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ChineseJournalofOceanologyandLimnology

    ;Vo1.26No.4,P.480485,2008

    ;DOh10.1007/s00343-0080480-Y

    ;Verticalfluxofparticulateorganiccarboninthecentral

    ;SouthChinaSeaestimatedfrom234Th_238Udisequilibria*

    ;MAHao(马豪)t.tt,tttZENGZhi(曾志),HEJianhua(何建华)”,

    ;CHENLiqi(陈立奇)”„,YINMingduan(尹明端)”,ZENGShi(曾实)”“,

    ;ZENGWenyi(曾文义)”

    ;~-KeyLaboratoryofGlobalChangeandMarine-AtmosphericChemistry(GCMAC).StateOceanicAdministration.Xiamen

    ;361005,China

    ;ffThirdInstituteofOceanography,StateOceanicAdministration,Xiamen361005,China ;fffDepartmentofEngineeringPhysics.TsinghuaUniversity,BeUing100084,China ;ReceivedJune29,2007;revisionacceptedNov.6.2007

    ;AbstractTh_Z3SUdisequilibriawereappliedtoexaminethepanicledynamicsintheeuphoriczone

    ;ofthecentralSouthChinaSeaduringthespring2002cruise.Theparticulateorganiccarbon(P0C1.Th

    ;fincludingbothdissolvedandparticulateandUinthewatercolumnatthreestationsweredetermined.

    ;TheprofilesofTh/238UactivityratioatthethreestationsallshowedconsistentThdeficitas ;comparedtoUintheupper100mwatercolumn.Basedontheprofilesofthedissolvedandparticulate

    ;Thandasteadystateboxmode1.thedissolvedThscavengingrates,theparticulatehremoval ;ratesandtheirresidenttimeswerequantified.ItwasfoundthatthePOCdownwardexpo~fluxesoutof

    ;theupperl00meuphoticzonerangedfrom9-4OtoI4.78mmol?m-2d.Theresultsfromthi

    sstudy

    ;providenewinformationforourunderstandin

    ofcarbonbiogeochemicalcycleintheSouthChinaSea.

    ;Keyword:Th_238Udisequilibria;POCexportflux;Euphoticzone;SouthChinaSea ;lINTRODUCTIoN

    ;Theeuphoriczoneisanactivewaterdepthinthe

    ;oceanformarinephytoplanktontoconvertinorganic

    ;carbonintoorganiccarbon.resultinginaCO,

    ;exchangeacrosstheatmosphere/oceaninterface.

    ;Moreover.itisanimportant1ayerfortheproduction

    ;ofbiogenicmatterandfortheverticaltransportof

    ;particlestothedeepsea.Exportnuxofparticulate

    ;organiccarbon(POC1fromtheeuphoriczonetothe ;deepoceanisregardedasacriticaindexofthe

    ;efficiencyofbiologicalpumpandithasoftenbeen ;usedasanecessarymeasurementtodeterminethe ;biogeochemicalcyclingratesofparticlereactive

    ;elementsandconstituentsintheocean(Eppleyand ;Peterson1979;Buesselerl998;Falkowskieta1. ;1998:Sarmientoeta1.2004).

    ;TheestimationofPOCexportfluxesintheocean ;isbasedonthreemainapproachesincluding ;sedimenttrapdeployment,particleabundance ;measurementandthe3Th238Udisequilibrium ;method.Intheupperocean,thequantityandquality ;ofsinkingmaterialscollectedbysedimenttrapsare ;largelybasedonthehydrodynamicsituationand ;ofteninvolve”swimmer”relatedissues(Buesseler.

    ;19911.Duetothewidelyvariablecompositionand ;sinkingratesofmarineparticles,themeasurements ;ofparticleabundancearenotpracticaltoestimate ;POCexportfluxesaswell(Caieta1.,2001). ;Thisaradioactivenuclidewithveryhigh

    ;particlereactivity,producedinsitufromthedecayof

    ;itsparent238Uwithasteadvrateintheocean. ;24Th

    ;canbescavengedandremovedrapidlywithsinking ;particulatematterresultingindisequilibrium ;between.ThandU

    ;,especiallyintheupperwater

    ;column.ThedeficiencvofThwithrespecttoU ;ref1ectsthecycle.exportandremineralizationof ;particles.DHetotherelativelyshorthalf-lifeofTh ;(24.1d),itiswel1suitedfortracingbiogeochemical ;processesinthetimescalesimilartoparticle ;dynamicsintheupperocean(Caieta1..2002a; ;SupportedbytheNSFC(keyprogramNo.40531oo7)andgeneral ;Programs(No.404060l4,40276001and107050191,theStateSociaI

    ;Commonwea1Grant(No.2OO4DIB5178).andthecommonweaIproject

    ;sponsoredbytheScientificResearchFoundationoftheMinistryof

    ;ScienceandTechnologyofChina(No.2007050228)andChineseIPY

    ;CampaignfoundbyMOF

    ;Correspondingauthor:lqchen@soa.govcn ;No.4

    ;Waples

    ;(2006),

;proved

    ;particle

    ;OCean.

    ;MAeta1.:VerticalfluxofparticulateorganicCinthecentralSCSestimatedfromTh33sUdi

    sequilibria481

    ;eta1.,2006).AsreportedbyBuesselereta1. ;the4Th238Udisequilibriumtechniqueis ;tobearobustmethodifltheestimationof ;exportfluxesfromtheeuphoticzoneinthe ;HerewereportexportfluxesofPOCfromthe ;euphoticzoneinthecentralSouthChinaSeabased ;on2Th_238Udisequilibriumdataandasteadystate ;boxmode1.

    ;2EXPERIMENT

    ;2.1Studyareaandsamplecollection

    ;TheSouthChinaSeafscs).westernarmofthe ;PacificOcean.isthelargestmarginalseaintheworld ;withanareaof3500000km‟andadeepbasinuDt0

    ;5490mindepthIthasawidenoHherncontinental ;shelf,theSundashelf,affectedbylargeinputsof ;freshwaterfromseveralmaiorriversincludingthe ;MekongRiverandthePear1River.Itislocated ;betweentheTibetanPlateauandthewarlnpoolof ;thewesternPacific.twoofthemostimportantfactors ;affectingtheglobalclimatepatterns.

    ;N

    ;l0.

    ;,

    ;N?5

    ;

    ;N$28?

    ;l_?一一一一一一一一一一_H

    ;?

    ;105.l10.115.120.E

    ;Fig.1MapofthesamplingstationsintheSouthChinaSea ;ThefilledcirclesindicatetheJocationofthethfeestatkms ;Comparedwiththeotheroceanecosystems,the ;SCSisnotableforitsveryshallowmixeddepth. ;whichis1essthan50mduringmosttimeofyear.The ;seaisaffectedbytheactionofstronginterna1waves. ;internaltidesandinertiawaves.Itissubiecttoannua1 ;alternatingmonsoon.Inaddition.morethanlO ;typhoonsoffaveragepassthroughthisregmnevery ;year.Oflalongertimescale,itscirculationand

    ;ventilationarenotablymodulatedbvElNifiocycles ;(Shiaheta1l999).

    ;Sampleswerecollectedatthreestationsonboard ;R/V‟Shiyan3‟duringaspringsurveycruisetothe

    ;SCSinMay,2002(Fig.1.Table11.Ateachstation, ;20Lofseawaterforeachsamplewascollectedusing ;aCTDRosettesystemineachwaterdepth.Upon ;samplecollection,alLportionofeachsamplewas ;filteredthrough0.7gmglassfiberfilters ;rWhatman”GF/F).driedandbroughtbacktothe

    ;laboratoryforPOCanalysis.Watersampleswere ;acidifiedwithconcentratedHCLandfilteredthrough ;0.45lammixedcelluloseestermembranestoseparate ;dissolvedandparticulateThforanalysisonboard. ;Table1Samplelocationsandmaximumwaterdepth ;2.2Analysisandmeasurement

    ;ThemethodusedfortheextractionofThfrom

    precipitationwithFe(OH1 ;seawaterinvolvestheco

    ;fromabout19Lofseawater.TheThwas

    ;subsequentlyseparatedandpurifiedonan ;anionexchangecolumn,eIectrodepositedona ;stainlesssteeldisc.IntheDrocedure,aU_228Th ;balancedsolutionwasintroducedasflyieldtracer.In ;otherstudies.activitiesofThanditsyieldtracer ;ThforTh..Th)wereusuallymeasuredusinga ;3-counterandspectrometer,respectively,sothe ;detectorintercalibrationwasnecessaryfortheresults ;(Busselereta1.,20o1:vanderLoeffeta1.,2006).In ;thepresentstudy,onlyonedetectorwasusedto ;measuretheactivjties0f24Thand22Th

    ;simultaneously;therefore.thedetectorintercalibra ;tionbetweenc【一spectrometerand3-counterwas

    ;avoidedandthemeasurementerrorwasthen ;reduced.

    ;However,thecharacteristicsandenergyof6and0c ;radiationaredifierentasusedtodistinguishing ;detectionefficiency.Thus.itisnecessarytocalibrate ;thecountingefficiencvbetween228Thand234Th.The ;specificstepsareasfollows:Theknownamountof ;.U_234Thequilibriumsolutionismixedwiththe ;eauilibriumtracerofU_228ThThethoriumsource ;diskismadeintheabovementionedprogramand ;detectedinourdetector.Withthesamechemical ;recycleratio,difierentdetectingefficienciesof13and

    ;cLradiationcanbedeterminedbythecountsofTh ;and”.Th.

    ;CTDdatawerecollectedbytheSouthChinaSea ;InstituteofOceanology,C:hineseAcademyof ;Sciencesusingstandardprocedures.

    ;Uactivitieswereestimatedfromthesalinity ;measurementsusingCTDandtherelationshipwas ;definedas:U(dpm?L)=0.07081xsalinity(Chen ;eta1.,1986).

    ;482CHIN.J.OCEANOL.LIMNOL.,26(4),2008 ;3RESUISANDDISCUSSION

    ;3.123Thscavengingandremovalrates

    ;HydrographicdataandPOCconcentrationsas ;wel1asThactivitiesatthethreestationsaregiven ;inTable2.DepictedinFig.2a.bandcarevertical ;profilesofthreeformsofthe.Ttl/U)ARratio.It ;ispronouncedthatactivitiesofdissolved.Thare ;deficienttothoseofUinthewatercolumn.The ;ratiosoftota1234Thre1ativetO23Uaresimilarto ;thoseofdissolvedThinallthewaterdepths. ;whereasitappearstobeanoppositetrendfor ;particulateTh.

    ;Estimatesofthescavengingandremovalratesof ;Thaswellastheresidenttimesofdissolvedand ;particulateThcanbedeterminedfroman ;VO1.26

    ;irreversiblesteadystatemodelofthorium ;scavenginginthewatercolumnusingthefollowing ;equation(CoaleandBruland1985;Cheneta1.,1996; ;Savoyeeta1.,2006):

    ;/=0=_[/4t(+Ae)]+

    ;(1)

    ;whereAu,ADThandApmrepresenttheactivities ;(dpm.L1of.U.

    ;dissolvedThandparticulate

    ;Th.andistheThdecayconstantf0.02876 ;d-i).ThetermJP(dpm?m,‟-d)representsthe

    ;removalrateofparticulateThduetoparticle ;sinkingandthecontributionsofadvectionand ;diffusiontotheThfluxes.

    ;Table2VerticaldistributionsofPOC,Th,temperatureandsalinityatthreestations

    ;dpm=1/60Bq

    ;Here,thecontributionsofadvectionanddiffusion ;areneglectedandEq.1isrewrittenfordissolvedand

;particulateTh.respectively:

    ;/af=0=??ADJ(2)

    ;aP/at=0=Jn-2?AP

    ;AccordingtoEqs.2and3

    ;J=?(A.)

    ;=J?Ap

    ;=

    ;AD/s

    ;Tp=Apn}

    ;(3)

    ;(4)

    ;(5)

    ;(6)

    ;(7)

    ;wherethetermJrh(dpm?m-J.d)representsthenet ;scavengingrateofdissolvedThontoparticles.”gD

    ;(d)and(d)aretheresidencetimesofThwith ;respecttoparticlescavengingandremoval, ;respectively.Jrh,PTh,Z”DandPwerethuscalculated

    ;andlistedinTable2.

    ;FromTable2.rOfallsintherangeof22.4-198.8d. ;withanaverageof69.5d.Andrprangesfrom2.8to ;80.3dwithanaverageof19.7d.Meanwhile,rOand ;rpatstationN$28aretheshortestamongthethree ;stations,whichisrelevanttothemostdeficiencyof ;ThrelativetOU(Fig.21.

    ;No.4MAeta1.:VerticalfluxofparticulateorganicCinthecentralSCSestimatedfromTh_2

    38Udisequilibria483

    ;2O

    ;j40

    ;6o.

    ;8O

    ;10o

    ;“111Uactivityratio‟111/2activityratio111/2.Uactivityratio

    ;o

    ;2O

    ;40

    ;6O

    ;8O

    ;lo0

    ;

    ;?DTh:o--PTh;一一rn1

    ;000

    ;20

;4O

    ;60

    ;80

    ;l0O

    ;Fig.2Verticalpro~es0fh,I7剌嘶ratio

    ;a.StationNS06;b.StationNS28;c.StationNS45,TTh,PThandDThrepresentactivityratio

    softotal

    ;„h.

    ;particulate*rhanddissolved*rhto.U ;,

    ;respectively

    ;3.2POCexport

    ;Thedepthoftheeuphoticzoneatthethree ;stationsisabout70-100m.Fromthevertical ;distributionsofthetemperatureandsalinitylistedin ;Table1.wecanseethatthemixedlayercanextendto ;30_40mdeepintheupperoceanandthemain ;thermoclineandthehaloclinelieinthedepthof ;3o-100m.Inthepresentstudy.theinterfacesof100 ;mdeepischosenasthebottom