Flux and Fate of Yangtze River Sediment Delivered

FluxandfateofYangtzeRiversedimentdeliveredtotheEastChinaSeaJ.P.Liua,⁎,K.H.Xub,A.C.Lic,J.D.Millimanb,D.M.Velozzia,S.B.Xiaod,Z.S.YangeaDepartmentofMarine,EarthandAtmosphericSciences,NorthCarolinaStateUniversity,Raleigh,NC,27695,USAbSchoolofMarineScience,CollegeofWilliamandMary,GloucesterPt.VA23062,USAcInstituteofOceanology,ChineseAcademyofSciences,Qingdao,266071,ChinadSouthChinaSeaInstituteofOceanology,ChineseAcademyofSciences,Guangzhou,510301,ChinaeCollegeofGeo-MarineSciences,OceanUniversityofChina,Qingdao266003,ChinaReceived2February2005;receivedinrevisedform22April2005;accepted29March2006Availableonline8September2006AbstractNumerouscoresanddatingshowtheYangtzeRiverhasaccumulatedabout1.16×1012tsedimentinitsdeltaplainandproximalsubaqueousdeltaduringHolocene.High-resolutionseismicprofilingandcoringinthesouthernEastChinaSeaduring2003and2004cruiseshasrevealedanelongated(∼800km)distalsubaqueousmudwedgeextendingfromtheYangtzeRivermouthsouthwardofftheZhejiangandFujiancoastsintotheTaiwanStrait.Overlyingwhatappearstobeatransgressivesandlayer,thisdistalclinoformthinsoffshore,from∼40mthicknessbetweenthe20and30mwaterdepthtob1–2mbetween60and90mwaterdepth,correspondingtoanacrossshelfdistanceoflessthan100km.Totalvolumeofthisdistalmudwedgeisabout4.5×1011m3,equivalentto∼5.4×1011tofsediment.MostofthesedimentinthismudwedgecomesfromtheYangtzeRiver,withsomeinputpresumablycomingfromlocalsmallerrivers.Thus,thetotalYangtze-derivedsedimentsaccumulatedinitsdeltaicsystemandEastChinaSeainnershelfhaveamountedtoabout1.7×1012t.Preliminaryanalysessuggestthislongshoreandacross-shelftransportedclinoformmainlyformedinthepast7000yrsafterpostglacialsealevelreacheditsmid-Holocenehighstand,andafterre-intensificationoftheChineselongshorecurrentsystem.Sedimentationaccumulationapparentlyincreasedaround2000yrsBP,reflectingtheevolutionoftheYangtzeestuaryandincreasedlanderosionduetohumanactivities,suchasfarminganddeforestation.Thesouthward-flowingChinaCoastalCurrent,thenorthward-flowingTaiwanWarmCurrent,andtheKuroshioCurrentappeartohaveplayedcriticalrolesintransportingandtrappingmostofYangtze-derivedmaterialsintheinnershelf,andhencepreventingthesedimentescapeintothedeepocean.©2006ElsevierB.V.Allrightsreserved.Keywords:EastChinaSea;YangtzeRiver;Clinoform;Delta;Mudwedge;Sealevel1.IntroductionMajorriversplayadominantroleworldwideintransferringbothparticulateanddissolvedmaterialsfromthelandtothecoastalocean.Thetotalsuspendedsedimentdeliveredbyallriverstotheoceanisabout15×109metrictonsannually,ofwhichAsianriversdischargenearlymorethan70%(MillimanandMeade,1983;MillimanandSyvitski,1992;Syvitskietal.,2005).Mostofthesesediments,however,aretrappedinestuariesordepositedonadjacentcontinentalshelves;Geomorphology85(2007)208–224⁎Correspondingauthor.E-mailaddress:[e-mail](J.P.Liu).0169-555X/$-seefrontmatter©2006ElsevierB.V.Allrightsreserved.doi:10.1016/j.geomorph.2006.03.023onlylessthan5–10%ofthefluvialsedimentspresentlyreachthedeepsea(e.g.Meade,1996).Theriver-dominatedoceanmarginsalsoplayasignificantroleinglobalbiogeochemicalcycles(Chenetal.,2003a,b;CaiandDai,2004;Daggetal.,2004;McKeeetal.,2004).Thestudyofestuaries,deltas,andinnercontinentalshelves,thereforehasparticularrelevanceasweseektounderstandthelinkbetweenpresent-dayterrestrialprocessesandcoastalmarinedeposition.Oneofthebestexamplesoftheriver-dominatedoceanmarginsistheepicontinentalshelfoftheEastChinaSea,whichtogetherwiththeYellowandBohaiseas,formsacontiguousshelfofabout0.75×1012m2inarea(Fig.1).Thiswideandshallowshelfreceivesalargeamountofterrigenousmaterialsfromtwoofthelargestriversintheworld,theYellowandYangtzerivers.TheYellowRiveralonehasdischargedtotallynear1.63×1012tonssedimenttotheBohaiandYellowseasduringthepast10kyr(Liuetal.,2002,2004).ThemodernYangtzeRiver'sannualsedimentloadhasbeenabout480milliontonshistorically.WhencombinedwiththeadjacentYellowRiver(about1billiontons),thetotalequals∼10%oftheglobalriverinesedimentfluxtotheocean(MillimanandMeade,1983).TheYangtzeRiveroriginatesfromtheQinghai-TibetanPlateauatanelevationof6600m,flowingFig.1.BathymetryandregionaloceancirculationpatternintheEastChinaSeaandYellowSea.JCC(JiangsuCoastalCurrent);TWC(TaiwanWarmCurrent);YSWC(YellowSeaWarmCurrent);ZFCC(ZhejiangFujianCoastalCurrent).209J.P.Liuetal./Geomorphology85(2007)208–224eastwardformorethan6300km.Thisriverdrainsanareaofmorethan1.94×106km2beforefinallydischargingintotheEastChinaSea.ItisthelargestriverinAsiaintermofwaterdischargeandthirdlongestriverintheworld(MillimanandMeade,1983).ThemodernYangtzeRiverdischargesmostofitsannualsedimentloadbet-weenJuneandSeptember,andshort-termmonthlydepositionratesneartherivermouthduringthisperiodareabout4.4cm,incontrasttolong-termaccumulationrates,whichareanorderofmagnitudeless,∼1–6cm/yr(McKeeetal.,1983;DeMasteretal.,1985;Chenetal.,2004).Thisdifferencebetweenshort-andlong-termaccumulationratessuggeststhatamajorportionoftheriver-derivedsedimentiserodedperiodicallyandistransportedelsewhere,presumablysouthwardtotheZhejiangandFujianProvinces.