Are Minor and Trace Elements Useful Indicators of

62ndEASTERNSNOWCONFERENCEWaterloo,ON,Canada200549AreMinorandTraceElementsUsefulIndicatorsofChemicalWeatheringProcessesandFlow-RoutinginSubglacialHydrologicalSystems?ANDREWC.MITCHELL,1GILESH.BROWN,2ANDRONFUGE3ABSTRACT:InAlpineglacierisedcatchments,snow-andicemeltarechemicallyenrichedandmodifiedduringtransitthoughsubglacialhydrologicalsystemsbydistinctbio-geochemicalweatheringreactions.Thenatureandextentofthesereactionsaredictatedbywaterssourceandmeltwaterrouting.Thischemicalmodificationofmeltwatershasbeenappliedinhydrochemicalandhydroglacialstudies,basedmainlyonthedeterminationofdissolvedmajorions.Inthisstudywepresentseasonalvariationsintheconcentrationofminorandtraceelementsinbulk-meltwatersandin-situmeltwatersfromHautGlacierd’Arolla,Switzerland,andinvestigatetheirutilityforsolute-provenanceandflow-routingstudies.Onseasonaltimescalesthedissolvedconcentrationofmanyminorandtraceelements(e.g.Sr,Rb,Al,Co)arecontrolledbyhydrologicalparameterssuchaswaterresidencetimeandwater-rockratios.However,Li,Be,Mn,Fe,AsandCdappeartobeaffectedbyphysico-chemicalcontrolssuchassorption,(oxi)hydroxideformationandco-precipitationreactions,ratherthanwaterresidencetimealone.Nevertheless,seasonalchangesofdissolvedminorandtraceelementconcentrationratiosrevealimportantinformationaboutsolutesources,thenatureofweatheringprocesses,andtemporalchangesinweatheringreactionswhichareassociatedwithhydrologicalconditions.Keywords:minorandtraceelements;chemicalweathering;meltwaterchemistry;soluteprovenance;subglacialhydrologyINTRODUCTIONChemicalcharacteristicsofglacialmeltwatershavebeenwidelyusedtoinvestigatetheconfigurationanddynamicsofsubglacialhydrologicalsystems(Brown,2002),basedupontheassumptionthatthecompositionofbulkoutflowreflectsdistinctbiogeochemicalweatheringreactionswhichareindicativeofthewatersourceandthehydrologicalsystemthroughwhichthemeltwaterflowed(Collins,1979;Raiswell,1984;Tranteretal.,1993;Brownetal.,1994a;Brownetal.,1996a;Tranteretal.,2002).Thecompositionoftheinputwaterstotheglaciershydrologicalsystemprovidestheinitialboundarycondition.Thereafter,catchmentgeologyandtheconfigurationandseasonalevolutionofthesubglacialdrainagesystemdictatesthemagnitude1DepartmentofGeology,UniversityofToronto,22RussellStreet,Toronto.M5S3B1.Canada.2SchoolofGeographicalSciences,UniversityofBristol,UniversityRoad,Bristol.BS81SS.UK3InstituteofGeographyandEarthSciences,UniversityofWales,Aberystwyth.SY233QW.UK50andprocessesofsoluteacquisition,bycontrollingparameterswhichaffectchemicalweatheringprocesses(e.g.waterresidencetime;sedimentavailabilityandreactivity;water-rockratio[Sharp,1991;Brown,2002]).Theintimatelinkbetweenglacierhydrologyandthechemicalevolutionofmeltwaters,andthephysicalinaccessibilityofsubglacialenvironmentshasdriventhestudyandapplicationofmeltwaterqualityvariationsasaremote-sensingtoolforgeochemicalandhydrologicalprocessesoperatinginglacialenvironments(Sharp,1991;Brown,2002).Thischemical‘fingerprinting’(Sharp,1991)ofdrainageenvironmentshaslargelyconsidereddissolvedmajorions(e.g.Ca2+,Mg2+,Na+,K+andSO42–)(e.g.Raiswell,1984;Tranteretal.,1993;Brownetal.,1994a;Brownetal.,1996a).However,minorandtraceelements(e.g.Sr,Rb,U,Mn,Zn,Pb,Cr,Co,Cu,Al,Ti,Fe)havereceivedlittleattention(BrownandFuge,1998a,b;Mitchelletal.,2001),despitetheirpotentialinsoluteprovenanceandhydrologicalflow-pathinvestigations.Untilrecently,agenericchemicalweatheringmodelwhereacid-hydrolysisreactionscoupledspecificprotonsources(CO2dissolutionorsulphideoxidation)withtheweatheringspecificmineralgroups(carbonatesorsilicates)wasappliedtomeltwatersintransitthroughsubglacialenvironments(Tranteretal.,1993;Sharpetal.,1995a).However,areappraisalofhydrochemicaldatafromboreholesdrilledtothebaseofHautGlacierd’ArollasuggestedweatheringbythedissolutionofCO2islimited,andthatthereisaprogressionofreactionsfrom(i)carbonateandsilicatehydrolysisto(ii)sulphideoxidation,firstbydissolvedoxygenandthenbythemicrobialcatalyticreductionofFe3+,whichdrivesfurthercarbonateandsilicateweathering(Tranteretal.,2002).Lithogeniccalciumisthedominantcationinmostglacialmeltwaters,whichisassumedtoderiveentirelyfromthedissolutionoftracecarbonatesinchemically-limitedweatheringregimes(DreverandHurcomb,1986;Sharpetal.,1995b;Whiteetal.,2005).Lithogenicsulphateisthoughttoderiveentirelyfromsulphideoxidation,andNa+andK+fromsilicateweathering(Tranteretal.,1993;Sharpetal.,1995a;Brownetal.,1996a;Hodson,etal.,2000;Tranteretal.,2002).Therapidityofcarbonatehydrolysissuggestscarbonateweatheringdominateswherewaterresidencetimesarelimited,andthereforeoccurspredominatelyinachannellisedsubglacialdrainagesystem.Converselytheslowerweatheringkineticsofsulphidesandsilicatessuggestschemicalenrichmentandmodificationwillmorepreferentiallyoccurwherethereisalongintimatecontactbetweensubglacialwatersandsediments,predominatelyinthedistributedsubglacialdrainagesystem(Brownetal.,1996a)(Figure1).Thisstudyaimstoinvestigatetheutilityofminorandtraceelementsasindicatorsofgeochemicalprocessesinfluencingmeltwaterchemistrybyutilizingseasonalvariationsineleme