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KENICHIKURIHARASEIICHITOSHIMITSUandHIROMICHIHIRANOKuriharaKToshimitsuS


Acta Palaeontol Pol57 4 749757 2012CTboundarybetweentheHakkinzawaRiversectioninHokkaidoJapanandthePueblosectionintheWesternInteriorUSATheysuggestedthatthetimingoftheextinctionandrecoveryphasesformollu

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Document on Subject : "KENICHIKURIHARASEIICHITOSHIMITSUandHIROMICHIHIRANOKuriharaKToshimitsuS"— Transcript:

1 KENICHIKURIHARA,SEIICHITOSHIMITSU,andHI
KENICHIKURIHARA,SEIICHITOSHIMITSU,andHIROMICHIHIRANOKurihara,K.,Toshimitsu,S.,andHirano,H.2012.AmmonoidbiodiversitychangesacrosstheCenomanian…TuronianAmmonoidbiodiversitychangesfromshallowtooffshoreenvironmentsacrosstheCenomanian…Turonian(C…T)bound Acta Palaeontol. Pol.57 (4): 749…757, 2012 C…TboundarybetweentheHakkinŠzawaRiversectioninHokkaido,Japan,andthePueblosectionintheWesternInteŠrior,USA.TheysuggestedthatthetimingoftheextinctionandrecoveryphasesformolluscanfaunaswasapproxiŠmatelysynchronousinbothsections.ThesescholarsconŠcludedthatthemolluscanfaunasinJapanwerestronglyinŠfluencedbytheOAE2.Theirapproaches,however,werereŠstrictedtoeventsinoffshorefaciesand/ordidnotexamineotherfactorsinsufficientdetail.Hence,itisnecessarytocomparefaunalchangesindifferentdepositionalenvironŠmentswithintheYezobasintounderstandtrulytheC…TmassextinctioneventinthenorthwesternPacificregion.Inthepresentpaper,ammonoidbiodiversitychangesfromshallowtooffshoreenvironmentsacrosstheC…Tboundaryin.„CIU,carbonisotopicunit;CTBE,Cenomanian…Turonianboundaryevent;FAD,firstappearancedata;TheCretaceousYezoGroup,interpretedasforearcbasinsediments(Okada1983),iswidelydistributedincentralHokkaido,northernJapanandSakhalinIslandinFarEastRussia(e.g.,Takashimaetal.2004;Yazykova2004).Thisgroupwasprobablydepositedatapproximately35…45alongawestwardsubductionmargininthenortheasternAsiancontinent(Kodamaetal.2002).Thestratayieldabundant,wellŠpreservedmacroŠandmicrofossilsandtheyshowconspicuouswestŠtoŠeastfacieschanges(e.g.,Takashimaetal.2004).Forthepresentpaper,thefollowingthreeareaswereinvestigated(Fig.1):theMikasaarea(shelf…coastalplainsetting),theObiraarea(incisedvalley…outershelfsetMikasaarea.„ThesuccessionacrosstheCenomanian…Turonian(C…T)boundaryiscomposedmainlyofshallowmarinesandyfacies(MikasaFormation),ofwhichthedepositionalenvironmentsareinferredtohaverangedfromshorefacetooutershelf(Fig.2).TheCenomanian…Turonianboundaryevent(CTBE)isinthemiddleportionoftheMikasaFormation(Kurihara2006).WeaklylaminatedmudstoneisonlyobservedjustbelowtheC…Tboundary.Obiraarea.„ThesuccessionacrosstheC…Tboundaryiscomposedofsandymudstone,mudstone,andalternatingbedsofsandstoneandmudstone(Fig.2).TheCTBEisinthelowerportionoftheSakuFormation(HasegawaandSaito1993;Uramotoetal.2007,2009).LaminatedmudstoneisdominantwithintheCTBE.GreenishbioturbatedmudstoneŠlikeburrowsisdominantabovetheCTBE.ThedepositionalenvironmentisinferredtohavebeenoutershelfbecauseofthepredominanceofoffshoremudstoneOyubariarea.„ThesuccessionthroughtheCenomanian…Turonianiscontinuouslyexposed.Thus,manypalaeontoŠlogicalstudiesconcerningfaunalturnoverinvarioustaxahavebeenproposed(KaihoandHasegawa1994;Hirano1995;Hasegawa1999;KuriharaandKawabe2003;TakaŠhashi2005).ThelithofaciesacrosstheC…Tboundaryiscomposedmainlyofalternatingbedsofturbiditicsandstoneandmudstone(SakuFormation).TheCTBEisinthelowerportionoftheSakuFormation(HasegawaandSaito1993;Hasegawa1997).WeaklylaminatedmudstoneandgreenishbioturbatedmudstonewithŠlikeburrowsaredomŠinantwithinandjustabovetheCTBE,respectively.Thedepositionalenvironmentisinferredtohavebeencontinentalslopebasedonfaciesanalysisandthemorphologyofcalcareousbenthicforaminifera(Kaihoetal.1993;TakashimaAmmonoidfaunasacrosstheCenomanian…Turonianboundarywe

2 reexaminedintheMikasa,Obira,andOyubariar
reexaminedintheMikasa,Obira,andOyubariareas.ThereisanabundanceofmegafossildataintheCenomanian…Turoniantransitionofallareas(e.g.,Hiranoetal.1977;Tanabeetal.1977;Futakami1986;KuriharaandKawabe2003;FunakiandHirano2004;Takahashi2005;Kuriharaetal.2007).Inthepresentstudy,thestratigraphicalrangesfromthesepublishedandoriginaldataareusedfortheexaminationofammonoidbiodiversitychangesineacharea.TocomparetheammonoidbiodiversitychangesamongintraŠandinterregionalbasins(e.g.,Europe,theWesternInterior,andTunisia),weusecarbonisotope(C)stratigraphy(Fig.2)becausetheavailableintraŠandinterregionalACTAPALAEONTOLOGICAPOLONICA57(4),2012 Fig.1.Mapofthestudyareas(Mikasa,Obira,andOyubariareasinHok correlationofCretaceousCstratigraphybetweenterrestrialorganicmaterialsandmarinecarbonatesiswidelyaccepted(e.g.,HasegawaandSaito1993;Hasegawa1997;Uramotoetal.2007,2009;Takashimaetal.2010).Thestratigraphyhasalreadybeendocumentedinthesuccessionsstudied.Theammonoidbiodiversity,then,iscountedasthenumberofspecies(speciesrichness)occurringwithinacarbonisotopicunit(CIU)basedoncorrelationsfrompreviousstudies(Fig.2).Theextinction(E)andorigination(O)ratesKURIHARAETAL.„AMMONOIDSANDCENOMANIAN…TURONIANBOUNDARYINHOKKAIDO751 Fig.2.CompositecolumnarsectionsintheMikasa(),Obira(),andOyubari()areasandthestratigraphicallevelsofmacrofossildatumplanesusedinthepresentstudy.TheshadedportionshowstheCenomanian…Turonianboundaryevent(CTBE).ThemolluscanfossildataarefromTanabeetal.(1977),Futakami(1986),KuriharaandKawabe(2003),FunakiandHirano(2004),Kuriharaetal.(2007)andunpublishedoriginaldata.TheplanktonicforaminzonationandtheCTBEarefromHasegawa(1997,1999),Nishietal.(2003),Kurihara(2006),andUramotoetal.(2007,2009).Abbreviations:Ka,KatsurzawaFormation;Hk,HikagenosawaFormation;mdst,mudstone;sdst,sandstone;A.nipponicusActinoceramusnipponicusHel.helveticaglobotruncanahelveticaI.hobetsensisInoceramushobetsensisnonsulcatusI.kamuyInoceramuskamuyW.arc.Whiteinellaarchaeocretacea Erate=(numberofpreexistingspeciesabsentaboveeachCIU)/(totalnumberofspeciesbeloweachCIU)×100(%)Orate=(numberofsuccessorspeciesnotpresentbeloweachCIU)/(totalnumberofspeciesaboveeachCIU)×100(%)Inthepresentstudy,sixcarbonisotopicunits(CIUs)acrosstheCenomanian…Turonian(C…T)boundarywereused(Fig.3):(i)CIU1,correlatedtotheCunningtonicerasinermelowerpartoftheAcanthocerasrhotomagensezonesinEurope(earlymiddleCenomanian);(ii)CIU2,correlatedtotheupperpartoftheA.rhotomagense…lowerpartofthezones(latemiddleCenomanian);(iii)CIU3,correlatedtotheupperpartoftheA.jukesbrownei…lowerpartoftheMetoicocerasgeslinianumzones(latestmiddleCenomanian…earlylateCenomanian);(iv)CIU4,correlatedtotheupperpartoftheM.geslinianum…upperpartofthecerasdevonensezones(neartheC…Tboundary);(v)CIU5,correlatedtotheupperpartoftheW.devonense…lowerpartoftheMammitesnodosoideszones(middleearlyTuronian);and(vi)CIU6,correlatedtotheupperpartoftheM.nodosoNoneoftheCIUsintheMikasaareawererecognisedexceptCIU4.However,asthetimingofthefirstappearancedata(FADs)ofCunningtonicerasCalycocerasspp.(ammonoids),ActinoceramusnipponicusInoceramuskamuy,andhobetsensisnonsulcatus(inoceramids)wasrecognisedwithinCIUs1,2,3,5,and6intheObiraandOyubariareas,respectively(Fig.3),theFADsofthesespecieswereusedasproxydatumplanesinthisstudy,andthebo

3 undariesofeachCIUweredefinedasthemiddlep
undariesofeachCIUweredefinedasthemiddleportionofeachspeciesintheMikasaarea.TheammonoidbiodiversitypatternswithineachCIUintheMikasa,Obira,andOyubariareasareshowninFig.4.Mikasaarea.„Theammonoidspeciesrichnessshowedpersistenthighvalues(12…13species)inCIU1andCIU2,thengraduallydecreased(6…7species),finallyreachingaminimum(4species)atCIU5.InCIU6,thespeciesrichnessincreasedagain(9species).Theextinction(Erate)andorigination(Orate)ratesrecordedhighvaluesbetweentheCIU1andCIU2intervals(Erate,67%;Orate,69%)andtheCIU2andCIU3intervals(EACTAPALAEONTOLOGICAPOLONICA57(4),2012 Fig.3.CorrelationofUpperCretaceousCstratigraphyofterrestrialorganicmaterialsintheMikasa,Obira,andOyubariareas,JapanandreferencestratigraphyofcarbonatesinEurope(afterJarvisetal.2006).Sixcarbonisotopicunits(CIUs)wereidentifiedinthepresentstudy.Abbreviations:e.,early;m.,middle;PDB,PeeDeeBelemniteStandard;AcanthocerasjukesbrowneiActinoceramusnipponicusAcanthocerasrhotomagenseCalycocerasguerangeriCunningtonicerasinertmeCollignoniceraswoollgariInoceramushobetsensisInoceramuskamuyMantellicerasdixoniMetoicocerasgeslinianumMammitesnodosoides rate,85%;Orate,71%),respectively.AlthoughthehighestvaluesofEandOrateswererecordedbetweentheCIU4andCIU5intervals,thecauseofthehighvaluesisinterpretedtoObiraarea.„Thespeciesrichnessshowedmoderatevalues(7…9species)inCIU1andCIU2;itthenabruptlydecreased(2species),reachingaminimum(1species)atCIU4.Subsequently,thespeciesrichnessincreasedagainatCIU5(8species)andpersistedwithamoderatevalue(9species)inTheErateshowedahighvaluebetweentheCIU2andCIU3intervals(77%)andtheCIU3andCIU4intervals(50%),respectively.ThehighestvalueofEandOratesduringtheCIU4andCIU5intervalsisinterpretedtobeduetothelowspeciesrichness.ComparativelyhighvaluesofEandOrateswererecordedbetweentheCIU5andCIU6intervalsOyubariarea.„Speciesrichnessshowedpersistentmoderatevalues(6…7species)duringtheintervalofCIU1toCIU3andthendecreasedatCIU4(4species).Subsequently,speciesrichnessincreasedabruptlyatCIU5(13species)andTheEandOrateswererelativelylowcomparedtothoseintheMikasaandOyubariareas.AhighOrate(85%)wasTaphonomy.„BeforecomparingtheammonoidfaunasamongtheMikasa,Obira,andOyubariareas,weshouldnotethetaphonomicattributesoftheammonoidfossils.AmmonoidshellsmayhavebeenpronetopostŠmortemdrift,justasmodernNautilusshellssometimesappearinareasoutsideoftheiroriginalhabitatareabecauseofoceancurrents(e.g.,Stenzel1964).Forinstance,Maedaetal.(2003)demonstratedthatLatePalaeozoicammonoidsweretransportedfromoneormoredistantoffshorebiotopesinTexas,USA.Kawabe(2003)examinedthetaphonomicattributesofammonoidsinlateAlbian…CenomanianstrataintheMikasaandOyubariareasandpointedoutthefollowing:(i)encrustationbyorganismsandcorrodedoutersurfaces,whichareindicativeofshellsthathavefloated,areabsentfromsmalltomoderateŠsized(1…30cmindiameter)ammonoids;and(ii)theattributesofshellbreakageindicatethattheshellsdidnotendurelongŠdistancetransportoutoftheiroriginalhabitat.ThepreservationalfeaturesoftheammonoidshellscollectedinthepresentstudywerealmostcoincidentwiththoseillustratedbyKawabe(2003).Consequently,weassumethatthemolluscanfossilsinthepresentstudydidnotundergolongŠdistancetransportexceptforbriefpostŠmortemmovement.KURIHARAETAL.„AMMONOIDSANDCENOMANIAN…TURONIANBOUNDARYINHOKK

4 AIDO753 Fig.4.Temporalchangesinammonoids
AIDO753 Fig.4.Temporalchangesinammonoidspeciesrichness,extinction,andoriginationratesintheMikasa,Obira,andOyubariareas.Abbreviations:CI AmmonoidbiodiversitychangesintheYezoGroupThetemporalchangesofammonoidspeciesrichnessacrosstheCenomanian…Turonian(C…T)boundaryindicatethefollowing(Fig.4):(i)persistentlyhighvaluesduringthemiddleCenomanian(thecarbonisotopicunit1(CIU1)andCIU2intervals);(ii)agradualdeclinefromnearthemiddle…lateCenomanianboundary(theCIU3andCIU4intervals),especiallyininshoreareas(MikasaandObiraareas);and(iii)abruptrecoveryjustabovetheCenomanian…Turonianboundaryevent(CTBE)(theCIU5interval),especiallyinoffMonnet(2009)demonstratedthetemporalchangesofammonoidspeciesrichnessduringthelateAlbiantoearlyTuronianinEurope,Tunisia,andtheWesternInterior,wherearealltyperegions(oldstratotypesandrecentboundarystratotypes)fortheCenomanianandTuronianstages.Althoughhisdataareofamuchhigherresolutionthantheonesinthepresentstudy,thusmakingitdifficulttocomparedirectlythebiodiversitychanges,thereseemtobesomesimilaritiesandcontrastsbetweentheYezoGroupandotherregions.ThetemporalchangesinspeciesrichnessintheYezoGroup,whichshowpersistentlyhighvaluesduringthemiddleCenomanianandasubsequentdeclinestepwisefromnearthemiddle…lateCenomanianboundary,resemblethoseinEurope(Monnetetal.2003;MonnetandBucher2007;Monnet2009)(Fig.4).ThisresemblanceindicatesthattheammonoidfaunasinJapanarecharacterisedbyasustaineddiversitydropdownsimilartothoseinEurope,showingthatthemassextinctionattheC…Tboundary,therefore,didtakeplace.Incontrast,speciesrichnessinTunisiaandtheWestŠernInteriorincreasesorremainsrelativelyhighacrosstheC…TboundarydespitethesynchronousdepositionoforŠganicŠrichsedimentscorrelatedtotheOAE2;therefore,noammonoidcrisisoccurred(Monnet2009).Hence,thesedifferencessuggestthattheCenomanian…TuronianmassextinctionŽisnotaglobaleventforammonoidsasMonnet(2009)pointedout,andmoreover,themassextinctionŽwasrestrictedtomidŠpalaeolatitudinalregions(EuropeandJaPossiblecauseofammonoidmassextinctionintheYezo.„IntheMikasaarea,highvaluesofextinction(Erate)andorigination(Orate)rateswerenotedforthemiddleCenomanian(CIU1…CIU2boundary),eventhoughhighvaluesofspeciesrichnesspersisted(Fig.4).ThisobservationimpliesthatamajorfaunalturnovertookplacewithinashorttimespanininshoreareasoftheYezoGroup.Atthattime,ammonoidfaunaswerereplacedatthegenuslevel;forinstance,theacanthoceratidsinCIU1werereplacedbyinCIU2,andthekossmaticeratidinCIU1byinCIU2(Kuriharaetal.2007).Subsequently,acanthoceratidsandkossmaticeratids,whichsofarcomprisedthemaincomponentsofammonoidspeciesrichness,disappearedwithintheCIU2interval(Fig.5).Asaresult,desmoceratidspeciesdominatedduringtheCIU3toCIU4interval.ThelithofaciesintheMikasaarearepresentsinterŠbeddedfineŠgrainedsandstonewithhummockycrossŠstratiŠficationinCIU1andbioturbatedmuddysandstoneinCIU2(Fig.2).Moreover,offshoremudstonesaredominantinCIU3toCIU4.Thedisappearanceofacanthoceratidandkossmaticeratidammonoids,therefore,mayhavebeenafŠfectedbyshallowtooffshorefacieschangeswithtransgresŠsion.IntheWesternInterior,Batt(1989)inferredammonoidhabitatsintheCenomanianandTuronianstagesbasedontherelationshipbetweenmorphotypesanddistributionpatterns.Withinhisinterpretation,CunningtoniŠ(Group1inBattsstudy)weredominant

5 anddiverseinshallowerareas,(Group2)inhab
anddiverseinshallowerareas,(Group2)inhabitedshorefacetodistaloffshoreareas,anddesmoceratids(Group7)weremostabundantinproximaltomedialoffshoremudfacies.Ourdata,therefore,conformtohishypothesis.However,inHokkaido,thefrequencyofacanthoceratidammonoidsismuchlowerthanthatoflessornateammonoids(i.e.,desmoceratids,gaudryceratids)invariousdepositionalenvironmentsduringthelateAlbiantoCenomanian(Kawabe2003).ItisthereforenecessarytoinvestigatetherelationshipbetweenspeciesdiversityandfrequencyoftheammonoidspeSpeciesrichnessstilldecreasedneartheC…Tboundary(CIU4)inallareas.LaminatedorweaklylaminatedfaciesweredominantintheYezoGroup.InanopenoceansuchasthePacificregion,highsurfaceproductivity,whichispromotedbycoastalupwellingandaninflowofnutrients,developsthemidŠwateroxygenŠminimumzone(OMZ)(Arthuretal.1987;Erbacheretal.1996).InacontinentalmarginsuchastheYezobasin,therewerehighcoastalupwellingsandaninflowofnutrientsbyamajorwidespreadtransgressionneartheC…Tboundary(Arthuretal.1987;KruijsandBarron1990).TheconspicuousabundanceofdinoflagellatecystssupportsACTAPALAEONTOLOGICAPOLONICA57(4),2012 Fig.5.TemporalchangesinfaunalcomponentsofammonoidspeciesrichnessintheMikasaarea.Abbreviations:CIUs,carbonisotopicunits;e., highproductivityneartheC…TboundaryintheYezobasin(KanekoandHirano2005),andthepredominanceofoffshoremudstonetowardstheC…TboundaryintheMikasaareareflectssealevelrise(Fig.2).Thesephenomena,therefore,suggestthatthemidŠwaterOMZwasexpandedandintensifiedupanddownthewatercolumnneartheC…TboundaryintheYezobasin.CalcareousbenthicforaminiferalmorphologyintheOyubariareasupportstheappearanceofalowoxygenenvironmentneartheC…Tboundary(KaihoandHasegawa1994).However,asmentionedabove,ammonoidspeciesrichnesshadalreadydecreasedfromnearthemiddle…lateCenomanianboundary,priortotheexpansionoftheOMZ.ThisfactdoesnotdenytheinfluenceoftheOAE2ontheammonoidspeciesdiversity,ratheritmeansthatthedecreaseofdiversityofammonoidsintheYezoGroupwasnotsolelycausedbytheOAE2.Thereisaquestion,then,astoprimarycauseofthereductionofdiversityinammonoidsintheYezoGroup.GiventhevariousabioticchangesduringtheCenomanian…Turoniantransition(e.g.,Haqetal.1987;Jenkynsetal.1994;Erbacheretal.1996;Braloweretal.1997;ClarkandJenkyns1999),itisunlikelythatasingle,simplecauseledtotheC…TboundarymassextinctionŽ(Monnet2009).However,amajortransŠgressionmaybeakeyfactorinammonoiddiversityreductionintheYezoGroupbecausespeciesrichnessdeclinedstepwisefromnearthemiddle…lateCenomanianboundarywithchangesoffaunalcomponents,especiallyininshoreareas,andthetimingofthediversitydropiscoincidentwiththeonsetofthetransgressionintheYezoGroup.TheAlbiantoTuronianintervalischaracterisedbyaglobal2ndordertransgressivetrend,andtheunusualtransŠgressionpeakedintheearliestTuronian,whichrecordsthemostmarkedhighstandoftheMesozoic(HancockandKauffman1979;Haqetal.1987).Sealevelchangecanseverelyalterthesurfaceareaofecologicallycriticalzonesoverthecontinentalshelvesandoceanographiccirculationpatterns(Smithetal.2001).TheunusualhighstandinducedashifttowarddeeperŠwaterenvironmentsduringthelateCenomanian,causingareducedpartitioningofepicontinentalseas(Monnet2009).Moreover,longŠtermOdatashowhighseasurfacetemperaturesduringtheCenomanianandTuronian(e.g.,Jenkynsetal.1994;ClarkandJenkyns1999

6 ;Wilsonetal.2002;Bornemannetal.2008).Thi
;Wilsonetal.2002;Bornemannetal.2008).ThisglobalwarmingisalsocharacterisedbyareductionoftheequatorŠtoŠpoleseasurfacetemperaturegradient(BiceandNorris2002).ThereducedpartitioningofepicontinentalseasandthemoreequableclimatecontributedtothehomogenisationofmarinehabitatsduringthelateCenomanian,whichinturncausedadecreasingendemismanddecreasingspeciesrichness(Monnet2009).IntheYezoGroup,thereisadiscrepancyinlongŠtermCstratigraphybetweenterrestrialorganicmaterialsandmarinecarbonatesduringtheCenomaniantomiddleTuronian(Hasegawa2003;Takashimaetal.2010).Thecauseofthediscrepancyisunderstoodasadrivingforceforaclimaticoptimum.Hence,sealevelandclimatechangesprobablyinfluencedtheammonoidfaunasinJapanaswellasthoseinEurope.ThisconclusionispresumablysupportedbythefactthatthemassextinctionŽwasrestrictedtomidŠpalaeolatitudinalregionsbecausemarinefaunasatmidŠpalaeolatitudinalregionsmustbemoresensitivetoclimatechanges(i.e.,transitionalrealm).SeveralphasesoflargeŠscalevolcanicactivityareknowninthelateCenomanianorTuronian,includingtheCaribbean…Colombianoceanicplateau,theKerguelen(BrokenRidge)plateau,andtheOntongŠJavaplateau(e.g.,Kerr1998).TheeruptionoftheselargeigneousprovinceswasresponsibleforthedisruptionofthebiosphereattheC…Tboundarybothdirectly(duetovolatileandtracemetalŠenrichedhydrothermalfluidrelease,sealeveltransgressionanddisruptedcirculationpatterns)andindirectly(duetoelevatedtemperaturesfromincreasedCO)(Kerr1998).IntheYezoGroup,frequentfelsictuffbeds,whichweregenerallyalteredtobentonites,areindeedobservableneartheC…Tboundary(especiallyintheearlyTuronian;Takashimaetal.2004),presumablyimplyingflourishingvolcanicactivity.However,nodetailedanalysisofvolcanicactivityhasbeendemonstratedintheYezoGroup,andthus,theinfluenceofvolcanicactivityonthechangesofammonoidbiodiversityisstillunknown.However,oneofthemostobviousphysicaleffectsofmantleplumeŠrelatedvolcanismintheoceanswillbetheriseinsealevel(Larson1991),andthesealevelriseprobablypredatestheonsetofsignificantvolcanism(Kerr1998).ThepossibilitythatlargeŠscalevolcanicactivityindiŠrectlyaffectedammonoidbiodiversityintheYezoGroup,AmmonoidbiodiversitychangesfromshallowtooffshoreenvironmentsacrosstheC…TboundaryarereconstructedintheYezoGroup,Hokkaido,Japan,whichwasprobablydepositedatapproximately35…45NalongawestwardsubductionmargininthenortheasternAsiancontinent.Thefol(i)TemporalchangesinspeciesrichnessintheYezoGroup,whichshowpersistentlyhighvaluesduringthemiddleCenomanian,thendeclinestepwisefromnearthemiddle…lateCenomanianboundary,resemblethoseinEurope,butnotthoseinTunisiaandtheWesternInterior.Hence,thesedifferencessuggestthattheCenomanian…TuronianmassextinctionŽisnotaglobaleventforammonoidsandthatthemassextinctionŽwasrestrictedtomidŠpalaeolatitudinalregions(EuropeandJapan).(ii)SpeciesrichnessstilldecreasedneartheC…TboundaryintheYezoGroup.Laminatedorweaklylaminatedfaciesweredominant,andoxygenŠdepletedconditionswidespread.However,ammonoidspeciesrichnesshadalreadydecreasedfromnearthemiddle…lateCenomanianboundary,priortotheexpansionoftheoxygenminimumzone.ThisfactdoesnotdenytheinfluenceoftheOAE2,butratherindiKURIHARAETAL.„AMMONOIDSANDCENOMANIAN…TURONIANBOUNDARYINHOKKAIDO755 catesthatthedecreaseinammonoiddiversityintheYezo(iii)IntheYezoGroup,sealeve

7 landclimatechangesprobablyinfluencedammo
landclimatechangesprobablyinfluencedammonoidfaunasaswellasthoseinEurope.However,itisunlikelythatasingle,simplecauseledtotheC…TboundarymassextinctionŽbecausevariousabioticchangesintheCenomanian…Turoniantransitionhavebeendetected,andthereareinterrelationsbetweenbioticandWeareverygratefultoClaudeMonnet(UniversityofZurich,Switzerland)andPaulWignall(UniversityofLeeds,UK)forhelpfulreviewsthatledtoimprovementofthemanuscript,toFumihisaKawabeandAkinoriTakahashi(bothfromWasedaUniversity,Tokyo,Japan),andRyojiWani(NationalScienceMuseum,Tokyo,Japan)forcriticalreadingofthemanuscriptandpertinentsuggestions,toHiroshiNishi,ReishiTakashima,andKazuyoshiMoriya((allfromKyushuUniver,Fukuoka,Japan)forconstructivepointersanddiscussionsandtoManabuKano(KochiUniversity,Japan)forassistanceduringourfieldArthur,M.A.,Schlanger,S.O.,andJenkyns,H.C.1987.TheCenomaŠnian…TuronianOceanicAnoxicEvent,II.PaleoceanographiccontrolsonorganicŠmatterproductionandpreservation.:J.BrooksandA.J.Fleet(eds.),MarinePetroleumSourceRocks,GeologicalSocietySpeŠBatt,R.1989.AmmoniteshellmorphotypedistributionsintheWesternInŠteriorGreenhornSeaandsomepaleoecologicalimplications.Batt,R.1993.AmmonitemorphotypesasindicatorsofoxygenationinaBice,K.L.andNorris,R.D.2002.PossibleatmosphericCOextremesoftheMiddleCretaceous(lateAlbian…Turonian).17:1070.Bralower,T.J.,Fullagar,P.D.,Paull,C.K.,Dwyer,G.S.,andLeckie,R.M.1997.MidŠCretaceousstrontiumŠisotopestratigraphyofdeepŠseasecBornemann,A.,Norris,R.D.,Friedrich,O.,Beckmann,B.,Schouten,S.,SinningheŠDamste,J.S.,Vogel,J.,Hofmann,P.,andWagner,T.2008.IsotopicevidenceforglaciationduringtheCretaceoussupergreenhouse.319:189…192.Clark,L.J.andJenkyns,H.C.1999.NewoxygenisotopeevidenceforlongŠtermCretaceousclimaticchangeintheSouthernHemisphere.Elder,W.P.1989.MolluscanextinctionpatternsacrosstheCenomanian…TuronianstageboundaryinthewesterninterioroftheUnitedStates.Erbacher,J.,Thurow,J.,andLittke,R.1996.Evolutionpatternsofradiolariaandorganicmattervariations:AnewapproachtoidentifyseaŠlevelchangesinmidŠCretaceouspelagicenvironments.24:499…502.Funaki,H.andHirano,H.2004.CretaceousstratigraphyinthenortheasternpartoftheObiraarea,Hokkaido,Japan[inJapanese,withEnglishabFutakami,M.1986.StratigraphyandpaleontologyoftheCretaceousintheIshikariprovince,centralHokkaido.PartI.StratigraphyandtheCretaceousinthesouthernareas.BulletinoftheNationalScienceMuseum,Gale,A.S.,Smith,A.B.,Monks,N.E.A.,Young,J.A.,Howard,A.,Wray,D.S.,andHuggett,J.M.2000.MarinebiodiversitythroughtheLateCenomanian…EarlyTuronian:palaeoceanographiccontrolsandsequencestratigraphicbiases.JournaloftheGeologicalSociety,LondonHancock,J.M.andKauffman,E.G.1979.ThegreattransgressionsoftheLateCretaceous.JournaloftheGeologicalSociety,LondonHaq,B.U.,Hardenbol,J.,andVail,P.R.1987.ChronologyoffluctuatingHarries,P.J.andLittle,C.T.S.1999.TheearlyToarcian(EarlyJurassic)andtheCenomanian…Turonian(LateCretaceous)massextinctions:similaritiesandcontrasts.Palaeogeography,Palaeoclimatology,PalaeoecolHasegawa,T.1997.Cenomanian…TuroniancarbonisotopeeventsrecordedinterrestrialorganicmatterfromnorthernJapan.Hasegawa,T.1999.PlanktonicforaminiferaandbiochronologyoftheCenomanian…Turonian(Cretaceous)sequenceintheOyubariarea,Hasegawa,T.2003.Cretaceousterrestrialpaleoenvironmentsofn

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