Organic Control of Dioctahedral and Trioctahedral Clay Formation in an Alkaline Soil System in the

23 Pages·2016·5.2 MB·English

RESEARCHARTICLE
Organic Control of Dioctahedral and
Trioctahedral Clay Formation in an Alkaline
Soil System in the Pantanal Wetland of
Nhecolândia, Brazil
LaurentBarbiero1☯¤a¤b*,GillesBerger2☯,AryT.RezendeFilho3☯,Jean-
FrançoisMeunier4‡,ElisângelaR.Martins-Silva5‡,SoniaFurian6‡
a11111 1 GéosciencesEnvironnementToulouse(GET),ObservatoiredeMidi-Pyrénées,UniversitédeToulouse,
CNRS,IRD,14AvenueEdouardBelin,F-31400Toulouse,France,2 InstitutdeRechercheen
AstrophysiqueetPlanétologie(IRAP),ObservatoiredeMidi-Pyrénées,UniversitédeToulouse,CNRS,IRD,
14AvenueEdouardBelin,F-31400Toulouse,France,3 FAENG—FaculdadedeEngenharias,Arquiteturae
UrbanismoeGeografia,UniversidadeFederaldoMato-GrossodoSul,CidadeUniversitárias/n,Caixa
Postal549,79070–900CampoGrande-MS,Brazil,4 LaboratoiredeChimiedeCoordination(LCC),
UniversitédeToulouse,CNRS,205routedeNarbonneF-31077,Toulouse,France,5 Departamentode
CiênciasAgráriaseAmbientais,UniversidadeEstadualdeSantaCruz,Salobrinho45662900,Ilhéus,Brazil,
6 DepartamentodeGeografia(DG)UniversidadedeSãoPaulo,AvenidaPr.LineuPrestes338,Cidade
OPENACCESS Universitária05508–900,SãoPaulo,Brazil
Citation:BarbieroL,BergerG,RezendeFilhoAT,
☯Theseauthorscontributedequallytothiswork.
MeunierJ-F,Martins-SilvaER,FurianS(2016)
¤a Currentaddress:CentrodeEnergiaNuclearnaAgricultura(CENA),UniversidadedeSãoPaulo,Avenida
OrganicControlofDioctahedralandTrioctahedral Centenário303,13400–970,Piracicaba,SãoPaulo,Brazil
ClayFormationinanAlkalineSoilSysteminthe ¤b Currentaddress:InstitutodeQuímica(IQ),UniversidadeEstadualdeCampinas,P.O.Box6154,13083–
PantanalWetlandofNhecolândia,Brazil.PLoSONE 970Campinas,SP,Brazil
11(7):e0159972.doi:10.1371/journal.pone.0159972 ‡Theseauthorsalsocontributedequallytothiswork.
*[email protected]
Editor:GangYang,SouthwestUniversity,CHINA
Received:March16,2016
Abstract
Accepted:July10,2016
Published:July27,2016
Recentstudieshavefocusedontheformationofauthigenicclaysinanalkalinesoilsystem
Copyright:©2016Barbieroetal.Thisisanopen
surroundinglakesoftheNhecolândiaregion,Pantanalwetland.Thepresenceoftrioctahe-
accessarticledistributedunderthetermsofthe
dralMg-smectites(stevensiteandsaponitetypes),whichrequireslowAlandFecontentsin
CreativeCommonsAttributionLicense,whichpermits
unrestricteduse,distribution,andreproductioninany thesoilsolutionforitsformation,contrastswiththeneoformationofdioctahedralFe-mica
medium,providedtheoriginalauthorandsourceare (glauconite,andFe-illite),whichinsteadrequiressolutionsrelativelyenrichedinAlandFe.
credited.
Thisstudyaimstounderstandtheconditionsofco-existenceofboth,Mg-smectiteandFe-
DataAvailabilityStatement:Allrelevantdataare micaacommonclayassociationinformerormodernalkalinesoilsystemsandsediments.
withinthepaperanditsSupportingInformationfiles.
Thestudywascarriedoutalonganalkalinesoilcatenarepresentativeoftheregion.The
Funding:ThisresearchwasfundedbytheSão soilorganizationrevealedthatMg-smectiteoccurintopsoilclosetothelake,whereasFe-
PauloStateResearchFoundation(FAPESP2011/
micadominateintheclayfractionofdeepergreenishhorizonsafewmetersapart.Wepro-
12770-0,2013/09192-0),theNationalCouncilfor
poseherethatthisspatialdistributioniscontrolledbythelateraltransferofFeandAlwith
ScientificandTechnologicalDevelopment(CNPq
405898/2012-6and443030/2015-4)andtheNational organicligands.Alkalineorganicrichsolutions(DOCupto738mgL-1)collectedinthe
InstituteofSciencesoftheUniverse(INSU-Ec2co).L. watertablewerecentrifugedandfilteredthroughmembranesofdecreasingporesize
BarbierowassupportedbygrantsfromtheFederal
(0.45μm,0.2μm,30KDa,10KDa,3KDa)toseparatecolloidalanddissolvedfractions.Fe,
AgencyfortheSupportandEvaluationofGraduate
Education(CAPES),theConsulateofFranceinSão Al,Si,MgandKwereanalysedforeachfraction.Althoughthefiltrationhadnoinfluenceon
PauloandtheSãoPauloandCampinasState SiandKcontents,almost90%ofFe(upto2.3mgL-1)andAl(upto7mgL-1)areretained
Universities.AuthorsareverygratefultoF.Fracassi,
PLOSONE|DOI:10.1371/journal.pone.0159972 July27,2016 1/23
ClayNeoformationinAlkalineWetland
G.Baldi,I.Lerco,S.Gouy,T.Aigouy,M.Thibautand atthefirstcutoffthresholdof0.45μm.Thetreatmentofthesamesolutionsbyoxygenperox-
F.Baronforperforminglaboratoryanalysis.Wethank idebeforefiltrationshowsthatalargeproportionofFeandAlwerebondedtoorganiccol-
J.Cuadros,B.Veldeandanonymousreviewersfor
loidsinalkalinesoilsolutionattheimmediatelakeborder,allowingMg-smectite
theirconstructivecomments.JimHessonof
AcademicEnglishSolutions.comrevisedtheEnglish. precipitation.Thefastmineralizationoftheorganicmatterafewmetersapartfromthelake
Thefundershadnoroleinstudydesign,data favorsthereleaseofFeandAlnecessaryforFe-micaneoformation.Incomparisonwith
collectionandanalysis,decisiontopublish,or
chemicalandmineralogicalcharacteristicsofalkalineenvironmentsdescribedinthelitera-
preparationofthemanuscript.
ture,thestudysuggeststhattheco-existenceoftrioctahedralMg-smectiteanddioctahedral
CompetingInterests:Theauthorshavedeclared
Fe-micashouldberegardedasastandardoccurrenceinalkalinesoilsystemswithorganic
thatnocompetinginterestsexist.
richwaters.
Introduction
Thehighchemicalandstructuralvariabilityofclaymineralsisanobstacletounderstanding
theirmechanismofformation,transformationandalteration.Thisalsoappliestothedetermi-
nationofthethermodynamicparametersthatarerequiredfordigitalmodelsbasedonthermo-
kineticlaws.Inaddition,itisdifficulttoestablishclearrelationsbetweenstructureandchemis-
tryforcrystalpopulationsofheterogeneousstructurehavingagrainsizeofafewhundred
nanometers.Thereforeinsoilsystems,thepresenceandspatialdistributionofclayminerals
aregenerallyinterpretedasafunctionoftheirthermodynamicstabilityinaqueoussolutions
accompanyingwaterpercolationthroughsoilprofilesandlandscape.Thisthermodynamic
visionissometimestemperedbyconsideringtherecyclingofcertainchemicalelementsbyfor-
estorcropsatthetopofthesoilprofiles[1],[2],[3],[4],[5].Manyreactionsatlowtempera-
ture,resultinginmineralogicalchanges,arenotcontrolledbythermodynamicsbutkinetics.
Claytransformationsuchas,forexample,smectitetoilliteconversion,requiresnotonlytimes
butalsoamineralprecursor.ReaderscanrefertotheworkofLansonetal.[6]forareviewin
sedimentarybasin,HuggetandCuadros[7]forpaleosoils,andAndradeetal.[8]formangrove
soils.Inaddition,verticalandlateralparticletranslocationwithinthesoilmayaltertheirdistri-
butionalongsoilsystems,andclaymineralsmayendupatdepthsandplacesdifferentfrom
thosewheretheyformed.Finally,livingorganismsmaymodifydeeplytheconditionsfrom
thosegeneratedbytheinorganicenvironmentalfactors.
Thisviewofclaystabilityinsoilsalsoappliestoevaporativeenvironmentswherethesolu-
tionconcentrationbyevaporationpredominatesandcontrolsthesoilformingprocesses.Evap-
orativeenvironmentscanbeclassifiedintotwomaintypesaccordingtothesignofthecalcite
residualalkalinity(RA ),definedasthedifferencebetweenalkalinityandtheconcentration
calcite
ofcalciuminsolution[9].Thesetwotypesareneutralenvironments(RA <0)andalkaline
calcite
environments(RA >0).Innaturallyalkalineenvironments,thepHiscontrolledbythe
calcite
carbonatesystemandincreasesthroughevaporativeconcentrationtoreachvaluesupto10or
11[10].Inthischemicalcontext,neoformation(withoutamineralprecursor)ofnon-orlow-
aluminoustrioctahedralsmectitesfromthesoilsolutionhasbeenreportedbymanyauthors
[11],[12],[13],[14],[15],[16],[17];[18],[19]).Mineralssuchasstevensite,sepiolite,andsap-
onitearefrequentlyidentified.TheyrequirehighpH,AlandFedepleted-andSirich-solution
fortheirformationthatusuallycontroldissolvedMgconcentrations[20].Ontheotherhand,
onlyafewstudieshavereportedauthigenicmicasinalkalineenvironments.Ithasbeenfirst
mentionedinthe1960s[21],[22],[23]asatransformationofsmectiticprecursors,usuallyby
wettinganddrying[24],[18],[7],whileotherstudiesrathersuggestaneoformationprocess
fromthesoilsolutionatleastforillite[25],[26],[27],[28]andglauconiteformation[29].
Whatevertheprocess(neoformationortransformation),theformationofAl-andFe-bearing
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ClayNeoformationinAlkalineWetland
dioctahedralclaymineralsrequiresahighavailabilityofSiandK,aswellasFeandAl.Asthese
latterelementsaregenerallyobservedatlowconcentrationsinnaturalwatersandsoilsolu-
tions,theiravailabilityappearsasaseriouschemicalconstraint.
InthesoilsystemaroundalkalinelakesinthePantanalwetlands,theoccurrenceofboth
trioctahedralMg-smectitesanddioctahedralFe-micashavebeenmentionedbyFurquimetal.
[19],[30],[31].Althoughtheyrequiredifferentchemicalcompositionofthesoilsolutionfor
theirformation,bothMg-smectitesandFe-micashavebeenidentifiedasauthigenicclaymin-
eralsbydirectprecipitationfromthesoilsolution.Inthiswork,wefocusonthedistributionof
theseclaymineralsinthesoilsysteminrelationwiththeseasonalhydrologicalfunctioningof
thelakesandwiththechemistryofcirculatingwaters,inordertounderstandtheconditions
thatcouldexplaintheco-existenceofsuchdi-andtrioctahedralclayminerals.Inparticular,we
exploretherelationshipbetweentheamountsoforganicmatter,apeculiarityofalkalineenvi-
ronments,andtheavailabilityofthelesssolubleilliteconstituents,AlandFe,asakeyforthe
neoformationofilliticmaterial.
PantanalofNhecolândia:thesitebackground
IntheBrazilianPantanalwetland,Nhecolândiaisaspecificsub-regionwithabout15,000
lakes,includingabout500saline-alkalineones[32].Itliesonthesouthernhalfofthevast
Taquarialluvialfan,oneofthelargestalluvialfansintheworld[33],[34].Withatotalsurface
areaofabout24,000km2,theregionisdelimitedinthenorthbytheTaquariRiver,inthe
southbytheNegroRiver,inthewestbyaportionoftheParaguayRiver,andintheeastbythe
MaracajúPlateau,whichcorrespondstothesoutheasternedgeofthePantanalwetland(Fig1).
Nhecolândiashowsrelativelycloseddrainagewithlittleconnectiontomajorfluvialsystems.
Semihumidclimatepatterns,classifiedastropicalhumidwithshortdryseason(“Aw”typein
theKöppenclassification),arecontrolledbytheseasonalmigrationoftheIntertropicalCon-
vergenceZone(ITCZ).Themeanannualairtemperatureis~25°Crangingfrom21°Cduring
drywintersto32°Cduringrainysummers[35].Themeanannualprecipitation(P)is~1100
mm,andtheannualevapotranspiration(ETP)isapproximately1400mm,providingahydro-
logicaldeficitof~300mm[36].Atthelocallevel,thestrongthermalcontrastbetweensaline
lakesandsurroundingforestedareascausesday–nightalternatingwindsthatenhanceevapora-
tion[37].QuaternarysedimentscoverNhecolândia,arisingfromsandstoneformationsthat
makeuptheMaracajúPlateau.
TheElectricalConductivity(EC)ofsurfacewatersrangesfrom0.02mScm-1to80mScm-1.
Furianetal.[32]havestudiedthewaterchemistryof147lakesincentralandsoutheastern
Nhecolândia.Thestudyrevealedthatthechemistryresultsfromevaporativeconcentrationof
freshwatersthatoriginatefromthesandstoneformationoftheuplandborderofthePantanal
[38],[39].Theevaporationofsuchwater,withpositivecalciteresidualalkalinity,resultsin
alkalinesolutionswithhighpHvalues,closetoorabove9.5.Thetrendsofthedifferentdis-
solvedionsalongtheevaporationprocessaresummarizedinFig2.Theprecipitationofcalcite
causesadecreaseofcalciumandconsequentlythesodicationofthesoilcation-exchangecom-
plex.Theincorporationof3to5%ofMginthecalcite[40]togetherwiththeneoformationof
Mg-smectiteclosetothelakeshoreprovokeadecreaseofMg.Finally,althoughthelevelsofK
increaseduringevaporation,thereisalossinK,whichcanlogicallybeattributedtoitsincor-
porationinthelayersduringtheformationofFe-mica[30].Furianetal.[32]haveshownthat
therangeinthesalinitylevelofsurfacewatersiscontrolledbythehydrologicalfunctioningof
eachlake.Thesalinelakesarepartiallyisolatedfromtheregionalfreshgroundwaterbysoil
horizonsthatlimitthefluxoffreshwaterintothesalinelakes,therebypreventingtheirdilu-
tion.AtthemaximumofthefloodinthePantanal,thefreshwaterlevelexceedstheupperlevel
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ClayNeoformationinAlkalineWetland
Fig1.LocationofthestudiedtransectattheborderofSalinaVerdeinthePantanalwetlandofNhecolândia.
doi:10.1371/journal.pone.0159972.g001
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ClayNeoformationinAlkalineWetland
Fig2.ConcentrationdiagramsbasedonNacontentsshowingtheregionaltrends(solidline)compiledfromFurianetal.[32]andthechemistry
ofwaterscollectedinaugerholesalongthetransectofSalinaVerde.NotethesigmoidalshapeofdissolvedKalongtheconcentrationprocess.
doi:10.1371/journal.pone.0159972.g002
ofthematerialwithlowpermeability,andonlyafractionflowsdowntowardthedepressions
ofthesalinelakes.MostofthesalinelakesinNhecolândiashowseasonalorpermanentcyano-
bacterialblooms,dependingontheintensityofrainfallduringthewetseason[41],[42],[43].
Thesoilssystemaroundthesaline-alkalinelakewasstudiedbyvariousauthorsinthewest-
ern[44],northern[30]andsoutheastern[45],[32]partofNhecolândia.Allthestudiesrevealed
asimilarsoilorganizationthathasbeensimplifiedonFig3,withmainly4contrastinghori-
zons.Closetothelake,agrey-browntopsoilloamysandhorizon(1)isobservedusuallywith
numerouscalcareousprecipitations.Theoccurrenceofthishorizonislimitedtotheoscillation
zoneofthelakeshorebetweenthewetanddryseasons.Itsclaycontentisabout10%.Below,
thereisalightbrownsandymaterial(2)colouredbyverticallyorientedorganicmatterstreaks.
Fig3.Standardsoillayoutandclayfractiondistributioncompiledfrompreviousstudiescarriedoutaroundsaline-alkalinelakesin
Nhecolândia.Seetextfordescriptionofhorizons1to4.
doi:10.1371/journal.pone.0159972.g003
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ClayNeoformationinAlkalineWetland
Theorganicmatterisalsoaccumulatedatthebase[32],[44]oratthetop[19]ofthishorizon
[46].Theclaycontentinthissandyhorizonislessthan1%.Subjacenttothishorizon,thereisa
massive(singlegrain),greyishloamysandmaterial(horizon3)withabout15%clay,andfur-
therbelowaloamysand,olivetolightolive-greycolouredhorizon(4),with15–20%clay,mas-
sivestructure(coherentandcemented)andlocallyextremelyfirmconsistency.Thetopofthis
horizon(4)iswavywithtwomainrisesusuallyseparatedby25to40m.
ThefineclayfractioninthisalkalinesoilsystemhasbeenstudiedindetailbyFurquimetal.
[19],[30][31]usingX-raydiffraction(XRD),transmissionelectronmicroscopy-energydisper-
siveX-rayanalysis(TEM/EDX),andinductivelycoupledplasma-massspectrometry(ICPMS).
Smectitesareconcentratedattwoplacesinthesoilsystem[19]:
(cid:129) Intheriseofhorizons3and4,thefurthestfromthelake,thesmectiteisdioctahedraland
wasclassifiedasferribeidellite.Accordingtotheseauthors,theinterstratificationofthisferri-
beidellitewithmicaandvermiculite,andtheFecontentsuggestthattheferribeidelliteorigi-
natesfromthetransformationofpreviouslyneoformedFe-illites(seebelow)andthat
vermiculiteisanintermediatephaseinthistransformation.
(cid:129) Inhorizon1,smectitesaredominantlyMg-richtrioctahedralsmectite.Theywereidentified
assaponiticandstevensiticmineralsresultingfromchemicalprecipitationinthesaline-alka-
linesoilsolution[30].
Thenon-expanding2:1phyllosilicatesaredioctahedralillite-micas,mainlyconcentratedin
horizons3and4,andinterpretedasneoformedclaybyFurquimetal.[31].Among8crystals
analysedbytheseauthors,glauconiteandFe-illitearethedominantmicasandhaveundergone
degradationintovermiculiteandferribeidellite.Thehigherpercentageofmicasinthefineclay
fractionreachedupto84%inhorizon4closetothelake.Si-richamorphousmaterialswere
associatedwithsmallcrystallitesinthemica-enrichedhorizon4.Low-frequencyelectromag-
neticinductionsurveyscombinedwithdirectobservations(augerholesandexcavatedpits)
revealedthatmica-enrichedhorizonsformacontinuousringaroundthesalinelakesattesting
totheirauthigeneticnature[40],[45],[32].
Fromthispedologicalframework,themajorissueistounderstandhowMg-smectiteand
Fe-micacanforminthesameenvironment.TheformationoftrioctahedralMg-smectitesin
thesoilhorizonsattheimmediateborderofthelakenecessarilyindicatesSi-andMg-richbut
Al-andFe-depletedsolutions.Thiscontrastswiththepresenceofdioctahedralmicasafew
metersaway,andforwhichthelocalsourceofFeandAlandtheirtransferinthelandscape
appearsasthemainchemicalconstraint.Noprecursorsthatcouldexplaintheabovechemical
variationswereidentified,andbothMg-smectiteandFe-micadominatedhorizonsareincon-
tactwiththesamewatertable.Therefore,inthepresentstudyweaddressthisparadoxicalclay
distributionbyfocusingonarepresentativesalinelake.Inafirststepweverifiedthroughmin-
eralogical,morphologicalandchemicalcharacteristicstheprevalenceofthisalkalinesoil
catenaintheNhecolândiaregion.Second,wefocussedontheseasonalfluxesofwaterand
chemicalelementswithinthesandofhorizon2,betweenMg-smectite-richhorizon1,andFe-
mica-richhorizons3and4.
MaterialandMethods
Thestudywasconcentratedaroundasalinelakecalled“SalinaVerde”(19°28’13”Sand56°
3’22”W)locatedonaprivatelandcalled“Centenario”farm(Fig1),andwasconductedwith
thepermissionoftheowner.Weacquiredthefielddataonlyatunrestrictedlocationswhereno
specialpermissionwasrequiredandavoidedthenationalparkandotherprotectedareas.The
fieldstudiesdidnotinvolveendangeredorprotectedspecies.
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ClayNeoformationinAlkalineWetland
Waterlevelandelectricalconductivitymonitoring
Thelakeissuppliedmainlybythelocalrainfall.Secondarily,duringthefloodpeak,itreceives
freshwaterbysubsuperficialflowfromdrainingfields,locallycalled“Vazante”[47].The
upperlevelofthesoilhorizonswithlowpermeabilitythatisolatethesalinelakewasassessed
bylowfrequencyelectromagneticinduction(EM31-MK2,Geonics,Ontario)accordingthe
relationshipestablishbyFurianetal.[32].Thewaterlevel(h)andelectricalconductivity(EC)
weremonitoredfromSeptember2010toOctober2011.Datawererecordedautomatically
every4husingSchlumbergerMini-andCTD-Diver1pressuretransducercombinedwitha
Baro-Diver1tocompensateforatmosphericpressure(bothhaving1.5mmaccuracy).The
Mini-Diverwasinstalledinapiezometerdrilledinthedrainingfieldsatabout300mfromthe
lake.CTD-DiverswereinstalledintheSalinaVerde,andinapiezometerdrilledonthebeach
betweenthetworisesofhorizon4.
Soilstudy
ThefirstobjectiveofthesoilstudywastoverifythatthesitechosenattheborderofSalina
VerdewassimilartothatcomprehensivelydescribedbyFurquimetal.[30].Thesoilwasstud-
iedalongatransectperpendiculartothelakeshorelocatedsoastomaximizethelateralchanges
inthesoil,whichisorganizedintoringssurroundingthelakes[32].Soilsamplesincontact
withthecollectedsolutionsweretakenduringthehigh-waterlevelperiod(June2011).The
sampleswerestoredinanairtightcontainerandcompleted(withoutair)withthecirculating
solutioninordertopreventoxidation.Theparticlesizewasmeasuredbythepipettemethod
[48].ThesoilsampleswereanalysedbyX-raydiffractiononorientedpowderpreparations
driedatnormaltemperatureandsaturatedwithethyleneglycol.Twofractionswereselected:
thebulkmaterialandthefinefractionisolatedbysonication/elutriation.Thespectrawere
obtainedwithangular(2θ)rangesfrom2.3°to34°andstepsof0.03°.Theanalyseswerecarried
outusingaG3000-INELdiffractometer(Cucathode)equippedwithadiodeSiLi.Quantitative
analyseswereperformedon6compactedfineclay(<0.2μm)patchesusingelectronicmicro-
probeSX50fittedwithwavelengthdispersivespectrometers(WDS).Fineclaysampleswere
alsoselectedforMössbauerspectrainordertocharacterizeFelocationintheclaynetwork.
Spectrawerecollectedonaconstant-accelerationconventionalspectrometerwitha0.30GBq
sourceof57Co(Rhmatrix)at80K,usingaMD306Oxfordcryostat.Thethermalscanningwas
monitoredbyanOxfordITC4servocontroldevice(0.1K).Theabsorberwasasampleofca.
100mgofpowderthatwasenclosedina20mmdiametercylindricalplasticsampleholder,the
sizeofwhichhasbeendeterminedtooptimizetheabsorption.Mössbauer’sparametersand
theirstandarddeviationsofstatisticaloriginwereobtainedbyleast-squaresfittingtoLorent-
zianlinesusingRecoilsoftware[49].Theisomershiftvaluesaregivenwithrespecttometallic
ironatroomtemperature.Inaddition,FTIRmeasurementswereperformedonorientedfine
claysamplesinthenearinfraredregionoverthewavenumberrangeof4000–8000cm−1,
usingaNicolet6700equippedwithaThermo-Scientificintegratingsphere.Whitesalineefflo-
rescentcrustswerecollectedduringthedryseasononthelakeshoreandthebulkmaterialwas
characterizedbyXRDandSEMusingaJeolJSM6360LVScanningElectronMicroscopefitted
withenergydispersivespectrometry(EDS).
Waterchemistry
Watersamplesweretakenfromaugerholesinthewatertableduringhighandlowwaterlevel
inJune2011andOctober2013,respectively.Theelectricalconductivity,pHandredoxpoten-
tialweremeasuredinthefield.Watersampleswerecentrifugedinthefieldimmediatelyafter
thecollectionfor30minat6000rpm(RelativeCentrifugeForce=3200g).Thesupernatant
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ClayNeoformationinAlkalineWetland
solutionwasstoredindarkand4°Cconditions,inHDPEbottlespreviouslywashedwithdilute
HCl.Thecentrifugedwatersampleswereanalysedbeforeandafterfiltrationatdifferent
threshold(0.45μm,30KDa,10KDaand3KDa)usingcelluloseacetatefiltersandAmicon1
ultrafiltrationunits,respectively.Membraneswerepreviouslyrinsedwithultrapurewaterin
ordertoavoidcontamination.Inasecondstep,thecentrifugedsolutionsweredilutedtwiceby
additionofhydrogenperoxide(H O 30%)inordertodecomposethepossibleorganiccolloids
2 2
andthenfilteredat0.45μm.Thecations(Na,K,Ca,Mg,Si,Al,Fe)wereanalysedbyhighreso-
lutionICP-MS(GETLaboratory,Toulouse,France)forsamplescollectedin2011andby
ICP-AES(SoilScienceandNUPEGELLaboratories,USP,Piracicaba,Brazil)forsamplescol-
lectedin2013.Anions(Cl,SO )weremeasuredbyHPLC,alkalinityby0.1NHCltitration,and
4
dissolvedorganiccarbon(DOC)concentrationsweredeterminedwithaShimadzuTOC-
5000A(CENA-USP,Piracicaba,Brazil).Inordertocomparethecollectedsoilsolutionswith
theregionalchemistrypresentedbyFurianetal.[32],theresultswereplottedintoconcentra-
tiondiagramsbasedontheirsodiumamount.
Additionaldata
AerialpictureswereselectedfromGoogleEarth™attheendofaseveredryseason(July11th.
2006andAugust15th.2010)inordertorevealthemorphologyofthebottomofthesealkaline
lakes.
Results
Soilmineralogyanddistribution
Foursoilhorizonshavebeenidentifiedandtheirdistributionalongthestudiedtransect(Fig4)
appearedverysimilartothatsummarizedinFig3fromotherstudiesintheregion.Inhorizon
1,XRDpatternsofbulkmateriel(notshown)displayamixturedominatedbyquartzand
smectite,secondarilymica,kaoliniteandoccurrenceofcalcite.Claycontentswereabout10%,
andorientedXRDspectraconfirmedapredominanceofsmectite,secondarilyofmica,I/S
interlayeredandkaoliniteinthefineclayfraction(Fig5).Themicroprobeanalysison3sam-
ples(claypatches)indicatedthatthisfractioniscomposedofSi(57.1to62.7%),andsecondary
ofMg(13.9to17.7%),Al(4.2to4.6%),Ca(4.1to4.4%),K(3.1to3.3%),andFe(2.9to3.7%).
Suchamineralogicalandchemicalcompositionissimilartothatreportedfromhorizon1by
Fig4.Crosssectionshowingthesoillayoutalongthetransect.AHareaugerholesforwatercollection,Electricalconductivity(EC)inmScm-1,
(cid:1)aresamplespresentedinFig5,Pzdenotesthepiezometerusedforwaterlevelmonitoringinthebeach.
doi:10.1371/journal.pone.0159972.g004
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ClayNeoformationinAlkalineWetland
Fig5.X-raydiffractionofglycoled-orientedclayalongthetransectshowingsmectite-dominant(1)andmica-dominanthorizon(4).
doi:10.1371/journal.pone.0159972.g005
Furquimetal.[19]usingthesamemethods.MgandAlarerelatedtothephyllosilicates,Kand
Careflectthepresenceofilliteandcalcite,respectively.Below,thesandymaterialofhorizon2
wascolouredbyverticallyorientedorganicmatterstreaks(Fig6A)andwithorganicmatter
accumulatedatthebaseofthehorizon.Theclaycontentwaslessthan0.5%andXRDonthe
bulkmaterialrevealedonlyquartz.Subjacenttothishorizon,greyishhorizon3andolivehori-
zon4wereobserved.Thetopofhorizon4formedtwomainrisesseparatedbyapproximately
25m.Thefirstrisewasreachedbythelakewaterlevelduringthedryseason,whilethesecond
onewasachievedduringthewetseasonsuggestingthattheyarisefromthepresentdayhydro-
logicalregimeofthelake.Theclaycontentswereabout10to14%,inhorizons3and4,respec-
tively,andthefineclayconsistedalmostexclusivelyofmicaandkaolinitefromthelaketothe
firstriseofgreenishhorizon(horizon4aonFig5),andofamixturedominatedbymica,sec-
ondarilysmectiteandkaoliniteinthesecondone(horizon4bonFig5),whereadiffusereflec-
tiononXRDspectrasuggeststheoccurrenceofinterlayeredmixedillite/smectite.Inhorizon4
closetothelake,themicroprobeanalysison3samplesindicatedSi(53.8to57.2%),Al(13.8to
17.4%),Fe(10to11.1%),K(4.7to5.6%)andMg(2.2to2.7%).Theclaypatchesappearhere
PLOSONE|DOI:10.1371/journal.pone.0159972 July27,2016 9/23
ClayNeoformationinAlkalineWetland
Fig6. (a)Streaksoforganicmatterinhorizon2;(b,candd)SEMpicturesofthewhiteefflorescentcrustconsistingofmatsofamorphousmaterial.
doi:10.1371/journal.pone.0159972.g006
moreenrichedinAl,FeandK,withmuchlessMg,achemicalcompositionconsistentwiththe
predominanceofmicawhencomparedtohorizon1.Again,thesechemicalandmineralogical
resultsaresimilartothosereportedinFurquimetal.[30].
Mossbauerspectroscopycarriedoutonfineclaymaterial(mainlymica)ofthegreenish
horizon4showedcomplementaryresults(Fig7).ThebestLorentziansiteanalysiswas
obtainedwiththreedoubletsofdifferentintensitywithdistinctisomershifts(δ)andquadru-
polesplitting(Δ)aspresentedinTable1.ThemostvisibledoubletscorrespondtoFe3+inthe
octahedralsite(Doublet1:δ~0.47mm/sandΔ=0.5mm/s)andFe2+intheoctahedralsite
(Doublet2:δ~1.3mm/sandΔ~2.8mm/s).Thevaluesofthesesparametersarecommonin
phyllosilicatesandcorrespondtoaFe2+:Fe3+ratioof1:4[50].Doublet3(δ~1.3mm/sandΔ~
2.8mm/s)hadintermediateparametersandismoredifficulttointerpret.Thisisduetothedif-
ficultytodistinguishdiamagneticFe(II)withlowspinfromFe(III).Accordingtosomeauthors
[51],[52]theintermediateparametersmaycorrespondtotetrahedralFe3+.Butotherstudies
PLOSONE|DOI:10.1371/journal.pone.0159972 July27,2016 10/23