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173

Material Transport Processes on the Continental Marginin the East China Sea

Masatoshi YAMADA

Nakaminato Laboratory for Marine Radioecology, Environmental RadiationProtection Research Group, Research Center for Radiation Safety, National Institute

of Radiological Sciences, Isozaki 3609, Hitachinaka, Ibaraki 311-1202, Japan

Abstract. Settling particles were collected from two locations in the OkinawaTrough by using time-series sediment traps and from three locations in the EastChina Sea continental slope by using both cylindrical and time-series traps.The Okinawa Trough samples were analyzed for 210Pb and the continentalslope samples were analyzed for 239+240Pu. Seawater samples were also collectedfrom the East China Sea continental shelf and analyzed for 239+240Pu. 239+240Puconcentrations in seawater were low in the surface layer and increased withdepth to show a maximum in the near-bottom layer. There was a clear tendencyfor 239+240Pu fluxes to increase with depth at every location and the highest239+240Pu fluxes were observed near-bottom. A high variability of 239+240Pu fluxoccurred in a very short time; such measurements have not been reportedpreviously. The large 239+240Pu fluxes might be attributed to episodic lateraltransport of particles that flow down the continental slope with the nepheloidlayer. 210Pb concentrations in settling particles showed a general tendency toincrease with depth. 210Pb fluxes showed large seasonal variations and increasedwith depth, with an especially large increase near-bottom. The annual mean210Pb fluxes in the near-bottom traps were 2.30 times higher at Stn. SST-1 and1.14 times higher at Stn. SST-2 than the 210Pb deficiency flux. These resultssuggest that the lateral transport process may play a significant role in materialtransport on the continental margin in the East China Sea.

Keywords: sediment trap experiment, 210Pb, 239+240Pu, lateral transport process,continental margin, East China Sea

1. INTRODUCTION

The continental margins occupy a mere 7% of the world’s ocean surface area,however, they are recognized as important areas for understanding the globalcarbon cycle because of their high biological productivity (Galloway and Melillo,1998; Wollast, 1998; Chen et al., 2003). The East China Sea, located in thewestern Northwest Pacific, is one of the largest marginal seas in the world. Largeamounts of nutrients and suspended loads are supplied from the Asian continentthrough rivers and the atmosphere (Walsh et al., 1981; Milliman et al., 1985). Themarginal seas act as transitional areas between the continents and the ocean,providing a good opportunity to investigate material transport processes for their

Global Environmental Change in the Ocean and on Land, Eds., M. Shiyomi et al., pp. 173–187.© by TERRAPUB, 2004.

174 M. YAMADA

impact on the open ocean.Transport processes of particulate matter in the marginal seas have been

investigated in the Middle Atlantic Bight during the SEEP (Shelf Edge ExchangeProcesses I and II) program (Walsh et al., 1988; Biscaye et al., 1988, 1994;Biscaye and Anderson, 1994), in the northeastern Atlantic and the MediterraneanSea during the ECOMARGE (ECOsystèmes des MARGEs continentales) program(Monaco et al., 1990; Heussner et al., 1990; Fowler et al., 1990; Heussner, 1995)and the ECOFER (ECOsystème du canyon du cap-FERret) programs (Monaco etal., 1999; Heussner et al., 1999; Radakovitch and Heussner, 1999), in the EastChina Sea during the MASFLEX (MArginal Sea Flux EXperiment in the westPacific) program (Tsunogai et al., 2003; Yamada and Aono, 2003a) and theMFLECS (Margin FLux in the East China Sea) program (Hu and Tsunogai,1999), in the East China Sea and the South Okinawa Trough during the KEEP(Kuroshio Edge Exchange Processes I, II, and III) program (Hung and Chung,1998; Hung et al., 1999; Chung and Hung, 2000; Chung et al., 2003), and inSagami Bay and off the mouth of Tokyo Bay (Kim et al., 1997; Noriki et al., 1997;Kato et al., 2003a, b).

The plutonium isotopes, 239Pu (half-life = 2.44 × 104 y) and 240Pu (half-life= 6.58 × 103 y), have been added to the Pacific Ocean mainly as a consequenceof global fallout from atmospheric nuclear weapons testing (UNSCEAR, 1982),while a second source has been close-in fallout from Bikini and Enewetak Atollweapons tests (Nevissi and Schell, 1975; Noshkin et al., 1997; Robinson andNoshkin, 1999). Plutonium has also been deposited in the upper layer of land soiland large inputs of weathered detrital material are added to the coastal sea areas.Plutonium is a reactive element, which is adsorbed by particles in seawater andscavenged from the water column, and is useful for tracing particulate transportprocesses (Higgo et al., 1977; Fowler et al., 1983; Livingston and Anderson,1983; Fowler and Knauer, 1986).

210Pb (half life = 22.3 y) is a naturally occurring uranium-series radionuclide.A significant source of 210Pb for shallow ocean waters is atmospheric input bydecay of 222Rn (half life = 3.8 d) to 210Pb and subsequent scavenging by wetdeposition. Another source for the ocean is in situ production by decay of 226Ra(half life = 1622 y). 210Pb is transported to the section of high productivity andparticle flux (Bacon et al., 1976) where its scavenging by particles acts as a majorremoval mechanism (e.g., Craig et al., 1973; Nozaki and Tsunogai, 1973; Baconet al., 1976, 1985; Spencer et al., 1978, 1981; Brewer et al., 1980; Harada andTsunogai, 1986; Moore and Dymond, 1988; Fisher et al., 1988; Nozaki et al.,1991: Colley et al., 1995). The 210Pb in settling particles is a useful natural tracerfor studying particulate transport and removal processes (e.g., Moore et al., 1981;Biscaye et al., 1988; Heussner et al., 1990; Huh et al., 1990; Narita et al., 1990;Biscaye and Anderson, 1994; Thunell et al., 1994; Heussner, 1995; Radakovitchand Heussner, 1999; Chung et al., 2003). This paper aims to provide a summaryof our recent investigations on 239+240Pu and 210Pb in the East China Sea (Yamadaand Aono, 2002, 2003a, 2003b) and to discuss the material transport processes onthe continental margin in the East China Sea.

Material Transport Processes on the Continental Margin in the East China Sea 175

2. MATERIALS AND METHODS

2.1 Sediment trap sampling

2.1.1 Time-series traps in the Okinawa TroughThree time-series sediment traps were deployed at each of two sites, Stns.

SST-1 (water depth: 1088 m) and SST-2 (water depth: 1070 m), in the OkinawaTrough (Fig. 1). The traps were deployed at depths of 464 m above the bottom(464 mab), 257 mab, and 51 mab. The sampling intervals were 16 days fromMarch 1993 to September 1993 and 10 days from October 1993 to February 1994.Details of the mooring strategy in the Okinawa Trough are given in Yamada andAono (2003a).

Fig. 1. Map showing sampling locations used in this study; settling particles were collected at Stns.SST-1, SST-2, F-8, F-6, and F-4 (solid black triangles) and seawater samples were collected at Stns.PN-8, PN-10, and PN-12 (solid black circles).

F-8

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176 M. YAMADA

2.1.2 Cylindrical and time-series traps on the East China Sea continentalmargin

Three moorings of sediment traps were deployed at Stns. F-4 (water depth:604 m), F-6 (water depth: 301 m), and F-8 (water depth: 132 m) on the continentalmargin of the East China Sea from 26 October to 4 November 1995 (Fig. 1). Threecylindrical traps were deployed at depths of 60 m above the bottom (60 mab), 35mab, and 12 mab at Stns. F-6 and F-8. Two types of five sediment traps were usedat Stn. F-4, three cylindrical sediment traps (CT) and two time-series sedimenttraps (TST) at depths of 110 mab (CT), 102 mab (TST: PARFLUX Mark 7G-21;MacLane Research Ltd.), 35 mab (CT), 30 mab (TST: SMD 21S-6000; Nichiyu-Giken-Kogyo Ltd.), and 12 mab (CT). Mooring durations of cylindrical trapswere 8.77, 8.93, and 8.99 days at Stn. F-4, F-6, and F-8, respectively. Thesampling interval of time-series traps was 12 hours except that of the last samplebottle which was only 6 hours. Details are given in Yamada and Aono (2002).

2.2 Water sampling

Seawater samples were collected at three stations, Stns. PN-8, PN-10, andPN-12, in the East China Sea during the K93-05 cruise from September toOctober 1993 (Fig. 1). The samples were collected with 20 liter Niskin bottleswhich were attached to a Rossette multi-bottle array system and transferred to 20-liter polyethylene containers. Each seawater sample was acidified with nitric acidand brought back to a land-based laboratory. Contents of eight 20-liter Niskinbottles had to be combined in order to achieve the large volume desired forplutonium analysis (Yamada and Aono, 2003b).

2.3 Analytical procedure

2.3.1 Settling particlesDried and weighed samples collected at Stns. SST-1 and SST-2 were

subjected to total dissolution spiked with stable lead carrier. Pb was purified byanion exchange and electrodeposited onto a platinum anode. The activities of210Bi were counted by a low-background α/β counting system (Tennelec LB5100)after radiochemically equilibrating with 210Pb (precipitated as PbSO4). Detailsare found in Harada and Tsunogai (1986) and Yamada and Aono (2003a).

The analytical procedure for 239+240Pu is the same as that described byAnderson and Fleer (1982). Settling particles collected at Stns. F-4, F-6, and F-8 were spiked with 242Pu as a yield monitor and dissolved with nitric, perchloric,and hydrofluoric acids. Pu was purified by anion exchange and electrodepositedonto a stainless steel disc. The activities of Pu isotopes were determined with αspectrometers (Tennelec TC-256) equipped with PIPS detectors (IPC-500-100-19) and a multichannel analyzer (Tennelec PCA-M).

2.3.2 SeawaterThe analytical procedure used to separate and purify 239+240Pu from seawater

was described in detail by Yamada et al. (1996). Briefly, Pu was coprecipitatedwith Fe(OH)3 by neutralizing the seawater with ammonia, and removed by


passing the sample through an anion exchange resin column, before beingelectrodeposited onto a stainless steel disc. The activities of Pu isotopes werecounted using the same α spectrometers as for the settling particles.

3. RESULTS AND DISCUSSION

3.1 239+240Pu in the East China Sea

3.1.1 239+240Pu in seawaterThe vertical profiles of 239+240Pu concentration and light transmittance are

shown in Fig. 2. Data of 239+240Pu concentrations and light transmittance werecited from Yamada and Aono (2003b) and Kusakabe et al. (1998), respectively.The 239+240Pu concentrations were low in the surface layers and increased withdepth to show a maximum in the near-bottom layers. The vertical profiles of239+240Pu had a similar tendency for all three stations. Light transmittanceincreased with depth to reach its maximum value in the middle layer, and then itdecreased sharply with depth (Fig. 2). The light transmittance can be convertedinto the beam attenuation coefficient which has a linear relationship to thesuspended particle concentration in seawater (Kusakabe et al., 1998). Namely,the suspended particle concentration had a minimum value in the middle layerand then increased with depth to have a high value in the near-bottom in the fallof 1993.

The vertical profiles of 239+240Pu concentration varied inversely with lighttransmittance (Fig. 2), that is, a good correlation was observed between the239+240Pu concentrations and the suspended particles concentrations. Hoshika etal. (2003) have observed that the development of bottom turbid layers in thecontinental shelf area of the East China Sea showed large seasonal variations with

Fig. 2. Vertical profiles of 239+240Pu concentration in seawater (solid black symbols) and lighttransmittance (solid lines) at: (a) Stn. PN-8; (b) Stn. PN-10; and (c) Stn. PN-12 (After Yamada andAono, 2003b).

178 M. YAMADA

their dramatic development from summer to fall. Yanagi et al. (1996) have shownfrom field observations and diagnostic numerical experiments that the suspendedparticles were transported from the continental shelf edge of the East China Seato the inner shelf in summer and from the inner shelf to the shelf edge in fall andwinter. Plutonium is a reactive element which is associated with particles inseawater and scavenged from the water column (Higgo et al., 1977; Fowler et al.,1983; Livingston and Anderson, 1983; Fowler and Knauer, 1986; Livingston etal., 1987). The high concentration of 239+240Pu presently found in the near-bottomlayer in the East China Sea continental shelf might be caused by high concentrationof suspended particles and such particles were transported from the inner shelf tothe shelf edge in fall and winter.

3.1.2 239+240Pu in settling particlesThe vertical profiles of 239+240Pu concentrations and fluxes obtained from the

cylindrical sediment traps at Stns. F-8, F-6, and F-4 are given in Fig. 3, togetherwith 239+240Pu concentrations in the surface sediments. Data for 239+240Puconcentrations and fluxes came from Yamada and Aono (2002). 239+240Puconcentrations in settling particles ranged from 3 to 4 mBq/g at depths below 120m. 239+240Pu concentrations at the East China Sea continental margin were

Fig. 3. Vertical profiles of 239+240Pu concentration in settling particles (open circles, open squares,and open rhombs), 239+240Pu flux (solid black symbols) at Stns. F-8, F-6, and F-4 (after Yamada andAono, 2002). Open triangles denote 239+240Pu concentrations in the surface sediments.


roughly comparable to those off the central California coast (Fowler et al., 1983)and in the equatorial northwest Atlantic (Livingston and Anderson, 1983).239+240Pu concentrations in the surface sediment increased gradually with seafloordepth. 239+240Pu concentrations collected in the near-bottom traps (12 mab) wereapproximately two times higher than those in the underlying surface sediments.The 239+240Pu fluxes in the East China Sea continental slope were approximatelyone order of magnitude higher than those in the Panama Basin (Livingston andAnderson, 1983), the North Pacific off central California (Fowler et al., 1983),and the Sargasso Sea (Bacon et al., 1985). The highest 239+240Pu fluxes wereobserved near-bottom (12 mab) at every station. The large fluxes of 239+240Puobserved at Stn. F-4 were the highest of any yet reported (Livingston andAnderson, 1983; Fowler et al., 1983, 1990, 1991, 2000; Bacon et al., 1985; Huhet al., 1990).

The time-series variations of 239+240Pu concentrations and 239+240Pu fluxes atdepths of 502 m and 574 m at Stn. F-4 are shown in Fig. 4. 239+240Pu concentrationshad little variation throughout the sampling period. However, no significantcorrelation was observed between the total mass flux and 239+240Pu concentrationsat depths of 502 m (r = –0.37) and 574 m (r = –0.21) (data not shown). Thisobservation indicated that 239+240Pu concentrations were not dependent on thedilution effect by total mass. The 239+240Pu fluxes showed large variations, similarto the trend of the total mass fluxes. There was a large variation of 239+240Pu fluxeseven during a very short period of time (1/2 day), which has not been reportedpreviously. The 239+240Pu concentration in surface sediment was significantlylower than that of the near-bottom trap at every station. The large fluxes of239+240Pu could not be explained as coming from resuspension of underlyingsurface sediments. The high variability of 239+240Pu fluxes which occurred in veryshort time periods might be attributable to episodic lateral transport of particlesthat flow down the continental slope with the nepheloid layer. This lateral

Fig. 4. Time-series of: (a) 239+240Pu concentration; (b) 239+240Pu flux at Stn. F-4 at depths of 502 m(solid squares) and 574 m (hatched squares) (after Yamada and Aono, 2002). The sampling intervalwas 12 hours except that of the last bottle (Period 17) which was 6 hours.

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180 M. YAMADA

transport is considered to be a key process for 239+240Pu transport on the continentalmargin in the East China Sea.

3.2 210Pb in the Okinawa Trough

3.2.1 Seasonal variations of total mass fluxes, 210Pb concentrations, and210Pb fluxes

The vertical profiles of seasonal and annual mean total mass fluxes at Stns.SST-1 and SST-2 are shown in Fig. 5. The period of sample collection wasdivided into four seasons: Spring, 1 March–3 June 1993 (95 days); Summer, 4June–7 September 1993 (96 days); Fall, 8–23 September and 15 October–3December 1993 (66 days); Winter, 4 December 1993–21 February 1994 (80 days)(Yamada and Aono, 2003a). The total mass fluxes showed large seasonalvariations and they increased with depth, including an especially large near-bottom increase. Total mass flux peaks (>1 g/m2/day) were observed in spring andSeptember at a depth of 50 mab (1036 m) at Stn. SST-1. Higher total mass fluxeswere observed in winter and spring at a depth of 50 mab at Stn. SST-2. There weresome differences between Stns. SST-1 and SST-2; total mass fluxes at 50 mab atStn. SST-2 were less than 700 mg/m2/day, whereas those at Stn. SST-1 were morethan 1 g/m2/day in spring and September; both winter and spring maximum totalmass fluxes were observed at 50 mab at Stn. SST-2, whereas there was no wintermaximum at Stn. SST-1; annual mean total mass flux at 50 mab at Stn. SST 1 (555mg/m2/day) was higher than that at Stn. SST-2 (303 mg/m2/day).

The vertical profiles of seasonal and annual mean 210Pb concentrations insettling particles are shown in Fig. 6 (Yamada and Aono, 2003a). All seasonal andannual mean concentrations of 210Pb increased gradually with depth at bothstations. Increasing 210Pb concentrations in settling particles with depth also havebeen reported for the Middle Atlantic Bight (Biscaye et al., 1988; Biscaye andAnderson, 1994), on the Mediterranean continental margin (Heussner et al.,

Fig. 5. Vertical profiles of seasonal and annual mean total mass flux at: (a) Stn. SST-1; (b) Stn. SST-2.

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1990), in the Santa Monica Basin (Huh et al., 1990), in the tropical northeastAtlantic (Legeleux et al., 1996), on the continental margin of the Bay of Biscay(Radakovitch and Heussner, 1999), in the western Arabian Sea (Borole, 2002),and in the western South Okinawa Trough (Chung et al., 2003). 210Pb concentrationsat Stn. SST-2 were higher in summer and fall than in spring and winter at depthsof 813 m and near-bottom at which concentrations were more than 200 dpm/g.

The vertical profiles of seasonal and annual mean 210Pb fluxes are shown inFig. 7 (Yamada and Aono, 2003a). The 210Pb fluxes showed large seasonalvariations at both stations, similar to the trend of the total mass fluxes. There weresignificant differences for 210Pb fluxes at 1000 m depth (50 mab) between Stns.SST-1 and SST-2; the largest 210Pb flux was observed in spring and 210Pb flux inwinter was approximately one-eighth as much as that in spring at Stn. SST-1,

Fig. 6. Vertical profiles of seasonal and annual mean 210Pb concentration in settling particles at: (a)Stn. SST-1; (b) Stn. SST-2.

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Fig. 7. Vertical profiles of seasonal and annual mean 210Pb flux at: (a) Stn. SST-1; (b) Stn. SST-2.

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182 M. YAMADA

whereas 210Pb flux was nearly the same and higher in spring and winter at Stn.SST-2; 210Pb flux in spring, summer, and fall at Stn. SST-1 was twice as large asthat in each season at Stn. SST-2. A remarkable increase of 210Pb flux in near-bottom traps has been reported (e.g. Biscaye et al., 1988; Heussner et al., 1990;Biscaye and Anderson, 1994; Radakovitch and Heussner, 1999; Chung et al.,2003; Kato et al., 2003b). There was also a clear tendency for 210Pb fluxes toincrease with depth in the Okinawa Trough, with an especially large increasenear-bottom. This is discussed in detail by drawing up a 210Pb budget in the watercolumn in the Okinawa Trough.

3.2.2 210Pb budget in the Okinawa TroughUnder the steady-state condition, the total supply of 210Pb to the water

column can be calculated as the sum of the 210Pb production rate by decay of 226Rawithin the water column and the input rate of 210Pb from the atmosphere. Theoutput flux is given by the radioactive decay rate of 210Pb calculated from the210Pb water column inventory. The difference between the total supply and outputflux represents the amount of 210Pb removed by settling particles (Cochran et al.,1990), and is called the 210Pb deficiency flux. The ratio of the observed 210Pb fluxto the 210Pb deficiency flux is called the 210Pb trapping efficiency.

The atmospheric 210Pb flux of 43.3 dpm/m2/day observed at Naha City in theOkinawa Islands (Tsunogai et al., 1985) is used for the calculation. The verticalprofiles of 226Ra in the water column have not been measured for Stns. SST-1 andSST-2. So, an approximate activity of 226Ra is estimated using an empiricalformula proposed by Nozaki et al. (1990) based on the GEOSECS 226Ra data inthe western North Pacific reported by Chung and Craig (1980):

[226Ra] = 0.062 + 0.00124[Si]

where [226Ra] is the activity of 226Ra in dpm/l and [Si] is the dissolved silica

Fig. 8. Vertical profiles of seasonal and annual mean 210Pb trapping efficiency at: (a) Stn. SST-1;(b) Stn. SST-2. Solid lines indicate the 210Pb trapping efficiency of one.

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concentration in µmol/l. The production rate of 210Pb in the water column iscalculated by multiplying the estimated 226Ra inventory by the decay constant of210Pb. The total supply of 210Pb to a given depth is calculated as the sum of theatmospheric flux of 210Pb and the production rate of 210Pb. The vertical profile of210Pb in the water column has been reported for Stn. SST-2 by Aono et al. (1995),but not for Stn. SST-1. Therefore, the 210Pb data sets at Stn. SST-2 are used tocalculate the 210Pb inventory at both stations. The decay rate of 210Pb is calculatedby multiplying the 210Pb inventory in the water column by the decay constant of210Pb. The deficiency flux of 210Pb can be obtained by subtracting the decay rateof 210Pb from the sum of the atmospheric flux of 210Pb and the production rate of210Pb to a given depth. This value represents the predicted flux of 210Pb removedfrom a given water column through particles.

The vertical profiles of the seasonal and annual mean 210Pb trappingefficiencies are shown in Fig. 8 (Yamada and Aono, 2003a). The 210Pb trappingefficiencies showed an especially large near-bottom increase. The 210Pb trappingefficiencies in the near-bottom traps showed large seasonal variations with thelargest efficiencies being observed in spring at both stations. The 210Pb trappingefficiencies of more than one were observed in spring, summer, and fall at Stn.SST-1 and in spring, winter and fall at Stn. SST-2. There were significantdifferences for 210Pb trapping efficiencies at 1000 m depth (50 mab) betweenStns. SST-1 and SST-2; the largest 210Pb trapping efficiency was observed inspring and 210Pb trapping efficiency in winter was approximately one-eighth as

Fig. 9. Schematic illustration of material transport processes on the continental margin of the EastChina Sea. The highest 239+240Pu fluxes were observed near-bottom (12 mab) on the continentalslope. The annual mean 210Pb trapping efficiencies in the near-bottom traps were 2.30 times higherat Stn. SST-1 and 1.47 times higher at Stn. SST-2 than the 210Pb deficiency flux, showing a large210Pb flux excess. These results suggest that settling particles in near-bottom traps are suppliedmainly by lateral transport. The lateral transport process may play a significant role in materialtransport on this continental margin (see text for more details).

Shelf Edge

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184 M. YAMADA

much as that in spring at Stn. SST-1, whereas 210Pb trapping efficiency was nearlythe same or higher in spring and winter at Stn. SST-2. The annual mean 210Pbtrapping efficiencies in the near-bottom traps were 2.30 times higher at Stn. SST-1 and 1.47 times higher at Stn. SST-2 than the 210Pb deficiency flux, showing alarge 210Pb flux excess at this depth. The large 210Pb trapping efficiencies in thenear-bottom traps could not be explained by a contribution from underlyingsurface sediments (Yamada and Aono, 2003a). The large 210Pb trapping efficienciesin the near-bottom traps and the increases of 210Pb concentrations in settlingparticles with depth may be attributable to lateral transport of particles that slowlyslide down on the continental slope nepheloid layer while scavenging 210Pb (Fig.9).

The rates of laterally transported 210Pb in the near-bottom layer can beestimated using a simple steady-state box-model (Yamada and Aono, 2003a).The highest net rates of laterally transported 210Pb were estimated in spring to be138 and 51.7 dpm/m2/day at Stns. SST-1 and SST-2, respectively. The annual netrates of laterally transported 210Pb were 3.5 times higher at Stn. SST-1 and 1.5times higher at Stn. SST-2 than 210Pb fluxes observed at 800 m traps. These resultssuggest that settling particles in near-bottom traps of the Okinawa Trough aresupplied mainly by lateral transport in spring. The lateral transport process mayplay a significant role in material transport on the continental margin in the EastChina Sea.

Acknowledgements—The author would like to thank the scientists, captain, officers, andcrew of the R/V Kaiyo and R/V Natsushima for their help in the sampling. He is gratefulto Dr. H. Kawahata and an anonymous reviewer for providing constructive comments. Healso thanks Dr. T. Aono for many valuable suggestions. This work was supported bySpecial Coordination Funds for Promoting Science and Technology (GCMAPS program)by the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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M. Yamada (e-mail: [email protected])

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