Tagus estuary salt marshes feedback to sea level rise over a 40-yearperiod: Insights from the application of geochemical indices

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Tagus estuary salt marshes feedback to sea level rise over a 40-yearperiod: Insights from the application of geochemical indices
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  This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institutionand sharing with colleagues.Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third partywebsites are prohibited.In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further informationregarding Elsevier’s archiving and manuscript policies areencouraged to visit:http://www.elsevier.com/authorsrights  Author's personal copy EcologicalIndicators34(2013)268–276 ContentslistsavailableatSciVerseScienceDirect Ecological   Indicators  journalhomepage:www.elsevier.com/locate/ecolind Original   article Tagus   estuary   salt   marshes   feedback   to   sea   level   rise   over   a   40-yearperiod:   Insights   from   the   application   of    geochemical   indices B.   Duarte a , ∗ ,   I.   Cac¸    ador a ,    J.C.   Marques b ,   I.W.   Croudace c a CenterofOceanography,FacultyofSciences,UniversityofLisbon,CampoGrande,1749-016Lisbon,Portugal b InstituteofMarineResearch–MarineandEnvironmentResearchCentre(IMAR-CMA),C/ODepartmentofZoology,FacultyofSciencesandTechnology,UniversityofCoimbra,3000Coimbra,Portugal c NationalOceanographyCentre,Southampton,UniversityofSouthampton,EuropeanWay,SouthamptonSO143ZH,UK  a   r   t   i   c   le   i   n   f   o  Articlehistory: Received11April2013Receivedinrevisedform17May   2013Accepted17May   2013 Keywords: SealevelriseSaltmarshesGeochemicalindices 137 CsdatingSedimentaccretionratesHeavymetalsElementalratio-basedindices a   b   s   t   r   a   c   t Sea   level   rise   (SLR)   has   been   evaluated   using   data   acquired   from   two   Tagus   estuary   saltmarshes.   Sedimentaccumulation   rates   over   a40-year   study   period   were   determined   using  137 Csalong   with   an   evaluationof    several   geochemical   indices   and   ratios   as   proxies   of    the   mechisms   underlying   these   SAR    variations.Correlating   SLR    data   from   1963   to   2001   with   the   sediment   accretion   rates   (SARs)   an   inverse   pattern   of interaction   wasobserved,   with   lower   SAR    associated   to   periods   of    higher   mean   sea   level   (MSL)   heights.This   pointed   out   to   an   erosion   effect   of    the   salt   marsh   during   higher   tidal   flooding.   Although   SLR    appar-ently   slows   down   SAR,   it   still   presents   a   positive   balance   with   SLR,   similar   to   that   identified   in   mostmesotidal   estuaries.   The   geochemical   analysis   of    sediments   and   chemical   alteration   index   (CAI)   alsosuggest   that   the   major   processes   inherent   to   the   SAR    vary   inversely,   being   mostly   based   by   physicaldisturbances.   Geochemical   ratio-based   indices   showed   that   both   salt   marshes   presented   enhanced   high-energytransport   driven   inputs   of    sediments,   although   in   Pancas   saltmarsh   there   is   a   slight   evidence   of chemical   weathering   of    the   sediments.   Anthropogenic   contamination   of    the   sediments   by   heavy   metalswas   identified   and   has   been   decreasing   from   1963   to   2001,   mostly   linked   to   amarked   reduction   of    indus-trial   activities   in   some   areas   surrounding   the   Tagus   estuary,   rather   thanthe   sedimentary   history   of    theestuary.©   2013   Elsevier   Ltd.   Allrights   reserved. 1.Introduction Climatechangeisnowadaysonemajorconcernspanningacrosstheenvironmentalsciencecommunity.ItiswidelyacceptedthatincreasingCO 2  levelsmay   induceclimatechangethroughthegreenhouseeffectandholdsthepotentialtoaffectmostecosystemstosomedegree.SomedirectcausesofCO 2  risingconcentrationsintheatmosphereareincreasingtemperaturevaluesandoceanacidification,butothermay   resultfromdirectchangestothegasandparticlecontentswithintheatmosphere.Theimpactofanincreasedgreenhouseeffecthasbeenwidelystudied(e.g.IPCC,1990;TitusandNarayanan,1995).ThereportsoftheIntergovern-mentalPanelonClimateChange(IPCC,1990),recentlyupdatedin1995,specifythattheglobalaverageairandseasurfacetem-peraturesareexpectedtorisebyabout2.5 ◦ C,withinarangeof 1.5–4.5 ◦ C(BernerandBerner,1996;Houghton,1999;IPCC,1999),althoughthereisadegreeofuncertaintyabouttheseestimationsrelatedtoissuesofgeographical,diurnalandseasonalvariability ∗ Correspondingauthor.Tel.:+351217500000x20319;fax:+351217500009. E-mailaddress: baduarte@fc.ul.pt(B.Duarte). (Gates,1993;Houghton,1999).Oneofthemajorside-effectsof globalwarmingissealevelrise,duetopolaricemeltdownandincreasingoceanwatersupply,butalsocausedbythethermalexpansionofthislargerwatermass.Inevitably,themostaffectedareasoftheglobeandhighlyvulnerabletosealevelrisearelocatedclosetotheseashore,includingcoastallagoons,estuariesandtheshoreline.Estuariesstandoutasareasofspecialconcern,sincetheyincludesomeofthemostdenselypopulatedareasinourplanet(DuarteandCac¸    ador,2012;Cohenetal.,2001).Estuarineecosys-temsholdthereforegreatscientificpotentialinthiscontextandwillhelpusunderstandingclimatechangedynamicsandtheirimpactsupontheseareas.Saltmarshesareoftenlocatedalongestuarinesidesandtheyareparticularlyvulnerabletosealevelrise(Dyeretal.,2002).Saltmarshesareconsideredtobeamongthemostproductiveecosys-temsontheplanetandtheyhaveanessentialroleasnursingareasformarinefishandinvertebratesofgreateconomicvalue.Theyareinvaluablehabitatsandfeedingsitestomanymigra-torybirdspecies,whileshelteringgreatbiodiversity(Reed,1990;VanDijkemanetal.,1990).Theslopeofthemarshinrelationtothetidalamplitude,andtheelevationoftheshorelineoffer-ingbetterprotectioninconditionsofincreasingstorminess(Dyer 1470-160X/$–seefrontmatter©2013ElsevierLtd.Allrightsreserved.http://dx.doi.org/10.1016/j.ecolind.2013.05.015  Author's personal copy B.Duarteetal./EcologicalIndicators34(2013)268–276 269 etal.,2002)willbedeterminingfactorsfortheinitialset-upandresilienceofthesaltmarshcommunitiesunderincreasingcondi-tionsofsealevelrise(Simasetal.,2001).TheseareasaredescribedastransitionalwaterbodiesintheWaterFrameworkDirectiveof theEuropeanUnion,andtheyarefurtherrecognizedasecologi-callysensiblezonesaccordingtotheBirdsandHabitatsdirectives.Lockedbetweenlandmassesandocean,risingseawaterlevelswillplayanimportantroleinthefutureecologyoftheseregions.Sealevelrisewillalsomeanhighererosionratesintheouterbound-aryofthesaltmarsh(Reed,1990).However,somemechanismsmay   counteracttheinfluenceoftheseclimaticinducedfactors,assaltmarshesalsohavetheabilitytokeeptheirrelativeelevationaboveseawaterthroughoutsedimentation(SalgueiroandCac¸    ador,2007).Saltmarshesactassinksforcontaminants(Cac¸    adoretal.,2000;Duarteetal.,2010),carbon(Cac¸    adoretal.,2004)andnitrogen(Cac¸    adoretal.,2007).Themajordepositionalprocessfortheseele-mentsisthroughsedimentationofparticulatematterduringtidalflooding(Silvaetal.,2009),whenthehalophyticvegetationactsasabaffleandsedimenttrap,leadingtothesettlingoffinesuspendedparticlestransportedonthewatercolumnthataredepositedonthemarsh(Silvaetal.,2009).Furthermore,saltmarshgrowthisoftenassociatedtoprocessesoccurringinestuarineareaswithhighlevelsofsedimentdischarge,likemudflats(Simasetal.,2001).Tocoun-teracttheeffectsofsealevelrise,themarshelevationmustkeeponatsuchapacethatiscompatiblewiththerise.Otherwiseflood-ingisinevitableandwillbefollowedbysubsequentmarsherosion.ThishasalreadybeenverifiedintheNileandMississippirivers(Gornitz,1991;BlumandRoberts,2009).Anotherimportantfac-tortoconsideristhetidalrangeofthesaltmarsh.Stevensonetal.(1986)suggestedthatareaswithhightidalrangeareassociatedwithhighersedimenttransportrates,thusbeinglessaffectedbysealevelrise.Inthiscaseanegativefeedbackmechanismisobserved,whereasmallincreaseinsealevelleadstohighermineraldeposi-tionduetolongersubmersionperiods.Thisisalsoassociatedwithlesssedimentcompactionduetoreduceddecompositionoforganicmatterinthesediment(Nymanetal.,1993;Allen,1994).However,rapidsealevelsrise,aspredictedinsomeclimatechangescenarios(IPCC,2007)increasessalt-marshlosscausedbytheincreasedsub-mersionperiodssincesaltmarshproductivity(organogenicinput)issuppressed(Nymanetal.,1993,1994).Recently,anincreasingnumberofnumericalmodellingstudieshaveappearedaimedatidentifyingandsimulatethemainprocessesofmarshelevationdynamicsinresponsetochangingsealevel(Allen,1990,1995,1997;Callawayetal.,1996;Chmuraetal.,1992;Dayetal.,1999;French,1993;Krone,1987;Morrisetal.,2002;Pontetal.,2002;RybczykandCahoon,2002;Rybczyketal.,1998;Temmermanetal.,2003a;VanWijnenandBakker,2001).However,asstatedbyAllen(2000),thesemodelsareataratherembryonicstageofdevel-opment.Importantinformationcanbeobtainedmeanwhilefromempiricalstudiesofsaltmarshsystems.Byinvestigatingsedimen-taryrecordsinthecontextoftheclimateconditionsthatproducedthemitshouldbepossibletounderstandhowsaltmarshgeochem-icalcharacteristicsandaccretionrateshavebeenrespondingtochangesinsealevel.Thistypeofinformationwillbeveryusefulinpractice,allowingforbetteradaptivemanagementofhumanactivities,andhopefullytoguideourpreparationandprotectivemeasuresagainstfuturescenarios.Caesium-datingtechniquesarewidelyappliedtoSLRandSAR studiesincoastalenvironments.AlthoughitsundeniableefficiencyprovidinggoodinsightsonSAR,theyonlytellhalfthestory.Thisway,thepresentworknotonlyaimstoassesssedimentaccumu-lationratesintheTagussaltmarshoverthelast40yearsusing 137 Csdating,butalsothepossibleapplicationofgeochemicalandelemental-basedindicesinordertoprovideinsightsonthepro-cessesbehindthetemporaloscillationsandspatialvariationsintheSAR.Inatimeofmajorconcerninwhichrespectstoglobalchanges, Fig.1. MapoftheTagusestuaryshowingPancasandS.JoãodaTalhasaltmarshsamplingstations. thisanalysiswillbeintegratedinthecontextofrisingsealevelcon-ditions.Thisway   isintendednotonlytoevaluatethepossibleuseofthisgeochemicalindicesbutalsotousethemasstorytellersof therecentpastofTagusestuary. 2.Materialandmethods  2.1.Studyareadescription TheTagusestuaryisthelargestestuaryonthewestcoastof Europe(38 ◦ 44  N,9 ◦ 08  W),   locatedinthemostpopulatedareaof Portugal(Fig.1).Itisashallowestuaryanditscirculationismainlydrivenbytides.TheTagusRiverdrainsatotalareaof86,629km 2 ,representingthesecondmostimportanthydrologicalbasinintheIberianPeninsula.Theriveristhemainsourceoffreshwaterintotheestuary.Inflowfluctuatesseasonallywithanaveragemonthlyvaluevaryingfrom120m − 3 s − 1 insummerto653m − 3 s − 1 inwin-ter(last30yearsofWaterNationalInstitutepublicdatabase,INAG).Estimatedwaterresidencetimerangesfrom6to<65daysforwin-terandsummeraverageriverdischarge,respectively(Martinsetal.,1984).ItisamesotidalestuaryaccordingtotheNOAAclassification,withsemi-diurnaltidesrangingfrom0.4matneaptideto4.1matspringtide.Seawaterenterstheestuarythroughadeepnarrowinletchannel.Thetidalinfluencereaches80kminlandfromLisbon.CoringtookplaceatPancas(PAN)saltmarsh,locatedinthesouthernsideoftheTagusestuarywithintheNaturalReserveof TagusEstuaryandinS.JoãodaTalha(SJT)saltmarsh,locatedinthenorthernsideinLisbonmetropolitanzone.Threesedimentcoresweretakenusingatubularprobe(6.7cmdiameter)in2001.Thecoresweretakenalongthesaltmarshinanareavegetatedby Sarcocorniaperennis .Appropriatemeasuresweretakentoavoidcompactionduringthecoring.ThepositionofthecoresandverticallevelofthecoringsiteswasattainedbyGPS.  2.2.Laboratoryanalysis Thecoresobtainedatthesamplingstationswerebroughtbacktothelaboratoryinrefrigeratedchambersandsliced.Thesam-plesweredriedtoconstantweightat60 ◦ C.Organicmattercontentwasdeterminedbylossonignition(LOI)afterslowlyashingsub-samplesat600 ◦ Cfor3h.Thesedimentaccumulationratewas   calculatedonthebasisof thetwo   pronounced  137 Cs-peaks,whichwerepresentinallthecores.Thelowerpeakcorrespondstothe1963maximumintest-ingofnuclearweaponsintheatmosphereandtheupperpeakis  Author's personal copy 270 B.Duarteetal./EcologicalIndicators34(2013)268–276 duetotheaccidentatthenuclearpowerplantatChernobylin1986.Thesetwopeakshavebeenusedasmarkersandtheaverageaccumulationrateduringtheperiod1963–1986hasbeencalcu-latedastheaccumulatedmassbetweenthetwo   peaksdividedbythetimebetweenthetwoevents(23years).Theaccumulationfrom1986tothetimeofcoring(2003–2005)hasbeencalculatedinasimilarway.Theorganiccontentinthecoresdecreaseswithdepthandconsistsprimarilyofrootsandmacrofragmentsofthesaltmarshvegetation.Inclusionofthisorganicmaterialinthecal-culationsofaccumulationrateswouldinducesystematicerrors,asthedensityofrootsandplantfragmentsishigherintopofsaltmarshsediments.Thesubjectofthepresentstudyistheprimarilyminerogenicallocthonoussedimentandcalculatedmasseswerethereforecorrectedfororganiccontent.TheminerogenicfractionofthesedimentwascalculatedbyreducingthetotaldryweightwiththeLOIpercentage.Sampleswereanalyzedfor  137 Csactivity,usinggamma-spectrometrybywayofitspeakat661keV.Depthof the1963peak(RitchieandMcHenry,1990a,b)was   usedtoinves-tigatetheaccumulationrates,whilethesedimentaccumulationrates(SAR)werecomputedtakingintoaccountthediameterofthecorerandanaveragesedimentdensityof1020kgm − 3 ,obtainedfortheTagussaltmarshesinpreviousstudies(SalgueiroandCac¸    ador,2007).Sedimentationvalueswerealsocorrectedfororganicmattercontent,consideringtheorganicpercentagefoundinthedifferentlayers.Thesevaluesweretakeninaccounttoeliminateforanypos-sibleartefactscausedbythefactthateachlayerinoursampleisatadifferentstageoforganicdecomposition.GeochemicalanalysisofthesedimentsandtheirheavymetalcontentwascarriedoutthroughX-rayfluorescencespectrome-try(XRF).Thesedimentswerefrozen-driedandcompressedintotablets.XRFwas   performedinaPhilipsMagix-ProWD-XRFunitattheNationalOceanographyCenter(NOC)inSouthampton,UK.AfterknowingtherelevantdatafromtheoxidescompositionitwaspossibletoworkouttheChemicalAlterationIndex(CAI):CAI = Al 2 O 3 Al 2 O 3 + CaO + K 2 O + Na 2 OThisindexprovidesinformationaboutthetypeofdisturbanceaffectingthesediments.Inpractice,CAIvaluesbelow40suggesttheabsenceofdisturbance,andvaluesfrom40to70aretypicalof physicaldisturbanceonly,whileindexvaluesabove70essentiallyareindicatorsofchemicaldisturbanceofthesediments(CoxandLowe,1995).  2.3.Sealeveldata ThetidalgaugeclosesttobothstudiedsaltmarshesistheCas-caistidalgauge(www.igeo.pt)locatedinthesametidalbasinatthemouthoftheestuary.Thisgaugeholdsrecordingsfrom1880topresent.Tidalheightsfortheperiodcomprehendedfrom1880tothetimeofcoring(2001),wereobtainedfromtheHydrographicInstitute(www.hidrografico.pt)andtheirdatasetfortheLisbonstation.Theannualheightswereplottedandalinearregressionwasmade,inordertoestablishasealevelriserate(Andersenetal.,2011). 3.Results  3.1.  137  Csdatingandsedimentaccumulationrates Theisotopicanalysisofthecoresshowedtwomajorpeaksof  137 CsconsistentwiththerecenthistoryofNWEurope.Thelowerpeakcorrespondstotheinputofbombtestmaterialthathaditshigheractivityperiodin1963.Theupperpeakisnormallyidentifiedas  137 CsdepositionresultingfromtheChernobylaccidentin1986(Andersenetal.,2000).Thesepeakswerechosenasmarkersfor Fig.2. Verticalprofilesof  137 CsactivityinsaltmarshsoilobtainedfromcoresamplescollectedatPancas(A)andS.JoãodaTalha(B). thoseeventsandallowedustoconsidertwo   differenttimeperiodsforthepurposeofthisstudy:from2001to1986andfrom1986to1963(Fig.2).UsingtheseradiometricmarkerstheaverageSARwereassessedforbothtimeperiods(Table1)andthisallowsforincreasedaccuracyinouranalysisandbetterdiscriminationofthesedimen-tationhistoryatthetwo   saltmarshstationsforthestudyyears(1963–2001).Theradiometricverticalprofileinourcoresshowsratherobviouspeaksofhigher 137 Csactivityatdifferentdepthsandthesepeakshavebeenobservedforthetwosaltmarshes.However,whilethe1986peakwasdetectedatadepthof9cminthePancas(PAN)saltmarsh,thesamepeakhasbeendetectedfurtherdowninSJT,occurringatadepthof21cm.   Asimilarpatternwas   foundalsoforthe1963peak,occurringatadepthof24cminPancasand56cmdeepinSJT.TheSARvalueshavebeenassessedfromthistypeofradiometricdating(Table1)andfortheyears1963–2001theaccretionwasalwayshigherintheSJTstationwhencomparedtoPAN.Therewasanaltitudinalincreaseof0.10cmassociatedtotheaccretionratesforthe1963–1986and1986–2001timeintervals  Table1 Calculatedtotalandmineralsedimentation(kgm − 2 y − 1 )andaccretion(cmy − 1 )ratesbasedon  137 Cspeaksinbothsaltmarshes.Pancas(PAN)S.JoãoTalha(SJT)  Apparentsedimentaccumulationrate 1963–19866.65 ± 0.3315.52 ± 0.781986–20015.44 ± 0.2713.60 ± 0.681963–20016.17 ± 0.3114.76 ± 0.74 Realsedimentationrate 1963–19866.03 ± 0.3013.17 ± 0.661986–20014.87 ± 0.2411.06 ± 0.551963–20015.60 ± 0.2812.25 ± 0.61  Accretionrate 1963–19860.65 ± 0.041.52 ± 0.001986–20010.53 ± 0.001.33 ± 0.011963–20010.61 ± 0.031.45 ± 0.00  Author's personal copy B.Duarteetal./EcologicalIndicators34(2013)268–276 271  Table   2 GeochemicalcharacterizationofthesedimentcoresofSJTandPANsitesatthedepthsdatedas1963,1986and2001(average ± standarddeviation).SJTsitePANsite196319862001196319862001SiO 2  54.5 ± 0.354.0 ± 0.445.3 ± 8.758.7 ± 0.757.4 ± 0.751.8 ± 4.1Al 2 O 3  19.4 ± 0.419.1 ± 0.316.8 ± 2.720.7 ± 0.520.7 ± 0.617.8 ± 1.8TiO 2  0.9 ± 0.10.9 ± 0.10.8 ± 0.10.9 ± 0.00.9 ± 0.00.7 ± 0.1Fe 2 O 3  7.8 ± 0.6 7.9 ± 0.5 6.6 ± 0.57.0 ± 0.57.7 ± 0.06.9 ± 0.6MnO   0.1 ± 0.00.1 ± 00.1 ± 0.00.1 ± 0.00.1 ± 0.00.1 ± 0.0MgO   3.1 ± 0.13.0 ± 0.12.9 ± 0.13.1 ± 0.23.1 ± 0.43.1 ± 0.2CaO   4.2 ± 0.24.4 ± 0.34.0 ± 0.41.1 ± 0.30.9 ± 0.51.0 ± 0.5K 2 O3.8 ± 0.33.6 ± 0.23.3 ± 0.23.7 ± 0.13.7 ± 0.13.3 ± 0.3Na 2 O2.2 ± 0.21.6 ± 0.41.7 ± 0.51.8 ± 0.21.7 ± 0.34.8 ± 2.5P 2 O 5  0.2 ± 0.0 0.3 ± 0.0 0.3 ± 0.0 0.2 ± 0.0 0.3 ± 0.1 0.3 ± 0.1Sum   96.2 ± 1.1 95.1 ± 0.381.7 ± 13.297.2 ± 0.896.5 ± 1.489.8 ± 4.1CAI   65.4 ± 1.266.5 ± 0.764.9 ± 0.875.9 ± 0.976.5 ± 0.866.2 ± 6.7Al 2 O 3 /SiO 2  2.8 ± 0.12.8 ± 0.12.7 ± 0.12.8 ± 0.12.8 ± 0.12.9 ± 01 observedinPancassaltmarsh,andthiscomparestosome0.20cmintheSJTstationforthesameperiod.Thisdifferencebecomesmoreevidentwhencomparingdataintermsofmassdepositionbysquaremetreofsaltmarshsoilarea,ortotalsedimentationrate.Dif-ferencesamongourstationswereintheorderof10kgofsedimentdepositedonasquaremetreofsaltmarsh,furtheremphasizingtheelevatedaccretionratesobservedintheSJTsite.Wheneverthecoresdidexhibitdifferentorganicmattervaluesduetovariationsindecompositionrateswithintheverticalprofile,thesevaluesof totalsedimentationwerecorrectedforitsorganicmatter(aslossonignition)andexpressedasmineralsedimentationrate.  3.2.Sealevelrisedata Thelocalvariationofmeansealevel(MSL)fortheevaluatedperiodsisplottedinFig.3.Plottingalineartrendforthewholestudyperiod(1880–2001)arateofincreaseoftheMSL    was   assessedatabout1.29mmy − 1 inthisarea.Theyearlymeanvaluesdidoscillatearound10.5mmduringthestudyperiod.Themeanwaterdepthinthoseareaswherethelowestpartofbothsaltmarsheswillbecov-eredbytheincomingtidestaysabout1mabovethenationalchartdatum–PortugueseHydrographicZero(PHZ).ThisMSL    risewillhavemajorconsequencesintheecologyofthesaltmarsh,associ-atedtolongerinundationperiodsduringthehightideperiods.  3.3.Elementalanalysis Thegeochemicalanalysisofthesediments(Table2)revealedthattheSiO 2  contenthasbeendecreasingfrom1963to2001, Fig.3. ChangesinMeanSeaLevelvalues(mm)   monitoredfrom1963to2001. remaininghigherincoresfromPANwhencomparedtoSJT.MajordifferenceswerealsoobservedforCaOinthetwosaltmarshes.InbothcasesthevaluesforCaOthroughoutthechronosequencedidremainratherstableduringtheperiodofouranalysis,butSJTshowedhighervaluesofCaOwhencomparedtoPAN.AsforNa 2 O,therewasadecreaseofthisoxidefrom1963to2001atSJT,whileinPANtherewas   anevidentincreasefrom1986to2001.Thisdifferenceinoxidecompositionatthetwositesisevi-dentalsowhenwelookintotheCAIvalues.InbothsaltmarshestherewasanincreaseoftheCAIvaluesfrom1963to1986,andasubsequentdecreasefrom1986to2001.Inthiscase,thedecreasewasmoreevidentinPancassaltmarsh.Thevaluesobservedforbothoxidesinthetwo   saltmarshesareconsistentwithcurrentinterpretationofpredominantphysicaldisturbancemechanismsinfluencingthegeochemistryofthesediments(CoxandLowe,1995),andthisismoreevidentinPancas(PAN).TheratiosSi/AlandSi/Karedependentupontheproportionof thecoarsematerials,mainlyformedbyquartziticsandscontaininghighpercentagesofsilicon,andfineclayswhichcontainSi,AlandK.Comparingtheseratios(Fig.4),itispossibletoobserveastrongrelationshipbetweenthetwo   ratiosinPancassaltmarsh,whileinSJTtheSi/KratioremainsratherstabledespitevariationsintheSi/Alratio.Ironandphosphorousareratherabundantelementswithhighimportanceonsedimentbiogeochemistry.AvariationonP/CaandP/Feratiosindicatesanaccumulationoforganicphosphorusandinfluencesthemechanismscontrollingtheamountoftotalphos-phorusinsediments.Althoughthemoreobviouspatternwas   foundagaininthePancassaltmarshsediments(withastrongcorrela-tionbetweenthetworatios),thereisalsoevidenceofacorrelationbetweenFeandP(Alnormalizedvalues)fortheSJTsaltmarsh,althoughwithsmallervariabilitythanfoundatPancas.Calciumandmagnesiumarethemostabundantalkaline-earthelementsinreservoirsediments,withCamainlypresentascarbon-atemineralsinthiskindofsediments.SimilartotheSi/AlandSi/Kratios,analysisofthevariationoftheseelements(Fig.4)showsalso  Table3 Elementalstandardshaleconcentrations(TurekianandWedepohl,1961)intheearthcontinentalcrust(ppm)andaverageconcentrationinbothstudiedsaltmarshes(average ± standarddeviation).Standardshale*SJTPancasAs1350.7 ± 10.134.5 ± 11.3Co   2418.1 ± 2.213.0 ± 1.5Cr   90110.8 ± 12.9100.5 ± 8.2Cu   4574.3 ± 15.754.7 ± 5.4Ni   6839.1 ± 9.231.3 ± 3.0Pb   20160.2 ± 48.793.4 ± 23.5Zn   95553.9 ± 99.6347.0 ± 92.4
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