Recent sediment accumulation and origin of shelf mud deposits in the Yellow and East China Seas

Progress in Oceanography Progress in Oceanography 73 (2007) 145–159 www.elsevier.com/locate/pocean

Recent sediment accumulation and origin of shelf mud deposits in the Yellow and East China Seas D.I. Lim

a,*

, J.Y. Choi b, H.S. Jung c, K.C. Rho a, K.S. Ahn

d

a

c

South Sea Research Institute, KORDI, 391 Jangmok-ri Jangmok-myun Geoje 656-830, Republic of Korea b Department of Oceanography, Kunsan National University, Kunsan 573-360, Republic of Korea China-Korea Joint Ocean Research Center, 6 Xianxialing Road, High-Tech. Park, Qingdao, 266061, China d Department of Earth Sciences, Chosun University, Gwangju 501-759, Republic of Korea Received 16 October 2006; received in revised form 26 February 2007; accepted 26 February 2007 Available online 12 March 2007

Abstract Modern (last 100 yr) accumulation rates of shelf mud deposits in the Yellow and East China Seas were investigated using the distribution of excess 210Pb (210Pbex) in sediment core samples. Compilation and merger of new and previously published data helped clarify sediment accumulation in these seas. The estimated accumulation rates, together with data of suspended sediment concentrations, provided findings on the sediment budget, origin, and transport pathway of the mud deposits. The overall accumulation distribution in the Yellow and East China Sea shelf revealed a general, cross-shelf decreasing trend along the sediment dispersal system away from the rivers, except for the South Sea (SSM) and southeastern Yellow Sea (SEYSM) mud patches found along the Korean coast. Notably, 210Pbex activity profiles within the SSM and the SEYSM yielded a relatively high accumulation rate of 2–5 mm/yr, implying a sedimentation rate of 4– 15 · 107 tons per year in this coastal zone. Such an annual accumulation rate is about one order of magnitude greater than the total sediment discharge (6–20 · 106 tons/yr) from Korean rivers, suggesting an additional offshore source. The distribution pattern of the well-defined suspended plume clearly showed the possible transport and exchange of fine-grained sediments between the ECS shelf and the coastal area of Korea, especially during winter. Such a high accumulation in Korean coastal areas is attributable to the sediments supplied from the mud deposit of the ECS (i.e., SWCIM), with origins in Chinese rivers. Therefore, the Korean coastal area may be an important sink for some of Chinese river sediments being transported from the south by the Yellow Sea Warm Current.  2007 Elsevier Ltd. All rights reserved. Keywords: Accumulation rate; Sediment budget; Shelf mud patches; Suspended sediment concentration; Yellow Sea; East China Sea

1. Introduction In the western Pacific region, the Yellow Sea and the East China Sea (ECS) between the contiguous landmass of Korea and China form a typical epicontinental shelf with strong tidal action and complex shelf *

Corresponding author. Fax: +82 55 639 8589. E-mail address: [email protected] (D.I. Lim).

0079-6611/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.pocean.2007.02.004

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circulation systems that exhibit characteristic seasonal and spatial variations (Chen et al., 1994). On an annual basis, these seas receive approximately 109 tons of terrigenous material from neighboring rivers but mostly from the Changjiang and Huanghe rivers, and about 0.5 · 109 tons from coastal erosion at the Jiangsu coast of the Old Hunghe delta (Milliman and Meade, 1983). The transportation of fine-grained sediments derived from these rivers is controlled by estuarine processes, tidal currents, shelf circulation, and episodic storm/ typhoon events. Because of the large discharge (up to about 10% of the world’s river sediment load) and complex sedimentation processes, the Yellow and East China Sea shelf has attracted many geological research projects, yielding a better understanding of sedimentary processes in this ancient epicontinental setting. Surface sediments in shelf areas of the Yellow Sea and the ECS are dominated by fine-grained muds except for relict sands on the outer-shelf zone and transgressive basal sands in the northeastern Yellow Sea. The most characteristic feature of the continental shelf is the existence of four isolated mud patches (>6–7 phi in mean grain size) associated with coastal mud belts and open shelf mud patches (Fig. 1a and b). The central Yellow Sea mud (CYSM) and the southwestern Cheju Island mud (SWCIM) are two isolated shelf mud patches located in deeper continental shelf areas of the Yellow Sea and the ECS. They are formed by specific sedimentation mechanisms (i.e., a cyclonic gyre upwelling/downwelling processes) and consist mainly of clay-sized material finer than other mud deposits of the region (Hu, 1984; Zhu and Chang, 2000; Gao and Jia, 2003; Shi et al., 2003a,b). Their sediments are considered to be derived primarily from the Huanghe or Changjiang Rivers, or both, based on the circulation pattern in the central Yellow Sea and spatial distributions of various sedimentological, clay mineralogical, and geochemical attributes (Milliman et al., 1985a,b; Lee and Chough, 1989; Alexander et al., 1991; Park and Khim, 1992; Lim et al., 2006). In contrast, the coastal mud belts consisting of southeastern Yellow Sea mud (SEYSM) and South Sea mud (SSM) are associated with abundant material supply and strong tidal currents (Lee and Chu, 2001). Along the inner shelf of Korea, these two coastal mud deposits clearly differ from the CYSM in texture (e.g., clay content) and clay mineral composition (e.g., smectite content) (Park and Khim, 1992; Zhao et al., 2001). In addition, they are derived primarily from Korean rivers, on the basis of sedimentological and clay mineralogical data (Lee and Chough, 1989; Park and Khim, 1990; Lee and Chu, 2001). Until now, various interpretations have been presented concerning the ultimate sources and budgets of the SEYSM and SSM. Lee and Chough (1989) reported that the sediment loads from Korean rivers may be sufficient to account for the formation of these mud deposits along the Korean coast. In contrast, Alexander et al. (1991) argued the results of sediment budget calculations, especially concerning the SEYSM, and estimated that annual sediment deposits in the

Fig. 1. (a) Mean grain-size (phi) distribution of surface sediments in the Yellow and East China Seas. Note that there are four different mud patches (>6 phi in mean grain size) in the Yellow and East China Seas. CYSM: central Yellow Sea mud, SWCIM: Southwestern Cheju Island mud, SEYSM: southeastern Yellow Sea mud, SSM: South Sea mud. Contour interval is 0.5 phi; (b) sites of sediment samples and/or grain size data used in this study and obtained from several institutes and universities in Korea. The distribution map of grain size (a) was based on the approximately 1500 grain size data points.

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SEYSM were 4.9–8.7 · 107 tons, an order of magnitude greater than a previous estimate of 5 · 106 ton/yr (Lee and Chough, 1989). In recent years, sediment cores and seismic profiles collected by the Korea Institute of Geology, Mining, and Materials (KIGAM, 1996) and Park et al. (1999) have indicated that the SEYSM and SSM are as thick as 50 m and mainly consist of Holocene mud younger than 6000–7000 yr BP in age, yielding a relatively high accumulation rate of approximately >7 mm/yr during the late Holocene. Sediment discharge from Korean rivers is considered to be insufficient for the formation of these thick mud deposits. In this context, some researches (Zhao et al., 1990; Gao et al., 1996; Park et al., 2000; Zhao et al., 2001) proposed other additional offshore sources for the mud accumulation along the Korean coast, even though ultimate origin and transport pathway of these additional sediments do not appear to be well understood. Over the last several decades, many marine geologists have examined the depositional environment on the continental shelf of Korea. Their efforts have focused particularly on identifying the source of shelf sediments, understanding the transport processes of suspended materials, and interpreting the formation processes of the mud deposits. In the present study, we investigate the accumulation rate of sediment and the distribution of suspended matters in the Yellow and East China Seas to better understand regional sediment transport, accumulation of shelf mud deposits, and the depositional environment of the Korean continental shelf. 2. Materials and methods Interdisciplinary oceanographic research projects were conducted in the area, including the southern Yellow Sea, the northern East China Sea, and the South Sea of Korea, during winter (January) and early spring (April) 1999 (Fig. 2). Each research cruise was completed within a 2-week period; thus the results provide a quasi-synoptic distribution pattern of suspended matter. Water temperatures and salinities were measured at each station using conductivity-temperature-depth (CTD) sensors (Model SBE-911). On board, surface and bottom water samples were filtered through pre-weighed 0.45 lm Millipore membrane filters to measure suspended particulate matter (SPM) concentrations. SPM concentrations were compared with water transparency measured by a transmissometer (Model Seatech 25) attached to the CTD (Fig. 3). The SPM concentrations correlated well with water transparencies, and the correlation coefficient (r2) was higher than 0.8 for the range of 1–80% water transparency. Water transparency of 50% corresponds to SPM concentrations of about 5–10 mg/l. A total of 26 box cores up to 40–60 cm in length were collected from the study area. In the laboratory, the sediment cores were sampled at 2–5 cm intervals, dried in the oven, and then powdered using an agate mortar. The 210Pb geochronologies were determined following the method proposed by Nittrouer et al. (1979). A 209Po spike (used to monitor chemical yield) was added to 5 g of powered sediments before acidic solution (HNO3, HCl, and HClO4) as internal tracer. After the sediment was leached, the sample was dried and then dissolved

Fig. 2. Map showing (a) the study area with bathymetry (m) and (b) box core locations in the study area.

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Fig. 3. Relationship between water transparency (%) and the concentration of suspended particulate matter (SPM) (%).

in 1 M HCl. The acid solution was separated from the particles by centrifuge, and the polonium isotopes were plated spontaneously onto a silver planchet for each sample. 210Pb activities were determined at Virginia Institute of Marine Sciences (VIMS) by counting the alpha decay of its granddaughter, 210Po. For comparison and cross-checking of the result, 137Cs and 210Pb activities were also measured by gamma spectroscopy for some core samples. 3. Results 3.1. SPM concentrations and distribution patterns Fig. 4 shows the spatial distribution patterns of temperature, salinity, and transparency measured in the surface and bottom layers of the water column. In January 1999, a coastal water mass with relatively lower temperature and salinity was found both in the ECS and in the coastal area of Korea. Warm and saline offshore waters with a high transparency (>50%) were located in the central and southeastern parts of the study area surrounding Cheju Island. The transparency decreased to 10–30% in the ECS and the Korean coastal areas. Notably, a turbid water mass with SPM concentrations >10 mg/l (<30% transparency) formed a plume-like body trending NE–SW across the central section of the study area, resulting in connection of the ECS shelf with the Korean coastal area. This trend was apparent in the bottom layer. Temperature and salinity distribution patterns in April 1999 were similar to those in January 1999 (Fig. 4). However, transparency distribution patterns were somewhat different in surface and bottom layers. In the surface layer, clearer offshore water was located in the central part of the study area, and turbid water only occurred in the ECS and Korean coastal areas. In the bottom layer, however, a turbid water plume with less then 10– 20% transparency (SPM concentrations of >20 mg/l) was present and showed a NE–SW trend. The plume further expanded northward into the southern Yellow Sea and eastward into the South Sea along the Korean coast (Fig. 4). Fig. 5 shows the temperature, salinity, and transparency structure of the water column along a transect line. In January 1999, the vertical structures of temperature and salinity were homogeneous, but transparency

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Fig. 4. Spatial distribution of temperature, salinity and transparency in surface and bottom layers of water column, investigated at January and April 1999, respectively. Note that the turbid water plumes (shaded areas) with high suspended sediment concentration connect the East China Sea with the Korean coastal area, especially in the bottom layer.

clearly decreased from >60% in the upper water column to 10–40% in the lower column, especially in the Korean coastal area, the ECS, and in the bottom waters of >50 m water depth. In April 1999, water structures showed vertical stratification in temperature and salinity profiles with the thermocline and halocline at about 10–20 m and 50–60 m water depth, respectively. Transparency increased to more than 70% in surface and middepth water columns in the central part of the study area. However, transparency was still lower than 10–30% along the Korean coastal area and in the ECS. The turbid bottom waters were connected through the central deep trough, forming strong turbidity stratification at the 50–60 m water depth. 3.2.

210

Pb profiles and accumulation rates of cores

Fig. 6 shows the vertical profiles of total and excess 210Pb (210Pbex) activities for 18 sediment cores. Most of the cores, except for Core B-1, exhibited a typical 210Pb profile characterized by three distinct zones: a surface mixed layer (SML) with uniform activity; a middle inclined zone with exponential radioactive decay; and a lower background zone of constant low activity. In most of the 210Pb profiles, the SML, a layer of relatively uniform activity, was observed in the uppermost 5–10 cm of the cores, although in some cores the zone was up to 20 cm thick and in others it was almost absent or less than a few centimeters thick. Generally, this layer is probably because of the result of strong physical and/or biological mixing. Sediment accumulation rates were determined from 210Pbex activities that decreased exponentially below a surface mixed layer with a zone of uniform activity. Activities of 210Pbex were determined by subtracting the supported levels of 210Pb activity from the total 210Pb measured. Supported activity was ascertained by averaging the near-uniform, low level 210Pb activities (background activity) reached at depth in each core, assumed to equal the 226Ra activity (Fig. 6). The background level in all cores ranges between 0.6 and 0.9 dpm/g, which is identical to that of in the Korean coastal deposits (Park et al., 1999, 2000). Assuming that sediment mixing is restricted to the surface mixed layer, i.e., the mixing coefficient is zero below the surface mixed layer (Nittrouer et al., 1984), a linear sediment

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Fig. 5. Vertical structures of temperature, salinity and transparency in January and April 1999. For location of the transect, see Fig. 4. Shaded areas indicate the turbid water plume.

accumulation rate was calculated from the gradient of 210Pbex activity with depth in the sediment column. In this calculation, we also assumed a steady-state supply of 210Pb to the seafloor and that the surface mixed layer coincided with the depth of steady-state 210Pb activity. In core K-1 from the South Sea of Korea (see Fig. 2), the attenuation slope of the regression curve for the 210 Pbex profile differed between the upper and the lower layers of the middle inclined zone, a characteristic of cores from this area. Such characteristic profiles have been found elsewhere in the ECS and Korean and Chinese coastal areas where sediment accumulation is very rapid (e.g., Alexander et al., 1991; Huh and Su, 1999; Park et al., 2000; Su and Huh, 2002). This may be because of the changes of depositional environment and/or sedimentation processes (accumulation, biomixing and others) on decadal and centennial time scales (e.g., Kato et al., 2003). Another interesting 210Pb profile was observed in core B-1 from the southernmost part of SEYSM. 210Pb activities from this core revealed very low inventories throughout the entire core depth, even in the uppermost part of the core, indicating the background 210Pb activities. The near absence of 210Pbex in the seabed indicates that no significant accumulation over a 100-yr time scale in this area is occurring, which matches that of the several cores analyzed for the same area (Park et al., 2000). Sediment accumulation rates were calculated from 18 core samples we analyzed for 210Pb. Table 1 summarizes the sediment accumulation rates of the cores from the study area, with gamma spectroscopy measurements of 137CS and 210Pb activities for verification of the results. Sediment accumulation rates were highest in the SWCIM of the ECS, at >5 mm/yr, and also along the inner shelf of the South Sea of Korea, ranging

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Fig. 6. Total and excess 210Pb activity profiles of 18 core sediment samples. Sediment accumulation rate (S.R.) is determined from the slope of excess 210Pb activity profiles. For additional details, please refer to the text. Table 1 Sediment accumulation rates derived from Core number

210

Pb (alpha and gamma counting methods) and

Pb by alpha spectroscopy

a

Cs analysis in the study area

Sediment accumulation rate (mm/yr) 210

A4 A6 A9 A12 B1 D1 F9 G10 H2 I3 I4 I5 J5 J7 K1 K3 L1 L3

137

1.3 0.3 1.1 0.9 a

1.4 5.9 5.4 3.5 1.3 3.0 4.9 1.0 2.4 23.8 1.0 2.4 9.6

210

137

1.9 0.7 0.6 0.3 – – 4.3 3.2 2.0 – – 4.1 – 2.3 33.0 – 1.9 –

2.3 – 0.8 0.8 – –

Pb by gamma spectroscopy

Cs by gamma spectroscopy

3.3 – – – 3.3 – 3.8 – – 5.0 –

Negligible accumulation rate.

from 2 to 5 mm/yr. The values decreased to <1 mm/yr in the outer shelf area and were nearly zero at station B1 in the southern part of SEYSM. Most sediment cores from the southern part of the area were characterized by low values of 210Pb inventories throughout the cores (Park et al., 2000).

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4. Discussion 4.1. Pattern of sediment accumulation in the Yellow and East China Seas Modern sediment accumulation rates for the entire shelf area of the Yellow Sea and the ECS were delineated by compiling our data and all available previous data for this area (Fig. 7). The highest accumulation rates (>10 mm/yr) were observed in the coastal zone (<20 m water depth) south of the Changjiang River mouth. The distribution pattern of accumulation in this area shows that most of the Changjiang river sediments are being transported southward along the dispersal system of a strong coastal current on a 100-yr time scale. The high accumulation rate decreases to about 4 mm/yr toward offshore and outer shelf areas of the East China Sea, probably resulting from an offshore depletion in the flux of suspended matter. Such an across-shelf decreasing phenomenon in sediment accumulation rate is common in most other deposition sites of fine-grained sediment on continental margin (Nittrouer et al., 1984/1985; Lesueur et al., 2001). Similarly, at the western part of the Yellow Sea shelf, the distribution of accumulation rates also reveals a general trend along the terrigenous dispersal system from the Huanghe River (Alexander et al., 1991); >5 mm/yr at the inner shelf of the northwestern part of the Yellow Sea, especially in areas flanking the southern margin of the Shandong Peninsula, and l–2 mm/yr in offshore areas where the sediment supply is reduced. On the other hand, the

Fig. 7. Distribution of sediment accumulation rates (mm/yr) in the Yellow and East China Seas. Accumulation data are based on DeMaster et al. (1985), Alexander et al. (1991), Hong (1991), Chung and Chang (1995), Saito (1998), Huh and Su (1999), Park et al. (2000), Su and Huh (2002) and this study.

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central Yellow Sea, including the CYSM patch, clearly exhibits slow accumulation of <1 mm/yr (Fig. 7). Generally, sediment accumulation is rapid in areas with muddy sediments and slow in areas of coarse-grained sediments in the shelf area. Although surface sediments of the central Yellow Sea are composed of fine-grained muds (>7 phi in mean grain size), such a slow accumulation in this area implies low levels of sediment input from the Chinese river dispersal system. Again, the sediment from the surrounding land masses does not reach this area at this time because of the stable Yellow Sea Bottom Cold Water, which acts as a ‘‘barrier’’ for the sediments of the Chinese rivers and coastal areas to enter directly the this area (Alexander et al., 1991). On the northern ECS shelf, accumulation rates are moderate (approximately 3 mm/yr), except for a maximum value of >5 mm/yr in the depocenter of the main mud patch (SWCIM). Sediments in this area are probably derived from both the Changjiang and erosion of the abandoned Huanghe delta along the Jinagsu coast (Milliman et al., 1985a; Alexander et al., 1991; Lin et al., 2002; Liu et al., 2003; Lim et al., 2006). The South Sea shelf of Korea including the SSM deposit shows relatively moderate accumulation with a range of about 5–1 mm/yr, decreasing from 3–5 mm/yr in the coastal area to about 1 mm/yr offshore where the sediment supply from Korean rivers is reduced. In contrast, special attention to the distribution pattern of the sediment accumulation rate should focus on the SEYSM patch along the southeastern Koran coast. Two distinct areas of sediment accumulation rates are identified in the SEYSM patch (Fig. 8) as documented in Park et al. (2000): the northern part showing rapid sedimentation with high accumulation rates (approximately 4–43 mm/yr) and the southern part with negligible sedimentation (accumulation rates close to zero), indicating that non-deposition or even significant erosion occurs in the area. In the northern part, high accumulation rates (>18 mm/yr) seem to be an overestimate because of the strong biological mixing effect (Alexander et al., 1991; Park et al., 2000). Another possible explanation for the high accumulation in this area is that fine-grained sediment eroded from the southern part may be redeposited in the northern part, together with the suspended sediments from Korean rivers (Park et al., 2000). The 14C ages of the deep-drilled core (YSDP 103) from this area provides an average long-term (1000-yr time scale) sediment accumulation rate of 3.3–5.4 mm/yr (Park et al., 2000; Kong et al., 2006), which is comparable with the short-term (100-yr time scale) sediment accumulation rates (3.9 mm/yr) determined from 210Pb profiles of cores. In fact, the Korean coastal zone is not part of the direct dispersal system of the Huanghe and Changjiang River. Sediments in this area, especially SEYSM, seem to be complex, and their origin is controversial (Yang et al., 2003; Lim et al.,

Fig. 8. Accumulation rates (mm/yr) in the Korean coastal mud deposits. Note that the accumulation rates are nearly zero in the southern part of SEYSM.

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2006). For example, some clay minerals and geochemical compositions in the sediments are those of the rocks in the Korean river basins (Chough and Kim, 1981; Park and Khim, 1990, 1992). However, it is unlikely that the Korean rivers are major sources of the supply for enough sediment to support these rates (also see Alexander et al., 1991; Park et al., 2000; Lim et al., 2006). Thus, this deposit probably consists of a highly complex mixture of sediments derived from Korean and Chinese rivers, even though a detailed sediment transport pathway is still not suggested. 4.2. Budget and origin of muddy sediments (SEYSM and SSM) along the Korean coasts Korean rivers contribute an estimated 11–39 · 106 tons/yr of suspended sediment to the shelves of the Yellow Sea and South Sea of Korea (Table 2). Schubel et al. (1984) estimated that the sediment filtering efficiency of the estuaries is 60–80%. In the case of the Keum River, the sediments from the river are mostly deposited near its mouth, with a maximum contribution to the coastal deposit of about 25%, i.e., only a fourth of the material flux to the seabed (Cheng, 2000). In our calculations, we assumed that at least half of the riverine materials escape from the estuary and coastal zone that form the wide tidal flats along the Korean Peninsula. Accordingly, about 6–20 · 106 tons/yr of sediments are transported into the inner continental shelf system of the Yellow Sea and the South Sea of Korea. On the other hand, if we assume average sedimentation rates of 2.0–5.0 mm/yr (the range of the accumulation rate calculated above, Fig. 7), an area of 15 · 109 m2 (area of mud deposits calculated from Fig. 1), and a dry bulk density of 1.4–2.0 g/cm3 (Cheng, 1991; KIGAM, 1996; Shi et al., 2003a) for the inner shelf area (SSM and SEYSM) of Korea, a total of 4–15 · 107 tons/yr of sediment should be supplied from the sources. This rate is about an order of magnitude greater than can be explained by the sediment input from Korean rivers. Korean rivers do not discharge enough sediment to support this rate alone. On the basis of the total volume (1.5–3.0 · 1011 m3) of Korean inner shelf deposits (it was assumed in the calculation that the mud deposit is 10–20 m in thickness, based on seismic and deep-drilling data; Park and Yoo, 1988; KIGAM, 1996; Park et al., 1999), the total accumulation of Korean coastal muds for the last 6000 yr is about 4–10 · 107 tons/yr. This estimate is at least twice the volume of the Korean riverine sediment supply. According to our estimation, consequently, the present-day sediment discharge by Korean rivers cannot account for the entire deposition of mud patches along the Korean coasts, suggesting an additional sediment source for the coastal mud deposits of Korea. Alexander et al. (1991) and Zhao et al. Table 2 General characteristics of Korean and Chinese rivers around the Yellow and East China Seas (modified from Yang et al., 2003) Drainage area (km2)

Water discharge (109 m3/yr)

Sediment discharge (106 tons/yr)

Chinese rivers

Huanghe a Changjiang a Huaihe a Other rivers b

0.752 · 106 1.8 · 106 0.26 · 106 1.9 · 104

49 900 64.4 30.6

1080 500 14 5.2

Korean rivers

Aprock River b,c,d Han River d,e,f,g Keum River d,e,g,h Yeongsan River g Seomjin Riveri Nakdong River j,k

6.1 · 104 2.6 · 104 9.9 · 103 2.8 · 103 4.9 · 103 2.4 · 104

34.7, 25, 28 25, 19 5.0, 7, 5.8 1.6, 2.1 3.0 15

1.13, 2.04, 4.8 2.0, 4, 12.4 1.3, 5.6, 3.95, 11 1.24 0.8 4.6, 10.0

a b c d e f g h i j k

Hay (1998). Qin et al. (1989). Wang and Aubrey (1987). Schubel et al. (1984). Chough and Kim (1981). Chang and Oh (1991). Hong et al. (2002). Lee and Chu (2001). Park et al. (1996). Park et al. (1999). Kim et al. (1986).

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(2001) suggested that the SEYSM consists of a highly complex mixture of sediments from the smaller Korean rivers, resuspended Huanghe sediments from the central Yellow Sea, and suspended sediments carried to the area from the ECS by the Yellow Sea Warm Current. The distributions of SPM concentration in this study clearly indicate that the sediment reworked from the mud of the SWCIM of the ECS is a potential source of the Korean coastal mud deposits. In fact, the turbid water masses from the ECS and from the coastal area of Korea are connected with each other in a NE–SW direction, forming a turbid plume during the winter season (Figs. 4 and 5). The turbid water plume continues until early spring, especially in the bottom water. The distribution pattern of the turbid waters was interpreted as representing the transport behavior of suspended sediments. Lee et al. (1998) analyzed coastal zone color scanner images for the winter seasons of 1980 and 1981 and also observed the presence of a turbid water plume connecting the southwest tip of Korea and the continental shelf of the ECS. Because of the lack of sea current data for the cruise period of this study, we are not sure whether the suspended matter was transported from the ECS northeastward into the coastal area of Korea, or vise versa. However, a SEAWIF satellite image taken in April 1999 clearly shows a branch of northeastward-moving turbid water from the continental shelf of the ECS (Ahn et al., 2004). Also, results from drifter buoys deployed in surface waters have shown the north and northeastward movement of water from the ECS into the southern Yellow Sea and the South Sea of Korea during both summer and winter (Beardsley et al., 1992). Bottom currents measured directly on the central shelf of the ECS were about 10 cm/s to the northeast about 70% of the time (Li et al., 1985). Although the mean current velocity was insufficient to erode and resuspend the bottom sediments, strong currents produced by typhoons in summer and storms in winter could cause resuspension of bottom sediments and promote the transport of suspended matters. Gao et al. (1996) also reported the possibility of the northward transport of suspended matter from the ECS into the southern Yellow Sea during the summer typhoon period. These results clearly support a previous conceptual model (Fig. 9) for seasonal variations in the turbid water mass formed by the surface layer of the Yellow Sea and the ECS. During the winter season with active hydrodynamic conditions, these two turbid water masses having high SPM concentrations (>20 mg/l) from the bottom sediments are connected through the narrow zone, thereby forming a turbid plume. After this time, the Yellow Sea Warm Water again intrudes into the Yellow Sea breaking up the turbid water plume. Thus, during fair weather conditions from spring to fall, turbid waters occur on both sides of the Yellow Sea. Fig. 10 shows a schematic for the transport pathways of sediment materials in the study area based on our research results. The fine-grained materials, originating primarily from the Changjiang and the old-Huanghe submarine delta, are transported into the continental shelf of the ECS and are trapped within a cyclonic upwelling gyre, resulting in the formation of the SWCIM. However, a branch of the turbid plume is consid-

Fig. 9. Conceptual diagram showing the seasonal evolution of the turbid water plume between China and Korea (after Lee et al., 1998).

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Fig. 10. Schematic diagram showing the sediment transport pathway between the East China Sea and the coastal area of Korea. Solid and dashed lines are the turbidity front between coastal waters and the offshore waters. Arrows indicate the movement of suspended sediments.

ered to shift northeastward through the ‘‘GAP’’ area into the coastal area (Fig. 10). The ‘‘GAP’’ area between the CYSM and SWCIM is, therefore, interpreted as the area of non-deposition and as the main pathway of exchange of suspended matter between the ECS and the coastal area of Korea. Consequently, on the inner shelf of Korea, suspended matter derived from the ECS is considered to join the turbid coastal waters. This suspended matter cannot escape the strong coastal currents and is transported eastward along the South Sea of Korea to form the SEYSM and SSM (Park and Choi, 1989; Lee and Chu, 2001). 5. Conclusions This study identifies an inconsistency between the estimates of sediment discharge from Korean rivers and the sediment budget of Korean coastal mud deposits, and suggests an important offshore supply from the ECS. The fine-grained sediments from either the Huanghe or Changjiang Rivers are considered to be transported into the central area of the ECS, forming the SWCIM deposit. Later, some of these sediments seem to be reworked and transported into the inner shelf system of the Korea by the Yellow Sea Warm Current, especially during winter. Ultimately, some of the Chinese river sediments, being transported through the East China Sea shelf by a complex shelf circulation system, are suggested as the additional source for the Korean coastal mud deposit. We show that the Korean coastal area is a significant sink for some Chinese river sediments being transported from the south by the Yellow Sea Warm Current. However, further study is needed concerning the imbalance between the sediment supply from Korean rivers and estimates of the sediment budget, which could be explained by the discrepancy between decadal and centennial time scales of sediment input and output systems. The sedimentation rates measured by 210Pb were on the time scale of approxi-

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mately a few decades or up to 100 yr. If the system has been in equilibrium during the last 100 yr, the results in this study are reasonable. However, if the system has varied due to either natural or anthropogenic activities, we are probably only looking at a transient-state, rather than a steady-state condition. Of special concern is the shift of the Huanghe River from the southern Yellow Sea into the Bohai Sea, north of Shandong Peninsula, about 150 yr ago. The system of sediment supply, transport, and deposition may have changed significantly because of this shift. Furthermore, the recent construction of the Three Gorges Dam in upper reaches of the Changjinag River has affected the supply of sediments into the Yellow and East China Seas. The construction may be one of the most important factors determining the future depositional system of the Yellow Sea and the ECS. For example, the dam’s construction will most likely reduce the supply of sediments to the East China Sea, altering not only the coastal environment but also the oceanographic system of the entire shelf. Accordingly, the difference in sediment dynamic patterns during the formation of mud deposits should be considered in budget calculations. Acknowledgments This study was supported by Grants to KORDI research programs (Grant No. PP07401 and PK06800) in Korea. The authors are grateful to S.M. Kang of the Korea Ocean Research and Development Institute (KORDI), who kindly assisted with the laboratory work. Critical comments by three anonymous reviewers on the original manuscript are highly appreciated. References Ahn, Y.H., Shanmugam, P., Gallegos, S., 2004. Evolution of suspended sediment patterns in the East China and Yellow Sea. Journal of the Korean Society of Oceanography 39, 26–34. Alexander, C.R., DeMaster, D.J., Nittrouer, C.A., 1991. Sediment accumulation in a modern epicontinental-shelf setting: The Yellow Sea. Marine Geology 98, 51–72. Beardsley, R.C., Limeburner, R., Kim, K., Candela, J., 1992. Lagrangian flow observations in the East China Sea, Yellow and Japan Sea. La Mer 30, 297–314. Chang, H.D., Oh, J.K., 1991. Depositional sedimentary environments in the Han River estuary and around the Kyunggi Bay posterior the Han River’s development. Journal of the Korean Society of Oceanography 26, 13–23. Chen, C., Beardsley, R.C., Limburner, C., Kim, K., 1994. Comparison of winter and summer hydrographic observations in the Yellow and East China Seas and adjacent Kuroshio during 1986. Continental Shelf Research 14, 909–929. Cheng, G.D., 1991. Depositional Processes and Model of the Modern Yellow River Delta. Geological Press, Beijing, pp. 77–93 (in Chinese). Cheng, P., 2000. Sediment characteristics and transport processes of fine-grained material over the northern Yellow Sea. Unpublished Ph.D. thesis, IOCAS, China. Chough, S.K., Kim, D.C., 1981. Dispersal of fine-grained sediments in the southeastern Yellow Sea: a steady-state model. Journal of Sedimentary Petrology 51, 721–728. Chung, Y., Chang, W.C., 1995. Pb-210 fluxes and sedimentation rates on the lower continental slope between Taiwan and the South Okinawa Trough. Continental Shelf Research 15, 149–164. DeMaster, D.J., Mckee, B.A., Nittrouer, C.A., Qian, J.G., Cheng, G.D., 1985. Rates of sediment accumulation and particle reworking based on radiochemical measurements from continental shelf deposits in the East China Sea. Continental Shelf Research 4, 143–158. Gao, S., Jia, J.J., 2003. Modeling suspended sediment distribution in continental shelf upwelling/downwelling settings. Geo-Marine Letters 22, 216–218. Gao, S., Park, Y.A., Zhao, Y.Y., Qin, Y.S., 1996. Transport and resuspension of fine-grained sediments over the southeastern Yellow Sea. In: Lee, C.B., Zhao, Y. Y. (Eds), Proceedings of the Korea–China international seminar on Holocene and late Pleistocene environments in the Yellow Sea Basin. pp. 83–98. Hay, W.W., 1998. Detrital sediment fluxes from continents to oceans. Chemical Geology 145, 287–323. Hong, S.K., 1991. Late Quaternary seismic stratigraphy of the inner shelf sediments in a part of the South Sea, Korea. Unpublished M.S. thesis, Chungnam National University, Korea. Hong, G.H., Zhang, J., Kim, S.H., Chung, C.S., Yang, S.R., 2002. East Asian marginal seas: river-dominated ocean margin. In: Hong, G.H., Zhang, J., Chung, C.S. (Eds.), Impact of Interface Exchange on the Biogeochemical Processes of the Yellow and East China Seas. Seoul 2002. BumShin Press, Seoul, Korea, pp. 33–260. Hu, D.X., 1984. Upwelling and sedimentation dynamics I. The role of upwelling in sedimentation in the Huanghe Sea and East China Sea – a description of general features. Chinese Journal of Oceanological Limnology 2, 12–19. Huh, C.A., Su, C.C., 1999. Sedimentation dynamics in the East China Sea elucidated from 210Pb, 137Cs and 239,240Pu. Marine Geology 160, 183–196.

158

D.I. Lim et al. / Progress in Oceanography 73 (2007) 145–159

Kato, Y., Kitazato, H., Shimanaga, M., Nakatsuka, T., Shirayama, Y., Masuzawa, T., 2003. 210Pb and 137Cs in sediments from Sagami Bay, Japan: sedimentation rates and inventories. Progress in Oceanography 57, 77–95. Kim, M.S., Chu, K.S., Kim, O.S., 1986. Investigation of some influence of the Nakdong River water on marine environment in the estuarine area using Landsat imagery. Report of Korea Ministry of Science and Technology, 93–147. Korea Institute of Geology, Mining and Materials (KIGAM), 1996. Yellow Sea drilling program for studies on Quaternary geology. KIGAM Research Report KR-96(T)-18, p. 595. Kong, G.S., Park, S.C., Han, H.C., Chang, J.H., Mackensen, A., 2006. Late Quaternary paleoenvironmental changes in the southeastern Yellow Sea, Korea. Quaternary International 144, 38–52. Lee, H.J., Chough, S.K., 1989. Sediment distribution, dispersal and budget in the Yellow Sea. Marine Geology 87, 195–205. Lee, H.J., Chu, Y.S., 2001. Origin of inner-shelf mud deposit in the southeastern Yellow Sea: Huksan Mud Belt. Journal of Sedimentary Research 71, 144–154. Lee, J.H., Yoo, S.J., Chang, K.I., 1998. Inflow of warm waters into the Yellow Sea observed by coastal zone color scanner. In: Brown, R.A. (Ed), Remote sensing of the Pacific Ocean by satellites. pp. 251–254. Lesueur, P., Jouanneau, J.M., Boust, D., Tastet, J.P., Weber, O., 2001. Sedimentation rates and fluxes in the continental shelf mud fields in the Bay of Biscay (France). Continental Shelf Research 21, 1383–1401. Li, C.Z., Zhang, F.A., Wang, X.C., 1985. Preliminary study on genetic environment of sediments of the East China Sea. Acta Oceanologica Sinica 4, 254–265. Lim, D.I., Jung, H.S., Choi, J.Y., Yang, S., Ahn, K.S., 2006. Geochemical compositions of river and shelf sediments in the Yellow Sea: Grain-size normalization and sediment provenance. Continental Shelf Research 26, 15–24. Lin, S., Hsieh, I.J., Huang, K.M., Wang, C.H., 2002. Influence of the Yangtz River and grain size on the spatial variations of heavy metals and organic carbon in the East China Sea continental shelf sediments. Chemical Geology 182, 377–394. Liu, J., Zhu, R., Li, G., 2003. Rock magnetic properties of the fine-grained sediment on the outer shelf of the East China Sea: implication for provenance. Marine Geology 193, 195–206. Milliman, J.D., Meade, R.H., 1983. World-wide delivery of river sediment to the oceans. The Journal of Geology 91, 1–21. Milliman, J.D., Beardsley, R.C., Yang, Z.S., Limeburner, R., 1985a. Modern Huanghe-derived muds on the outer shelf of the East China Sea: identification and potential transport mechanisms. Continental Shelf Research 4, 175–188. Milliman, J.D., Shen, H.T., Yang, S.Z., Meade, R.H., 1985b. Transport and deposition of river sediment in the Changjiang estuary and adjacent continental shelf. Continental Shelf Research 4, 37–45. Nittrouer, C.A., Sternberg, R.W., Carpenter, R., Benett, J.J., 1979. The use of Pb-210 geochronology as a sedimentological tool: application to the Washington continental shelf. Marine Geology 31, 297–316. Nittrouer, C.A., DeMaster, D.J., McKee, B.A., 1984. Fine-scale stratigraphy in proximal and distal deposits of sediment dispersal systems in the East China Sea. Marine Geology 61, 13–24. Nittrouer, C.A., DeMaster, D.J., Kuehl, S.A., McKee, B.A., Thorbjarnarson, K.W., 1984/1985. Some questions and answers about the accumulation of fine-grained sediment in continental margin environments. Geo-Marine Letters 4, 211–213. Park, Y.A., Choi, J.Y., 1989. Mechanism and distribution patterns of the fine-grained suspended materials off the southwest coast of Korea. Acta Oceanographical Taiwanica 24, 52–64. Park, Y.A., Khim, B.K., 1990. Clay minerals of the recent fine-grained sediments on the Korean continental shelves. Continental Shelf Research 10, 1179–1191. Park, Y.A., Khim, B.K., 1992. Origin and dispersal of recent clay minerals in the Yellow Sea. Marine Geology 104, 205–213. Park, S.C., Yoo, D.G., 1988. Depositional history of Quaternary sediments on the continental shelf off the southeastern coast of Korea (Korea Strait). Marine Geology 79, 65–75. Park, S.C., Hong, S.K., Kim, D.C., 1996. Evolution of late Quaternary deposits on the inner shelf of the South Sea of Korea. Marine Geology 131, 219–232. Park, S.C., Yoo, D.G., Lee, K.W., Lee, H.H., 1999. Accumulation of recent muds associated with coastal circulations, southeastern Korea Sea (Korea Strait). Continental Shelf Research 19, 589–608. Park, S.C., Lee, H.H., Han, H.S., Lee, G.H., Kim, D.C., Yoo, D.G., 2000. Evolution of late Quaternary mud deposits and recent sediment budget in the southeastern Yellow Sea. Marine Geology 170, 271–288. Qin, Y.S., Zhao, Y.Y., Chen, L.R., Zhao, S.L., 1989. Geology of the Yellow Sea. China Ocean Press, Beijing. Saito, Y., 1998. Sedimentary environment and budget in the East China Sea. Bulletin on Coastal Oceanography 36, 43–58. Schubel, J.R., Shen, H T., Park, M J., 1984. A comparison of some characteristic sedimentation processes of estuaries entering the Yellow Sea. In: Park, Y.A., Pilkey, O.H., Kim, S.W. (Eds.), Marine geology and physical processes of the Yellow Sea. pp. 286–308. Shi, C., Zhang, D.D., You, L., 2003a. Sediment budget of the Yellow River delta, China: the importance of dry bulk density and implications to understanding of sediment dispersal. Marine Geology 199, 13–25. Shi, X., Shen, S., Yi, H.I., Chen, Z., Yi, M., 2003b. Modern sedimentary environments and dynamic depositional systems in the southern Yellow Sea. Chinese Science Bulletin 48, 1–7. Su, C.C., Huh, C.A., 2002. 210Pb, 137Cs and 239,240Pu in the East China Sea sediments: sources, pathways and budgets of sediments and radionuclides. Marine Geology 183, 163–178. Wang, Y., Aubrey, D.G., 1987. The characteristics of the China coastline. Continental Shelf Research 7, 329–349. Yang, S.Y., Jung, H.S., Lim, D.I., Li, C.X., 2003. A review on the provenance discrimination of sediments in the Yellow Sea. Earth Science Reviews 63, 93–120. Zhao, Y.Y., Qin, Z.Y., Li, F.Y., Chen, Y.W., 1990. On the source and genesis of the mud in the central area of the south Yellow Sea. Chinese Journal of Oceaonological Limnology 8, 66–73.

D.I. Lim et al. / Progress in Oceanography 73 (2007) 145–159

159

Zhao, Y.Y., Park, Y.Y., Qin, Y.S., Choi, J.Y., Gao, S., Li, F.Y., Cheng, P., Jiang, R.H., 2001. Material source for the Eastern Yellow Sea mud: evidence of mineralogy and geochemistry from China–Korea Joint Investigation. The Yellow Sea 7, 22–26. Zhu, Y., Chang, R., 2000. Preliminary study of the dynamic origin of the distribution pattern of bottom sediments on the continental shelves of the Bohai Sea, Yellow Sea and East China Sea. Estuarine, Coastal and Shelf Science 51, 663–680.