生物活动及DMS

CarbonisotopechemostratigraphyofaPrecambrian/CambrianboundarysectionintheThreeGorgearea,SouthChina:Prominentglobal-scaleisotopeexcursionsjustbeforetheCambrianExplosionTomokoIshikawaa,,,YuichiroUenob,c,TsuyoshiKomiyaa,c,YusukeSawakia,JianHand,DeganShud,YongLie,ShigenoriMaruyamaa,candNaohiroYoshidac,faDepartmentofEarthandPlanetaryScience,TokyoInstituteofTechnology,Meguro-ku,Tokyo152-8551,JapanbGlobalEdgeInstitute,Meguro-ku,TokyoInstituteofTechnology,Tokyo152-8551,JapancResearchCenterfortheEvolvingEarthandPlanets,TokyoInstituteofTechnology,JapandDepartmentofGeologyandKeyLaboratoryforContinentalDynamics,NorthwestUniversity,Xi’an710069,ChinaeSchoolofEarthSciencesandResourcesManagement,Chang'anUniversity,Xi'an710054,ChinafDepartmentofEnvironmentalScienceandTechnology,TokyoInstituteofTechnology,Midori-kuYokohama152-8551,JapanReceived13July2007;revised18October2007;accepted23October2007.Availableonline21November2007.AbstractCarbonisotopechemostratigraphyhasbeenusedforworldwidecorrelationofPrecambrian/Cambrian(Pc/C)boundarysections,andhaselucidatedsignificantchangeofthecarboncycleduringtherapiddiversificationofskeletalmetazoa(i.e.theCambrianExplosion).Nevertheless,thestandardδ13CcurveoftheEarlyCambrianhasbeenpoorlyestablishedmainlyduetothelackofacontinuousstratigraphicrecord.Herewereporthigh-resolutionδ13CchemostratigraphyofadrillcoresampleacrossthePc/CboundaryintheThreeGorgearea,SouthChina.ThissectionextendsfromanuppermostEdiacarandolostone(DengyingFm.),throughalowermostEarlyCambrianmuddylimestone(YanjiaheFm.)toamiddleEarlyCambriancalcareousblackshale(ShuijingtuoFm.).Asaresult,wehaveidentifiedtwopositiveandtwonegativeisotopeexcursionswithinthisinterval.NearthePc/Cboundary,theδ13Ccarbincreasesmoderatelyfrom0to+2‰(positiveexcursion1:P1),andthendropsdramaticallydownto−7‰(negativeexcursion1:N1).Subsequently,theδ13Ccarbincreasescontinuouslyuptoabout+5‰attheupperpartoftheNemakit–Daldynianstage.Afterthispositiveexcursion,δ13Ccarbsharplydecreasesdowntoabout−9‰(N2)justbelowthebasalTommotianunconformity.Thesecontinuouspatternsoftheδ13Cshiftareirrespectiveoflithotype,suggestingaprimaryoriginoftherecord.Moreover,theobtainedδ13Cprofile,exceptforthesharpexcursionN2,iscomparabletorecordsofothersectionswithinandoutsideoftheYangtzePlatform.Hence,weconcludethatthegeneralfeatureofourδ13Cprofilebestrepresentstheglobalchangeinseawaterchemistry.Theminimumδ13CoftheN1(−7‰)isslightlylowerthancarboninputfromthemantle,thusimplyinganenhancedfluxof13C-depletedcarbonjustacrossthePc/Cboundary.Hence,theoceanatthattimeprobablybecameanoxic,whichmayhaveaffectedthesurvivalofsessileorbenthicEdiacaranbiota.Thesubsequentδ13Criseupto+5‰(P2)indicatesanincreaseofprimaryproductivityoranenhancedrateoforganiccarbonburial,whichshouldhaveresultedinloweringpCO2andfollowingglobalcooling.Thisscenarioaccountsforthecauseoftheglobal-scalesea-levelfallatthebaseoftheTommotianstage.Thesubsequent,veryshort-term,andexceptionallylowδ13C(−9‰)inN2couldhavebeenassociatedwiththereleaseofmethanefromgashydratesduetothesea-levelfall.Theinferreddramaticenvironmentalchanges(i.e.,oceananoxia,increasingproductivity,globalcoolingandsubsequentsea-levelfallwithmethanerelease)appeartocoincidewithoroccurjustbeforetheCambrianExplosion.Thismayindicatesynchronismbetweentheenvironmentalchangesandrapiddiversificationofskeletalmetazoa.Keywords:Precambrian/Cambrianboundary;Carbonisotope;EarlyCambrian;CambrianExplosion;SouthChinaArticleOutline1.Introduction2.Geologicalsetting3.Method4.Results5.Discussion5.1.Diagenesis5.2.Characteristicsoftheδ13Ccarbprofile5.2.1.P1(positiveexcursion1)5.2.2.N1(negativeexcursion1)5.2.3.P2(positiveexcursion2)5.2.4.N2(negativeexcursion2)5.2.5.δ13CprofileabovethebasalTommotianunconformity5.3.Correlationoftheδ13Ccarbprofilewithothersections5.3.1.YangtzePlatform5.3.2.Globalcorrelation5.4.Originofthecarbonisotopeexcursions6.ConclusionAcknowledgementsReferencesFig.1.(a)GeologicalmapoftheThreeGorgearea,SouthChina.Boxindicatesthestudyarea(Fig.1b).(b)GeologicalsectionacrossAnjiaheregionintheThreeGorgearea.Theblackbarlineindicatesthepositionofthedrillingsiteinthisstudy.Fig.2.Lithostratigraphyandcarbonandoxygenisotopeprofile(δ13Ccarbandδ18Ocarb)ofthedrillcore,spanningfromtheEdiacarantotheEarlyCambrian.ThebiostratigraphyofthissectionisfromChen(1984)andQian(1999).Fig.3.Theenlargedδ13Ccarbandδ18OcarbprofileacrosstheYanjiahe/Shuijingtuoboundary.Fig.4.a:anorganic-walledacritarchfossil(303111,Table1),around79mdepthintheYanjiaheFormation,b:arelativelywellpreserved,thin,limestonesamplewithfine-grainedcarbonate,recordingtheN2excursion(303111,Table1),c:thelimestonesamplerecordingtheP2excursion,around94mdepthintheYanjiaheFormation.Fig.5.Proposedcorrelationoftheδ13CcarbprofilesofthePc/CboundarysectionswithintheYangtzePlatform.ND;Nemakit–Daldynianstage;DH,DahaiMember;BYS,BaiyanshaoMember;XWTS,XioawaitoushanMember;ZY,ZhongyicunMenber;A,ChinaMarkerAatthefirstappearanceofskeletalfossils;B,ChinaMarkerBatthemassappearanceofamorediverseassemblagerichinmicro-molluscs;C,ChinaMarkerCatamajordisconfo