Phosphorus(P)fractionationandsorptionbehaviorwerestudiedinthesedimentsoftwocalcium-sulfate(Ca-SO4:Ca2t170mgL-1;SO42-400mgL-1)waterreservoirsinthesoutherncentralArgentineAndes-ElCarrizalReservoir(ECR),whichstratifiesinsummerandElNihuilReservoir(ENR),whichisverticallymixedthroughouttheyear.Sedimentsizeclassesreflectedthelithologyofbothbasinsandthereservoirsrelativelocation:ECR(downstreamfromavalleyadjacenttoamountainenvironment)hadhigherclayproportionswhilethesandfractionwasmoreimportantinENR(justnexttoamountainenvironment).Inbothreservoirs,thechemicalcompositionrevealedlowalterationandcalciumenrichment.TotalPcontentwasrelativelyhigh(1.1-1.6mgPkg-1dryweight)asexpectedfromthecontributionofmarineformations.PfractionationwasdominatedbyCa-P(ECR,69%andENR,63%)followedbyorganic-P(23%)insummer-anoxicECRandbyiron/aluminum(Fe/Al)-P(27%)inoxicENR.BatchexperimentsshowedthatsorptionbehaviorinECRhadatypicalLangmuirisothermplotwhileinENRthatmodelfittedonlyatlowdissolvedPvalues(o5mgPL-1)andco-precipitationevidencewasobservedathigherPconcentrations.ItisproposedthatlowerFe/Al-Pcontentinanoxic(ECR)vs.oxic(ENR)sedimentsresultsfromthePreleasedfromiron-boundphosphorusduetosulfatereduction-ferroussulfideformationinananoxicenvironment.ThisconditionshouldenableECRsedimentstoadsorbPintofreshoxidizediron,andENRsedimentstoco-precipitatePwithcalciumduetoFe/Al-Psaturation,asobservedinbatchexperiments.ThispaperconstitutesthefirstdescriptionofphosphoruscontentanddynamicsinreservoirsedimentsinaridcentralwesternArgentina,wherewatersulfateconcentrationsarenaturallyhighduetobasinlithology.
