A 300-million-year record of atmospheric carbon dioxide from

通过古植物学来反演3亿年来二氧化碳的变化

Acknowledgements

WethankA.NichollsfordiscussionsandhelpwithTEManalysis.TheworkdoneatUICwassupportedbytheUSNSF,andtheworkdoneatDrexelUniversitywassupportedbyDARPAviaONRcontract.TheelectronmicroscopesusedinthisworkareoperatedbytheResearchResourcesCenteratUIC.TheJEM-2010FpurchasewassupportedbytheNSF.CorrespondenceandrequestsformaterialsshouldbeaddressedtoY.G.(e-mail:gogotsi@drexel.edu).

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A300-million-yearrecordofatmosphericcarbondioxidefromfossilplantcuticles

GregoryJ.Retallack

DepartmentofGeologicalSciences,UniversityofOregon,Eugene,Oregon97403-1272,USA

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TounderstandbetterthelinkbetweenatmosphericCO2concen-trationsandclimateovergeologicaltime,recordsofpastCO2arereconstructedfromgeochemicalproxies1±4.AlthoughtheserecordshaveprovideduswithabroadpictureofCO2variationthroughoutthePhanerozoiceon(thepast544Myr),http://doc.guandang.netingtheknownrelation-shipbetweenleafstomatalabundanceandgrowingseasonCO2concentrations5,6,IreconstructpastatmosphericCO2con-centrations.Forthepast300Myr,onlytwointervalsoflowCO2(,1,000p.p.m.v.)areinferred,bothofwhichcoincidewithknowniceagesinNeogene(1±8Myr)andearlyPermian(275±290Myr)times.ButformostoftheMesozoicera(65±250Myr),CO2levelswerehigh(1,000±2,000p.p.m.v.),withtransientexcur-sionstoevenhigherCO2(.2,000p.p.m.v.)concentrations.TheseresultsareconsistentwithsomereconstructionsofpastCO2(refs1,2)andpalaeotemperaturerecords7,butsuggestthatCO2reconstructionsbasedoncarbonisotopeproxies3,4maybecom-

promisedbyepisodicoutburstsofisotopicallylightmethane8,9.Theseresultssupporttheroleofwatervapour,methaneandCO2ingreenhouseclimatewarmingoverthepast300Myr.

AsatmosphericCO2levelshaverisenoverthepast200yearsofindustrialfossilfuelconsumption,plantshaverespondedbydecreasingthedensityofstomatesontheirleaves5,6.HereIusethewell-knowninverserelationshipbetweenatmosphericCO2concen-trationandstomataldensityasapalaeobarometerofatmosphericCO2duringgrowthoffossilplantleaves.Stomataldensityoflivingplantshas,however,beenshowntoberelatedtodifferencesininsolation,waterstressandstomatalpositionwithinleaves,whichalsoaffectcellsize,andthusstomataldensity.Theeffectsofthesecompetingvariablesareminimizedbyusingstomatalindex(per-centagestomatesoverstomatesplusepidermalcells)ratherthanstomataldensity(stomatesperunitarea6).Furtherlimitstosuchcompetingvariablesaresuggestedbythehabitatsofthefossilleavesstudied:theseweremostlyhumid,lowland,¯uvialandswampenvironmentssuitableforpreservationoffossilplantcuticles10.Theplantshadnutrient-poorpeatyandsiliceoussubstrates,sufferedperiodic¯ooddisturbanceandwereinopenvegetationearlyintheecologicalsuccessionafterdisturbance,asisindicatedbysedimen-tologicalandtaphonomicstudiesofseveralofthestudiedfossilsites10,11.AnotherpotentiallymisleadingeffectisthedifferencesbetweentreesofdifferentsexinGinkgo:pollen-producing

trees

Permian Triasssic JurassicStomatal index

Cretaceous Cenozoic

Millions of years ago

Stomatal index

Figure2Stomatalindexhasvariedconsiderablyoverthepast300millionyears,butwasashighduringtheearlyPermianiceageasithasbeenduringtheNeogene.DataarefromfossilleavesofRhachiphyllum(earlyPermian,295±265Myrago),

Lepidopteris(mid-PermiantolateTriassic,258±200Myrago),Tatarina(latePermian,252±250Myrago)andGinkgo(lateTriassictorecent,229±0Myrago).Filledsymbolsandthesolidlinearereliabledataasestablishedbyrarefactionanalysis(Fig.1);un®lledsymbolsarestatisticallyinadequatesamples,andareplottedtoshowprovendatadensityofthecuticularrecordofthesetaxa.AlldataareavailableasSupplementary

Information.

Carbon dioxide (p.p.m.v.)

Stomatal index

Number of epidermal cells counted

Figure1Rarefactionanalysisindicateshowmanycellsneedtobecountedforreliabledeterminationofstomatalindex(SI).Shownhereareleavesofafemale(circles)andmale(diamonds)treeofGinkgobilobacollectedfromEugene,Oregon,5September2000,andofLepidopterisstormbergensisfromthelateTriassic(Carnian)MoltenoformationatLittleSwitzerland,SouthAfrica12.TheGinkgosampleswereideal:imagesofthesamesizefromdifferentleavesoftwotrees.TheLepidopterissampleswerenumerousbutfarfromideal:imagesofirregularshapeandvariedsize,withgreatnaturalvariationinSI,andfromanunknownnumberofleavesor

plants.

NATURE|VOL411|17MAY2001|http://doc.guandang.net

Figure3PalaeoatmosphericCO2canbeinferredfromfossilleafSIbyusingthistransferfunctionderivedfromGinkgoleavesgrowningreenhouseexperiments5andtakenfromherbariumsheetsdatingbackto1888(seeSupplementaryInformation).

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通过古植物学来反演3亿年来二氧化碳的变化

havebranchesreachingupwardsandsoaremoreevenlylit,whereashorizontalbranchestendtoshadelowerbranchesinovule-producingtrees.CountsofstomatalindexfromSEMimagesofleavesofamaleandfemaletreefromEugene,Oregon,showedidenticalstomatalindex(meanSIwas8:660:4;Fig.1).

ThemostseriousobstacletotheuseofSIasaCO2palaeo-barometerhasbeenthedifferentSIamongspeciesgrowinginthesamelocation.Forthisreason,applicationsofthistechniquehavefocusedonparticularspecies,suchasQuercuspetraea,knowninthefossilrecordbacktotheMiocene(10Myrago)6.HereIusedprimarilythegenusGinkgo,whichhasafossilrecordofwell-preservedleafcuticlesatleastbacktothelateTriassicperiod(229Myrago)12.ToextendthisrecordbackintothePalaeozoic,thepteridospermgeneraLepidopteris(Permian±Triassic),Tatarina(latePermian),andRhachiphyllum(Permian)werechosenforseveralreasons.TheyhavesimilarSIwheretheyoccurinthesameplaces:forexample,Ginkgotelemachus(SI7:661:4)andLepidopterisafricana(SI6:561:7)atLittleSwitzerland(lateTriassic,SouthAfrica12),andTatarinaconspicua(SI5:561:1)andLepidopterissp.(SI6:360:9)atB …… 此处隐藏：15144字，全部文档内容请下载后查看。喜欢就下载吧 ……