Drexler, J.Z., Paces, J., Alpers, C.N., Windham-Myers, L., Neymark, L., and Taylor, H.E. 2013. 234

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AppliedGeochemistry40(2014)164–179
ContentslistsavailableatScienceDirect
Applied Geochemistry
journal homepage: www.elsevier.com/locate/apgeochem
234U/238U and d87Sr in peat as tracers of paleosalinity in the
Sacramento-San Joaquin Delta of California, USA
J.Z. Drexlera,⇑, J.B. Pacesb, C.N. Alpersa, L. Windham-Myersc, L.A. Neymarkb, T.D. Bullenc, H.E. Taylord
aU.S.GeologicalSurvey,CaliforniaWaterScienceCenter,6000JStreet,PlacerHall,Sacramento,CA95819,UnitedStates
bU.S.GeologicalSurvey,GeosciencesandEnvironmentalChangeScienceCenter,Box25046,DenverFederalCenter,MailStop963,Denver,CO80225,UnitedStates
cU.S.GeologicalSurvey,Building15,McKelveyBlg.,MailStop480,345MiddlefieldRoad,MenloPark,CA94025,UnitedStates
dU.S.GeologicalSurvey,3215MarineStreet,SuiteE-127,Boulder,CO80303,UnitedStates
a r t i c l e i n f o a b s t r a c t
Articlehistory: Thepurposeofthisstudywastodeterminethehistoryofpaleosalinityoverthepast6000+yearsinthe
Received16May2013 Sacramento-SanJoaquinDelta(theDelta),whichistheinnermostpartoftheSanFranciscoEstuary.We
Accepted25October2013 usedacombinationofSrandUconcentrations,d87Srvalues,and234U/238Uactivityratios(AR)inpeatas
Availableonline1November2013
proxiesfortrackingpaleosalinity.PeatcoreswerecollectedinmarshesonBrownsIsland,FranksWet-
EditorialhandlingbyM.Kersten
land,andBaconChannelIslandintheDelta.Coresweredatedusing137Cs,theonsetofPbandHgcon-
taminationfromhydraulicgoldmining,and14C.Aproof ofconceptstudyshowedthatthedominant
emergentmacrophyteandmajorcomponentofpeatintheDelta,Schoenoplectusspp.,incorporatesSr
andUandthattheisotopiccompositionoftheseelementstrackstheambientwatersalinityacrossthe
Estuary.ConcentrationsandisotopiccompositionsofSrandUinthethreemainwatersourcescontrib-
utingtotheDelta(seawater,SacramentoRiverwater,andSanJoaquinRiverwater)wereusedtocon-
structathree-end-membermixingmodel.Deltapaleosalinitywasdeterminedbyexaminingvariations
inthedistributionofpeatsamplesthroughtimewithintheareadelineatedbythemixingmodel.
TheDeltahaslongbeenconsideredatidalfreshwatermarshregion,butonlypeatsamplesfromFranks
WetlandandBaconChannelIslandhaveshownaconsistentlyfreshsignal(<0.5ppt)throughtime.There-
fore,theeasternDelta,whichoccursupstreamfromBaconChannelIslandalongtheSanJoaquinRiver
anditstributaries,hasalsobeenfreshforthistimeperiod.Overthepast6000+years,thesalinityregime
at the western boundary of the Delta (Browns Island) has alternated between fresh and oligohaline
(0.5–5ppt).
PublishedbyElsevierLtd.
1.Introduction are not common or well distributed in tidal freshwater marshes,
which are found in the inner reaches of estuaries. Therefore, a
Peatsoilsandwetlandsedimentsarewellknownasarchivesof different proxy is needed to determine paleosalinity in tidal
environmental change and contamination (e.g., Shotyk, 1996; freshwatermarshes.
Byrne et al., 2001; Charman, 2002). Within these archives there Thenaturalchemistryofpeatmay,inandofitself,beauseful
exist numerous proxies that can be used to address questions proxy for paleosalinity, if certain conditions are met. First of all,
regarding climate change, hydrological processes, environmental thetracerbeingconsideredmustdifferinconcentrationbetween
contamination, and paleosalinity (Ingram et al., 1996; Charman, freshwaterandseawater.Next,thetracerneedstobeconservative
2002; Van der Putten et al., 2012). With regard to paleosalinity, in naturesuch that, oncesequestered,there islittletendencyfor
severalproxiessuchaspollen,stratigraphicseeddata,d13Cindia- subsequent mobility in peat, or be present in sufficient quantity
tomsandforaminifera,andB,Br,Na,S,Ge,andUconcentrationsin thatexchangewithambientwaterisnegligible.Ideally,thetracer
peathavebeenusedtocharacterizesalinityregimesthroughthe should be immune from modification due to changes in state,
millennia (Goman and Wells, 2000; Malamud-Roam and Ingram, evaporativeconcentration,or chemicalprecipitationunderambi-
2004;DiRitaetal.,2011).Inaddition,d87Srinfossilbivalvesfound entconditions.Finally,thetracermustchieflyresideintheorganic
withinsalineandbrackishestuarinesedimentshasbeenshownto componentofthepeat,whichisderivedfromtheplantsthatgrew
beaparticularlyeffectivetracer(IngramandSloan,1992;Ingram in a site under a particular salinity regime (López-Buendia et al.,
and DePaolo, 1993; Ingram et al., 1996). However, fossil bivalves 1999).Iftheprovenanceofanelementinpeatismostlyfromsed-
imentwashedinfromthewatershed(lithogenicsources),theele-
mentcannottracesalinityconditionsatthetimetheplantswere
⇑
Correspondingauthor.Tel.:[phone];fax:[phone]. growing.Previousresearchhasshownthatcertainhighlyinsoluble
E-mailaddress:[email protected](J.Z.Drexler).
0883-2927/$-seefrontmatterPublishedbyElsevierLtd.
http://dx.doi.org/10.1016/j.apgeochem.2013.10.011
Author's personal copy
J.Z.Drexleretal./AppliedGeochemistry40(2014)164–179 165
elements,suchastitanium,arepredominantlyfoundinthelitho- fluctuationswould helpmanagersbetterpredict the resilienceof
genic fraction of peat and remain highly immobile once incorpo- theDeltasubsequenttotheproposedmajorchangesinwatercon-
rated into the peat matrix (Shotyk, 1996; López-Buendia et al., veyance (California Department of Water Resources et al., 2013).
1999; Novak et al., 2011). Such highly insoluble elements have Paleosalinityresearchhasbeencarriedoutinsalineandbrackish
beenusedto‘‘normalize’’thesedimentvariabilityinthepeatma- parts of the San Francisco Estuary but in the upstream reaches,
trix.Thisallowsforclearidentificationofelementschieflyfoundin only the western periphery of the Delta (at Browns Island) has
theorganicfractionofpeat(i.e.,thoselackingarelationshipwith beenstudied(GomanandWells,2000;Goman,2001).Theresults
Ti),whichconstitutethepotentialproxiesforpaleosalinity. ofthispreviousresearchshowedthat,between6200and3800cal-
Thepurposeofthisstudywastodeterminethehistoryofpale- ibratedyearsbeforepresent(calyrBP),paleosalinityatBrownsIs-
osalinityoverthepast6000+yearsintheSacramento-SanJoaquin landwascomparabletopresentvalues(slightlybrackish).Aperiod
Delta(theDelta),whichistheinnermostpartoftheSanFrancisco offreshersalinitywasnotedbetween3800and2000calyrBPfol-
Estuary.WeusedacombinationofSrandUconcentrations,d87Sr lowedbyareturntotheslightlybrackishconditionsoverthepast
values, and 234U/238U activity ratios (AR) in peat as proxies for 2000years.Inordertocharacterizepaleosalinityrangeswithinthe
tracking paleosalinity. We chose U and Sr because, similar to Ca, greaterDeltasystem,westudiedpeatfromBrownsIslandaswell
Mg,K,andNa,theyarereadilysolubleacrossthesalinityspectrum astwoothersitesonawest-eastgradientwithsalinitiescurrently
andhavedistinctconcentrationsinfreshwatervs.seawater(Table ranging from oligohaline (0.5–5 practical salinity units (psu);
1).Furthermore,theyeachhaveheavy,radiogenicisotopecompo- wherepsuistheunitlessPracticalSalinityScaleonwhichseawater
sitionsthatmaybeuniquetoeachwatersourceandcanbeused is(cid:2)35)tofresh(<0.5psu).
concomitantly as hydrologic tracers. The benefit of this approach
isthat,unlikeelementalconcentrations,whichundernear-surface
conditionsareaffectedbyavarietyofprocessesandchemicalreac- 2.Studysites
tions that can result in complex geochemical behavior, 87Sr/86Sr
and 234U/238U are largely immune from fractionation caused by The Sacramento-San Joaquin Delta of California was once a
near-surface physical, chemical, and biological processes (Faure 1400km2 region of marshes, channels, and mudflats. Beginning
and Powell, 1972; Hart et al., 2004). Furthermore, Sr is plentiful in the mid-1800s, the Delta was largely drained for agriculture
inoceanwaterrelativetoriverwaterandSrisotopeshaveprevi- (Thompson, 1957), resulting in its current configuration of more
ously been used for tracking paleosalinity (Ingram and Sloan, than100islandsandtractssurroundedby2250kmofman-made
1992;IngramandDePaolo,1993;Ingrametal.,1996).Inaddition, leveesand1130kmofwaterways(Prokopovich,1985).Tidesinthe
Srisotopeshavebeensuccessfullyusedasatracerofprovenancein Delta are semidiurnal and microtidal, with a normal range of
organicmaterials(Shandetal.,2007;vonCarnap-Bornheimetal., approximately one meter (Shlemon and Begg, 1975; Atwater,
2007). Uranium, which is commonly present at much lower 1980).TheclimateintheDeltaischaracterizedasMediterranean
concentrationsthanSr,hastheextrabenefitofbeingtakenupvery withcoolwintersandhot,drysummers(Thompson,1957).Mean
efficientlyandheldtightlybythepeatmatrixunderreducingcon- annualprecipitationisapproximately36cm,butactualyearlypre-
ditions(Szalay,1964;Idizetal.,1986;Johnsonetal.,1987;Owen cipitationvariesfromhalftoalmostfourtimesthisamount.Over
et al., 1992; Zielinski et al., 2000; Novak et al., 2011). Therefore, 80% of precipitation occurs from November through March
234U/238U isotopic composition of peat can be used as a tracer (Thompson,1957).
andindependentcheckonSrisotopebehavior,whichmaybemore Currently,thewaterflowintheDeltaishighlyregulatedsothe
susceptibletopost-depositionalmobility. saltwater wedge originating in the San Francisco Bay has little
We applied our approach using peat cores collected in tidal chancetotravelmuchfurthereastthanthewesternboundaryof
marshesintheDelta.Wefirstconductedaproofofconceptstudy theDelta.However,priortoconstructionofdamsonnearlyallma-
in the greater San Francisco Estuary to determine whether living jor tributaries to the Delta during the 1940s to 1970s, salinity
plants,whichultimatelybecomethebulkofthepeatmatrix,track incursionsintotheDeltacouldextendasfarastheeasternDelta,
the salinity signal with regard to these tracers. Then we con- especiallyduringadryyear(Ingebritsenetal.,2000).Furthermore,
structedathree-componentmixingmodelforwaterusingacom- inanygivenyear,salinitylikelyrangedfromfreshduringtherainy
bination of Sr and U concentrations, d87Sr values, and 234U/238U seasontoslightlybrackishorverybrackishduringthedryseason,
activityratios(AR)inthethreehydrologicend-members:seawa- dependingontheamountofannualprecipitation.Currently,how-
ter, Sacramento River water, and San Joaquin River water. The ever, salinity is regulated for the purpose of maintaining water
resultingmixingmodelwascomparedtothed87Srand 234U/238U qualityaswellasmanagingaparametercalledX ,whichisthedis-
2
compositions of marsh peat samples, nearly all of which plotted tancefromthemouthoftheestuaryattheGoldenGateuptheaxis
withinthe‘‘mixingtriangle’’delineatedbytheendmembers.Delta towherethetidally-averagedbottomsalinityis2psu(Kimmerer,
paleosalinitywasdeterminedbyexaminingpatternsinthedistri- 2002).
butionofpeatsamplesthroughtimewithinthemixingtriangle. Marshstudysiteswerechosenalongthehistoricfloodplainof
TodaytheSacramento-SanJoaquinDeltaisclassifiedasatidal theSacramentoRiveraswellastheglacialoutwashareaalongthe
freshwater delta, yet whether this has been the case throughout SanJoaquinRiver(Fig.1).Inaddition,siteswereselectedfromhigh
its (cid:2)6700-year history (Drexler et al., 2007) remains largely energyenvironmentssuchastheconfluenceoftheSacramentoand
unknown. Knowledge of the extent and timing of past natural SanJoaquinRiverstomorequiescentenvironmentssuchasdistrib-
Table1
Concentrations(mgl(cid:3)1)ofthemajorcationsCa,Mg,K,andNa,andminorcations,SrandUaswellaspHinfreshwaterfromtheSacramentoandSanJoaquinRiversandseawater.
RiverdatawereretrievedfromtheNationalWaterInformationSystemoftheU.S.GeologicalSurveyfortheperiodfrom1950to2010andrepresentmedianconcentrationsofall
measurements,whichincludeallchemicalspecies.SeawaterconcentrationsarefromTurekian(1968).
Ca Mg K Na Sr U pH
SanJoaquinRiver 32 15 3 69 0.389 0.005 7.7
SacramentoRiver 12 6.1 1.2 9.1 0.091 0.00013 7.7
Seawater 400 1290 392 10800 8.1 0.0033 7.9–8.2
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166 J.Z.Drexleretal./AppliedGeochemistry40(2014)164–179
3.Methods
3.1.Samplecollection
In the summer of 2005, peat cores were collected from each
marsh site using a modified 5-cm-diameter Livingstone corer
(Wright, 1991). At Browns Island, the first core, BRIC4, unlike all
theothercoresinthestudy,didnothavegoodrecoverynearthe
surface due to a particularly dense root mat. In addition, BRIC4
didnotreachtheunderlyingmineralsubstrate,eventhoughmuch
clay was already present below 700cm. Therefore, an additional
core(BRIC5)thatincludedtheentirepeatcolumnof922cmwas
collectedinMarch2007within2moftheBRIC4site.Asoilmono-
lithofapproximately50(cid:4)50(cid:4)50cm(BRIP)wasexcavatedfrom
thesurfacetoimproverecoveryoverthatachievedwiththeLiving-
stone corer. Further details about coring can be found in Drexler
etal.(2009a).
Real-timekinematicgeographicpositioningwasusedtoestab-
lish the elevations and coordinates of the coring locations. Tidal
benchmarkLSS13(NationalOceanicandAtmosphericAdministra-
tiontidalstation9415064locatednearAntioch,CA)withastatic
surveyed ellipsoid height of (cid:3)28.75 meters was used to adjust
the elevations of the coring sites to local mean sea level. Details
concerning peat coring and survey procedures can be found in
Drexleretal.(2009b).Overallcharacteristicsofpeatateachmarsh
siteaswellasmarshsurfaceelevationsareincludedinTable2.
DuringNovemberandDecember2009,weconductedaproofof
conceptstudytoevaluatewhethermarshplantsreflectthesalinity
andisotopicsignalofthehydrogenicversuslithogeniccomponents
withintheirambientgrowthenvironment.Wechose4marshsites
intheSanFranciscoEstuaryrangingfrombrackishtofresh:Gali-
Fig.1. Mapofpeat coringandplantcollectionsites(triangles)andriverwater nasCreekinSanPabloBay,RushRanchinGrizzlyBay,andBrowns
collectionsites(circles)intheSanFranciscoEstuaryanditswatershed,California, IslandandFranksWetlandintheDelta(Fig.1).Wecollectedentire
USA.
specimens of Schoenoplectus californicus (dominant in Galinas
utariesoftheSanJoaquinRiver.Thethreesites,BrownsIsland(BRI, Creek)orS.acutus(dominantintheothersites)atthreedifferent
268ha,highenergy),FranksWetland(FW,28ha,lowenergy),and places within each marsh. These plant species were chosen be-
Bacon Channel Island (BACHI, 10ha, low energy) are relatively cause(1)theyareubiquitousinmostbrackishandfreshwatertidal
undisturbed marshes, which, unlike those in the vast majority of marshes in the San Francisco Estuary, and (2) they are emergent
the region, were not converted for agriculture (Drexler et al., macrophytes with large roots and rhizomes, which are the main
2009a).Vegetationonthesenaturalmarshislandsishighlyproduc- organiccontributorstothepeatmatrix.Entireplantsampleswere
tiveanddominatedbytheemergentmacrophytesSchoenoplectus collected as close as possible to water quality gauges for which
americanus(Americanbulrush),S.acutus(hardstembulrush),Phrag- long-termsalinitydatawereavailable.Plantsampleswererinsed
mitesaustralis(commonreed),andTyphaspp.(cattail)andshrub- in the field with ambient water to remove sediment and placed
scrub wetland species including Salix lasiolepis (arroyo willow) inlabeledplasticbags.
andCornussericea(red-osierdogwood)(Drexleretal.,2009b).Peat WatersampleswerecollectedattheU.S.GeologicalSurveyriv-
depositsatthesesitesrangefromseventooverninemetersindepth ergaugesontheSacramentoRiveratFreeportandtheSanJoaquin
(Table2)andcontainabundantorganicmaterialincludingdecaying RiveratVernalis(Fig. 1)onMarch15and16,2011,respectively.
plantroots,rhizomes,andstems,someofwhichareverywellpre- Sampleswerefilteredwitha0.45lmfilterandstoredin500-ml,
served(GomanandWells,2000;Drexler,2011).Suchpreservation pre-cleaned high-density polyethylene bottles until analyzed for
oforganicmaterialdemonstratesthatreducingconditionsarepre- SrandUconcentrationsandisotopiccompositions.
valentinthepeatmatrix,causingdecompositiontoproceedalong
slow,anaerobicpathways(GambrellandPatrick,1978).Reducing 3.2.Coreprocessing
conditionsaretypicalintidalmarshsoilsduetothehighwaterta-
ble,whichismaintainedbytheperiodicityofthetides(particularly In the laboratory, core stratigraphy was documented, cores
inmicrotidalregionssuchastheDelta)aswellasthelowratesof weresplitlengthwiseandphotographed,andonelongitudinalhalf
hydraulic conductivity that keep soils at or near saturation, even ofthecorewasarchivedforfutureuse.Bulkdensitywasobtained
duringebbtides(Rabenhorst,2001). by sectioning cores into 2-cm-thick blocks, measuring each
Table2
ElevationsandbasicdescriptionsofpeatcoresfromBACHI,BRI,andFW.
Core Elevationoftopofcore(mMSL) Depthofpeatcolumn(cm) Meanbulkdensity(gcm(cid:3)3,±sd) Mean%organicmatter(±sd)
BACHI 0.21 726 0.12±0.05 76±16
BRI 0.51 922 0.31±0.15 40±18
FW 0.27 718a 0.14±0.08 72±19
a Onlythetop608cmofcorewereusedbecausedeeperpeatlayerscontainedinversionsinradiocarbondates,precludingaccuratedating.
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J.Z.Drexleretal./AppliedGeochemistry40(2014)164–179 167
dimension,obtainingwetweightofthesample,dryingovernight Concentrationsof Sr, U, Ti, Al, and Zr in freeze-driedpeat and
at 105(cid:2)C, and then weighingagain to obtain dry weight (Givelet modernplantrootsweredeterminedbyinductivelycoupledplas-
et al., 2004). Core data were examined for compression and/or mamassspectrometry(ICP–MS)usingaPerkinElmerElanModel
expansion,butnomathematicalcorrectionswereneeded.Thesoil 6000andinductivelycoupledplasmaatomicemissionspectrome-
monolithremovedfromthesurfaceofBrownsIslandwascorrected try (ICP-AES) using a Perkin Elmer Optima Model 3300DV. For
forexpansion. theseanalyses,approximately100mgofsolidmaterialwascom-
pletelydissolvedinanHCl–HNO –HFacidmixtureusingamicro-
3
wave total-digestion procedure (Roth et al., 1997; Barber et al.,
3.3.Plantprocessing
2003; Hart et al., 2005). The digested samples were diluted at
1:10 (volume:volume, digest:water) with 18MXcm deionized
To remove as much sediment as possible, live roots and rhi-
water and were preserved with distilled nitric acid. Aerosols of
zomes,whicharethemainorganiccontributorstopeatformation,
acidified aqueous samples were introduced into both spectrome-
were rinsed briefly in deionized water and blotted dry. Further
ters using a high-solids Burgener pneumatic nebulizer. Multiple
cleaning involved immersing rinsed samples in deionized water
internal standards covering most of the mass range (In, Ir, and
and placing them in a vibrating disrupter/homogenizer (Vortex).
Rh)wereusedtocorrectforsystemdrift.Standardreferencemate-
This process was repeated until the deionized water ran clear.
rials(SRM)fromtheNationalInstituteofStandardsandTechnol-
Balchtubescontainingthesampleswerethenfilledwithdeionized
ogy (NIST) (Buffalo River Sediment, NIST SRM 8704 and Marine
water,placedinasonicbathandsonicatedat51kHzfor20min.
SedimentNISTSRM2702)weredigestedandanalyzedwitheach
Afterparticlessettledfor1h,samplesweregentlyretrievedfrom
analyticalbatch.Additionaldetailsregardingthespecificanalysis
thetubesandexaminedat40(cid:4)magnificationforthepresenceof
techniques,procedures,andinstrumentalsettingsforICP-MSanal-
mineral particles. When particles were no longer observed, sam-
yses can be found in Garbarino and Taylor (1996) and Taylor
ples were freeze-dried for 12–24h until all liquid was removed.
(2001);andforICP–AESinGarbarinoandTaylor(1979),Bossand
Driedsampleswerestoredinsealedvialsuntilanalysis.
Fredeen(1999),andHartetal.(2005).
All concentrations were determined in triplicate on a single
3.4.Watersalinitydata digestionofeachfreeze-dried,homogenizedsample.Thestandard
deviation(sd)forthetriplicateanalyseswasdeterminedforeach
Surface-watersalinitydataforeachofthemarshsitesusedfor digest. In addition, replicate digestions were done every 10 sam-
theplantstudywereobtainedfromlong-termgaugestationsmon- plestotesthomogeneity.
itoredbytheU.S.GeologicalSurvey,theCaliforniaDepartmentof
WaterResources,andtheNationalEstuarineResearchReservePro-
3.6.SrandUisotopeanalyses
gram. Mean annual surface salinity values, data sources, years
available,andlong-termsalinitytrendsareprovidedinTable3.
IsotopiccompositionsofSrandUweredeterminedintheUSGS
DenverRadiogenicIsotopeLaboratorybythermalionizationmass
3.5.Chemicalanalyses spectrometryonasubsetofthepeatandplantdigestionsusedfor
elementalanalysis,and onsamplesof river water.Both Srand U
Thepercentageoforganicmatterindriedbulkdensitysamples were chemically purified using ion chromatography (AG1(cid:4)8 for
was determined by standard loss on ignition procedures (Heiri U;Sr-Spec™forSr),loadedontorheniumfilaments(doubleassem-
etal.,2001)at4-cmintervalsnearthetopandbottomofeachcore bliesforU;singlefilamentsloadedalongwithtantalumoxidefor
andotherwiseat10-cmintervals.Onaverage,duplicateswererun Sr),andrunonaThermoFinniganTritonmulti-collector,thermal
every9–10samples.Reproducibilityofduplicateanalysesranged ionization,isotoperatiomassspectrometer.
from0%to4.8%withanaveragevalueof0.74%. Srisotoperatiosweredeterminedineitherstaticormulti-dy-
Two-cmsectionsofcorewerefrozenondryiceandshippedto namicanalyticalmode,and87Sr/86Sratomicratioswerecorrected
theUSGSlaboratoryinBoulder,Coloradowheretheywerefreeze- for mass fractionation using 88Sr/86Sr measured in the same run.
dried and milled in polyethylene vials with acrylic balls using a Reported 87Sr/86Sr values are also corrected for inter-laboratory
SPEX(cid:3) Model 8000 mixer mill. In the monolithic block from the biasbynormalizingtothemeanvalueobtainedforSrisotopestan-
BRI surface, inorganic (clay lenses) and organic material (‘‘rela- dardNIST987analyzedalongwithsamplesandassumingavalue
tivelypurepeat’’)in4contiguous2-cmsectionsrangingindepth of 0.710248 (McArthur et al., 2001). Replicate analyses of Sr in
from0.220to0.171mMSLwereseparatedusingaceramicknife modern-marine carbonate standard, EN-1, run during this time
to compare the chemistry of these different components of the gaveameanvalueof0.709176±0.000014(2(cid:4)sd;N=48)which
peatmatrix. is within error of the accepted value of 0.709174±0.000002 for
Table3
Surfacewatersalinitydataforgauges(deployedbelowmeanlowerlowwaterata15-mintimestep)intheSanFranciscoEstuary.Alldatawerecollectedassalinity,exceptfor
BrownsIslandsiteswhichwerecollectedasspecificconductanceandcorrectedtosalinitybymultiplyingby0.0006364(Ganjuetal.,2005).
Site Meansalinity Source Yearsavailable
(psu,(sd))
GalinasCreek(SanPabloBay) 23.2(4.2) SanFranciscoBayNationalEstuarineResearchReserve 5/1/2008–12/31/2009
underNOAA’sNationalEstuarineResearchReserve
System’s(NERRS)NationalMonitoringProgram
RushRanch(GrizzlyBay) 6.1(2.0) NERRSNationalMonitoringProgram 5/20/2008–12/31/2009
BrownsIsland(theDelta) 1.8(1.8) TwoUSGSgauges:sitesdescribedinGanjuetal.(2005) 3/27/2002–5/20/2002;11/9/2002–12/17/2002;
3/12/2003–4/16/2003;9/5–28/2005;10/12–26/2005;
1/11/2006–2/1/2006
FranksWetland(theDelta) 0.7(0.4) SJOgaugeoftheCaliforniaDepartmentofWaterResources 1/1/2007–12/31/2008
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168 J.Z.Drexleretal./AppliedGeochemistry40(2014)164–179
modernseawater(McArthuretal.,2006). Deltavaluesin permil (Hornbergeretal.,1999;AlpersandHunerlach,2000;Domagalski,
(‰)usedinthisreportarecalculatedwiththefollowingequation: 2001;Bouseetal.,2010).Therefore,weidentifiedthiscriticaltime
d87Sr=((87Sr/86Sr /87Sr/86Sr )(cid:3)1)(cid:4)1000, using the periodinCaliforniahistorybysearchingformajorincreasesinPb
sample seawater
87Sr/86Sr value given above. Analytical errors determined and/or Hg in the peat record. By normalizing these elements to
seawater
by replicate analyses of standards and unknowns are better than Ti to account for variable sediment content (Alpers et al., 2008),
±0.00002(=±0.03‰ford87Sr)andrepresent95%confidencelevels wefoundrapidincreasesinPbcontaminationbeginningatapprox-
(2(cid:4)sd). imately(cid:3)0.153(±0.12)mMSLatBRI,(cid:3)0.29(±0.04)mMSLatFW,
U isotope ratios were determined in multi-dynamic peak- and(cid:3)0.17mMSLatBACHI.
hoppingmodeusingasinglesecondaryelectronmultiplier.Ratios Radiocarbondatingofachenes(Schoenoplectusfruitingbodies),
were correctedfor mass fractionation using theknown236U/233U seeds, charcoal, and other terrestrial macrofossils were used to
isotope ratio in an added double spike. Replicate analyses of estimate ages greater than 250years. Radiocarbon samples were
U-isotope standard (NIST4321B run between January 2009 and analyzedbyacceleratormassspectrometryattheCenterforAccel-
July2010) yieldeda mean234U/235Uatomic ratioof0.0072921± erator Mass Spectrometry, Lawrence Livermore National Labora-
0.0000135 (=±0.19% based on 2(cid:4)sd; N=134), which is within tory in Livermore, California. Ages were calibrated using CALIB
analytical uncertainty of the accepted value (0.007294± (version5.0.1;StuiverandReimer,1993)withtheINTCAL04curve
0.000028).Correctionsforinstrumentdriftweremadebynormal- (Reimeretal.,2004).Age-depthsplinefitmodelsforthecomplete
izing234U/235Uvaluesmeasuredforunknownsbythesamefactor peat profiles (all 2-cm sections) were constructed following the
needed to correct measured 234U/235U ratios for the SRM 4231B procedureinHeegaardetal.(2005).Additionalinformationabout
standardruninthesamemagazine.Measured234U/235Uatomicra- the 14C dating of the peat cores, including all data, is reported
tios were converted to 234U/238U activity ratios (AR) using decay elsewhere(Drexleretal.,2009a).
constantspublishedbyChengetal.(2000)(k =2.8262(cid:4)10(cid:3)6yr(cid:3)1)
234
andJaffeyetal.(1971)(k =1.55125(cid:4)10(cid:3)10yr(cid:3)1),assumingthat
238
all U has a 238U/235U composition of 137.88 (Steiger and Jäger, 3.8.Three-componentisotoperatiomixingmodels
1977). Replicate analyses of a solution of 69 million year old U
Water in the Sacramento-San Joaquin Delta is derived from
ore (Ludwig et al., 1985) assumed to be in radioactive secular
freshwater inputs from the Sacramento and San Joaquin Rivers
equilibrium were typically within analytical uncertainty of
1.000. However, the long-term average 234U/238U AR value of andseawaterfromtheSanFranciscoEstuary.Eachofthesesources
hasadistinctchemicalandisotopicfingerprint.Therefore,thedis-
0.9983±0.0025(2(cid:4)sd;N=93)indicatesasmallsystematicbias,
solvedionloadinthewatercolumnatanygivenplacewillmostly
implying that the ore ‘‘standard’’ may not completely satisfy the
consist of a mixture of these three sources, which reflect the ion
assumptionofclosed-systemevolution.Analyticalerrorsformea-
sured 234U/238U AR values are given at the 95% confidence level concentrations of each end member. The d87Sr and 234U/238U AR
compositionsofthemixturedependonboththeisotopiccomposi-
(2(cid:4)sd) and include contributions from within-run uncertainties
tionsandtheSrandUconcentrationsofendmembers.Becausethe
(counting statistics) plus uncertainties propagated from blank,
isotopic compositions of these constituents are not strongly
spike,andmassfractionationcorrections,aswellasexternalerror
affected by near-surface chemical reactions or processes, simple
derivedfrommultipleanalysesoftheUisotopestandard.
binary mixing equations were used to describe the resulting
d87Sr and 234U/238U AR compositions of the mixed water. The
3.7.Peatdating
three-componentmixingmodelsemployedinthisstudyarebased
Peat profiles were analyzed for 137Cs in order to estimate the ontheoryandequationsgiveninFaure(1986,chapter9)andFaure
depth of the 1963 layer. The activities of 137Cs in core sections andMensing(2005,chapter16).
Inasystemconsistingoftwoend-membercomponents,Xand
withinthetopmeterofpeatwerecountedusingagammadetector
Y,theconcentrationofanyelementinthemixture,C ,isdepen-
(low-background germanium detector) and a multi-channel ana- M
dentontheconcentrationsofthatelementinbothendmembers,
lyzeratUSGSlaboratoriesinDenver,COandMenloPark,CA.Sam-
C andC ,andthefractionofmixing,f,suchthat
ples were analyzed for 24–48h to achieve the desired precision. X Y
Thismethodhasbeenusedextensivelytodatepeatandsediments
C ¼C f þC ð1(cid:3)f Þ ð1Þ
inwetlandsandlakes(ArmentanoandWoodwell,1975;DeLaune M X X Y X
et al., 1983; Ritchie and McHenry, 1990; Van Metre and Fuller,
wheref =X/(X+Y)andvariesfrom0to1.Mixturesofthetwocom-
2009). 137Cs is a product of atmospheric fall-out from nuclear X
ponentswillresultinpointsthatdefineastraightlinebetweenthe
weapons testing and power plant accidents. Significant levels of
end-membercompositionsonplotsofC versusC thataredirectly
this isotope first appeared in the atmosphere in the early 1950s, X Y
proportionaltothevalueoff(SupplementalFig.1a).However,the
withpeakquantitiesdetectedin1963.Thepeatorsedimentlayer
isotopiccompositionofthemixture,R ,dependsonboththeiso-
from 1963 can be identified based on the maximum activity of M
tope ratios of end members, R and R , as well as their elemental
137Csinaprofile.Itisimportanttonotethatthereissomeuncer- X Y
concentrationssuchthat
taintyaboutthelong-termimmobilityof137Csinpeatduetopro-
cessessuchasdiffusionandadvectionthroughporewater,uptake
R ¼R C f þR C ð1(cid:3)f Þ ð2Þ
byvegetation,andflushingbywatersofhighersalinity(Turetsky M X X x Y Y X
et al., 2004; Foster et al., 2006). However, because research has Because values of R are most strongly influenced by the
M
shown lower mobility of 137Cs in peat containing clay minerals componentwiththehigherconcentration,mixingcurvesnolonger
(MacKenzie et al., 1997), which is the case in the Delta, and definestraightlinesonplotsofR versusC ,butinsteadafamily
M M
becausewearemostinterestedinvariationsonamillennialtime ofhyperboliccurveswhosedegreeofcurvaturedependsonthedif-
scale, we decided that using 137Cs dating to estimate the 1963 ference in concentration between C and C (Faure and Mensing,
X Y
horizonsatisfiestheneedsofthisstudy.Weestimatedthelocation 2005, Fig. 16.5). Furthermore, mixing fractions defined by f are
ofthe1850peathorizon,whichrepresentstheinitiationoftheCal- no longer distributed proportionally along the mixing curve, but
ifornia Gold Rush period, by examining peat geochemistry. arecompressedtowardtheendmemberwiththehigherconcen-
Hydraulic gold mining, which began in 1852, resulted in a sharp tration (Faure and Mensing, 2005, Fig. 16.6). Binary mixtures of
rise in Pb and/or Hg contamination in the San Francisco Estuary two elements (Sr and U) with different isotope ratios can be
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J.Z.Drexleretal./AppliedGeochemistry40(2014)164–179 169
treatedinthesamewayusingtwosetsofmixingequations.Values (a)
ofd87Sr and234U/238UAR foranygivenvalueoffwillplotalong
M M
thecurvedependingonvaluesofSr ,Sr ,U ,andU .
X Y X Y
Three-component mixing of d87Sr and 234U/238U AR between
components X, Y, and Z, can be treated as a series of separate
twocomponentmixtures.Ifallthreecomponentsarepresent,mix-
tureswillplotwithinthepolygonalspacedefinedbythree2-com-
ponent mixing curves between X–Y, X–Z, and Y–Z. To quantify
proportions of each component, mixing webs can be calculated
using the two-component mixing curves between intermediate
endmembersconsistingoftheR andC determinedat0.1inter-
M M
valsoffformixturesalongallthreetwo-endmembercurves.
Forthisstudy,d87Srand234U/238Uendmembersconsistofcom-
positionsobservedinseawater,theSacramentoRiver,andtheSan
JoaquinRiver(SupplementalFig.1b).Valuesfortheseawaterend
memberswereobtainedfromMcArthuretal.(2006)andDelanghe (b)
etal.(2002).DatafortheSacramentoRiverendmemberwereob-
tainedfromwatersamplescollectedforthisstudyanddataavail-
able from the U.S. Geological Survey National Water Information
System(NWIS).DatafortheSanJoaquinRiverendmemberwere
obtained from water samples collected for this study, NWIS, and
Westcot et al. (1992). Values of d87Sr for river water determined
in this study are similar to those reported by Ingram and Sloan
(1992). For the purposesof this study,we assume that d87Sr and
234U/238U AR values of all water sources within the Estuary have
remained relatively constant throughout the Holocene, which is
justified as a first approximation because the lithogenic compo-
nents of the watershed have not changed for well over
100,000years(IngramandSloan,1992).
4.Results
Fig.2. Meansurfacewatersalinityvs.(a)d87Srand(b)234U/238Uactivityratio(AR)
determinedinlivingSchoenoplectusroots/rhizomescollectedfromfoursitesinthe
4.1.Proofofconceptstudy SanFranciscoEstuaryrepresentingasalinitygradientfromfreshtosalinemarshes.
DataarefromSupplementalTable1.Simplebinarymixingmodelsareshownfor
The results of the proof of conceptstudy illustrate that the Sr mixturesofseawaterandSacramentoRiverwater(dottedlines)andSanJoaquin
Riverwater(solidlines)usingdatafromSupplementalTable3.Italicizednumbers
isotopiccompositionsofmodernplantsreliablyintegratethesalin-
in(b)areUconcentrationsinroot/rhizometissuefromSupplementalTable1.
itysignalthroughouttheEstuary.SrandUconcentrationsandiso-
tope data for Schoenoplectus roots/rhizomes are provided in
Supplemental Table 1. d87Sr values in Schoenoplectus roots/rhi- Sr concentrations (median value for 11 samples in Supplemental
zomescompriseanarrowrangeateachsite,butawiderrangebe- Table1is15lgg(cid:3)1forSr),makingthemmoresusceptibletoinflu-
tween sites that is consistent with location within the Estuary encebyfactorsotherthansimplemixing.
(Fig. 2a). Seawater dominates d87Sr compositions at salinities Unlike Sr isotopes, U isotopes can be fractionated as a conse-
aboveafewpsu.Althoughthed87Srcompositionofwaterpresent quence of recoil processes associated with alpha-decay of 238U.
ateachsitewasnotmeasured,dataforplantstendtofollowthe Duringthisprocess,234Uisphysicallydisplacedfromthelocation
mixing relationship expected between seawater and freshwater oftheparental238Uatomduetotheejectionofthealphaparticle
represented by Sacramento and San Joaquin River water (solid (Kigoshi, 1971). The resulting 234U decay products are more
anddashedcurvesinFig.2a).Thehyperbolicnatureofthemixing susceptible to mobilization than 238U due to factors such as
curvesistheresultofthenon-lineareffectswhenlargedifferences radiation-induced ionization and damage of the crystal lattice
inSrconcentrationarepresentbetweenthemixingendmembers. (Gascoyne, 1992; Chabaux et al., 2008; Porcelli, 2008). Direct
Theobservedpatternofd87Srversussalinityinplantsisverysim- implantationofrecoil234Uis particularlynoticeableinsituations
ilar to a plot of surface-water salinity and d87Sr measurements where adjacent phases have high and low U concentrations
shownbyHobbsetal.(2010). (Neymark and Amelin, 2008), such as the case here with low U
Incontrast,asimilarplotof234U/238UARversussalinityshows root/rhizometissuegrowinginhigherUpeat.Directimplantation
that compositions of roots/rhizomes from Franks Wetland and of recoil 234U into root/rhizome tissue may be further enhanced
GalinasCreekarebroadlyconsistentwithamixingmodelinvolv- because recoil distances increase substantially in low density
ingseawaterandfreshwaterfromtheSacramentoRiver.However, materials (Chabaux et al., 2008). Roots/rhizomes from BRI and
samples from Browns Island and Rush Ranch have much greater RushRanchareparticularlysusceptibletoinfluencebythisprocess
variationsthatdeviatetohighervaluesthanthoseexpectedfrom becauseoftheirlowUconcentrationscomparedtoroot/rhizomes
simplemixingofseawaterandfreshwater(Fig.2b).Wenotethat analyzedfromFWandGalinasCreek.Itislikelythatallroots/rhi-
roots/rhizomeswiththehighest234U/238UARvaluestendtohave zomesincorporatealimitedamountofrecoil-generated234Ufrom
substantially lower U concentrations ((cid:2)0.11lgg(cid:3)1 or less) than thepeatinwhichtheygrowinadditiontosolubleUderivedfrom
samples that conform to the salinity mixing pattern (typically theoverlyingwatercolumn.However,atlowconcentrations, the
>0.2lgg(cid:3)1) or samples of peat from the Delta (median value of additionofrecoil234Umayhaveanoticeableeffectontheisotopic
3.8lgg(cid:3)1 for 150 analyses). We also note that U concentrations compositionofUincorporatedintothelivingroot/rhizometissue.
inroots/rhizomeswithelevated234U/238UARaremuchlowerthan Consequently, root samples with very low U concentrations are
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170 J.Z.Drexleretal./AppliedGeochemistry40(2014)164–179
likelytohave234U/238UARvaluesthatarehigherthanvaluesinthe water. However, starting around 350BCE, 234U/238U AR values in
overlyingwatercolumn. peatsamplesfromBRIgraduallydecreasetowardsvaluesobserved
inseawaterandSanJoaquinRiverwater,reachingalowof(cid:2)1.15at
around1850–1900CE.Duringthistimeperiod,peatsamplesfrom
4.2.Depthprofilesinchemicalandisotopedata
FWandBACHIshiftsubtlytowardSacramentoRivercomposition
intermsofboth234U/238UARandd87Sr.
Profiles of Sr and U concentrations show a large amount of
scatterwithtime(ordepth)atindividualsites(Fig.3aandb).This
is particularly the case at Browns Island, which is located at the 4.3.RelationshipswithTicontent
confluenceofthetworivers.Correlationofshort-termfluctuations
isnotreadilyapparentbetweensites.Furthermore,bothSrandU ThereisastrongrelationshipbetweenTiandashcontentinthe
concentrations show substantial overlap between different loca- peatfromallthreesitesintheDelta(Fig.4).Thisstrongrelation-
tionswithintheDeltaanddonotclearlyreflectasalinitygradient ship justifies the use of Ti normalization to account for varying
fromBRItoBACHI. amounts of sediment being incorporated into the peat through
Incontrast,profilesofSrandUisotopiccompositionsshowless time.
scatterovertimeandgreaterseparationbetweensites(Fig.3cand Plots of various elements vs. Ti suggest different processes by
d). All peat samples have d87Sr compositions that range between which elements have become incorporated into the peat matrix
valuesmeasuredforSacramentoRiverwater((cid:3)3.83‰)andseawa- (Fig. 5). Because a pure organic component could not be isolated
ter(0‰).ValuesforBRIpeatsamplesaretypicallyequaltoorsub- from peat samples prior to analyses, a means of discriminating
stantially higher than values observed in FW or BACHI samples, between contributionsfrom lithogenic and hydrogenic sources is
whichisconsistentwiththegreatercontributionsfromseawater needed. A number of elements are highly insoluble in typical
inthewesternDeltathanfurtherupstream. surfacewaterincludingTi,Al,andZr.Thesourcefortheseelements
Unliked87Sr,234U/238UARvaluesfortwoofthethreesourcesof is the inorganic sediment incorporated into the peat. This is
water (San Joaquin River and seawater) are nearly identical evident in plots of Al and Zr versus Ti from peat samples at BRI
(Fig.3d).Valuesof234U/238UARforolderpeatsamplesclearlydis- (Fig.5aandb)whereallthreeelementsdefinelithogenicmixing
tinguishBRIfromthetwofreshersites,BACHIandFW(Fig.3d).FW lines that pass through the origin as well as through clay-rich
andBACHIsamplesplotclosertothevalue forSanJoaquinRiver samples from either near the top of the profile or the basal clay.
waterandBRIsamplesplotclosertovaluesforSacramentoRiver Noadditionalsourcefortheseelementsisrequired.
Fig.3. PlotsofSrconcentrations(a),Uconcentrations(b),d87Srvalues(c),and234U/238UAR(d)versuselevationrelativetomeansealevel(andpeatageforBRI)inpeat
samples.
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J.Z.Drexleretal./AppliedGeochemistry40(2014)164–179 171
100 digestionsofpeatsamples(U ).ValuesforU rangefromnear0%
T N
to 89%; however, median values of 55% and 73% are obtained
dependingon whetherbasal clay or clay-richlayers near thetop
80
of the profile are used to represent the lithogenic component.
TheseresultsindicatethatmostoftheUpresentinpeatsamples
%) 60 isderivedfromnon-lithogenicsources.
wt. UnlikeU,Srconcentrationsinpeatsamplesshownorelationto
h ( 40 FW: y = 171.91x + 14.003 lithogenicmixingmodelsandcanhavevalueseitherlowerorhigh-
s
A R2 = 0.7617, p < 0.0001 er than those expected based on Ti concentrations (Fig. 5d). Fur-
BRI: y = 119.91x + 26.274 thermore, peat samples show a crude negative correlation
20 R2 = 0.8309, p < 0.0001 betweenSrandTi(graybandinFig.5d).Theserelationsindicate
BACHI: y = 174.41x + 2.7613 that lithogenic Sr does not provide a significant contribution to
R2 = 0.8421, p < 0.0001
0 peatsamplesdespitesimilarconcentrationsinpeatandclay-rich
0 0.1 0.2 0.3 0.4 0.5 0.6 samples.Instead,samplesofclay-richmaterialsmaybecontami-
Ti (wt. %) natedwithsmallamountsoforganicmatterthatcontributemuch
oftheSrinthebulkanalysis.
BRI FW
ThesedataindicatethatSrandUinpeatsamplesarenotpartic-
BACHI Linear (BRI)
ularlysensitivetocontributionsfrominorganicsedimentincluded
Linear (FW) Linear (BACHI)
inthepeatmatrix.UnlikeTi,bothUandSraresolubleinoxidizing
surfacewatersincludingseawaterandriverwater.Therefore,itis
Fig.4. RelationshipsbetweenTiconcentrationsandashcontentofpeatsamplesat
likelythatvariationsobservedintheseelementsareduetoshifts
BRI,FW,andBACHI.
incontributionsfrommultiplehydrogenicsources.Minimalinflu-
ence by lithogenic components in Delta peat samples is further
The lithogenic source also contains some U and Sr; however, supported by isotope data. Four of the five samples of clay-rich
neither of these elements has a strong correlation with Ti as do materialfromBRIhavemeasured234U/238UARvaluesthatarelow-
Al and Zr (Fig. 5c and d). U concentrations of most peat samples erthanthoseobservedinpeat(Fig.6a).Theseresultsareconsis-
aregreaterthanvaluespredictedbycontributionsfromthelitho- tent withthe conceptthat most lithogenic source material is old
genic source based on Ti concentrations (i.e., they fall above the enough to have reached radioactive secular equilibrium
lithogenicmixinglinesinFig.5c).TheproportionofUderivedfrom (234U/238U AR=1.0). However, the fact that all clay-rich samples
non-lithogenicsources(U )canbecalculatedbysubtractingtheU have 234U/238U AR values greater than 1 indicates a strong
N
attributabletolithogenicsources(U)fromtheUmeasuredintotal likelihoodthatthefinesedimentconstitutingthelithogenicfaction
L
Fig.5. RelationshipsbetweenTiconcentrationsandaluminum(a),zirconium(b),uranium(c),andstrontium(d)atBRI.Lithogenicmixinglinesareshownforplotsof
concentrationandarebasedontheassumptionthatclay-richmaterialsrepresentthelithogeniccomponentofpeatsamplesandthatconcentrationsattributabletothe
lithogeniccomponentarezerowhenTiiszero.R2andp-valuesprovidedforsimplelinearregressionsfororganic-richpeat(thickgraylines).SeetextforexplanationofUN
andULin(c).
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172 J.Z.Drexleretal./AppliedGeochemistry40(2014)164–179
includesUadsorbedfromthewaterinwhichitwastransported. component drops to less than about 70%, 234U/238U AR in mixed
Thesorptioncapacityoffineclays,ferrihydroxidesandamorphous water becomes dominated by U from the San Joaquin River,
silica,canvaryfromminimaltosubstantialdependingonmineral- obscuringfurtherquantification.
ogy and grain size as well as physical conditions and chemical Because Sr and U present in peat samples are dominated by
compositions of the fluids (Borovec, 1981; Ames et al., 1983; hydrogenic rather than lithogeniccomponents,isotopic composi-
Anderssonet al., 2001;Chabaux etal., 2003, 2008). U adsorption tionsofwholepeatdigestionsshouldreflectmixturesofthethree
onto surfaces of fine, inorganic particles during transport further mainwatersourceswithintheDelta.Nearlyallpeatsamplesrep-
decouples the connection between lithogenic and hydrogenic resenting different time periods in the Delta fall within or very
sourcesforU.Consequently,valuescalculatedforU likelyinclude closetothedistorted3-membermixingtriangle(Fig.7).Samples
L
asubstantialcomponentofU whose234U/238UARismorelikelyto that fall outside the limits of the mixing model consist of one of
N
reflect the composition of the hydrogenic source rather than the 37 peat samples from BRI (from the (cid:2)1850 to 1963 period), all
original lithogenic source from which Ti is derived. This scenario the clay samples from BRI (which are dominated by lithogenic
is supported by the lack of a correlation between 234U/238U AR ratherthanhydrogeniccomponents),and4of23FWpeatsamples
andTiinpeatsamplesthatwouldberequiredifthelithogenicfrac- (Fig.7bandc).PeatsamplesfromBACHIshowthetightestcluster
tionmadeasignificantcontributiontoU (Fig.6a).Likewise,d87Sr ofd87Srand234U/238UARvalues,probablybecausethissiteisgeo-
T
valuesinpeatdonotshowanoverallcorrelationwithTi,although graphically the least likely to be influenced by hydrologic inputs
samples do show an intriguing V-shaped pattern that reflects from either seawater or Sacramento River water. Peat samples
changesinbothTiandd87Srwithtime(Fig.6b).Variationsinthese fromFWshowsubstantiallygreaterscatter,consistentwithitspo-
constituentsshift more or less systematicallythroughouttheen- sitionclosertobothseawaterandSacramentoRiversources.d87Sr
tire 6000+year period of Delta evolution (see profile in Fig. 3c). compositions indicate that seawater was a minor constituent
Thus,theycannotreflectsimplemixingtrendsbetweenlithogenic remaining<(cid:2)5% throughout the last 6000years. Variations in U
andnon-lithogeniccomponentsatanygiventimeandmorelikely isotopes in FW peat indicate that freshwater sources fluctuated
reflectevolvingdepositionalandenvironmentalconditions. greatly, alternating between domination by the San Joaquin vs.
the Sacramento River (Fig. 7c). Peat samples from BRI show the
4.4.PaleosalinitymixingmodelbasedonSr–Udata greatestamountofvariabilitybetweenfreshwatersourcesbecause
of its position at the confluence of both rivers and the greatest
TheresultsofsimplemixinginSr–Uconcentrationspacecanbe influence from seawater due to its location at the western
represented by an ordinary ternary plot (Supplemental Fig. 1a), (seaward)boundaryoftheDelta(Fig.7b).
however in d87Sr–234U/238U AR space, the ternary plot becomes Over time, there is considerable variability in d87Sr and
warpedandstretchedduetothelargedifferencesinSrandUcon- 234U/238U AR in BRI peat samples (Fig. 7b), which can be related
centrations between end members and the non-linear nature of tochangingpatternsofpaleosalinityintheDelta.Theoldestpeat
mixing relations (Supplemental Fig. 1b). Mixtures of the three samples(4350–1550BCEshownbyblacksymbols)havethelow-
end members are greatly influenced by addition of seawater be- estd87Srvaluesandlikelyrepresentconditionswiththeleastsea-
causeofitsrelativelyhighconcentrationofSr.Additionofaslittle water (a mean of approximately 1%, which is equivalent to
as10%seawatercausesd87Srtoincreasefromlowvaluesinfresh- (cid:2)0.35psu).Thesesamplesshowawiderangeof234U/238UARval-
watersources((cid:3)2.62‰to(cid:3)3.84‰forSanJoaquinandSacramento ues and may represent frequent shifts between the San Joaquin
Riverwater,respectively,SupplementalFig.1a)tovaluesashighas andSacramentoRiversprovidingthedominantsourceofdissolved
(cid:3)0.35‰.Furthermore,thelargedifferenceinUconcentrationbe- ionstothesite.Duringtheperiodfrom1550to350BCE(darkblue
tweenthetworivers(0.13and5.0lgl(cid:3)1forSacramentoandSan symbols),samplesclusteraroundd87Srof(cid:3)2.0,whichisveryclose
JoaquinRiverwater,respectively,SupplementalFig.1b)resultsin tothemeansalinityoftheoldest(black)groupofsamples.These
substantial variation in 234U/238U AR values even if water in the samples also have 234U/238U AR values between 1.30 to 1.35 and
Delta remains dominated by freshwater sources. Mixtures of up plot close to the mixing line between seawater and Sacramento
to 20% San Joaquin River and 80% Sacramento River waters can River water, indicating that the Sacramento River dominated the
be readily discriminated; however, once the Sacramento River freshwater component during this time. Peat deposited during
(a) (b)
Fig.6. RelationshipsthroughtimebetweenTiconcentrationsand234U/238Uactivityratio(a)andd87Sr(b)inpeatsamplesfromBRI.