布朗大学同位素地球化学applicationsinpaleoclimatestudies-1.ppt

Applicationofoxygenandhydrogenisotopesinpaleoclimatestudies:anicproxiesforpaleoclimatestudiesbasedonOandHisotoperatiosδ18Oofhardfossilsinmarinesediment,coral(annualbands)δ18OandδDoficecoresObjectivesUnderstandthebasicprinciplesofpaleoclimatereconstructionusingstableisotopesAgeneralunderstandingofdifferent(anic)approachesforpaleoclimatereconstructionsIsotopicfractionationfactorα=f(T)Equilibriumisotopefractionationisstronglydependentonthetemperatureofthereaction,becauseitisathermodynamicreactionEmpiricalrelationship(overleafofthetext)1000lnαx-y≈dx-dy=a106/T2+b103/T+cBasisI:temperaturedependencyofequilibriumisotopefractionationTwophases,onereservoirWhentwophases,monoxygenreservoir(.,H2O)ataspecifictemperature,thedifferencesintheirδ18Ovaluesisafunctionoftemperature,youalsohave 1000lnαx-y=A*106/T2+B≈δx-δySo,positionswerenotalteredaftertheestablishmentofequilibriumα=f(T),isknownfromtheexperimentaldataThreebasicpre-requirements“Theestimationofthepaleotemperaturesoftheancientoceansbymeasurementoftheoxygenisotopedistributionbetweencalciumcarbonateandwater,(1947),isoneofthemoststrikingandprofoundachievementofmodernnucleargeochemistry”─HarmonCraig,1965PaleothermometryintheoceansBasisII:ChangesintheisotopicratiosofthesourcereservoirwhenclimatechangesTemperaturedependenceofisotopicratiosofOandHisotopesinprecipitationEffectofprecipitationamount(Amounteffect)Effectofaridity/plicationsarisewhenclimatechangescausebothvariationsinfractionationfactorα,ycleIsotopicexchangereactionbetweenCaCO3andH2Oduringtheformationofshellcarbonates,withα=Rc/Rw=(18O/16O)calcite/(18O/16O)waterAt25°,α=,‰relativetoseawaterat25oC[Δ≈103(α-1)],andtheαisafunctionoftemperature(notea=)Foraminiferashells