Exticntion - 6 June 1980 Volume 208 Number 4448...

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Unformatted text preview: 6 June 1980, Volume 208, Number 4448 Extraterrestrial Causefor Cretaceous-TertiaryExtinct Experimental resultsandtheoretical interpret. LuisW.Alvarez,Walter Alvarez,FrankAsaro,HelenV.A In the 570-million-year period for whichabundantfossilremainsareavail- able,therehavebeen fivegreatbiologi- calcrises,duringwhichmany groupsof organismsdiedout.The mostrecentof thegreatextinctionsisusedtodefinethe boundary between the Cretaceous and Tertiaryperiods,about65millionyears microscopicfloatingai both the calcareous p niferaandthecalcarec were nearly extermin. fewspeciessurviving otherhand, some groi fected,includingthe1; diles,snakes,mammal Summary. Platinummetalsaredepletedintheearth'scrustrelat abundance;concentrationsoftheseelementsindeep-seasedim dicateinfluxesofextraterrestrialmaterial.Deep-sealimestonesexp mark,andNewZealandshowiridiumincreasesofabout30,160, spectively,abovethebackgroundlevelatpreciselythetimeofth tiaryextinctions,65millionyearsago.ReasonsaregiventoindicatE ofextraterrestrialorigin,butdidnotcome fromanearbysupernov suggestedwhichaccountsfortheextinctionsandtheiridiumobsei a largeearth-crossingasteroidwouldinjectabout60timesthe obj4 atmosphereaspulverizedrock;afractionofthisdustwouldstayii forseveralyearsand bedistributedworldwide.Theresultingdar pressphotosynthesis,andtheexpectedbiologicalconsequencesn theextinctionsobservedinthepaleontologicalrecord.Onepredicti sishasbeenverified:thechemicalcompositionofthe boundarycla tocomefromthestratosphericdust,is markedlydifferentfromthat theCretaceousandTertiarylimestones,whicharechemicallysirr Fourdifferentindependentestimatesofthediameteroftheastero lieintherange 10 ± 4kilometers. S C I E N C E extinctions(3,4),and two recent meet- ings on thetopic(5,6)produced no sign of a consensus. Suggested causes in- cludegradualor rapidchangesinocean- ographic, atmospheric, or climaticcon- ditions(7)due toa random (8)or a cy- clical (9)coincidence of causative fac- the t o r s ; m a g n e t i c r e v e r s a l ( 1 0 ) ; n e a r b y supernova (11);and thefloodingofthe tion ocean surfacebyfreshwaterfroma pos- tulatedarcticlake (12). A major obstacle to determining the ation causeoftheextinctionisthatvirtuallyall the available information on events at thetimeofthecrisisdealswithbiological 4ichel changes seen inthepaleontological rec- ordand isthereforeinherentlyindirect. Littlephysicalevidenceisavailable,and italso isindirect. This includes varia- nimalsandplants; tionsinstable oxygen andcarbon isotop- Alanktonic forami- icratios across the boundary inpelagic us nannoplankton sediments,which may reflectchangesin ated, with only a temperature, salinity,oxygenation, and thecrisis.On the organicproductivityoftheocean water, ups were littleaf- and whichare not easy tointerpret(13, andplants,croco- 14).Theseisotopicchangesare notpar- Is,and many kinds ticularly striking and, taken by them- selves, would not suggest a dramatic crisis.Smallchangesinminorand trace tivetotheircosmic elementlevelsattheC-Tboundaryhave...
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Exticntion - 6 June 1980 Volume 208 Number 4448...

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