古海洋学及古气候替代性指标.docx
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古海洋学及古气候替代性指标.docx
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古海洋学及古气候替代性指标
Paleoceanography
Introduction
Paleoceanographyisthestudyofthehistoryoftheoceans.Itencompassesaspectsofoceanography,climatology,biology,chemistryandgeology.Themainsourcesofinformationarebiogenicandinorganicmarinesediments,aswellascorals.Biogenicsedimentincludesplanktonicandbenthicfossilswhereasinorganicsedimentincludesice-rafteddebrisanddust.Onland,paleo-shorelinesanderosionalfeaturesaswellasoutcroppingsofpaleomarinesedimentsaretheprincipalsourcesofproxydata.Glaciologicalrecordscanalsogiveindirectinformationaboutpaleoceans.Theocean'shighheatcapacityanditsabilitytotransportenergyandtosequesterandreleasegreenhousegasesgiveitanimportantroleinhelpingtodeterminethestateoftheplanet'sclimate.Thus,paleoceanographicresearchisalsointimatelylinkedtopaleoclimatology.
Methods
Thereconstructionofpaleoceancharacteristicsanddynamicsrequiresclimaticdetectivework.Itinvolvesthedatingandinterpretationofpaleoclimaticrecordsaswellasthedefinitionofphysicalanddynamicalconstraintswhichspecifypossiblecirculationpatternsandcharacteristics.
Reconstructions(ProxyData)
Directmeasurementsofthequantitiesofinteresttooceanographersextendonlyintotherelativelyrecentpastandinmostcasesdonotgofurtherbackthanthemid-nineteenthcentury.Tostudytheoceanduringperiodsforwhichtherearenodirectmeasurementsonemustrelyonindirectevidence.Historicaldocumentscanbeusedassourcesofdata.Shiplogbooksandsailingtimesacrossfrequentlytraveledrouteshaveprovidedestimatesofthedirectionsandstrengthsofpastprevailingwinds(Brazdiletal.,2005).ThefrequencyandintensityofElNiñoeventssincethe1500shavebeenreconstructedbasedpartiallyonhistoricalaccountsoflargefloodsandcroplosses(QuinnandNeal,1987).Thistypeofanalysisfurnishesqualitativedescriptionsofthepast.
Quantitativereconstructionsarepossiblebyproxy,whereaquantitywhichispreservedinanaturalarchiveandcanbemeasured,standsasasurrogateoftheparameterofinterest.Abasicrequirementisthattherelationshipbetweentheproxyparameterandthequantityofinteresthastobeknown.Whenthisisthecase,thehistoryoftheproxyvariablecanbeconvertedintothehistoryofthevariableofinterestbytheuseofmathematicalexpressionsofthetype:
(
(1))
whichstatethattheparameterofinterest,Int,isafunctionoftheproxyquantity,Prx.Thetindexreferstotime.Equationsofthiskindarecommonlycalledtransferfunctions.TheconfidencewithwhichIntcanbeestimatedwilldependonaseriesoffactors,startingwiththequalityoftheproxymeasurements.Alsorelevant,andacommonsourceofuncertainty,ishowwellfrepresentstherelationshipbetweenPrxandInt.
Inmostcases,transferfunctionsareobtainedempiricallybycomparingdirectlymeasuredvaluesofthequantityofinteresttoapertinentsetofproxydata.Apotentialsourceoferroristhatthefunctionobtainedbythisproceduremightnotbegeneral,butinfactcouldrepresentarelationshipbetweenPrxandIntthatispeculiartothedatasetsusedtogeneratef.Thisproblemcanbeminimizedbyexpandingthespatialandtemporalcoverageofthedatausedtoestablishthetransferfunction.Still,evenassumingthatfisaperfectrepresentationofhowtheproxyandthequantityofinterestareconnectedtoeachotherinthepresent,thereisnoguaranteethattherelationshipbetweenthemwasthesameinthepast.
Anothersourceoferrorcanbeeasilyunderstoodbyre-writingEquation
(1)sothatitexpressestheproxyquantityasafunctionofthevariableofinterest.Itisreasonabletoassume(andinmanycasesithasbeendemonstrated)thatIntisnottheonlyfactorcontrollingPrx,sothatinfactweendupwith:
(
(2))
whereE1,E2,…,Enrepresentenvironmentalparametersthatalsoinfluencetheproxyvariablebutareindependentofthequantityofinterest.AnimmediateconclusionisthatreconstructionsofIntbasedonPrxwillbe"contaminated"byotherfactorssothatpartofthevariabilityobservedintheproxyquantityisnotrelatedtochangesintheparameterofinterest.Comprehensiveanalysisoftherelationshipsbetweenproxiesandaseriesofobservedparameterscanoffersomeinsightintohowtoremovepartoftheundesiredinfluenceofotherfactorsfromthereconstruction.
Giventhecomplexityinvolvedindevelopingskillfultransferfunctionsaswellasinidentifyingandcorrectingforpotentialsourcesoferror,acommonstrategyistoreconstructthesameparameterofinterestusingdifferentproxies.Suchanalysesareknownasmulti-proxyreconstructions(FischerandWefer,1999).
Typesofproxies:
Thesystematicuseofproxiesinquantitativereconstructionsofpastoceanicenvironmentsoriginatedinthesecondhalfofthetwentiethcentury.Sincethen,alargenumberofproxytechniqueshavebeenestablishedandmoreareconstantlybeingdeveloped.Proxiescanbegroupedinsixbroadcategories,basedonthetypeofdirectmeasurement(FischerandWefer,1999).Thesearelistedbelow,togetherwithbriefdescriptionsofthemainvariablesofinterestassociatedwitheachproxy.Thefollowingchaptersonpaleotemperature,paleoproductivityandpaleocirculationpresentinmoredetailtheproxiesrelevanttoeachofthesefields.Comprehensivediscussionsofoceanographicproxiescanbefoundintheliterature(Bradley,1999;FischerandWefer,1999;Henderson,2002).
∙Microfossilassemblages.Therelativeabundanceofplanktonicandbenthicspeciesofforaminifera,coccoliths,radiolaria,diatomsandotherorganismscanbeusedtoestimatepastoceantemperature,productivityandseaicedistribution.ThisproxytypewasusedfortheCLIMAPproject,whichproducedthefirstglobaldistributionofseasurfacetemperaturefortheLastGlacialMaximum(CLIMAP-ProjectMembers,1976).
∙Stableisotopesarebasedontheratiobetweendifferentisotopesofanelement.Theratiosareusuallystandardizedbyareferencevalueandnamedaftertheheavierisotope.Theratiobetween18Oand16O,forexample,isrepresentedbyδ18O.Isotopereadingsareretrievedmainlyfromforaminiferaskeletons(tests),organicmatterorothersources(e.g.,watermoleculesincontinentalicesheets).Theamountof18Oincorporatedbyorganismslikeforaminiferaandcoralsincreasesastemperaturedecreases.Continentaliceisrelativelydepletedin18Ocomparedtoseawater.Thismakesδ18Oaproxyforbothtemperatureandtheextentofcontinentalicesheets.δ11BisusedasaproxyforpH.Productivity,nutrientconcentrationandpastcirculationcanbereconstructedfromδ15Nandδ13C(12Cistakenupwithslightpreferenceto13Cduringphotosynthesis).Togetherwithmicrofossilassemblages,δ18Oandδ13Carethepaleoceanographicproxieswiththemostwidespreaduse.
∙Radiogenicisotopes.Thedifferentsolubilitiesofuraniumandtwoproductsofitsnaturallyoccurringdecay,thorium(Th)andprotactinium(Pa)canbeusedtoestimatetherateofdeepwaterflowandthefluxofparticlesfromthewatercolumntothesediments.Thisfluxcanalsobeusedasaproductivityestimate.14Cpreservedinorganicmatterisusedtoestimatetheagedifferencebetweennearsurfaceanddeepwatersandhence,ventilationrates.
∙Biogeniccompounds.Theconcentrationsofsomecompounds,mainlyorganiccarbon,calciumcarbonateandopal,areusedasestimatesofpastproductivity.Calciumcarbonateisalsoanindicatorofthecalcitecompensationdepth.Alkenonesarelongchainedorganicmoleculesresistanttodegradation.Thealkenonesproducedbysomecoccolithophorscanhavetwoorthreedoublebondsintheirstructure.Theratiobetweenmoleculeswithtwoandthreedoublebondsreflectthetemperatureatthetimeofsynthesis.
∙Elements.Theconcentrationsandratiosofcertainelementsinthesediment,organicremains,testsandcoralsarealsousedasproxies.Theratiosofstrontiumtocalcium(Sr/Ca)andmagnesiumtocalcium(Mg/Ca)inbiologicallyprecipitatedmarinecarbonatesaretemperaturedependent.Thecadmiumtocalcium(Cd/Ca)ratioisusedfornutrientreconstructions.BariumconcentrationandtheBa/Caratioareproxiesforproductivityandalkalinity,respectively.
∙Sedimentology.Grainsizedistributioncanprovidequalitativeinformationaboutbottomcurrentspeedsandactasanindicatoroficerafteddebris.Informationaboutpasttidescanbeinferredfromlayeredsedimentscalledrhythmites.Themineralogyofthesedimentscanbeusedtoestablishsourceareasanddirectionoftransportforbothwaterandwindbornesediments.
Conspicuouslyabsentfromthequantitiesofinterestlistedaboveissalinity,afundamentalparameterwhichinfluencesmanyaspectsoftheoceanenvironment.Thereis,atthemoment,noindependentproxyforthisquantity.Assalinityalsoinfluencesδ18O,anindirectmeasurementcanbeobtainedbyusingindependentestimatesoftemperature(alkenones,forexample)toremovethetemperaturesignalfromexistingδ18Oseries.Attemptshavealsobeenmadetoinfersalinityfrommicrofossilassemblages.Unfortunately,bothapproachesgenerateerrorsof∼1psu,verylargecomparedtotherangeofsalinityvariabilityintheoceans(FischerandWefer,1999).
Reconstructions(Models)
Thetheoreticalapproachtopaleoceanographyusesquantitativeoceanandclimatemodelstoreconstructpaleoconditionsoftheoceanandinterpretobservations.Anumberofdifferentoceanandcoupledclimatemodelshavebeenusedinthepastandwewillgiveaveryshortoverviewofthehierarchyanduseofthesemodelsinthebroadresearchareaofpaleoceanography.
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- 关 键 词:
- 海洋学 古气候 替代 指标