores were taken from the central south Yellow Sea for geochemical analysis and sedimentological study. The muddy deposits are mostly distributed in the central and western parts, and have higher contents of total organic carbon and CaCO 3. The sandy sediments are dominated in the eastern area and bear lower concentrations of organic carbon and CaCO 3. Particularly, the CaCO 3 contents in the central western part are significantly higher than those in the east, which implies that the carbonate-rich Huangheborne sediment might contribute significantly to the central western area of the south Yellow Sea. The sediment accumulation rate of the muddy sediment in the east off Shandong Peninsula is 0.68 cm/yr, decreasing to cm/yr in the central Yellow Sea mud area, along the transport pathway of the modern Huanghe sediment to the CSYS (central south Yellow Sea). Most of the element concentrations in the CSYS sediments show large variations, especially in the surface sediments, ranging between those of the Changjiang, Huanghe, and Keum river sediments. The muddy sediments in the central western part are characterized by higher concentrations of Ca, Mg, and

13 240 S. Yang, J.-S. Youn / Marine Geology 243 (2007) most transition elements than those sandy sediments in the eastern part. A discrimination diagram of Sc/Al vs Cr/Th demonstrates that the muddy surface sediments in the central western part are mostly supplied by the modern and/or old Huanghe through transport by the Yellow Sea Coastal Current, whereas the postglacial muddy sediments in the CSYS were derived from diverse sources varying between the Changjiang and Huanghe origins. In contrast, the sandy sediments in the eastern part primarily came from the Keum River during the postglacial transgression. Acknowledgements We thank S. C. Lee and S. Y. Bae at Cheju Applied Radioisotope Research Institute for their helpful guidance in the use of isotopic facilities. Thanks extend to Z. B. Wang and Y. G. Mei for sampling process and analytic work. G. de Lange, W. W.-S Yim and two anonymous reviewers are also greatly appreciated for their constructive comments on the original draft. 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