DSR II Oguri2003.pdf

Deep-Sea Research II 50 (2003) 513–528

Sediment accumulation rates and budgets of depositingparticles of the East China Sea

Kazumasa Oguria,*,w, Eiji Matsumotob, Masatoshi Yamadac, Yoshiki Saitod,Kazuo Isekie

a Institute for Hydrospheric–Atmospheric Sciences, Nagoya University, Furo, Chikusa, Nagoya, 464-8601 Japanb Graduate School of Environmental Studies, Nagoya University, Furo, Chikusa, Nagoya, 464-8601 Japan

c Nakaminato Laboratory for Marine Radioecology, National Institute of Radiological Sciences, Isozaki 3609, Hitachinaka,

Ibaraki, 311-1202 Japand MRE, Geological Survey of Japan, AIST Central 7, Higashi 1-1-1, Tsukuba, 305-8567 Japan

e National Institute of Fisheries and Environment of Inland Sea, Fisheries Research Agency, Maruishi 2-17-5, Ohno, Saeki, Hiroshima,

739-0452 Japan

Accepted 13 July 2002

Abstract

To understand sedimentary processes of depositing particles in the marginal sea, we measured sediment accumulation

rates of 11 cores collected from the East China Sea. Sediment accumulation rates were calculated using excess 210Pb

profiles in the sediment column. The results show that sediment accumulation rates decrease from 1.8 in the inner shelf

to 0.049 g cm�2 yr�1 to the Okinawa Trough. Organic carbon (org. C) accumulation rates ranged from 0.42 to

3.1mg C cm�2 yr�1 and were higher in the shelf edge and the slope areas. Compared with the results of the sediment trap

experiments, these results indicate that lateral transport plays an important role in particle accumulation in the trough

area.

r 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction

At present about 1� 109 t of sediments per yearare supplied into the East China Sea (ECS) by theChangjiang (4.8� 108 t yr�1; Milliman and Meade,

1983) and the Huanghe and from coastal erosionof the old Huanghe River mouth (5� 108 t yr�1;Saito and Yang, 1995). These sediments aredeposited in the region south of Cheju Island,around the Changjiang mouth and along thesouthwestern coast of the Changjiang mouth(Milliman et al., 1985a,b; Chin, 1979; Sternberget al., 1985). In the continental shelf of the ECS,Niino and Emery (1961) first showed the distribu-tion of bottom sediments, and DeMaster et al.(1985) reported that the accumulation rates arerelatively high in muddy areas and low in sandyareas. Narita et al. (1990) reported accumulation

*Corresponding author.

E-mail addresses: [email protected] (K. Oguri), matsu-

[email protected] (E. Matsumoto), m yamada@nirs.

go.jp (M. Yamada), [email protected] (Y. Saito), kise-

[email protected] (K. Iseki).wPresent address. Institute for Frontier Research on Earth

Evolution, Japan Marine Science and Technology Center,

Natsushima 2-15, Yokosuka, Kanagawa, 237-0061 Japan.

0967-0645/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 9 6 7 - 0 6 4 5 ( 0 2 ) 0 0 4 6 5 - 4

rates in the central part of the Okinawa Trough,and Chung and Chang (1995) reported rates in thetrough off Taiwan. These studies suggest thatthe accumulation rates of the trough are low butthose of the slope area are relatively high. Theseprinciple data are very important to understandmaterial transport in marginal seas. In the westernMediterranean Sea, the ECOsyst"emes de MARGEcontinentale (ECOMARGE) Project (Monacoet al., 1990) was carried out to study carbon cyclesfrom the shelf edge to the deep sea, and in theMiddle Atlantic Bight, shelf edge exchange pro-cesses (SEEP) I (Walsh et al., 1988) and SEEP IIProjects (Biscaye et al., 1994) obtained muchinformation about material transport from theshelf to the abyssal plain. These studies also havediscussed the relationship between sediment accu-mulation and carbon burial rates (Courp andMonaco, 1990). According to the studies, sus-pended particles are supplied from the shelf to theslope area (Buscail et al., 1990; Biscaye andAnderson, 1994), sediment accumulation variesgreatly with location (Biscaye and Anderson,1994), and organic carbon (org. C) is preservedpoorly in slope sediments (Buscail et al., 1990;Anderson et al., 1988). In the ECS such sedimen-tary processes are poorly known, although theECS is one of the largest marginal seas inthe world. The objectives of our study, as a partof the MArginal Sea FLux EXperiment in theWest Pacific (MASFLEX) Project, were to in-vestigate sediment accumulation rates and tounderstand sedimentary processes in the ECScontinental margin.

2. Materials and methods

2.1. Locations and core samples

Sampling sites are shown in Fig. 1 and Table 1.The ECS continental margin can be divided intothree areas by water depth and surfical sediments:(1) the shelf area with water depths above 150 mand surfical sediments consisting of muddy sand,(2) the slope area with water depths from 150 to800 m and surfical sediments consisting of sandymud in the upper slope and silt to clay in the lower

slope, and (3) the trough with water depths greaterthan 800 m and surfical sediment consisting ofsilt to clay. Eleven short cores were collectedusing a multiple core sampler from the innercontinental shelf to the Okinawa Trough areaduring cruises of K92-09 and K95-09 of the R/VKAIYO of the Japan Marine Science and Tech-nology Center. These cores were cut on board at1-cm intervals from surface to 10 cm deep, and2-cm intervals from 10 cm deep to the bottom.Sediment samples were then stored in the freezeruntil analyzed.

2.2. 210Pb activities in sediment

To measure excess 210Pb activities of sediments,samples were dried, powdered, weighed, and storedin sealed plastic vessels. These samples were left atleast for 2 weeks in order to establish radioactiveequilibrium between 226Ra and 222Rn. Thenspecific gamma rays of 210Pb (46.5 keV) and 214Pb(351.9 keV) were measured using both an EG&GORTEC model GMX-25190-S planer germaniumdetector and an EG&G ORTEC model GWL-120230 well germanium detector and a SEIKOEG&G model 7800 multi channel analyzer. Excess210Pb activities were calculated by subtracting214Pb activities on the assumption that the sup-ported 210Pb from 226Ra is equal to 214Pb. Samplesweights were 50 g for the planer detector and 3.0 gfor the well detector, respectively. As a reference,uranium–thorium ore DL-1a from Canada Centrefor Mineral and Energy Technology was used. Thecounting error was one standard deviation ofcounting, and counting times were from 24 to 96 h.

2.3. Sediment-trap experiments

Sediment-trap experiments were carried outduring K92-09 and K95-09 cruises. These trapswere moored at SST-2, F-4, F-6 and F-8 (Fig. 1).The traps deployed at F-4, F-6 and F-8 were madeof quadruplets of cylindrical tubes with a singlecup; the trap at SST-2 was funnel-shaped time-series trap with multiple cups. All the traps exceptat SST-2 were moored at 12 m above sea bottom toavoid occasional intrusion of resuspended sedi-ment. The sediment trap at SST-2 was moored at

K. Oguri et al. / Deep-Sea Research II 50 (2003) 513–528514

50 m above sea bottom. The mooring sites andperiods of sediment-trap deployments are shownin Table 2.

The samples obtained by the sediment-trapexperiments were analyzed for both 210Pb activitiesand org. C contents. Samples were dried and weighed

120°E35°N

30°

25°

125° 130°

20

20

40

40

60

60

80

80

100

100

100

200

200

200

200

500

500

1000 1000

1000

1000

PN-12

PN-11

PN-9

PN-5PN-4

F-8F-6F-4F-2

SST-2

PN-8PN-Line

F-Line

Okinawa Is.

Cheju Is.

Kyushu Is.

Okinaw

a Tro

ugh

East China Sea

Fukien-R

einan

Mass

if

Changjiang

Yellow Sea

Fig. 1. Sampling sites of sediment cores and sediment-trap deployments. Sediment cores were collected from all the sites. Sediment

traps were deployed in the shelf edge and the trough areas (SST-2, F-4, F-6 and F-8).

Table 1

Locations and water depth of sediment core collection

Area Site Latitude (N) Longitude (E) Water depth ðmÞ Cruise

Shelf PN-12 31112.2930 123105.0700 50 K92-09

Shelf PN-11 30136.6820 123139.5830 58 K92-09

Shelf PN-9 29159.8320 124130.0280 68 K92-09

Shelf PN-8 29135.5510 125105.4410 87 K92-09

Shelf PN-5 28142.0220 126125.9570 127 K92-09

Slope PN-4 28124.5350 126153.9290 284 K92-09

Trough SST-2 28108.2670 127112.8860 1095 K92-09

Shelf F-8 28155.7920 126146.1890 133 K95-09

Slope F-6 28144.5600 126158.4050 297 K95-09

Slope F-4 28141.6100 127104.5730 590 K95-09

Trough F-2 28137.9300 127112.8660 1000 K95-09

K. Oguri et al. / Deep-Sea Research II 50 (2003) 513–528 515

to measure total mass flux. For 210Pb measurement,samples were decomposed using HNO3, HF andHClO3. Dissolved lead was purified by ion exchangeand recovered as sulfate. These samples were storedfor about 6 months, and 210Pb activities weredetermined to measure beta ray emitted by 210Biwhich grows from 210Pb (Yamada and Aono, 2003).Sediment-trap samples also measured the org. Ccontent using CHN analyzer (Iseki et al., 2003).

2.4. Org. C content of the surfical sediments

Org. C contents of the surfical sediments (fromcore top to 1 cm depth) were measured using aHORIBA EMIA-820 C/S Analyzer. Carbon con-tent was measured for untreated and decalcifiedsediments by 1 N hydrochloric acid. Standarderror for the analysis was 0.2%.

3. Results

3.1. Sediment cores

Descriptions of sediment cores are shown inTable 3. In all cores, fine sedimentary structuressuch as lamination could not be found and smallchimneys were observed in the sediment surface,suggesting biological mixing.

3.2. Accumulation rate from excess 210Pb profile

Excess 210Pb profiles in the sediments are shownin Fig. 2 and Table 4. The core depth in the graphis represented both cumulative mass and cm-scalesfrom core top. The cumulative mass of sediment

can be calculated from the following equation:

W ¼Z z

0

r dz; ð1Þ

where W is the cumulative mass of the sediment(g cm�2), r is the dry bulk density of the sediment(g cm�3), and z is the depth from core top (cm). Inthe PN-4, PN-8, PN-9 and PN-11 profiles, distinctsurface homogeneous layers indicating sedimentmixing were observed. We excluded data fromthese layers to calculate sediment accumulationrates. Sediment accumulation rates were extremelyhigh in the inner shelf area (PN-12) and decreasedoffshore (Fig. 3).

3.3. Excess 210Pb inventory

The distributions of excess 210Pb inventories (i.e.total amount of excess 210Pb in the sedimentcolumn) are shown in Fig. 4. The highest excess210Pb inventory was in the trough (F-2). Highvalues also were noted at the shelf edge (PN-5 andF-8) and the slope (PN-4 and F-6). In the innershelf (PN-12), excess 210Pb inventory was also highbecause of the high accumulation rates.

3.4. Org. C accumulation rates

Org. C produced in the surface seawater isremineralized rapidly in the seabed (e.g., Andersonet al., 1994). Thus org. C seems less preserved insediment. We determined the org. C accumulationrate by knowing the org. C content of the surficalsediments and sediment accumulation rate ob-tained by excess 210Pb profiles. Org. C contents ofsurfical sediments increased offshore (Fig. 5),

Table 2

Locations, mooring depth, water depth, type of trap and period of the sediment trap experiments

Site Latitude

(N)

Longitude

(E)

Mooring

depth ðmÞWater

depth ðmÞType of trap Period of deployment Cruise

From To

F-8 28153.2910 126141.4010 120 132 Single cup 10/27/1995 11/4/1995 K95-09

F-6 28143.6910 126158.0360 289 301 Single cup 10/27/1995 11/4/1995 K95-09

F-4 28140.8600 127104.4730 592 604 Single cup 10/27/1995 11/4/1995 K95-09

SST-2 28140.8600 127104.4730 1020 1070 Multiple cup 3/1/1993 2/22/1994 K92-09


whereas org. C accumulation was high in the innershelf or slope and relatively low in the middle shelf(Fig. 6).

4. Discussion

4.1. Accuracy of the accumulation rate from excess210Pb profile

In areas where biological or physical mixing areactive, accumulation rates based on excess 210Pbprofiles can be overestimated (DeMaster et al.,1985; Alexander et al., 1991). In such cases, theaccumulation rates from excess 210Pb profiles areregarded as ‘apparent accumulation rates’. How-ever, assuming that an inventory of excess 210Pb inthe sediment column and 210Pb activities of thedepositing particles are constant over severaldecades, a mean accumulation rate can becalculated by the following equation (Oguri et al.,1997):

wP ¼ lS; ð2Þ

where w is the mean sediment accumulation rate(g cm�2 yr�1), P is the 210Pb activity in depositingparticles (dpm g�1), l is the decay constant of210Pb (0.031 yr�1), and S is the inventory of excess210Pb in the sediment column (dpm cm�2). Table 5shows the difference between the apparent and themean accumulation rates obtained by the equa-

tion. To calculate the mean accumulation rates, weused the 210Pb activities of the sediment-trapsamples as 210Pb activities in depositing particles.This result indicates that the apparent accumula-tion rates seem to be overestimated. However, inmarginal seas, there is doubt as to whether theinventories of excess 210Pb are in steady state ornot. Nevertheless, in our discussion we usedapparent accumulation rates as sediment accumu-lation rates because we did not have enoughinformation about the seabed environment in theECS.

4.2. Excess 210Pb inventory

The high excess 210Pb inventory on the innershelf may reflect the higher accumulation rates. Inthe shelf edge and the trough areas, the excess210Pb inventories were higher although sedimentaccumulation rates were lower. In the ECS, 210Pbactivities of depositing particles are increasedoffshore (Table 5) because of the boundaryscavenging process (Nozaki et al., 1991). There-fore, higher excess 210Pb inventories in these areasseem to be a result of the supply of depositingparticles having higher 210Pb activities. In thetrough area (SST-2 and F-2), excess 210Pb inven-tories are quite different between the two sites.Perhaps this may reflect differences of 210Pbactivity of depositing particles; however, thereason is not clear any more.

Table 3

Descriptions of obtained sediment cores. These data are contained in cruise reports of MASFLEX K92-09 and K95-09 (Ikehara et al.,

1992; Matsumoto and Oguri, 1996

Site Core length (cm) Description

PN-12 25.0 Brownish black to olive black colored homogenous very fine sand.

PN-11 38.0 Olive black to dark olive gray colored homogenous very fine sand.

PN-9 28.0 Grayish olive to dark olive gray colored homogenous very fine sand.

PN-8 28.5 Grayish olive to olive gray colored homogenous fine to very fine sand.

PN-5 36.0 Grayish olive to olive gray colored very fine to fine sand.

PN-4 46.5 Grayish olive to olive gray colored homogenous muddy very fine sand.

SST-2 40.5 Brownish black colored silty clay (0–5.0 cm)/grayish olive colored silty clay (5.0–35.5 cm).

F-8 27.0 Homogenous muddy sand with shell fragments.

F-6 31.5 Homogenous sandy mud (0–6.0 cm)/homogenous silt (6.0–24.0 cm)/homogenous clayey silt (24.0–31.5 cm).

F-4 26.0 Homogenous silt (0–8.0 cm)/homogenous clayey silt (8.0–26.0 cm).

F-2 40.5 Homogenous clayey silt.


SST-2

PN-4PN-5

PN-8PN-9

PN-11PN-12

Exc

ess

210 P

b (d

pm/g

)

Cumulative Mass (g/cm2)

Exc

ess

210 P

b (d

pm/g

)


Exc

ess

210 P

b (d

pm/g

)


Exc

ess

210 P

b (d

pm/g

)


Exc

ess

210 P

b (d

pm/g

)


Exc

ess

210 P

b (d

pm/g

)


Exc

ess

210 P

b (d

pm/g

)


Core depth (cm)

Core depth (cm)

Core depth (cm)

Core depth (cm)Core depth (cm)


0.1

1

10

100

0 5 10 15 20 25 30 35

0.1

1

10

100

0 5 10 15 20 25 30 350.1

1

10

100

0 5 10 15 20 25 30 35

0.1

1

10

100

0 5 10 15 20 25 30 350.1

1

10

100

0 5 10 15 20 25 30 35

0.1

1

10

100

0 5 10 15 20 25 30 350.1

1

10

100

0 5 10 15 20 25 30 35

0 5 10 15 20

0 5 10 15 200 5 10 15 20

0 5 10 15 20 0 5 10 15

0 5 10 15 20 0 5 10 15 20

0.22 g cm-2 yr-1 0.11 g cm-2 yr-1

0.28 g cm-2 yr-1 0.21 g cm-2 yr-1

1.81 g cm-2 yr-1 0.40 g cm-2 yr-1

0.055 g cm-2 yr-1

Fig. 2. Excess 210Pb profiles of the sediment cores collected from the East China Sea continental margin.


4.3. Sediment and org. C accumulation in the ECS

In the inner shelf area (PN-12), sediment andorg. C accumulation rates showed the highest inthe area of ECS. Our sampling site is in outer edgeof the Changjiang dispersal system where muddysediment accumulates with extremely high accu-mulation rate (DeMaster et al., 1985). Therefore, itis possible that such high accumulation is causedby high depositing particles supply from theChangjiang River and/or adjacent submergeddelta by strong tidal current (Sternberg et al.,1985).

In the middle shelf area (PN-8, PN-9 and PN-11), both sediment and org. C accumulation ratesdecreased, suggesting that the supply of depositingparticles seem to be also decreased offshore. Hamaet al. (1997) mentioned that the primary produc-tivity from the inner to the middle shelf area is thehighest in the ECS continental margin. However,org. C accumulation rates in the middle shelf aresmaller. It suggests that org. C produced in the

area may be remineralized and/or transportedoffshore.

From the shelf edge to the trough areas (SST-2,PN-4, PN-5, F-2, F-4, F-6 and F-8), the sedimentaccumulation rates increased a little in thePN-5 and F-8, and decreased again in the trougharea. On the other hand, the org. C contentsincreased offshore and the org. C accumulationrates are higher from the shelf edge to theslope areas, although primary productivity inthese areas are lower than that in the middleshelf area (Hama et al., 1997). The depositingparticles having higher org. C from the middleshelf may increase the org. C accumulation rates inthis area.

4.4. Processes of lateral transport in the ECS

For previous studies, most mud fractions fromthe Changjiang River mouth are accumulatedalong the southwestern coast of China (Chin,1979; Yang et al., 1984) and the sediments

0.1

1

10

100

0 5 10 15 20

0 5 10 15 2520

25 30 35

F-2

Exc

ess

210 P

b (d

pm/g

)


Core depth (cm)

0.049 g cm-2 yr-1

0.1

1

10

100

0 5 10

0 5 10 15

15 20 25 30 35

F-4

Exc

ess

210 P

b (d

pm/g

)


Core depth (cm)


0.14 g cm-2 yr-1

0 5 10 15 20

0.1

1

10

100

0 5 10 15 20 25 30 35

F-6

Exc

ess

210 P

b (d

pm/g

)


0.14 g cm-2 yr-1

0.1

1

10

100

0 5 10 15 20 25 30 35

F-8E

xces

s 21

0 Pb

(dpm

/g)


0 5 10 15

0.31 g cm-2 yr-1

Fig. 2 (continued).


Table 4

Total 210Pb, 214Pb and excess 210Pb activities in the sediment cores

Core Depth (cm) Total 210Pb (dpmg�1) 214Pb (dpmg�1) Excess 210Pb (dpmg�1)

PN-12 0–1 4.4670.43 0.6970.07 3.7770.44

2–3 5.5270.56 1.5570.09 3.9770.56

4–5 4.2970.50 1.6270.08 2.6870.61

6–7 4.4970.44 1.5970.08 2.9070.45

8–9 4.2770.55 1.4870.08 2.7970.56

10–12 5.1970.45 1.6570.07 3.5570.46

14–16 3.6170.46 1.8470.08 1.7770.47

18–20 4.1670.47 1.6670.07 2.5070.47

PN-11 0–1 4.2070.28 1.5570.58 2.6570.19

2–3 4.1770.25 1.5270.53 2.6570.23

4–5 3.3370.23 1.5370.50 1.8070.23

6–7 3.2870.20 1.4370.43 1.8570.18

7–8 3.2670.19 1.4170.43 1.8570.17

8–9 3.0670.26 1.4370.56 1.6370.24

9–10 2.6670.17 1.4370.39 1.2370.16

10–12 2.3870.19 1.4070.43 0.9870.17

12–14 1.9870.26 1.4170.57 0.5770.24

14–16 2.0670.17 1.3270.40 0.7470.16

16–18 1.9270.18 1.4470.43 0.4870.16

20–22 1.7270.16 1.3270.39 0.4070.14

PN-9 1–2 3.6570.26 1.3270.06 2.3270.27

3–4 4.2570.26 1.3170.06 2.9470.27

5–6 4.5470.26 1.3570.06 3.1970.27

7–8 4.2070.26 1.3070.06 2.9070.27

8–9 3.2270.18 1.4070.04 1.8270.18

9–10 3.4370.20 1.3970.05 2.0470.21

10–12 2.6070.19 1.2870.04 1.3270.19

14–16 2.2670.18 1.2970.04 0.9770.18

18–20 1.8670.15 1.3670.04 0.5070.16

20–22 1.5570.17 1.4070.04 0.1570.17

22–24 1.5570.12 1.3770.03 0.1870.12

F-8 0–1 15.9370.68 1.5370.09 14.4070.68

2–3 12.3270.66 1.5770.09 10.7570.66

4–5 12.3770.61 1.4970.09 10.8770.62

6–7 8.0270.58 1.5270.09 6.4970.58

8–9 6.4970.52 1.5670.07 4.9370.52

10–12 5.7670.42 1.5870.08 4.1870.43

14–16 3.6770.41 1.6670.08 2.0170.41

F-6 0–1 21.3371.14 1.4670.09 19.8771.14

2–3 18.0371.06 1.5870.09 16.4671.06

4–5 13.7070.93 1.6070.08 12.1070.93

6–7 10.3071.32 1.3370.10 8.9771.32

9–10 4.0770.48 3.1670.10 0.9170.49

10–12 3.8770.39 1.6670.07 2.2170.40

14–16 4.4870.46 2.1570.08 2.3270.47

F-4 0–1 32.4270.72 1.9670.09 30.4670.72

2–3 17.3170.63 1.8270.09 15.4970.64


Table 4 (continued)

Core Depth (cm) Total 210Pb (dpmg�1) 214Pb (dpmg�1) Excess 210Pb (dpmg�1)

4–5 7.5570.59 2.0170.09 5.5470.59

6–7 16.5070.60 2.0070.09 14.5070.61

8–9 8.5370.47 1.6370.07 6.9170.48

10–12 5.5970.37 1.7970.06 3.8070.38

14–16 3.7570.36 1.7570.06 2.0070.37

F-2 0–1 134.4972.16 3.7370.17 130.7672.16

2–3 87.2271.28 3.9770.12 83.2471.28

4–5 53.4071.42 2.5270.13 50.8871.43

6–7 29.9371.49 2.9370.15 26.9971.50

8–9 18.1071.62 2.9570.15 15.1571.63

10–12 7.7070.76 2.9670.09 4.7470.77

14–16 5.1470.89 3.3470.09 1.8070.89

PN-8 0–1 7.0370.88 1.8370.10 5.2070.88

2–3 7.5070.51 1.9170.09 5.5970.52

4–5 7.8070.48 1.6170.08 6.1870.48

6–7 5.0470.61 1.7070.08 3.3470.62

8–9 5.5470.63 1.5470.08 4.0070.64

10–12 2.9570.59 1.7170.08 1.2470.60

14–16 2.3370.71 1.5970.08 0.7470.71

PN-5 0–1 17.6471.05 1.6270.10 16.0271.05

1–2 14.0671.38 1.4370.11 12.6371.39

3–4 11.8371.28 1.4770.11 10.3671.29

4–5 11.4571.17 1.5170.09 9.9371.17

5–6 6.9370.91 1.3070.08 5.6370.92

8–9 4.9071.29 1.4870.09 3.4271.29

10–12 3.3370.80 1.5770.08 1.7670.81

12–14 3.8970.75 1.4170.08 2.4870.75

16–18 3.1570.67 1.7670.08 1.3970.67

PN-4 0–1 22.9670.64 1.1070.10 21.8670.27

2–3 19.7070.53 1.0770.09 18.6370.27

4–5 18.8670.53 1.0570.08 17.8170.27

6–7 15.1270.53 1.2570.09 13.8770.27

8–9 12.9770.48 1.1770.09 11.8070.18

10–12 7.8370.33 1.1570.06 6.6870.21

14–16 5.5570.30 1.1870.06 4.3770.19

18–20 2.4370.19 1.1170.04 1.3270.18

22–24 1.4570.14 1.1970.04 0.2670.16

SST-2 0–1 36.0271.34 6.5870.17 29.4471.35

2–3 28.5770.94 8.1570.16 20.4270.95

4–5 15.8271.06 14.6770.22 1.1571.09

6–7 11.1270.99 3.6870.14 7.4471.00

8–9 4.5670.68 3.1870.08 1.3870.69

10–12 4.0770.76 3.1170.09 0.9670.77

Activities in sediment cores of PN-4, PN-9, PN-11 and PN-12 are measured using a planer detector, and the others are measured by a

well detector.


distributed in the shelf area are transported bothnorthward and southeastward by strong tidalcurrents (Sternberg et al., 1985). However, asseen in these sediment cores, sediment structure

exhibited homogeneous muddy sand with shellfragments. If only muddy sediments are suppliedto the sandy bottom, mud content should beincreased near the core top. So it is reasonable to

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

PN-12 PN-11 PN-9 PN-8 PN-5PN-4

SST-2

PN-Line

F-LineSedi

men

t ac

cum

ulat

ion

rate

(g

cm-2

yr-

1 )

F-8

F-6 F-4F-2

Shelf areaSlope area

Trough area

1.80

0.28

0.310.14 0.14

0.049

0.40

0.21 0.22

0.11 0.050

Fig. 3. Sediment accumulation rates calculated from excess 210Pb profiles in sediments.

PN-12 PN-11 PN-9 PN-8 PN-5 PN-4SST-2

PN-Line

F-Line

F-8

F-6

F-4

F-2


Trough area

Exc

ess

210 P

b in

vent

ory

(dpm

cm-2

)

0

50

100

150

200

250

300

68

105>90

38 40

171122

88

60

257

56

Fig. 4. Excess 210Pb inventories in sediments.


assume that both mud and sand particles areaccumulated there (Nittrouer et al., 1984). In theshelf edge area, depositing particles are trans-ported northeastward by the Kuroshio Current

(Chin, 1979). However, as seen the sediments, bothsand and mud are accumulated. Perhaps mudfractions come from around the Changjiang Rivermouth. Li et al. (1997) carried out water sampling

PN-12 PN-11 PN-9 PN-8 PN-5PN-4

SST-2

PN-Line

F-Line0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8O

rgan

ic c

arbo

n co

nten

t (%

)

F-6

F-4

F-2


Trough area

0.3 0.3 0.4

0.8

1.4

1.8

0.6 0.40.7

0.8F-8

0.2

Fig. 5. Org. C contents in surfical sediments.

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

PN-12 PN-11 PN-9 PN-8 PN-5 PN-4SST-2

PN-Line

F-Line

F-6

F-4

F-2


Trough area

Org

.C a

ccum

ulat

ion

rate

(mgC

cm-2

yr-1

)

3.1

1.1

0.77

1.3

1.21.1

2.0

0.880.85

0.79

0.42

F-8

Fig. 6. Org. C accumulation rates calculated from sediment accumulation rates and org. C contents in surfical sediments.


in the shelf area of the ECS and measured theamount of suspended particles and concluded thatthe suspended particles are transported offshore inwinter half-year. This offshore transport mechan-ism is also supported by results of the sediment-trap experiment deployed in the Okinawa Troughduring the MASFLEX project (Iseki et al., 2003)and a numerical model (Yanagi et al., 1996).Perhaps offshore transport is triggered by winterstorm (Milliman et al., 1985a). On the other hand,most sand fractions are no longer supplied to theslope and the trough areas because these particleshave not been seen in the seabed. For mudfraction, a part of them seems to be transportedto the trough area and the rest would betransported by the bottom turbidity at the shelfbreak (Okamura et al., 1997). Schematic illustra-tions of particle transport in the ECS continentalmargin are summarized in Fig. 7.

4.5. Accumulation and transport rate of depositing

particles from the shelf edge to the trough areas

From the shelf edge to the trough areas, we canestimate the fraction of both sediment and org. Caccumulations of the depositing particles tocompare accumulation rates with fluxes obtainedby sediment traps. The budgets of the depositingparticles on the sea bottom are shown in Fig. 8.

The total mass fluxes observed by sediment trapswere higher than the sediment accumulation rates.As mentioned in the Table 2, the sediment-trapdeployments except SST-2 were carried out duringa short time in the autumn. So these total massfluxes may reflect a larger value than the centuryaverage which is the same time scale as theaccumulation rates from the excess 210Pb profilesbecause offshore transport is somewhat enhancedin this season (Yanagi et al., 1996; Li et al., 1997).However, we used the sediment-trap data becausethere are no other data in these areas.

In the shelf edge area (F-8), total mass fluxshowed 0.33 g cm�2 yr�1 and the accumulationrate was 0.31 g cm�2 yr�1. Thus most depositingparticles seems to be accumulated in this area. Inthe upper slope area (F-6), total mass flux showed0.21 g cm�2 yr�1 and the accumulation rate was0.14 g cm�2 yr�1. So 67% of the total mass flux isaccumulated there. In the lower slope area (F-4),total mass flux showed 0.85 g cm�2 yr�1 whichwas the highest in the slope to the trough area.On the contrary, the accumulation rate was0.14 g cm�2 yr�1, which is only 16% of the totalmass flux. The fractions that were not accumulatedwould be retransported by the Kuroshio Currentas intermediate turbid layer (Okamura et al.,1997). In the lower slope, the fraction of accumu-lation was relatively low compared to the shelf

Table 5210Pb activities and its fluxes of depositing particles, excess 210Pb inventories in sediment and accumulation rate from the inventory and

from the profile

Site 210Pb

activity of

depositing

particle

(dpmg�1)

210Pb flux into

the bottom trap

(dpm cm�2 yr�1)

Excess 210Pb

inventory in

sediment

(dpmcm�2)

210Pb decay in

sidement

(dpmcm�2 yr�1)

Accumulation

rate from

excess 210Pb

inventory

(g cm�2 yr�1)

Sediment

accumulation

rate from

excess 210Pb

profile

(g cm�2 yr�1)

Total mass

flux into

the bottom

trap

(g cm�2 yr�1)

F-8 47.9 15.9 144 4.5 0.093 0.31 0.33

F-6 63.0 13.5 91 2.8 0.045 0.14 0.21

F-4 80.9 68.8 98 3.0 0.038 0.14 0.85

F-2 N.D. N.D. 203 6.3 N.D. 0.049 N.D.

SST-2 214 2.3 56 1.7 0.008 0.050 0.011

N.D.: No data.

Data of SST-2 is time-weighted mean value.


edge and the upper slope area. This is because thetotal mass flux from the trap was extremely high.Okamura et al. (1997) mentioned that the turbidityin the shelf edge and the slope area is affected byfluctuations of the Kuroshio pathway, which flowsjust above the upper slope area of the ECS. So it ispossible that the large flux in F-4 may reflect sucha Kuroshio fluctuation. However, this hypothesesis not still clear because time series sediment-trapdata from the shelf edge to the slope area were notobtained during the MASFLEX project. Inthe trough area, total mass flux in SST-2showed 0.011 g cm�2 yr�1 in annual average. Onthe other hand, the accumulation rates were0.049 g cm�2 yr�1 (F-2) and 0.050 g cm�2 yr�1

(SST-2), respectively. Therefore about 460% ofthe total mass fluxes accumulated in the trougharea. This extremely high value means that thelateral transport at the seabed plays an importantrole in the trough area. Although the accumula-tion rate of the trough area is lower than those ofthe other areas, the trough seems to be a storage ofthe depositing particles.

4.6. Org. C accumulation and transport rate from

the shelf edge to the trough areas

The org. C budget in marginal seas is veryimportant for understanding global carbon cycles(Walsh et al., 1981). So org. C accumulation rates

Fig. 7. Schematic illustration of depositing particles transport in the ECS continental margin. Arrows represent directions of particle

transport. Depositing particles are transported both northward and southeastward by strong tidal current (Sternberg et al., 1985), and

are transported northeastward by bottom part of the Kuroshio Current (Chin, 1979). However, these particles are supplied as bottom

turbid layer from the Changjiang River mouth in winter half-year (Li et al., 1997) triggered by winter storm (Milliman et al., 1985a).

Most particles composing bottom turbid layer are detached in the shelf break (Okamura et al., 1997).


on the sea floor have been discussed in variousshelf environments (e.g., Buscail et al., 1990;Anderson et al., 1994). However, in the ECS,org. C accumulation rate into the sediment has notbeen estimated. Org. C budgets from the shelf edgeto the trough area are shown in Fig. 9. Org. Ccontent and its fluxes are from Iseki et al. (2003)and Iseki et al. (unpublished data). At all the sitesexcept SST-2, the org. C contents of the depositing

particles were higher than those of the surficalsediments, and their values were lower in the shelfedge area and higher in the slope area.

The org. C accumulation rate was calculatedto be 1.2 mg C cm�2 yr�1 in the shelf edge area(F-8), 1.1 mg C cm�2 yr�1 in the upper slope area,2.0 mg C cm�2 yr�1 in the lower slope area(F-6), and 0.88 mg C cm�2 yr�1 in the trougharea (F-4). In the trough area, the org. C

0.330.02 0.21

0.07 0.85

Shelf edge(F-8, 133m)

Upper slope(F-6, 297m)

Lower slope(F-4, 590m)

200

0

400

Water depth(m)

600

800

1000

Deposition rate fromwater column

Transportationrate

Accumulation rate

Depositing particles(g cm-2 yr-1)

0.71

0.31

0.14

0.0490.050

0.14

Trough(F-2, 1000m)

(SST-2, 1095m)

0.039? 0.011

Fig. 8. Budgets of depositing particles on sea floor from the shelf to the trough areas.

7.3

Shelf edge(F-8, 133m)

Upper slope(F-6, 297m)

Lower slope(F-4, 590m)

Trough(F-2, 1000m)

(SST-2,1095m)

200

0

400

Water depth(m)

600

800

1000

Deposition rate fromwater column

Remineralization and transportation rate

Accumulation rate

Org. C(mgC cm-2 yr-1)

1.1

4.4

3.3

2.0

0.88

>0.17

17

15

?

6.1

1.2

0.42

0.25

Fig. 9. Budgets of the org. C on sea floor from the shelf to the trough areas. Org. C fluxes in F-4, F-6 and F-8 area from Iseki et al.

(unpublished data) and flux in SST-2 is from Iseki et al. (2003).


accumulation rates were 0.88 mg C cm�2 yr�1

(F-2) and 0.42 mg C cm�2 yr�1 (SST-2), respec-tively. Fig. 9 indicates that most of the org. Cfrom water column is retransported and/or remi-neralized. These rates were 6.1 mg C cm�2 yr�1 inthe shelf edge, 3.3 mg C cm�2 yr�1 for the upperslope, and 15 mg C cm�2 yr�1 for the lower slopearea. On the other hand, in the trough area (SST-2), more than 0.17 mg C cm�2 yr�1 of org. C istransported by lateral transport and accumulated.So this area seems to be a storage of org. C as wellas depositing particles.

In the ECS, annual average of the primaryproductivity in the shelf, the shelf edge, and theKuroshio area (i.e. above the slope area) are 27.4,12.4 and 10.2 mg C cm�2 yr�1, respectively (Hamaet al., 1997). However, the primary productivity inthe shelf area increases up from 36.5 to54.8 mg C cm�2 yr�1 in summer and autumn sea-son (Hama et al., 1997). So, it is difficult to thinkthat most org. C deposited from the slope to thetrough area is supplied only from the Kuroshioarea. Some org. C fractions deposited there mustbe supplied from the shelf area as resuspendedmatter (Anderson et al., 1994). As mentionedabove, depositing particles are transported off-shore in the winter half of the year (Li et al., 1997).So a fraction of org. C produced in summer andautumn in the shelf area also seems to betransported to the slope area in this season andaccumulated there.

5. Conclusion

Both sediment and org. C accumulation rateswere the highest in the inner shelf area. Sedimentaccumulation rates decreased offshore, whereasorg. C accumulation rates were relatively high onthe shelf edge and the slope. From the shelf edge tothe trough, most of the depositing particlesaccumulated except at lower slope area. On theother hand, org. C is less preserved, suggestingthat most is retransported and/or remineralized. Inthe trough area, however, both sediment and org.C accumulation rates are higher than the fluxes bythe sediment trap. This indicates that the lateral

transport plays an important role on the materialtransport into the trough sediments.

Acknowledgements

We thank for the cooperation of the researchersand the crew of K92-09 and K95-09 cruises. Dr. Y.Kato in Tokai University provided dry bulkdensity data of sediments. Drs. K. Ikehara, Y.Tanaka and H. Katayama at National Institute ofAdvanced Industrial Science and Technologyassisted core sampling and gave us useful com-ments. This research was supported by SpecialCoordination Funds for Promoting Science andTechnology of the Science and TechnologyAgency of Japan.

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