Sedimentary features of the Yangtze River-derived along-shelf clinoform deposit in the East China Sea

Sedimentary features of the Yangtze River-derived along-shelf clinoform deposit in the East China Sea

ARTICLE IN PRESS Continental Shelf Research 26 (2006) 2141–2156 www.elsevier.com/locate/csr Sedimentary features of the Yangtze River-derived along-...

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ARTICLE IN PRESS

Continental Shelf Research 26 (2006) 2141–2156 www.elsevier.com/locate/csr

Sedimentary features of the Yangtze River-derived along-shelf clinoform deposit in the East China Sea J.P. Liua,, A.C. Lib, K.H. Xuc, D.M. Velozzia, Z.S. Yangd, J.D. Millimanc, D.J. DeMastera a

Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695, USA b Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China c School of Marine Science, College of William and Mary, Gloucester Pt., VA 23062, USA d College of Marine Geoscience, Ocean University of China. Qingdao 266003, China Available online 6 September 2006

Abstract A predominant sigmoidal clinoform deposit extends from the Yangtze River mouth southwards 800 km along the Chinese coast. This clinoform is thickest (40 m) between the 20 and 30 m isobaths and progressively thins offshore, reaching water depths of 60 and 90 m and distances up to 100 km offshore. Clay mineral, heavy metal, geochemical and grain-size analyses indicate that the Yangtze River is the primary source for this longshore-transported clinoform deposit. 210 Pb chronologies show the highest accumulation rates (43 cm/yr) occur immediately adjacent to the Yangtze subaqueous delta (north of 30 1N), decreasing southward alongshore and eastward offshore. The interaction of strong tides, waves, the China Coastal Current, winter storms, and offshore upwelling appear to have played important roles in trapping most Yangtze-derived sediment on the inner shelf and transporting it to the south. r 2006 Elsevier Ltd. All rights reserved. Keywords: East China Sea; Yangtze River; Deltaic sedimentation; Clinoform; Along-shelf

1. Introduction The East China Sea (ECS) is one of the best examples of a river-dominated ocean margin. This wide (4500 km) and shallow (o130 m) marginal sea receives a large amount of terrigenous sediment from two of the largest rivers in the world, the Yangtze River (Changjiang) and Yellow River (Huanghe) (Fig. 1). The Yangtze River’s annual sediment load has averaged about 480  106 tons over the past 100 yr, which when combined with the Corresponding author.

E-mail address: [email protected] (J.P. Liu). 0278-4343/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2006.07.013

adjacent Yellow River (1  109 t/yr), equals 10% of the global riverine sediment flux to the ocean (Milliman and Meade, 1983). The Yangtze River, the largest Asian river and third longest river in the world, discharges most of its annual sediment load between June and September. Short-term deposition on the adjacent shelf during this period reaches about 4 cm/month, in contrast to long-term accumulation rates, which are an order of magnitude less, 1–5 cm/yr (McKee, et al., 1983; DeMaster et al., 1985). This difference between shortand long-term accumulation rates suggests that a major portion of the river-derived sediment is eroded seasonally and is transported elsewhere, presumably

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Fig. 1. Bathymetry and regional ocean circulation pattern in the East China Sea and Yellow Sea. JCC—Jiangsu Coastal Current; KC— Kuroshio Current; TWC—Taiwan Warm Current; YSWC—Yellow Sea Warm Current; ZFCC—Zhejiang Fujian Coastal Current.

southward along the Zhejiang and Fujian Provinces (McKee et al., 1983; DeMaster et al., 1985; Nittrouer et al., 1984). The alongshore deposits are commonly referred to as the ‘‘mud belt deposit on the inner shelf of the ECS’’ (Fig. 2) (Qin, 1979). In the early 1980s, a group of multi-institutional American and Chinese marine scientists, including three co-authors of this paper (ZSY, JDM, DJD), conducted a series of cooperative field studies of the Yangtze River estuary and East China Sea. As a part

of this field program, Sternberg et al. (1985) obtained the first instrumented tripod data off China, confirming the previous presumption that a major portion of river-derived sediment is transported predominantly southward along the Chinese coastline. Because most previous research was restricted to the Yangtze estuary and ECS middle and outer shelf, knowledge about this along-shelf ‘‘mud belt’’ remained limited. Little information existed concerning its distribution, thickness, budget and depositional

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34°

2143

Jiangsu

Yangtze River

32° Profile1

China DEB4 DEB5

30°

Qiantang Jiang

East China Sea

EB6 Zhejiang

Ou Jiang

28°

Profile2

DE15

DEB7

DEB10

Okinawa Trough

Fujian

Profile3

26°

DEB11 Min Jiang

DEB13

Taiwan 0 km

24° 118°

120°

Sand

Silt

122°

Clay

124°

100 km

200 km

126°

128°

Box core Gravity core Seismic Profile

Fig. 2. Distribution of surface sediments in the ECS and Okinawa Trough (modified after Qin et al., 1987). Black lines signify selected seismic profiles (Fig. 3), triangles indicate sites of selected box cores, and the black dot represents the site of a gravity core. The clay and silt deposit along the Zhejing and Fujian coast is the so-called ‘‘mud belt’’.

history, let alone the transport, deposition, resuspension, and accumulation processes. It was only with the recent acquisition of high-resolution seismic Chirp Sonar profiles that the presence of a longshore sediment body off the Zhejiang–Fujiang coasts was confirmed (Liu et al., 2006). This study presents three across-shelf seismic profiles together with some new analyses of the grain-size distribution, clay mineralogy, radio-geochemistry (210Pb), and sedimentary dynamics to help better understand the sediment transport and sedimentary processes involved in the formation of this major clinoform deposit. 2. Study area The Yangtze estuary is a large fluvial and tidedominated depositional basin, with a mean tidal

range of 2.7 m and a maximum range of 4.7 m. Average tidal currents are about 1.0 m/s, but can reach more than 2.0 m/s during spring tide (Chen et al., 1988). Historical data collected at the Datong Hydrological Gauging Station (see location in Fig. 1) over the past 100 yr show that the annual water discharge to the Yangtze estuary has averaged 955  103 km3, and until recently the annual fluvial sediment input was 450–480  106 tons/yr (Chen et al., 2001). In the last decade, the Yangtze-derived sediment discharge has decreased nearly 40%, due to dam and reservoir construction (Yang et al., 2006). Based on numerous shallow borings, the modern Yangtze delta has been shown to begin accreting after postglacial sea level reached its mid-Holocene highstand 7000 yr ago. Following this event it developed a large subaqueous delta up to 60 m thick

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near the river mouth (Stanley and Warne, 1994; Chen and Stanley, 1993). Deceleration of sea-level rise and regression in the mid-Holocene-induced fluvial sediments to accumulate along the periphery of the delta plain (Stanley and Chen, 1996). Recent seismic profiling and 14C dating also indicate the longshore-distributed clinoform began to form around 7000 BP (Liu et al., 2006). The late Quaternary stratigraphy on the ECS inner shelf consists of late Pleistocene terrigenous material (fluvial and lacustrine) overlain by Holocene transgressive silt, which in turn are overlain by prodelta clay and delta front fine sand and silt (Chen et al., 2000; Wang et al., 2005). Coastal oceanography is dominated by the southward-flowing China Coastal Current (CCC), a relatively cold and brackish counter current, which includes the Jiangsu Coastal Current (JCC) in the north and the Zhejiang–Fujian Coastal Current (ZFCC) in the south (Fig. 1). This current intensifies in the winter, carrying the Yangtze’s brackish water and sediment southward along the inner shelf. Offshore, there is a northward flow of warm and saline water, the Taiwan Warm Current (TWC) (Fig. 1), which intensifies in the summer in response to prevailing southeast monsoon and as the southward ZFCC weakens (Beardsley et al., 1985; Lee and Chao, 2003). 3. Methods and data Two cruises were conducted in the East China Sea in 2003 and 2004, during which 22 subbottom seismic profiles (1200 km) were acquired using the EdgeTech 0152i Chirp sonar system with frequencies 0.5–6.0 kHz, at a ship speed of 4–5 knots. All profiles were post-processed using an acoustic velocity of 1500 m/s to calculate depths and sediment thickness. Three cross-shelf seismic profiles— one near the river mouth and two far from the river mouth in the inner shelf—have been selected to discuss in this paper (Fig. 2). Seven box cores and one gravity core (see Fig. 2 and Table 1) were obtained for grain-size, 210Pb, and clay mineral measurements. Grain size was analyzed in sediments sampled at 2-cm vertical intervals from the box cores and the gravity core at the Institute of Oceanology, Chinese Academy of Science (IOCAS). Sediment samples were pre-treated according to the following procedures: (1) 10 ml of H2O2 (10%) were added to 0.2 g of sample at room temperature (28 1C) and

Table 1 Core locations, length, and water depth Core

Type

Latitude (N)

Longitude (E)

Water depth (m)

Length (cm)

DEB4 DEB5 DEB6 DEB7 DEB10 DEB11 DEB13 DE15

Box Box Box Box Box Box Box Gravity

30121.700 30121.840 29147.820 27150.140 27104.980 26125.910 26100.800 28104.300

123100.520 122154.200 122140.320 121124.350 120134.360 120106.760 120107.150 122130.380

56 50 41 19 19 24 42 72

24 32 32 38 38 56 28 82

allowed to react for 30 min; and then (2) 10 ml of 2 N HCl were added to the solution prior to the sample being placed in a 60 1C bath for 1 h. Solutions containing the sediment samples were centrifuged at 3500 rpm for 6 min. Subsequently, samples were dispersed and homogenized using ultrasound before passing through the CILAS laser particle size analyzer (Model: 940L). 210 Pb activities were determined at North Carolina State University (NCSU) laboratory on sediments sampled at 2-cm intervals from all box and the gravity cores. Sediments were weighed, dried, and reweighed to determine water content. Approximately 5 g of dried sample were ground, homogenized, and spiked with a 209Po tracer (50 dpm/ml) for yield determination. The samples were then sequentially leached in 6 N HCl and concentrated HNO3. After the sediments were leached, the samples were taken to dryness and picked back up in 6 N HCl. For each sample, the polonium isotopes were plated onto a silver planchet (1  1 cm) by spontaneous electro-deposition in 1.5 N HCl, in the presence of ascorbic acid, at 80–90 1C for 4–5 h. 210Pb activity was determined by counting the a decay of its granddaughter, 210Po. Excess 210Pb activities were determined by subtracting the supported levels of 210Pb activity from the total 210Pb measured. In this study, supported levels were determined from the average total 210Pb activity reached at depth in each core, assumed to equal the 226Ra activity. On average, three depth intervals were used to calculate the supported levels. The supported values ranged from 1.4470.05 to 1.670.06 dpm/g. Clay mineral analysis was conducted on surface sediments (0–2 cm) from all seven-box cores and at depth (2-cm intervals) from the gravity core (DE15). Clay minerals were prepared at IOCAS with

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analysis at NCSU. Oriented clay mineral slides were prepared according to the methods outlined in Berry (1987). Identification of clay minerals was made mainly according to the position of the (0 0 1) series of basal reflections on the two XRD diagrams. Semi-quantitative estimates of peak areas of the basal reflections of the main clay mineral groups of smectite (17 A˚), illite (10 A˚), and kaolinite/chlorite (7 A˚) were carried out on the glycolated curve using the Jade 5 software. Kaolinite and chlorite were separated by relative proportions according to the ratios of the areas of the 3.57/ 3.54 A˚ peaks. Kaolinite/chlorite proportions were used to compute proportions of the 7 A˚ peak area for the respective mineral.

4. Results 4.1. Along-shelf clinoform revealed in the seismic records Three across-shelf profiles have been selected near and away from the river mouth over the ECS inner shelf mud belt (Fig. 2). Profile 1 is located seaward of the Yangtze estuary, between water depths of 20–50 m, where the clinoform overlies a prominent

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late Pleistocene subaerial surface (Fig. 3). At the end of the profile, we can see the easternmost boundary of the Yangtze subaqueous deltaic deposit. Profiles 2 and 3 are located in the inner shelf, offshore of the Zhenjiang and Fujian provinces, about 300 and 600 km south of the Yangtze estuary, respectively. These two profiles also show a 30–50-m thick clinoform with typical sigmoid shape and gently dipping topset beds, prograding foreset beds, and flat bottomset beds thinning offshore, pinching out at water depths of 60–90 m. Overall, the elongated clinoform is contained within 100 km of the shore, and extends at least 800 km southward to the middle of the Taiwan Strait (see Fig. 4).

4.2. Sediment compositions and particle size distributions Surface sediments in the ECS inner shelf are characterized by gray mud and generally contain 40–45% clay, 40–60% silt, and less than 5% sand. Grain-size analyses (cores DEB5, DEB6, DEB7, DEB10, DEB11) show silt, clayey-silt, and silty-clay (6.0–8.0 f mean grain size), generally wellsorted and symmetrically skewed with a ‘‘quasibimodal’’ grain-size distribution. The majority of

Fig. 3. Selected chirp sonar subbottom profiles located near the Yangtze subaqueous delta (Profile 1) and away from the river mouth (Profiles 2 and 3) in the inner shelf. All show seaward (eastward) progradation of the clinoform overlying the uneven subaerial surface or postglacial transgressive surface.

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33° N

10

Yangtze River

Yangtze Delta Plain and Subaqueous Delta

20 40

32°

60 31°

0

Profile 1

40 10 Hangzhou Bay

CHINA

20 10

30° 0 30 29°

Profile 2

Zhejiang Province

28°

0

20

10 30

27°

East China Sea

40

Profile 3

20 10 26°

0

Fujian Province

Alongshore Clinoform Deposits

20

N

25° 0 Taiwan Strait

W

Taiwan 0

50

24° 118°E

119°

120°

121°

122°

100 km

E S

123°

124°

Fig. 4. Isopach map (in meters) of the Yangtze-derived sediment deposited over the last 7000 yr in the inner shelf of the ECS (after Liu et al., 2006). Solid black lines are locations for selected seismic profiles in Fig. 3.

the grain-size distributions show a single dominant mode at 6–7 f, with a minor second mode at 8.5–9.5 f (Fig. 5a). The grain size of sample DEB4, from mud-belt margin, has a broader distribution; in addition to a minor ‘‘quasi-bimodal’’ distribution at 7 and 9 f, there is a dominant second coarser mode at 2.5 f (Fig. 5b). Both core DE15 and DEB13 show a single mode at 4 and 7 f. However, in comparison with fine sediments from the mud belt, which predominantly exhibit a major singlemode distribution (6–7 f up to 30%) (Fig. 5a), the sediments in DE15 and DEB13 are relatively well mixed with fine and coarse grains (Fig. 5b).

4.3. Clay mineral analysis Clay mineral assemblages of our samples from the inner shelf sediments of the ECS are composed primarily of illite (generally more than 70%), with small amounts of chlorite (12%), kaolinite (9%) and smectite (3%) (Table 2). The high illite concentrations and low smectite concentrations (o5%) are a good indicator of Yangtze River-derived material, although there are probably some inputs from the smaller coastal rivers, specifically the Qiantang, Ou and Min (Fig. 2). Samples collected offshore of the Ou Jiang mouth in the west Wenzhou Bay, an area

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40 DEB5

35

DEB6

Percent

30

DEB7

25

DEB10 20

DEB11

15 10 5 0 0

2

4

6

8

10

12

14

Phi (φ)

(a)

Percent

40 35

DEB4

30

DE15 DEB13

DEB4

25 20

DEB13

DE15

15 10 5 0 0

2

4

6

8

10

12

14

Phi (φ)

(b)

Fig. 5. (a) Representative grain-size distribution curves of the fine-grained sediments sampled from the ‘‘mud belt’’ on the inner shelf of the ECS. (b) Representative grain-size distribution curves of the sandy-silts, silty-sands in cores DEB 4 and DE 15 from the outside of the ‘‘mud belt’’.

strongly influenced by southward flow of the Yangtze dispersal system, indicate a similar clay mineral assemblage as the rest of the clinoform mud (Yang, 1995; Table 2). A ternary diagram of smectite, chlorite+kaolinite, and illite concentrations shows that the concentrations of the samples collected on the inner shelf are quite similar to the clay concentrations derived from the Yangtze River plume and Yangtze delta deposits, but are very different from Yellow River-derived sediment (Table 2, Fig. 6). 4.4.

210

Pb accumulation rates

In most of the 210Pb profiles, a layer of relatively uniform activity (probably the result of strong physical and biological mixing) was observed in the upper 5–10 cm (Fig. 7). Data from these layers were excluded when calculating sediment accumula-

tion rates. Compared to the low supported 210Pb activity in the tropical ocean, like Amazon shelf (1.0 dpm/g) (e.g., Kuehl et al., 1986; DeMaster et al., 1986), the surface sediment on the ECS shelf shows a little bit higher supported 210Pb activity (1.5 dpm/g), which are identical to those in the Yellow Sea shelf (Mckee et al., 1983; Alexander et al., 1991). The difference may be caused by differing source rock or weathering intensity. The exponential decay of excess 210Pb activity yields apparent accumulation rates (presuming no deep sediment mixing) of 0.16–2.14 cm/yr, or 0.19–2.17 g/ cm2 yr (Fig. 7, Table 3). If deep mixing (i.e. below the intensely mixed surface layer) occurs, these accumulation rates represent maximum values for the area. Two cores, collected from foreset beds (DEB5, DEB6) (Fig. 7), display relatively high accumulation rates of 2.14 and 0.86 cm/yr, respectively. Core

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Table 2 Clay mineral assemblages and their relative content (%) in sediments derived from the Yangtze and Yellow rivers Location

Station

Illite

Chlorite

Kaolinite

ECS inner shelf mud

DEB 5 DEB 6 DEB 7 DEB 10 DEB 11 DEB 13 DE15

73 83 83 84 68 71 81

16 10 10 9 14 15 9

10 7 7 6 16 11 8

1 0 0 1 2 3 3

Average

77

12

9

3

Hangzhou Bay Zhou Shan Xiang Shan Jiao Jiang Dong Tou

60.1 59.9 61.5 62.6 61.7

20.4 20.9 18.8 18.6 18.4

16.5 16.4 15.6 16.5 17.9

2.9 2.7 4.0 2.3 2.0

Yangtze River delta

68 77 65 66

13.9 20 11 12

12.7 20 14 16

5.5 3 10 6

Xu (1983) Ren and Shi (1986) Yang (1988) Yang et al. (2003b)

Offshore Yangtze diluted water

63

14

13

10

Guo et al. (1995)

Offshore Oujiang mouth Yellow River

74 59 67 62 62

13 9.3 12 12 16

9 8.5 8 10 10

4 23.2 13 16 12

Yang (1995) Xu (1983) Ren and Shi (1986) Yang (1988) Yang et al. (2003b)

Central YS and North ECS Yellow River-derived mud

53 54 52 56 50 55 53 54 49

12 13 14 14 11 11 12 13 16

11 11 9 10 10 11 10 11 10

24 22 24 20 29 23 24 22 25

Guo et al. (1995)

DEB5 reveals a profile that is nearly vertical and fairly linear. Accumulation rates in bottomset strata either display negligible accumulation rates (DEB4, DE15), or significantly reduced rates (0.33 cm/yr, DE13). 210Pb profiles from DEB4 and DE15 reveal background 210Pb activities below 4 cm depth in the cores. These data indicate that no significant accumulation (on a 100-yr time scale) is occurring, which matches the Holocene isopach map derived from seismic observations (Fig. 4). 5. Discussion Both the seismic profiles and sample analysis show an extensive along-shelf and limited across-

Smectite

References This study

Zhou et al. (2003)

shelf mud deposit along the Zhejiang–Fujian coasts, extending down to Taiwan Strait (Fig. 4). The selected seismic profiles indicate that the longshoretransported clinoform is also a typical across-shelf sigmoid-type clinoform. AMS 14C and stratigraphic sequence analyses indicate the Yangtze-derived sediment began to be transported southward after the middle Holocene sea-level highstand, around 7 ka BP, mainly by the re-intensified Chinese coastal currents (Liu et al., 2006). This Holocene mud wedge directly overlays a pre-Holocene transgressive surface that marks a maximum landward shift of the shoreline at the end of the last rapid rise of post-glacial sea level. Above this unit, there is an extensive clinoform, with a depocenter (440 m

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0

2149

100

Yangtze delta and along shore clinoform 80

20

Distal Yellow River mud 60

40

Ka o

lin ite +

e Illit

Ch

lor ite

Yellow River delta

40

60

20

80

0

100

0

20

40

60

80

100

Smectite Fig. 6. Ternary diagram of kaolinite+chlorite, illite and smectite concentrations in surface sediments collected from the inner shelf of the ECS, and the suspended sediments from the Yangtze River plume, Yellow Sea current (see data in Table 2). Note the small percentages of smectite, which is indicative of Yangtze River suspended sediment loads.

thick) located between the 20–30 m isobaths in the north, progressively thinning offshore, and diminishing at 60–70 m water depths in the south (Fig. 4). 5.1. Sedimentary features of the longshore dispersed Yangtze River sediment The distribution of fine-grained sediments south of the delta area indicate that most sediments from the Yangtze River were transported southward along the inner shelf, similar to the ‘‘mud belt’’ (Fig. 2) described by Qin (1979). The predominant unimodal size distributions of the sediments are a further indication that the sediments are derived from a single source, the Yangtze River. The coarsegrained surface sediments near the seaward margin of the deltaic region (Core DEB4) are reflective of the energetic and erosive physical conditions that serve to resuspend and transport the Yangtzederived fine-grained sediment southward along the inner shelf (Core DE15), exposing coarse relict and

transgressive sands beneath. The bimodal distributions of the sediment grain sizes indicate a mixture of fine-grained Yangtze River sediment and coarsegrained relict sediments. The mixture of the fine and coarse sediments in the southernmost core DEB13 suggests that, besides the predominant Yangtze sediment, there might be some local inputs from the adjacent Min Jiang (see location in Fig. 2). The clay mineral assemblage in the surface sediments in the ECS and north shelf of the SCS is predominantly by illite and chlorite (Chen, 1978). The illite content increases southward from about 65% at the Yangtze River estuary (Guo et al., 1995) to 73% at the northeastern Taiwan Strait, to 84% at the southwestern Taiwan Strait (You et al., 1993). Although the Yellow River also delivers its suspended sediments into the ECS middle shelf, its higher smectite concentrations (410%) indicate that it is not a major supplier of mud to the Yangtze subaqueous delta or southward-trending mud wedge. Heavy metal, organic carbon and carbonate concentrations over this clinoform

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0.1

1

Excess 210Pb Activity (dpm/g) 0.1 1 10

10

0

Depth (cm)

DEB 5

DEB 6 5

5

10

10

10

15

15

15

20

20

20

25

25

25

30

30

30

10

S = 0.16 cm/ yr

S=0.86 cm/yr

S= 2.14 cm/yr

1

DEB 7

5

35

0.1

0.01 0

0

35

35 Excess 210Pb Activity (dpm/g)

0.1

1

0.1

10

0

10

0.1

1

10

0 DEB 11

DEB 10

Depth (cm)

1

0

DEB 13

5

5

5

10

10

10

15

15

15

20

20

20

25

25

S = 0.33 cm/yr

25 30

30

35 S= 1.6 cm/yr 40

35 40 S =0.47 cm/yr 45

210

Fig. 7. Excess Pb profiles of the sediment cores collected from the inner shelf of the East China Sea. The trend lines indicate the best-fit profiles, ignoring the top 5–10 cm of intensively mixed surface sediments. These values are ‘‘apparent’’ accumulation rates because the effects of deep bioturbation are assumed to be negligible.

surface also indicate that the Yangtze is the major source (Lin et al., 2002). When combined with previously published 210Pb accumulation rates (Table 3), the depositional pattern of the Yangtze’s sediment longshore transport can be inferred (Fig. 8). The highest accumulation rates (43 cm/yr) on the inner shelf are found immediately adjacent to the Yangtze subaqueous delta (north of 301N). There is a general decrease southward alongshore and eastward offshore (Fig. 8).

5.2. Transport and formation of the Yangtze alongshelf clinoform Our grain-size analysis shows the sediments discharged by the Yangtze River are dominated by silt- and clay-sized material. The suspended sediment load at the Datong Hydrology Station accounts for 99.96% of the total load, with a median grain size being 0.027 mm (Yang et al., 2003a). More than 67% of the accumulated sediment near the river mouth has a grain size finer than

ARTICLE IN PRESS J.P. Liu et al. / Continental Shelf Research 26 (2006) 2141–2156 Table 3 Sedimentation rates derived from

2151

210

Pb analysis in the study area

Cores

Latitude

Longitude

Water depth (m)

Rate (cm/yr)

References

DEB4 DEB5 DEB6 DEB7 DEB10 DEB11 DEB13 DE15

30.36 30.36 29.80 27.84 27.08 26.43 26.01 28.07

123.01 122.90 122.67 121.41 120.57 120.11 120.12 122.51

56 50 41 19 19 24 42 72

0 2.14 0.9 0.16 1.6 0.47 0.33 0

This This This This This This This This

study study study study study study study study

MJ114 DS2 XS2 SM30 TD136 WD277 ND227 TD399

30.67 30.28 29.65 29.06 28.68 27.48 29.10 28.15

122.42 122.23 122.02 121.71 121.75 121.04 121.77 121.30

30 5 9 12 10 12 0 2

2.93 2.57 1.3 0.9 0.57 0.69 1.9 5.6

Xia Xia Xia Xia Xia Xia Xia Xia

et et et et et et et et

al. al. al. al. al. al. al. al.

(1999) (1999) (1999) (1999) (1999) (1999) (1999) (1999)

FG17 LH80 HN108 TX259 YS1 YS2 YS5 YS6 YS3 YS4

30.87 30.83 30.87 30.68 30.64 30.60 30.60 30.60 30.55 30.54

122.20 122.64 122.69 121.70 121.97 122.04 122.19 122.28 122.26 122.20

8 10 11 7 9 10 11 15 12 14

3 0.31 0.34 3.02 1.43 1.01 1.58 1.26 0.29 0.35

Xia Xia Xia Xia Xia Xia Xia Xia Xia Xia

et et et et et et et et et et

al. al. al. al. al. al. al. al. al. al.

(2004) (2004) (2004) (2004) (2004) (2004) (2004) (2004) (2004) (2004)

I-2 II-1 II-2 P10 P11 P13 P14

27.90 27.89 27.88 27.87 27.83 27.78 27.81

120.90 120.93 120.88 120.89 120.95 120.93 120.88

0.31 0.58 0.31 2.97 1.43 0.97 1.23

Xie Xie Xie Xie Xie Xie Xie

et et et et et et et

al. al. al. al. al. al. al.

(1994) (1994) (1994) (1994) (1994) (1994) (1994)

G8004 G8005 G8000

31.00 31.00 30.90

122.50 122.75 122.50

5.4 3.1 3.5

DeMaster et al. (1985) DeMaster et al. (1985) DeMaster et al. (1985)

Y5 Y6 Y7 Y8

31.12 31.07 31.02 30.99

122.54 122.69 122.83 122.91

3 3 6 1

Chen Chen Chen Chen

BC8 BC11 493-5 493-6 493-7 493-8 493-9

26.51 26.83 27.50 28.01 28.53 29.01 29.49

121.00 121.50 121.76 122.01 122.18 122.35 122.51

68 68 66 47 39 33 26

0.59 1.5 0.26 0.7 0.82 1.8 0.88

Huh Huh Huh Huh Huh Huh Huh

499-13 499-14 499-15 499-16 499-17 499-27

30.01 30.33 30.67 30.83 30.83 31.00

122.84 122.84 122.95 122.83 122.27 123.33

48 34 49 25 46 56

0.2 1.1 0.5 1.8 0.1 0.2

Su Su Su Su Su Su

13 21 15

et et et et

al. al. al. al.

and and and and and and and

and and and and and and

(2004) (2004) (2004) (2004)

Chen (1999) Su (1999) Su (1999) Su (1999) Su (1999) Su (1999) Su (1999)

Huh Huh Huh Huh Huh Huh

(2002) (2002) (2002) (2002) (2002) (2002)

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2152 Table 3 (continued ) Cores

Latitude

Longitude

Water depth (m)

Rate (cm/yr)

References

499-28 499-29 499-31 499-32 499-37

31.00 31.00 31.17 31.33 31.33

123.00 122.83 122.67 122.67 123.00

51 32 38 42 53

0.1 0.6 0.7 0.4 1

Su Su Su Su Su

and and and and and

Huh Huh Huh Huh Huh

(2002) (2002) (2002) (2002) (2002)

32° Yangzte River 3.0

31° CHINA

30°

2.0 DEB5

Qiangtang Jiang

DEB4

1.5 DEB6 1.0 0.5 29°

Zhejiang East China Sea Ou Jiang

28°

DE15 DEB7 1.5

27° Fujian

26°

DEB10 DEB11

1.0

0.5 Min Jiang

DEB13

Taiwan

25° 118°

119°

120°

121°

122°

0 km

100 km 123°

200 km

124°

Fig. 8. Distribution of the sedimentation rate (cm/yr) in the inner shelf of the ECS (data from Table 3; large circles are data from this study, small circles are data from previous studies).

0.05 mm. Most of finer sediments escape the river mouth and disperse further seaward (Chen et al., 1988). Suspended matter, temperature and salinity surveys conducted in the northwestern ECS show that near-bottom concentrations of suspended matter are much higher during the winter than during the summer, due in large part to winter storms and the well-mixed water column (Milliman et al., 1985, 1989; Yang et al., 1992; Guo et al., 2002, 2003). Hu’s (1984) study suggests that sediment

distributions are affected by coastal upwelling and downwelling. In winter, monsoon-driven coastal currents flow southward and cause downwelling of nearshore waters, whereas the northward-flowing Taiwan Warm Current causes upwelling. With these two vertical circulation cells, fine sediment transport is constrained to areas shoreward of the upwelling regime (Fig. 9). During the winter, strong winter monsoon winds drive the sea-surface currents southward along the Zhejiang and Fujian coast. The Coriolis effect causes the surface current to veer

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Yangtze River

Zhejian-Fujian Coastal Current

Downwelling Topset Foreset

Upwelling Bottomset

Clinoform

Taiwan Warm Current

Fig. 9. Conceptual model of sedimentary and oceanographic processes affecting the sediment dispersal at both the subaqueous delta and alongshore clinoform deposits (after Liu et al., 2006).

to the right toward the coast. This onshore component causes coastal set-up, the buildup of which causes a pressure gradient that drives an offshore current. This current extends throughout the entire water column and veers to the right, producing a geostrophic flow oriented parallel to the shore. In the bottom Ekman layer, flow is oriented obliquely offshore (Cookman and Flemings, 2001). Thus, the onshore transport in the surface Ekman layer and offshore transport in the bottom produces a downwelling regime (Fig. 9). At the same time, the Taiwan warm current flows northeasterly at 35–40 cm/s and transports at least 1Sv to the ECS during the summer (Katoh et al., 2000). The offshore northward-flowing TWC causes a landward moving upwelling at mid and bottom levels. The velocity of the upwelling is about 6.5  103 cm/s (Luo, 1998). Numerous field observations and modeling efforts have demonstrated existence of this nearshore downwelling and offshore upwelling circulation system (e.g., Guan, 1999; Liu et al., 2004b; Pan and Sha, 2004). Due to monsoonal shifts in wind direction and intensity, the northward TWC is relatively stronger in the summer and weaker in the winter, as is the landward upwelling, in contrast to the ZFCC and induced downwelling. This upwelling/downwelling in the

overlying water column apparently increases seabed accumulation (Gao and Jia, 2003). Like other large river systems, the Yangtze-derived sediments are transported to the sea mainly by the across-shelf diffusion and a strong along-shelf advection, as a 3-D quantitative model shows (Driscoll and Karner, 1999). Furthermore, most excess 210Pb activity profiles display significant fluctuations in with depth (Fig. 7), which may indicate either periodic deposition/erosion winnowing, or a variable 210Pb input. This may reflect the interactions between riverine sediment supply and 210Pb input from the open ocean i.e. the seasonal or decadal variations in the coastal Yangtze River plume and offshore Taiwan Warm Current. Similar phenomena have also been observed on the Amazon continental shelf, where riverine supply and lateral transport from offshore waters are the main sources of excess 210Pb activity (DeMaster et al., 1986; Dukat and Kuehl, 1995). 5.3. Sediment budget Based on numerous borings as well as seismic profiles, the Yangtze River has delivered an estimated 1.7  1012 tons of material over last 7000 yr (Liu et al., 2006), an average of 0.24  109 t/yr, the

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same as Saito et al.’s (2001) earlier estimate. Nearly 47% of the Yangtze-derived sediment has accumulated in the subaerial delta and estuarine system, and about 21% has been deposited in the nearshore subaqueous deltaic systems. The remaining 32% is believed to have been resuspended and transported southward, accumulating in the inner shelf along the coasts. Interestingly, the ratio of the southward transport is larger than Shen’s (2001) estimate of 11%, but nearly identical to Milliman et al.’s (1985) estimate of 30%. Previous studies in the middle shelf of ECS also show that very little of the Yangtze sediment presently escapes the shelf to the deep sea (Shen, 2001; Hu and Yang, 2001). As in other large coastal mud wedges derived primarily from a single sediment source, e.g., the Amazon (Allison et al., 2000), Po (Cattaneo et al., 2003), and Yellow (Liu et al., 2004a) rivers, smaller coastal rivers may have local or even regional importance. Along the Zhejiang and Fujian Provinces, three major rivers, Qiantang (10  106 t/yr), Ou (2.73  106 t/yr), and Min (5.7  106 t/yr) rivers may provide local sediment input (Qin et al., 1987; Zhu, 1993). Collectively, however, they deliver only 4% of the present Yangtze’s annual total sediment load, but may account for 12% of the alongshore clinoform budget. To quantify the different sources, further study is clearly needed

to the Yangtze subaqueous delta (north of 301N), decreasing offshore and to the south. The interaction of strong tides, waves, the China Coastal Current, winter storms, and offshore upwelling appear to have played important roles in transporting and trapping most of Yangtze-derived materials in the inner shelf, and thereby, facilitating longdistance transport of the clinoform sediments far from the Yangtze River mouth. This oceanographic environment also has helped block the escape of any Yangtze-derived sediment to the Okinawa Trough.

Acknowledgments Many thanks to our Chinese and American colleagues for their help on the cruises and postcruise sample and data processing. Financial support for this joint research came from the China NSF, International Office of the US NSF, ONR, IOCAS and NCSU. We also thank Dr. Alan Orpin and an anonymous reviewer for their comments, critical reviews, and suggestions. Special thanks to Dr. Andrea Ogston’s in improving the article for this special issue. Finally, we acknowledge the career and contributions of Dick Sternberg, a long-time colleague and friend to many of this paper’s authors.

6. Conclusions References Selected seismic profiles show a predominant clinoform deposit stretching from the Yangtze River mouth southward along the inner shelf of the East China Sea. This elongated clinoform deposit is up to 40 m in thickness, 100 km in width, and nearly 800 km in length. Surface sediments are composed generally of wellsorted, positively skewed silts, clayey-silts, and siltyclays (6.0–8.0 f mean grain size). Clay mineral assemblages are dominated by illite (generally more than 70%), with small amounts of chlorite (12%), kaolinite (9%) and smectite (3%). The high illite concentrations and low smectite concentrations (o5%) are a good indicator of Yangtze Riverderived material. In addition to the alongshore sediment transport, the accumulation also has an across-shelf development that shows a typical sigmoid-type clinoform. 210 Pb chronologies reveal that the highest accumulation rates (43 cm/yr) occur immediately adjacent

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