布朗大學同位素地球化學Hydrogen and oxygenisotopesin precipitation Oxygen and hydrogen isotopes 2_第1頁
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Hydrogenandoxygenisotopes

inprecipitationTopicsinthisclasssectionFactorsaffectingtheisotopicratiosinprecipitationGlobalscale(Temperature,Latitude,Continent)Regionalscale(Altitude,Season,amount)Isotopiccompositionofoceanwaters,anditseffectonisotopicvalueofprecipitationObjectivesUnderstandinghowandwhyhydrogenandoxygenisotopicratioschangeunderdifferentenvironmentalconditionsandacrossdifferenttimescales(seasonal,annual,longertime)CausesforthevariabilityofoceanwaterisotopicratiosFactorscontrollingisotoperatios

inprecipitationTemperature(differentgloballyandregionally)LatitudinaleffectContinentaleffectAltitudeeffect(localeffect)Seasonaleffect(localeffect)Temperatureandδ18OandδDinprecipitation

Globally,thereisalinearrelationshipbetweensurfaceairtemperatureandd18OordDformeanannualprecipitation(Dansgaard,1964)δ18Om=0.695*Tannual-13.6δD=5.6*Tannual-100Inreality,itisin-cloudtemperaturethatdirectlyaffectsisotopicratiosofprecipitation,butbecauseofthemeasurementdifficulties,surfaceairtemperatureisusedVariabilityoftemperature–d18OrelationshipGNIPstationsVariabilityoftemperature–d18OrelationshipT–d18OrelationshipforworldwideprecipitationcomprisesdifferentcurvesorlinesforspecificgeographicregionsMarinestationscorrelatepoorlywithglobaldataduetothedampingofseasonalvariationsintemperatureandprecipitation,andclosenesstotheocean(moisturesource)Higherlatitude,continentalstationshavegreaterslopethanmarinestationsStableisotopedistributioninmeteoricwaterStableisotopedistributioninmeteoricwaterDeuteriumexcessmapNorthAmericaNorthAmericad18ONorthAmericadDLatitudeeffectBasedonδ-Trelationship,onecanexpectthatprecipitationathigherlatitudestendstohavelowerδ18OvaluesPolarregionsaresituatedattheendoftheRayleighrainoutprocess.So,theδ18Ogradientshouldbecomeincreasinglysteep-0.6‰forδ18OperdegreeoflatitudeforcontinentalstationsofN.America,andEurope-2‰forδ18OperdegreeoflatitudeforcolderAntarcticStationLatitudinalgradientLatitudinalgradientisrelativeflatinthetropics,particularlyovertheoceansLatitudinalgradientincreasesploeward,distortionsofthelatitudinalgradientisduetocontinentaleffectsAmazonBasin,nocontinentallatitudinalgradient.LittlePacificrainentersthebasinduetotheAndes.RecyclingofwatervaporintheAmazonBasinistheculpritContinentaleffectLandmasseshavetheeffectofforcingrainoutfromvapormassesRainremovesheavierisotopes,Dor18Opreferentially,leavingalighterisotopicresidualvaporCostalprecipitationsareisotopicallyenriched,whilecolderinnercontinentalregionsreceiveisotopicallydepletedprecipitation,withstrongseasonalvariations.Continetality:Conrad’sindexkKisanempiricalrelationshiprelatingtheaverageannualrangeintemperature(DTinoC)andlatitudeangleφNorthAmerica:kandd18Od18OchangealongvaportrajectoryCanadianCordilleraDeparturefromglobalT-d18OContinentalitycanimpartsignificantdepartureinglobalT-d18OrelationshipGlobal d18O=0.7T–13.6WestCoast(Victoria) d18O=0.18T–11.2WesternInteriorCanada d18O=0.49T–17.3EasternCanada d18O=0.43T–13.6LocaleffectsonT-d18OStudiesofgroundwaterrechargerelyonlocalratherthancontinentalscalevariationsintheisotopiccompositionsofprecipitationLocaltopographyProximitytosurfacewaterbodiesSeasonalchangesAltitudeeffectTdecreaseswithincreasingaltitude,ascloudrisesuptoamountainside,theheavierisotopes,Dor18Oarelostfirst,residualvaporairmassbecomesprogresslighter0.10~0.50‰δ18O/100mofelevationchange1.2~5.0‰δD/100mofelevationchangeItalianAlpsVariabilityingradientSeasonaleffectsTheamplitudeofseasonalvariationsintemperatureincreaseswiththecontinentalityofthesiteGreaterseasonalextremesintemperaturegeneratestrongseasonalvariationinisotopesofprecipitationUsefultodetermineratesofgroundwatercirculation,watershedresponsetoprecipitation,andthetimeduringtheyearwhenmostrechargeoccursSeasonaleffectLatitudinalseasonaleffectAmounteffectTheheaviertherainfallevent,thelowertheδDandδ18Ovaluesofwater,becauseTheheavierrainfallisassociatedlargeTdecreaseincloudsInlighterrain,thereismoreevaporationfromraindrops,hencehigherδD,δ18Oofrain.Forexample,Seasonalvariationofδ18OfortropicalmarinestationscorrelatespoorlywithT,butcorrelateswithamountofrain(monsoonseasonhaslowerδ18O,δDvalues).AmounteffectCondensationofcoastalfogCondensationoffogonthelandscape(occultprecipitation)canbeanimportantsourceofrechargeinsomearidregionsIncoastalregions,vaporfromtheoceancancondensedirectlyontovegetationasitrisesoverthecoastalmarginLittlemixingintheuppertroposphere,minimalisotopicevolution(fractionationduringtheevaporationandcondensationcancelseachother)Condensationofcoastalfogδ18O,δDvaluesofmostpartofseawaterareclosetozero,andvaryonlywithinnarrowlimits(δ18O=0±0.5‰,δD=0±5‰)δD=M*δ18Oforsurfaceseawater,co-efficientvarieswithsalinity:Salinity↑M↓

SalinityM=7.5,No

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