Olsen 1999 ScienceGiant Lava Flows Mass Extinctions and Mantle Plumes0.docx

Olsen 1999 ScienceGiant Lava Flows Mass Extinctions and Mantle Plumes0.docx

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Olsen1999ScienceGiantLavaFlowsMassExtinctionsandMantlePlumes0

Science23April1999:

Vol.284no.5414pp.604-605

DOI:

10.1126/science.284.5414.604

∙Perspective

GEOSCIENCE

GiantLavaFlows,MassExtinctions,andMantlePlumes

1.PaulE.Olsen[HN20]*

+AuthorAffiliations

1.TheauthorisattheLamontDohertyEarthObservatoryofColumbiaUniversity,Palisades,NewYork10964,USA.E-mail:

polsen@ldeo.columbia.edu

WhataretheconsequencesandoriginsofthelargestvolcaniceventsknownonEarth?

Theseincludetheso-calledlargeigneousprovinces[HN1]（orLIPs）thatcompriseenormousedificesofbasalticlavaandassociatedigneousrocks[HN2]formedoverarelativelybrieftimeinterval

（1）.TwoofthelargestLIPs,theSiberianTraps[HN3]（∼2.5×106km3）andDeccanTraps[HN4]（∼2.6×106km3）,wereextrudedontothelandsurface

（2）andareoftentermedcontinentalfloodbasalts.Eachisalsoassociatedwithamassextinction,theSiberianTrapswiththeextinctionattheendofthePermian（250millionyearsago）andtheDeccanTrapswiththeextinctionattheendoftheCretaceous（65millionyearsago）.Inrecentyears,athirdgiantcontinentalLIPassociatedwithamassextinctionhasbeenidentifiedinthelong-studiedTriassic-Jurassic（∼201millionyearsago）lavasandigneousintrusionsthatmarktheriftingofthesupercontinentofPangeaandtheformationoftheAtlanticOcean.AsreportedbyMarzolietal.[HN5]onpage616ofthisissue（3）,thisCentralAtlanticMagmaticProvince（CAMP）maybethelargestLIPofall,atleastinarea.BeforetheformationoftheAtlanticOcean,itextendedover7millionkm2,fromFrancetosouthernBrazil,coveringsubstantialportionsoffourtectonicplates（seethefigure）.Andyetthisigneousactivityprobablyoccurredoverlessthanafewmillionyears.TheoriginofthisLIPbearsonthemechanismsofmassextinction[HN6],continentalbreakup,andthemotiveforcebehindcontinentaldriftitself[HN7].

Biglava

（Left）Basaltflow（brown）intheCAMPontopoftheTriassic-Jurassicboundary（white）onTriassicriftlakesediments（reddishbrown）.（Center）PangeaduringtheLateTriassic-EarlyJurassicwithfourterrestrialLIPs（northtosouth:

SiberianTraps,CAMPbasalts,DeccanTraps,andKarroolavas）.（Right）ThePalisadesSill,anintrusivepartoftheCAMPevent,exposedalongtheshoresoftheHudsonRiver,nearNewYorkCity.

RecognitionofthisTriassic-JurassicLIPhasbeenlongincoming,perhapsdelayedbythefactitwasdismemberedduringformationoftheAtlanticOcean,eitherdeeperodedordeeplyburied,andisdifficulttopreciselydate.However,aslongagoas1971,May（4）showedthatthenumerouslineardikes[HN8]ofbasalticcompositionineasternNorthAmerica,Africa,southwesternEurope,andSouthAmericamadeupagiantradiatingdikeswarmwhenplacedintheirpredriftpositions,theirfocusbeingnearFlorida.Thisisinfactthelargestradiatingdikeswarmknowninthesolarsystem（5）.Bythe1980s,itwasbecomingclearthatatleastsomeofthesedikesfedthevoluminousbasaltflowsandsills[HN9]intheTriassic-Jurassicriftbasins[HN10]ontheAtlanticMargin（6）andthattheageofthisdikesystemisabout201millionyears（7）.PaleontologicaldataestablishedthattheflowswereverycloseintimetotheTriassic-Jurassicboundaryandhencetoitspurportedmassextinctioneventandthattheflowsinfactdatetheboundary（8）.AswastruefortheeventsattheendofthePermianandtheCretaceous,thetemporalassociationoftheextensivebasaltflowsandamassextinctionhasledtospeculationthattheeruptionofthelavastriggeredecologicallycatastrophicclimatechangethroughmassiveinputofvolatilesintotheatmosphere[HN11]（9）.

Aswiththeothertwoevents,however,theproposedlinksbetweentheCAMPLIPandmassextinctionremainverycontroversial,withsubstantialvolume,timing,andmechanismproblems.AlthoughthepreerosionalextentoftheDeccanandSiberianlavasprobablyexceeded2.0×106km3,thepresentvolumeofCAMPlavasintheriftbasinsismorethananorderofmagnitudesmaller.However,alloftheexposedriftsaredeeplyerodedtodepthsofseveralkilometers（10）.Assumingthatthedistributionofdikesandotherintrusionsisaguidetothepreerosionextentoftheassociatedlavas,Marzolietal.（3）estimatetheoriginalvolumeofflowsataboutthesameasthoseoftheDeccanorSiberianTraps.Thereiscircumstantialevidence（11）thattherecentlyrecognizedseaward-dippingreflectors（12）offtheeasternUnitedStatesmayalsobepartoftheCAMP,althoughtheirageisverypoorlyconstrained.Thevolumeofthesebasaltic,purportedlyterrestrial,flowswouldraisethetotaltoabout4millionkm3.ThisexcludesthevolumeofintrusiveCAMProcks,whichcouldaddaboutthesameamountagain.

SoitisplausiblethattheactualvolumeofCAMPbasaltswasasgreatorgreaterthanthatoftheSiberianorDeccanTraps.Superficially,however,thetimingoftheCAMPeventhasseemedwrongfortheextinctionsattheendoftheTriassic.Inrecentyears,theTriassic-Jurassicboundary,markingtheextinction,hasbeenidentifiedwithconsiderableprecision,butatalloftheplacesinwhichtheboundaryandthelavashavebeenseeninsuperposition,fromVirginiatoNovaScotia,theboundaryalwaysliesbelowtheoldestlavas（8）.AlthoughtheseobservationscoveronlyasmallpartoftheCAMP,itishardtoarguecausationifthecauseoccursafterthesupposedeffect.However,themostlynormalmagneticpolarityoftheigneousrocks（13）andthestatisticalpropertiesofpolaritystratigraphy[HN12]aroundtheTriassic-Jurassicboundary（13,14）argueforabrief（lessthan2millionyears）igneousepisodewithsomemagmaticactivityprecedingtheboundary,andhalfcouldhaveoccurredbeforeandcouldhavecausedtheextinctions.

AnumberofplausiblemechanismslinkingthemassextinctionsattheendoftheTriassicwithLIPshavebeenproposed,withstrongparallelsbeingdrawnwiththeeventsattheendofthePermianandtheCretaceous（15）.TheseincludemassiveemissionsofCO2orSO2aerosolsfromLIPs（16）,rapidsea-levelchange（17）,andabolideimpact（18）.

CO2inputcouldhaveproducedsubstantialglobalwarmingandSO2globalcoolingifintroducedoverasufficientlybriefinterval.Bothshouldleaveadiscerniblegeochemicalrecordinmarineandlacustrinesediments,fossilsoils（19）,andfossilplantanatomy（20）.Thusfar,theavailabledatafromtheterrestrialrecordareconsistentwithanabruptandstrongdisruptionofthecarboncycle[HN13],specificallywithanincreaseinCO2,althoughexplanationsofitsoriginarediverse.ThereisasyetnoevidenceofanSO2-inducedcooling.

Inthesea-levelchangescenario,thermaldomingassociatedwiththeinitiationofPangeanriftingfollowedbycollapseduringCAMPeruptionproducedaveryrapidregression-transgressioncoupletthatpresumablydisruptedmarinecommunities,resultingintheobservedmarinemassextinction（17）.HallamandWignall（17）haveindeedarguedthatsea-levelchangewasthemajorcauseofallofthemajorextinctionsofthepast500millionyears.Itisdifficulttoseehowthesea-levelchangewouldcausetheterrestrialextinctions,however.

AviablealternativeforaCAMP-inducedextinctionattheendoftheTriassicisabolideimpact[HN14].TheoverallextinctionpatternandassociatedfloraleffectsattheTriassic-Jurassicboundary[HN15]doparallelthoseassociatedwiththeCretaceous-TertiaryChicxulubimpact[HN16]（21）.EvidenceincludesthepresenceofamassiveincreaseofsporesattheTriassic-Jurassicboundary（8）,whichhasbeenattributed,asinthecaseoftheverysimilar“fernspike”attheCretaceous-Tertiaryboundary,toimpact-inducedmassiveecologicaldisruption（22）.Inaddition,shockedquartz[HN17]hasbeenreportedfromtwomarineboundarysections（23）,althoughbotharedisputed（17）.However,concertedeffortstofindshockedquartzincontinentalsectionsattheTriassic-Jurassicboundaryhaveyettobecarriedout.

ConventionaltheoriesoftheoriginofLIPsinvolvemantledynamics,inparticular,mantleplumesandhotspots[HN18]

（1）.Inthesemodels,thedeepmantlegeneratesahot,upwellingplumeofrelativelylow-densitymaterialthatrisesandeventuallypondsatthebaseofthelithosphere,generatingmelt.RiftingintheregionoftheCAMPwasoccurringforatleast30millionyearsbeforethemagmaticepisode,sothecrustwasthinninginadvance.AccordingtoMarzolietal.（3）,theirnewageandgeochemicaldataareconsistentwithaplumeheadseparatedfromitstailandspreadoveranextremelylargearea.StructuraldatafromriftbasinsinthesoutheasternUnitedStatessuggestthattheyceasedsubsidence[HN19]andunderwenttectonicinversion（undercompressioninthesamedirectioninwhichtheywerepreviouslyunderextension）closetothefocusoftheCAMPbeforeoratthetimeofitsemplacement（11）.Simultaneously,riftbasinsfarthernorthactuallyacceleratedinsubsidence.Sequentially,overthenext145millionyears,thepatternofinitialseafloorandseaward-dippingreflectorformationandconcomitantriftbasininversionseemstohaveproceedednorthandprobablysouthfromthelocusofCAMPigneousactivity（11）.Allthissuggestsstrong,ifpoorlyunderstood,linksbehindtheCAMP,core-mantleprocesses,andplatemotions.

Presently,virtuallyallaspectsoftheCAMPLIParecontroversial.Majorproblemsthatneedtobeaddressedinthenearterminclude（i）establishingthepreciseagerangeoftheigneouseventbydirectdatingofCAMPdikesineasternNorthAmericabymultiplemethods;（ii）determiningthemagneticpolarityofalargesuite