生物活动及DMSWord格式.docx
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生物活动及DMSWord格式.docx
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,
YuichiroUenob,c,TsuyoshiKomiyaa,c,YusukeSawakia,JianHand,DeganShud,YongLie,ShigenoriMaruyamaa,candNaohiroYoshidac,f
aDepartmentofEarthandPlanetaryScience,TokyoInstituteofTechnology,Meguro-ku,Tokyo152-8551,Japan
bGlobalEdgeInstitute,Meguro-ku,TokyoInstituteofTechnology,Tokyo152-8551,Japan
cResearchCenterfortheEvolvingEarthandPlanets,TokyoInstituteofTechnology,Japan
dDepartmentofGeologyandKeyLaboratoryforContinentalDynamics,NorthwestUniversity,Xi’an710069,China
eSchoolofEarthSciencesandResourcesManagement,Chang'
anUniversity,Xi'
an710054,China
fDepartmentofEnvironmentalScienceandTechnology,TokyoInstituteofTechnology,Midori-kuYokohama152-8551,Japan
Received13July2007;
revised18October2007;
accepted23October2007.
Availableonline21November2007.
Abstract
CarbonisotopechemostratigraphyhasbeenusedforworldwidecorrelationofPrecambrian/Cambrian(Pc/C)boundarysections,andhaselucidatedsignificantchangeofthecarboncycleduringtherapiddiversificationofskeletalmetazoa(i.e.theCambrianExplosion).Nevertheless,thestandardδ13CcurveoftheEarlyCambrianhasbeenpoorlyestablishedmainlyduetothelackofacontinuousstratigraphicrecord.Herewereporthigh-resolutionδ13CchemostratigraphyofadrillcoresampleacrossthePc/CboundaryintheThreeGorgearea,South
China.
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,weconcludethatthegeneralfeatureofourδ13Cprofilebestrepresentstheglobalchangein
seawater
chemistry.Theminimumδ13CoftheN1(−
7‰)isslightlylowerthancarboninputfromthemantle,thusimplyinganenhancedfluxof13C-depletedcarbonjustacrossthePc/Cboundary.Hence,the
ocean
atthattimeprobablybecameanoxic,whichmayhaveaffectedthesurvivalofsessileorbenthicEdiacaranbiota.Thesubsequentδ13Criseupto+
5‰(P2)indicatesanincreaseofprimaryproductivityoranenhancedrateoforganiccarbonburial,whichshouldhaveresultedinloweringpCO2andfollowingglobalcooling.Thisscenarioaccountsforthecauseoftheglobal-scalesea-levelfallatthebaseoftheTommotianstage.Thesubsequent,veryshort-term,andexceptionallylowδ13C(−
9‰)inN2couldhavebeenassociatedwiththereleaseofmethanefromgashydratesduetothesea-levelfall.Theinferreddramaticenvironmentalchanges(i.e.,
anoxia,increasingproductivity,globalcoolingandsubsequentsea-levelfallwithmethanerelease)appeartocoincidewithoroccurjustbeforetheCambrianExplosion.Thismayindicatesynchronismbetweentheenvironmentalchangesandrapiddiversificationofskeletalmetazoa.
Keywords:
Precambrian/Cambrianboundary;
Carbonisotope;
EarlyCambrian;
CambrianExplosion;
South
China
ArticleOutline
1.
Introduction
2.
Geologicalsetting
3.
Method
4.
Results
5.
Discussion
5.1.Diagenesis
5.2.Characteristicsoftheδ13Ccarbprofile
5.2.1.P1(positiveexcursion1)
5.2.2.N1(negativeexcursion1)
5.2.3.P2(positiveexcursion2)
5.2.4.N2(negativeexcursion2)
5.2.5.δ13CprofileabovethebasalTommotianunconformity
5.3.Correlationoftheδ13Ccarbprofilewithothersections
5.3.1.YangtzePlatform
5.3.2.Globalcorrelation
5.4.Originofthecarbonisotopeexcursions
6.
Conclusion
Acknowledgements
References
Fig.1.
(a)GeologicalmapoftheThreeGorgearea,SouthChina.Boxindicatesthestudyarea(Fig.1b).(b)GeologicalsectionacrossAnjiaheregionintheThreeGorgearea.Theblackbarlineindicatesthepos
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