REVIEWARTICLE Charlgs L, Orake, Department01Earth Scrences DartmouthCollege,Hanover,New Hampshre 03755 and Yvonne Herman, Departmentof Geology,Washngton Stale Un versity,Pu lman, Washngton 99164.2A12 Did the DinosaursDie or Evolveinto Red Herrings? Introduction 0n February2 of 1986Halley'scornetreached the perihelion,its closestbrush with the sun.The last time it approached us wasin 1910,whenit causedquite a stir, and this time there was an international effort, combining spacecraftfrom a numberof countries,to take a closelook at it and determineits properties. In our presentview of the natureof nature we accept comets and other extratenestrial bodies as natural phenomena,but this was not alwaysthe case.For centuies it wasbelievedthat all aspectsoflife on Earth wereruled by the posi, tionsofthe starsand the planets.0nce the locations of the stars and the orbits of the planets were known,the appearanceof a new star of any kind seemeda violation of the natural order and henceto be a harbingerof disaster.For example, the passageof Halley'scomet in 1066 is recordedon the Bayeuxtapestry,which depicts King Harold'sfollowersassuringhim that this was not a promiseof good things ro come; a prediction that proved to be accuratein view of his defeatby William the Conqueror. A portrait of the samecometis found in Giotto's fresco,Adorationof the Magi, in the Arena Chapelin Paduawhereit assumesthe role of the Star of Bethlehem.The comet made an appearance in 1301,and the frescosappearto have beenstartedin 1303,so the recollectionwould havebeenfreshin his mind. The frescosin the chapel were commissionedby the Florentine businessman, Enrico Scrovegni,perhapsto expatiate the sins of his Father vho wasidentified by Dante as the archusererin his Inferno. In Dante's day the Earth was viewed as flawed,but as one went towardsthe heavens,one encounteredperfection;celestialspheres,unblemishedand pure, stretchingto the farthest star. The spherewas the perfect shapeand the individualheavenlybodieswerepresumedto be perfectlyspherical.The Earth stoodat the center of rhis perfect universeand only on Earth was there imperfection,irregularity,and sin; possibly Earth wasthe only interestingplacein this whole system. Padua rras a center of mathematics,the disciplinethat would eventuallyraise astronomy from its roots in the superstitionsof astrology. An important contributor to this was Galileo Galilei,who becamea lecturerin mathematics at the Universityof Paduain 1589.In Venicein 1609,he learnedof the recentinventionof the telescope and on his return homehe madeone, the first capableof astronomicalobservations, and turned it on the Moon. He discoveredthat it was not the perf€ct, smooth sphere of the acceptedcosmological models.He identifiedmountainscastingshadows so long that their height necessarilyexceededthe height of thosein his native Italy. He also observed circular depressions but producedno hypothesisto explainthem (Drakeand Komar, l984). Galileo went on to observethe phasesof Venus and the moons of Jupiter, and then ran into problemswhen he espousedthe Copernican model in which the Sun, not the Earth, wasat the centerofthe SolarSystem.This ran counter to the acceptedcosmology,which was tied into the catechismof the Church, and he wasforced to recant. In principle his observationof irregularitiesin the Moon had equalpotentialfor disturbingmedievalcosmology, for it indicated NorthwestScience,Vol. 62, No. 3, l9BB l3l that the Moon,aswell asthe Earth,lackedperfection, But perhaps this was too subtle for the Inquisition. Sixty-fiveyears after Galileo's initial obseron vations,RobertHooketrainedhis telescope the lunar area calledMons Olympus,sketched what he saw, and describedpits shapedlike dishesin the text accompanyingthe sketches publishedin his Micrographia(1665).Hooke advanced two theories to explain the lunar pits that he made.In the first basedon experiments experimenthe bombardeda mixtureof clayand waterwith a hearl body,and producedpits not unlike thoseon the Moon. But he could not think of a physical explanationfor such bodies striking the Moon and,therefore,he couldnot accept that this could be the crater-formingmechanism. He tried a secondexperimentinvolvinga pot When he removedit from of boiling alabaster. the fire, he noticed that the surfacewascovered to the with smallpits whichbore a resemblance pits that he observedon the Moon.He concluded that the Moon, like the Earth, contained subterraneanfires which on contact with aerial producegreat quantitiesof vapor that substances explodeand shatterthe crust;in otherwords,a steamexplosion. Meteorsand Meteorites Hooke'sexperimentstook place at a time of changein the perceptionofthe natureofnature. In the Age of Reason,supernaturalcausesfor natural phenomenawere rejectedand rational causes,following natural laws,were sought.The iesultswere not alwaysrational althoughthey were different. For example, the Swiss mineralogistJ. A. Deluc statedthat if he saw a fall of a meteoritehimself,he would not believe his own eyes.When a meteoritefell on Barbaton by in Francein 1790and the fall waswitnessed the mayorand the city council,the Frenchphysician and chemist,ClaudeLouis Berthollet,wrote, "How sadit is that the entire municipalityenters folk tales upon an official record. presenting them as somethingactuallyseen,while they cannot be explainedby physicsnor by anything reasonable."(Nininger 1933) The father of modernchemistry,Antoine Laurent Lavoisier, joined with scientistsof the Academyof Sciences of Paristo sign a memorandumin 1772which tried to put an end to irrationalinterpretations 132 Drake and Herman of the origin of meteoites.It concludedthat stones to which a cosmic origin had been assignedwere actuallyordinary terrestrial rocks "rhe that had beensrruck by lightning and thar is physically inposfalling of stonesfrom the sky sible." When Yale professorsSilliman and Kingsley describeda meteorite fall in Weston, Connecticutin 180?,ThomasJeffersonis alleged to have remarked that he would rather believe that thosetwo Yankeeprofessorswould lie, than lhat stoneswould fall from heaven(Hartrnann l982). The reality of meteoriteswasfinally establishednear the end of the l8th Centuryby the work of the physicistsE.F.F.Chladniof Germany and E, Biot of France,and evenmoreby a shower of severalthousandstoneson the town of I'Aigle in Francein 1803(Nininger1933).It is hard to ignoreexperience,evenwhentheoryties to deny it, althoughnoble effo s are sometimesmadeto do so. lmpact Craters With the origin of meteoritesaccepted,many r,ritherperimentsto scicntists busiedthemselves explainthe Moon's cratem.Notableamongthese wasthe distinguishedAmerican geologist,G. K. Gilbert. Gilbert had participated in the earlier Wheeler and Powellsurveysin the westand was the best known of the early GeologicalSurvey geologists. By the late 19th Century uniformitarianismhad gainedmany supportersand the Earth wasviewedasthe product of slowand continuous changethrough the samekinds of processes that are affectingit today.The Moonwas consideredalongsimilar linesand most of its surfacefeaturesrere viewedas the resultof internal forces.Gradualand prolongedvolcanismwas creditedwith the formationof the lunar surface features,especiallyits craters.Becausethese cratersdifferedin size,abundance,and shape from terrestrialcraters,they r'eerethought to representa specialtype of volcanicactivity that resulted from the Moon's peculiar physical condition. Survey, Whenthe Directorof the Geological John Wesley Powell, transferred him to Washington to help with administrative tasks, Gilbertsoughtan outletfor his scientificcuriosity by making observationsof the Moon through the refiacting telescopeof the Naval 0bservatory (El-Baz1980).From theseobservations he constructedthe form of thesecraters;a gentleslope up to a steeprim, a flat floor which lay at a lower elevationthanthe surroundingplain,and a central peak.Althoughtherewere someterrestrial volcaniccratersthat bore a little resemblance to theselunar features,mostwerevery differentand he concludedthat "through the expressionof everyfeaturethe lunar crateremphaticallydenies kinship vith the ordinary volcanoes on the Earth." (Cilbert 1893).In a New York hotel room.during a seriesof lectureshe wasgiving at ColumbiaUniversity,Gilbert performedbombardment experimentswith clay balls on a clay surface.producingmanyof the featuresseenin lunar craters, and he challenged the volcanic theory favoredby many geologists(El-Bazl9B0). Having satisfied himself with the impact origin of the lunar craters,Gilbertthen turned to the Earth to seeka terrestrialanalogue.He wasawareof a crater discoveredin l89l in central Arizona,then calledCoonButte, a landmark for locating Coon Springs(Darton l9l0), bur subsequently namedBarringercrateror Meteor Crater. This feature stands out as a strikins a n o m a l li n t h e f l a t - l v i n gr o c k so f t h e p l a i n i n whichit is located.The NativeAmericansofthe areahaveseverallegendsabout the crater which representit as the site wherethe Creat Spirit descendedin a blazing fire and disappeared beneath the ground (Nininger 1933). Large numbersof fragmentsof meteoriticiron had beenfound in the vicinity of the crater,chiefly within two miles of its rim, and Gilbert began his studvwith the convictionthat it wasthe result of the impactof an extraterrestrialbody. His New York hotelroom experiments led him to believe that he shouldfind evidenceof the largeimpacting body in the crater, but he could nor. Therefore,he reluctantlyconcludedthat the crater was the result of an internal exolosion. rather than an impact. N. H. Darton madea reconnaissance of the areafor the GeologicalSurveya few yearslater and visited the crater. He concluded,"The hypothesisthat it was causedby impact of a meteor . . . is in accordance with someof the featuresbut doesnot accordwith the all important fact that no meteoris present,as has been demonstrated by manyborings.. . . It is agreed that if there wasa meteo.,it must havebeenat least500 feet in diameter.The occurrenceof a few tons of meteoriteiron in the vicinity and mingled with sorneof the debris on the rim and in the centeris an enigma.The hypothesisof Gilbert that the crater and rim were produced by a volcanicsteamexplosionappealsto me most strongly, notwithstandingits purely gratuitous character"(Darton l9l0). The weight of an iron meteor 500 feet in diameterwould be about 7 million tons. Some preliminaryassays of the meteoriticiron in the areasuggested that it containedalluringquantities of plarinum and iridium so a stock company, Meteor Crater Explorationand Mining Company,wasformedby D. M. Barringerto exploit the property.A number of excavationsand boreholes were rnade, but they failed to encounterthe expectedlarge body. In addition, subsequentassaysdid not confirm the significant amountsof iridium and platinum.Nonetheless, Barringer wasstill convincedthat the crater had been created by meteor impact. The official GeologicalSurveyline wasthat it wasthe result of an internal explosionand Barringer was infuriatedwhenthe Surveyrefusedto call it Meteor Crater on its maps. In the 1950's,Gene Shoemaker,with the CeologicalSurvey,was studyingthe explosive ventsand volcanoes of the ColoradoPlateauin an effort to find the ultimate sourceof the abundant sedimentaryuranium deposits.At Los Alamos,Ted Taylor,then a bomb designerand latera professorat Princeton,wantedsomebody to studycratersproducedby nuclearexplosions. He askedthe Surveyfor help,and they assigned Shoemaherto the task. He looked at some of the craters at the Nevadatest site and then comparedthesevith Meteor Crater,finding many similarities.The generalcharacterofthe craterswassimilarand, in addition,he and his colleagues found that a high pressureform of quartz,coesite,produced by shockpressuresgreaterthan 20 kilobars{2 gigapascals], waspresent(Chaoet dL t960).Bur wherewasthe large meteor?It had remainedhidden despitegeophysical investigations, excavations and drilling. Shoemaker(1960)pointed out that Gilbert and earlierinvestigatorshad assumeda relatively large body travelling at modest speeds.The kinetic energyof a meteorite is equal to rnv2,so if the meteoriteis travelling at high speed,it will Did the DinosaursDie or Evolveinto Red Herrinss? 133 haveenormousenergy.The energyincreasespropoitionallyto the mass,but asthe squareof the velocity.Using craterscreatedby nuclear explosions of known strength for calibration, Shoemakercalculated that the impact energy that createdMeteorCraterwasequivalentto 1.7 megatonsof TNT. If the meteor r'eeretravelling at 10-15km/sec,within the range of observed velocities,then the massof the meteoriteneed be only 63,000tons and its diametera mere25 mete$. It is not surprisingthat a massthis small consideringthe couldnot be Iocated,especially breakup that must have resulted when it hit. Gilberthad underestimated the velocityof the impactingbody and this in turn meantthat he had to grossly overestimateits size Subsequentlya number of ancient craters were identified, some of very large size like Manicouaginin 0ntario, 100 km in diameter, some found during oil exploration buried in sedimentarybasins.Others,once called cryptovolcanic and attributed to steam explosions (Bucher 1933,1963),rere reidentified as impact suuctutes. There was someresistanceto acceptanceof many of thesestructuresas having been formed by meteorites,but the Apollo program,which produceda largebody of informationaboutlunar craters,generatedgreat enthusiasmfor the idea and producedevidencethat lunai, terrestrial,and explosioncratershad manygenericsimilarities. By 1970the concepthad been pretty well acceptedand peopleturned their attentionto other matrels. lmpactsand Extinctions lridiumanomalies The subjectcameto light againin the late 1970's when Walter Alvarez, a young geologist from Berkeley,ran into a stratigraphicproblem in Gubbio, Italy, and was discussing it with his father, Luis, a Nobel Prize-winning physicist. Walter Alyarez was working in a section which consistedof relatively thick limestone units separatedby thin shales.One of these shales markedthe boundarybetveen the Cretaceous and the Tertiary [K/T], a fundamental discontinuity in Earth history about 65 million years ago,He wantedto find out how much time the shaleunit representedand his father pointed out 134 Drake and Herman that meteodticdust is comingin all the time and that it wasnot unreasonableto assumethat the iate is constant.If one couldfind an elementthat is rare in the crustal rocks on Earth but present in meteoritesand meteoric dust, one could measure the relative concentrations in the limestoneand the shaleunits and calculatethe relativedepositionrates.This wasa cleveridea and the element irid.ium, one of the platinum metals,nas chosenas the indicator. When the iridium in the clay layer at the boundaryr,vasmeasured,it appearedro be higher by a factor of 30 than backgroundvalues;too high, it vas thought, to be explainedby changes r ant e .T h i sl e d l o l h es u g g e s l i o n in sedimentatio that the excesscamefrom the impacr of a l0 km diameter asteroid on the Earth's surface (L, Llvarezet al. l9B0).Such a collision would have an impact energy of about 103 megatons [equivalentto about 100 million MX missles]. This much energy,releasedin a shorttime, might well be expectedto have an effect on life on Earth. The eud of the Cretaceousr,vasa time of major faunal changes as indicated by the fossil record,including disappearance of the dinosauis and expansionof the mammals,so it \,yasan easy step to supposethat the impact was the cause, an idea previouslysuggestedby de Laubenfels (1956).Perhapsearlierimpactscausedearlierextinctions(Mclaren, 1970,l9B3; Urey, 1973).A scenaio wasdevelopedin which it wassuggested that the dust from the asteroid impact carried around the Earth reducing solar insolation,and cutting back on the plants with it photosydrhesis, and starving the animals(L. Alvarez et al. 1980, L. Alvarez 1987).When it was pointed out that dustfrom volcaniceruptionsdoesn'tstayin the atmospherevery long the computermodelers that the impact causedthe particles suggested to be blownout of the atmospherein ballistictrajectories and be distributed around the Earth to reenterlater. Doubts and Objections This modelwasmet with enthusiasmby someand dismayby others,Few doubt that extraterrestrial objects of considerablemagnitude have struck the Earth and, similarly,major paleontological discontinuitiesin the geologicalrecord are universallyaccepted.What is questionedis whetherthere is a direct linkage betweenthe two. It should be noted that K/T time was geologicallyvery noisy: there was almost unprecedentedvolcanic activity; the Laramide orogenywas beginningto raise the mountains in the West;the great Cretaceousseawayextending from the Gulf of Mexicoto the BeaufortSea wasdryingup in rcsponseto a major regression of the seafrom the land: continentswere splitting and volcanismwasrampant. Oceancirculation waschangingas the continentalland masses shifted their positions and there were changes in climate. As a result, simple causeand effect relationships are not clear cut nor easy to qetermlne. Our ability to measuregeologicallyshort time intervals in the past is limited and it is very difficult to determinewhetheran event wasinstantaneousfrom the geologicalrecord of65 million years ago.A study of the availableevidenceappearedto favor activityspreadover a time period of the order of l0a-106years(Officer and Drake 1983).Then, the clay layer was presumedto be the fallout of the dust cloud createdby the impact. If so the clay mineralogyof the layer might be expectedto be differentfrom that in the rocks immediatelyaboveor belo*. Measurementsthat were made suggestedthat the clays were predominantlydetdtal,rdth little or no difference in clay mineralogybetweenthose in the boundary clayand thosein the bedsimmediatelyabove and below although they differed from area to area(Rampinoand Reynolds1983).In rhe Caspian area, for example,the clays above,below, and at the boundaryare attapulgites(Herman et aL, in press);there is kaolinire ar the Gubbio boundary,but spreadover 2 metersof section representingseveralhundred thousandyears (Johnson a n d R e y n o l d s1 9 8 5 )O . thersections havesimilar characteristics. Perhapsthe amounts Ieft by the proposedimpact are mixedwith these, but were too small to leave a significant record. Maybenone are there. Then,too, latestCretaceous dinosaurswere found to have inhabited the North Slope of Alaska,at paleolatitudes of 70-85'N.Although the climate at theselatitudeswas much more equitablethan at present,thesesaurianswould havehad to endure annualvariationsin solar insolatiorr,temperatureand food supplyof more extreme and of longer duration than that proposedby the impact dust cloud theory of extinc- tions. Perhapsthey werernigratory,althoughthe presenceofjuveniles suggeststhat they occupied high latitudesyearround(Brouwerset al 1987). Bolidesor Volcanos? In a paperin E0S WalterAlvarez(1986)spelled out the evidencefavoringan impact.He cited iridium, spherules,shockedquartz, soot, and worldwide distribution as reasonsfor accepting impactsand rejectingvolcanismas a causeof the K/T extinctions.If we look at eachof these,we find that the evidencemay not be as compelling as suggested. l r l d i u mE n r i c h m e n t Initial descriprionof the iridium anomalydescribedit as a spikeconfinedto a very short time interval at the K/T boundary;the distribution ro be expected from an inslantaneousevent reworked by bioturbation (L. Abatez et al. l9B0). Somequestions wereraisedaboutthe uniqueness of the anomaly becausethe measurementsare not easyto makeand mosthad beenmadeat or near the boundary. Subsequentmeasurementsshowed anomalousiridium overa considerablylongersedimentary interval,sometimeswith multiple peaks,and sometimes not synchronouswith the paleontological boundary(Crockettet al. 1986,Hansen et al. 1986a,Kyte and Wasson 1986,Michel er al. 1985,Nazarov et aL. 1983, Preisinger et aL 1986,Rastand Graup 1985,Rocchiael ol 1984). This distriburion doesnot conform so well wirh the conceptof an instantaneous event. Schrnitz (1985) suggesred that redoxcontrolledprecipitation processes r.rere important in determining metal enrichment at the K/T boundaryin Denmarkand suggestedthat similar processesmay have affected the boundary in Spain and New Zealand.In a more recent study, Schmitz et al, (submitredfor publication)found iridium concentrationsin kerogen separated from rhe fish Clay at StevnsKlint to be t 100ppb and similar high concentrationsin the K/T boundary clay at Caravaca,Spain and suggestedthat bacterialactionhasinfluencedmetalaccumulation. Rucklidge et al. (1982\ found that the iridium in the K/T of Denmarkwasconcentrated in the organicfraction (dominantlydinoflagellate cycts), Did the DinosaursDie or Evolveinto Red Herrines? 135 It is true that iridium is rare in lhe crustal rocks of the Earth, but it is not so rare in the Earth's mantle.Observations at Kilauea(Zoller et al. 1983, Olmez et al. 1986) indicate that volcanismfrom deepin the mantle hasproduced an iridium enrichmentin the volcanicplume of l0a-l0s over that in Hawaiian basalts.The iridium appearsto be carriedas a fluorideand in this volatilephasehasthe opportunityfor wide distribution.Thushot spotvolcanismfrom deep in the Earth's mantle is a strong candidateas a possiblesourceof the anomalousiridium. Luck and Turekian(1983)suggested that the 18711860s ratio might be used to differentiate betweenextraterrestrial or terrestrialsourcesand ro identify an impact.Their results,and others to date,indicatelhat a crustalsourcedoesnot fit the data,but that either a mantlesourceor multiple impacts,rather than a singleimpact, would. Thusit is hard to concludethat the presence of iridium uniquely demonstratesan impact.The osmium isotopesand their distdbution would seem to require more than one, but both the osmiurnand the iridium couldalsobe explained in terms of a sourcein the Earth's mantle. Many ofthe K/T sections are incompletedue to the major regressionof the seathat $as tak' ing placebut in some,that appearto be complete,the transitionfrom Cretaceousnannofossils to Tertiary,like the iridium anomaly,doesnot interval occurabruptlybut over a considerable of tirne (Perch-Nielsonet al. 1982. Theirstein 1981).Maybethe iridium wasnot depositedin a geologicalinstant;maybeit has beenspread or maybe,as has aroundby geologicalprocesses; been suggestedrecently.it came from comel showerslasting 10'-105years(Hut et ol 1987). M crospherules Spherules,generallyranging in size between 100-1000g.m,havebeencited asevidencefor the impact(Smithand Klaver1981,Montanariet al. 1983).Somespherulesfound in deepseacores seemto be clearlyof cosnic origin (Claytoner a l I 9 8 6 )a. l t h o u g h t h e i rr a t eo I d e p o s i t i osne e m s to vary over time by severalordersof magnitude. interestin spherulesin Therewasconsiderable the 1960's,when the first Moon data were appearing,and comparisons were made between spherulesfrom explosiveand shieldvolcanoes 136 Drake and Herman and thosepresumedto be extraterrestrial(Hodge and Wright 1965,Heiken and Lofgren 1971). They seemto be presentin both shieldand explosivevolcanoesand they all appearto be quite similar in major element chemistry. Most of rhe spherulesat the K/T boundary are consideredto be secondary-replacements of something-andit is not alwayseasyto guess what that somethingwas(De Paolo et al. 1983, Montanari et al. 1983 (Varekampand Thomas 1982).Hansenet ol (1986b)concludedthat K/T microspherules from Denmark,Nev Zealand, and Spain were diageneticreplacementsof prasinophytealgae.In New Zealandgoethite spheruleswere found, probably derived from pyrite{Broofts Bhandari(writtencome/ al 1985). munication)found that the microspherulesat the K/T boundary in Mangyshlak,USSR, are glauconite.At Caravacathe K-spar spherules (possiblefrom alterationof glauconite)have a coreof goethite(Smitand Klaver l98l, Hansen et al. 1986b). Naslunder aL (1986)foundthat rhile K-spar spherulesweremorenumerousat the K/T boundary at Gubbio, they are also found in the clay layersboth aboveand belowthe boundary,covering a time span of at least 20 million years. If this is true, they are hard to relateto a single impact,or evena cometshower.Vanucciel al (l98l) also found an extendeddistributionat Cubbio and from their compositionconcluded that the spheruleswereof volcanicorigin. If they are primary, and not the result of diagenesis, perhapsextendedvolcanicactivityvould be a more plausiblesourcethan a sho term event. Recentlythe situation hasbecomemore confusing as lzett (1987)has concludedthat the spherulesin the Raton Basinand at Caravaca, Spain, were unrelated to a possibleimpact becausethey fall belowthe iridium horizonwhile Bohor and Triplehorn (1987)think that the spherulesstayedin the Ioweratmosphere after the impact and fell to Earth before the iridiumrich layerwhichwasblownout ofthe atmosphere. Hidebrandand Boynton(1987)found that the absolute abundancesof rare earth elementsin the "non-impactor" part of the boundarylayer can bestbe explainedby a mixture of thick sediments and upper mantle materials and suggest an oceanicimpact site near a continentalmargin. One might wonder what kind of rare earth element abundances rould be expected from volcanism from deep in the mantle beneath a sedimentary basin. ShockedN4inerals Common visdomirnplies thatshocked minerals are indicativeof impactsand they have been found in some abundancein boundary clays at a numberof localitiesin westernNorth America (Bohoret al. 1984,1985;Izett and Pillmore 1985; Lerbeckmoet al 1979;Lerbeckmoand St. Louis 1986).The distibution and the sizeof the grains [up to 0,58 mm] suggestsa continental source in the Western United States although rhe isotope work of DePaolo(1983)suggestsan oceanic impact site. The Manson structure in Iowa was suggestedas a possiblecandidatesite sinceit is about the right age and may be an impact structure, but it seemsto be too small (32 km) to be the record of the global extinction eveDr, In the 1960'sit wasconcludedthat shocked mineralscould not be producedby volcanoes because the modelsof volcanoes suggested that pressureswere not sufficientlyhigh, There is little evidence,however,that volcanicdebriswas ever examinedfor shockedminerals.Cartere, aL (1986,l9B7) examinedvolcanicash from the major eruption of Toba 75,000years ago which Kent (1981)concludedhad 40Toof the energy of the proposedimpact, and found shocked minerals,including quartz. They concludedthat pressures reachedat least l0 Gigapascals. Bohor et al. (1987)disclaimsToba because amountsof shockedquartz are so small.They report 25Toat BrownieButte, 40% in GPC-3, 307ofrom the Raton Basin.In this paper they claim that a commonaveragefor all K/T boundary claysstudiedis about25% althoughearlier (Bohorand Izett 1986)reportedlow or very low ratiosof shockedto non-shocked quartzat sites in Europeand New Zealand.If both statements are true, the "common average" is deceiving. Investigation of the Bishop tuff from the eruptionof the Long Valley calderain California 600,000years ago also revealedshocked minerals including feldspar,mica and some quartz (Carter et aL, in press).Most of the shockedquartzgrainsfrom volcanicexplosions found to date have only single planar features while many of thosedescribedin Western North Americacontainmultiple planes.This is taken as an argument in favor of impact and this may be the casein and nearwesternNonh America, but abundances elsewhere are too low to allow definitive statements. Still more recentlyCarter examinedan extendedpart of the Gubbio sectionand found shockedminerals over a considerablerange (Hallam, 1987).The distribution is difficult to relateto a singleimpact,but might be more appropriatelyassociated with extendedand extensivevolcanism.Maybeshockedmineralsare more abundant in the sediments than we think; perhapswe shouldlook for them. Soot A paperin Scienceby Wolbacket aL (1985)suggestedrhar graphitic carbon found at the K/T boundaryrepresentedsootfrom wildfiresstarted by an impact that producedan asteroidalwinter. It was claimed thar the flux of "soot" ar the boundarywas103-l0a times"normal" values.If you read the fine print in this a icle, the reason that the sool flux is in excessis becauseit was assumed that the boundary clay layer was depositedin one year while the layersaboveand belowvere takenasdepositedat ratesof cm/1000 years.The only comparisonsgiven were for the StevnsKlint sectionin Denmarkwherethereis about4 times as much carbonin the K/T layer as there is in layers above and below. This is a messysectionin which there hasclearlybeencarbonatesolution at the boundary(Ekdaleand Bromley 1984). If one makes a different a s s u m p l i o n - t h atlh e r " l a 1k e p tc o m i n gi n a t a constantrate- andthen compares the claycontent of the limestoneswith that of the boundary clay, one would actually get a lower carbon flux at the boundary. Global Distribution Finally, we come to the question of worldwide distribution.In the asteroidmodel,the energy is reputed to have blown a hole through the stratospheieand allowedfairly large particlesto circle the Earth. Could volcanoesdistribute things as widely? We know from satelliteobservationsthat aerosolsfrom explosivevolcanoeslike El Chichon may reach elevationsof 40 km and circulate aroundthe world(Clancyl986),but we haven't Did the DinosaursDie or Evolveinto Red Herrines? 137 foundmuchiridium associated with this type of volcanoas yet. A reasonablequestionthat might be askedis whetheriridium from hot spot or flood basalt volcanismcould circulate very far. Zoller et al. (7983)and Olmezet aL (1986)believe that iridium is in a volatile phase,possiblya fluoride,but flood basaltsare quite different from explosivevolcanoes. RecentlyStothersel ol (1986)concludedfrom plumetheory,appliedto observational estimates of the rate of thermalenergyreleasefrom large fire fountains.that flood basaltfissureeruplions that produceindividual flows with volumesof > 100km3at very high rateshavethe potential to inject aerosolsinto the lower stratospherewith potentiallydrasticshort-termconsequences. If the iridium is carriedasa volatile,onecouldexpect it to be carriedconsiderabledistancesin the a t m o s p h e rTeh. e c o n f i g u r a t i oonf t h ec o n t i n e n t s at K/T time permitted rapid global circulation of the oceansurfacewatersin low latitudes,and one couldanticipatefurtherdistributionby rhis means.This model would suggestthat iridium from hot spot volcanisrncould have beenwidely circulatedat K/T time. Volcanoes or Bo des? Thus it would appear that rnost of the features attributedto a bolide can alsobe explainedin terrnsof purelyterrestrialphenomenaand that prolongedvolcanismwould better fit the data than an instantaneous event. The very large ciater that would be expectedfrorn the irnpact of a l0 km bolide has been very elusive,But maybeit's buried somewhereor hasbeenmashed processes. up by geological The isotopedata(De Paoloet ol. la83)supportthe idpaof an oceanic impactand maybethe craterhit there and then wassubducted,although an oceanirnpact would presentsomeproblemswith the shockedquartz which suggestsa continentalsite. Maybe we aren't lookingfor the right thing; maybeit producedvolcanismwhich has hidden it, although the volcanismassociatedwith impactson the moon seemsto be pretty much confinedto the interiors of the craters. WhatWas The OuestionAgain? In the skirmishingthesedaysaboutphenomena that may be associatedwith impacts or with rol,'anism t h ep r i m a r l q u e s t i o sn o m c t i m egse t s 138 Drake and Herman lost. It is wherethere is any relationshipbetween impacts and the extinctionsof organismsfound in the fossilrecord.The answeris mostlikelyro comefrom paleonecrology, the studyof the extinctionsthemselves. What Happened at the K/T Transition? T h e K ' T b o u n d a r yr a s d e f i n e da l o n gt i m ea g o by the stratigraphersfrom disappearance of some fossilsand appearanceof others.The K/T extinctionsdo not all seemto be instantaneous, but are spreadover 103-l0syearsin the geologicalrecord and not necessarily synchronous. They wereselective; obviouslyeverythingdidn't die or you wouldn'tbe readingthis. Most of the largeand nakeddied,but mostofthe small,the furry, and the feathered survived despite their higher metabolism rates.The lastofthe dinosaurs seem to have died at about that time, but not the turtles, salamanderso , r c r o c o d i l e s .T h e moootremessufferedgrievously,but not the other mammals. It is very difficult to figure out exactlywhen the dinosaursdied out becauselre have found few of them left by the end of the Cretaceous. This was geologically a very spirited time in Earth historyand therehasbeenno shonageof ideas about the causesof their extinction; senescence, sealevelor climatechange,competition from mammals,starvationafter the impact, volcanicactivity,etc. Many organisms,including the dinosaurs,werein a long periodof decline beforethe extinctioncameand may havedisappeared through other causes. Many other organismsdid nor die at the end of Cretaceous time. Why did they surviveand othersthrow in their hands (or fins or hoovesor paws)? Selective Fxtlnctons A sweepingextinctionof ecologically unrelared organismsresultingfrom a protractedperiodof darkness,as postulatedby the asteroidimpact hypothesis(Alvarezet aL 1980),apparentlydid not occurat the closeof the Cretaceous. In fact, land, fresh rater, and rnarine noncalcareous photosyntheticorganismswhich should have been most affectedby an extendedblackout crossedthe boundaryunscathed(Archibaldl98l, iz Russelland Rice,eds.,l9B2;Van Valen1984, iz Berggren and van Couvering,eds., 1984; Hickey l98l; Hickey et al. 1983;Van Valen and r Sloan l9?7; etc). Extinctions were selective,affecting approxirnately90Vo of the warm-waler calcareousphyto-and zooplanktongenerain the Tethyan-Paratethyan regions(Tappan1982;Herman et al., in press).These highly diverse taxa vith manyendemicrepresentatiyes wereat the peakof their evolutionarydevelopment.The cocc o l i t h o p h o r e B r a a r u d o s p h a e r aa n d t h e c a l c a r e o u sc y s t - p r o d u c i n g d i n o f l a g e l l a t e Thoracosphaera survivedthe late Cretaceousenvironmentalcrisis;both have living representativesand are consideredtolerant of a wide rangeof habitats.These"survivors" are most commonin the basalDanian in mid and high latitudedeposits,but are scarcein low latitude sediments(Tappan 1979,Perch-Nielsen er aL l9B2). A similar pattern was observed in planktonic foraminifera. Following the nearly completeannihilationof the entiregroupat the end of the Cretaceous, one survivorwas found in the earliestTertiary sediments(Smit l9B2). This specieswaseithertolerantto a widerange o f p n v i r o n m e n tcaol n d i t i o nosr w a "a " u b s u r f a i e waterinhabitant.Higherlatitudeformsseemto havecaried on with lessattrition; noncalcareous ones,particularlythe diatoms,silicoflagellates and radiolarians, diversified at the end of the M e s o z o i ce r a . O t h e r g r o u p " o f o r g a n i s m s . graduallydecliningthroughourthe Cretaceous, diedout at the K/T boundary.They includethe ammonites, belemnites.inceramidbivalvesand rudists(e.g.Emiliani er aL 196l; Russelland Rice,eds.,1982;Berggrenand van Couvering, eds.,1984).Benthonicforaminifera,scaphopods, gastropods, nautiloids,bryozoans, brachiopods, marine turtles and irregular echinoidsexhibit only gradualextinctionacrossthe boundary(Tappan, 1982;Hermanet al, in press).Extinctions (Russell,1979). of freshwaterfish wereselective Arthur et al. (1987\ noted a decreasein marineprimary productivityfor a period of as long as 1.5MY near the K/T boundary,Accompanyingthis wasselectiveattritionoforganisms that dependeddirectly upon the flux of organic matter as a food sourceas oDDosed to detritus or depositfeederswho "ould-liu" off the accumulated organic carbon. It's fair to ask rrhetherthis wasthe resultof a short lived dusr cloud frorn an impact or of an eventor events of longer duration. Palon-nnoennarrnhv While dinosaurshavegreat PR value,they are not the most definitive creaturesto use for describingthe event becausethey were already in declineand becausewe havefound so few of them. Marine organisms are rnuch more numerous and widely distributed and have a fasterturnover time, so they shouldbe better indicators. Therefore it is useful to examine the paleooceanography of K/T time. The positionsof the continentsin K/T tirne wouldpermit readyglobalexchangeof surface waters at low latitudes in the Northern hemisphereBerggrenand Hollisterl98l). The paleooceanographerstell us that bottom temperatures in the deepoceanwerewarm and deepcirculationwassluggish,probablysalinity driven rather than temperaturedriven like today (Berger 1979).It vould seem possible,if the relativetime constantsrrereright, to introduce somethinginto the surfacewatersat low to mid latitudes that would haveits major effect on the calcareous-shelledplanktonic organisms, but would have lesseffect on the benthic ones,the siliceous-shelledforms, or the higher latitude organisms. In the modernoceanthe surfacewatersare supersaturated with calciumcarbonate,but in the oceandepththe watersare undersaturated and at the carbonatecompensationdepth [CCD], which dependsupon temperatureand the partial pressureof COz,the CaCO3content of the sedimentgoesto zero;the CaCO.dissolves out. Ar K/T time, there is confusionabout what happensto this CCD at K/T time. Somehave arguedthat it stayeddeepwhile othershavesuggestedthat it cameup to the surfaceand then rapidly descendedagain(Worsleyl9?4).This iatt e r \ ^ o u l dr e q u i r et h c a d d i t i o no f a v e r l l a r g e quantitlof H- ion becausp the bufferingcapacity of the wholeoceanis very large.But if the oceanwerestronglystratifiedat that time (Berger 1979)with sluggishdeepcirculation,a short term acidifying event would only have to affect the mixed layer of the surfaceoceanto makelife difficult for the calcareousshelledorganismsthat live in thesesurfacewaters,but to havea lesser effect on the benthic organismswho live below the mixedlayer.The reductionofcarbonateproductionin near surfacewaterswould result in an apparent upward excursionof the CCD (Arrhur et al. 1987). Did the DinosaursDie or Evolveinto Red Herrines? 139 Smit and Romein(1985)havenotedthat the planktonicforaminiferatend to disapCretaceous pear first, followedby the coccolithsover a more extendedperiod of time. Srudiesindicatethat the growth of modernforaminiferais inhibited when the pH reaches 7.6-7.8 while modern cochaveproblemsproducingtests colithophoridae whenthe pH reaches?.5 and do not survivebelow a pH of 7.0-7.3(Phlegerand Bradshaw1966; Boltovskyand Wright 1976;McLean 1985;Sikes and Wilbur 1980,1982;Sikeset al 1980).It has beenshownthat an abrupt increasein the rate of calcite dissolutionin oceanwater occursat a ApH of 0.08 (Takahashi1975). If thereis a loweringof the pH, we wouldexpect the forams to go first and the coccoliths later. The replacing Tertiary foraminifera are small and simple. (Perch-Nielsenet al. 1982, Russelll9B2).For modernplanktonsmallforms survive better in a lower pH environmentthan large forms (Boltovskyand Wright 1976),and an environmentwhich is undergoingchangeis selective towardsmallerspeciesas comparedwith a more stableenvironment(Kilham and Kilham le80) Organismsnot dependentupon carbonate, such as dinoflagellates, silicoflagellates, or diatomswould be Iessaffected(Gensemerand Kilham 1984).Some dinoflagellateslive in lake waterswith a pH of 4.0 (Tappan l9B2).We have to be a little careful,of course,whenwe dealwith paleonecrology, becausewe are rooting around i n l h e g r a v e y a r dn.o l e \ a m i n i n gl h e o r g a n i s m s themselves. lsotopic Indicators MacDougall(l9BB)interpreteda significantexcursionin ratio of 3'Sr to suSras an indication of greatlyincreasedcontinentalweatheringdue to acid precipitation following a large bolide impact.He basedhis conclusionon datafrom Hess et al. (1986\,but did not use the detailed data from Koepnicket oL (1980)which indicatethat the excursionbeganwell beforethe K/T boundary. The completedata make it difficult to associatethe data with an impact at K/T time (0fficer er aL, in preparation). OrTgenisotopiccompositionof the carbonate shellsof foraminifersand coccolithplates in marinesedimentsprovidethe combinedrecord 140 Drake and Herman of pastoceanrater temperaturesand the isotopic compositionof seawater (e.g,Duplessy1978). Among the factors conditioning and modifying s e a w a l e r c o m p o s i t i o nt e . m p e r a t u r es,a l i n i t ) . precipitation,river runoff, evaporationand convective mixing are considered important. In marine depositsthat contain continuoussequencesof latestCretaceous and earliestTertiary sediments,the oxygenisotopiccompositionof planktonicforminiferalshellsand bulk sediments (made up principally of nannoplankton and microplankton skeletalfragmerts)at the boundary showremarkablysharpoxygenisotopeshifts (Perch-Nielsonet al. l982l, Herman e, aL, in press).Although diageneticeffectscannot be ignored (Hallam 1987),this signal can be interpretedin termsof paleotemperature and suggests that the isotopicenrichmentat the K/T boundary can be attributed primarily to a sudden temperature drop while the laO depleted sedimentsin the overlyingEarly Danian suggesr a warming trend of the surface ocean water. The distribution of 6'3C is controlled by the primary productivity in near surfacewatersancl oxidationof organicmatter and generationof CO' in deepvaters (Kroopnik 1980,Williams er al. 19771.Alarge andrapid negativeshift in 6'3C valuesis found for planktonicforaminiferaat the K/T boundary although the magnitude varies with location(Perch-Nielson el al 1982,Scholle and Arthur 1980),Benthic fonns do not indicate this negativeshift and, indeedsometimesexhibit the oppositetrend (Boresmaand Shackleton l98l) extendingover a longer time period. T h e e v i d p n cien d i c a t easr a p i dd i m i n u t i o n in primary productivitynearthe K/T boundary(Hsu et al. 1982\.Bramlette(1965)and Tappan(1968) suggest nutrient starvation as a cause;others have related this to climate changeas indicated (Emilianiet al. 1981, by the paleotemperatures Fisherand Arthur 1977,Stanley1984)although Arthur et aL (1987)do not feelthat thereis consistent, irrefutable evidence for major temperaturechangesacrossthe boundary.They also find it difficult to reconcilethe changesin productivity with an irnpact becausethe lowproductivity epidode has a duration of at least I million years.One possibility(Sepkoskil9B4) is thar the characteristic recovery time of organisrnsto a shocking event may be of that order of masnitude. Alternate Extinclion Mechanisms lmpact Related Hsu(1980,l98l) notedthat the dust cloudfron an asteroidfailed to accountfor selectiveextinctions and preferred a cometary impact. He postulatedthat the extinctions were due to two causes;the larger animals died in responseto heatstressfollowingthe impact while calcareous planktonweredonein by cyanidepoisoningfrom the comet.Hsu e, a/. (1982)modifiedthe scenario to include the possibility of heary metal poisoning as well. Prinn and Fegley(198?)calculated that an asteroidcoming in normal to the surface wouldhavea modesteffecton the atmosphere, but that a comet travelling through the atmosphereat grazing incidencecould create enoughoxidesof nitrogento produceacid rain and affect the pH of the surface ocean. The lifetimeof N:O in the atmosphereis about 150 years(Crutzen,1987)so geologicallythe effect vrould be short term. Both of these mechanismsrespondto the s e l e c t i v i tp y r o b l e ma n d a r e m o r e p r o m i s i n g mechanisms than a dust cloud if a cometcan produce the iridium and other effects ano a crarer can be found. They both have problemsif the extinctionstook placeover a considerable span of time. Volcanism N u m e r o u sw e l l d o c u m e n t ehdi s l o r i ce r u p t i o n s havebeendiscussedin the literature(e.g.Budyko 1977,Offlcet et al. 1986),Resultsof stuaressummarizedin thesepublicationsindicatethat strong positive correlationsexist between volcanic emanationsand reductionin temperatureas well as atmospherictransparency. During volcanic eruptions CO, and sulphureous gases are dischargedinto the atmosphere;the latter reduce atmospheric transparency by increasingaerosol p a r t i c l e isn t h e l o n e r a t m o s p h e r cea. u s i n gc o n siderableair and sea-surfacetemperaturedrops (Budyko1977).The processis followedby settling of the particles,with reestablishment of "normal transparency"and increasedconcentratrons of C0r, which resuhs in rapid temperature increase. Late Mesozoicand Early Cenozoicglobal volcanismwas considerablymore widespread than it is today(Campsieet al 1984;Officerel aL l9B7)reachinga peakin K/T time. Bv analogy to present day effects of volcanic activity one would expecta depressionof surfacewater pH. Increasedacidity would dissolvethe calcareous shellsand temporarilyprohibit calcitenucleation leading to the massmortality of the warm water calcareousphyto and zooplankton(Takahashi 1975).The dissolutionof CaCO3wouldresultin the additionof bicarbonateto the oceanwarer which would ultimatelybuffer its pH (Weyl,written communication),so for long term effectsto be realized volcanic activity would have to persist,not be instantaneous. Keith (1980,l9Bl) emphasizedthe role of volatile emissionsin reducingatmosphericozone and alteringsurfaceoceanchemistry.Ekdaleand B r o m l e y ( 1 9 8 4 )c a l l o n v o l c a n i ce m i s s i o n s , especiallyCO, as a possibility.DeweyMclean (l9B5a)suggested that the cumulativeeffectsof COuemissionsover 500,000yearsfrom the Deccan traps of India at K/T time might reduce oceanalkalinity,but this assumesthat COzwould continueto accumulate. Feedbackmechanisms o v e rl h i s t i m e p e r i o d s. u r " ha " " h a n g e si n r i v e r flux of carbon,exchangesbetweenupper and lowerocean,changesin ratesof removalin the sediments, biologicalactivity,or othersare difficult to assessand could seriouslyinhibit this mechanism. Anothercandidateis sulphuricacid, Detrimental effectsof presentfossil fuel S0, emrnission havebeenreportedfrom local areason land, but the hydrogen ion flux per unit area of the oceanfrom this is too small to reduceoceanpH significantlyevenif it all went there.However, an unusualeventproducinga larger amountof S0, over geologicallyshort time periods of 10'?-l0s yearsthat wasconcentrated in the mix, ed layer of the surfaceoceancould havea significant effect. The Deccantrapsof India representsuchan unusualevent(Subbaraoand Sukheswala, eds., 1981. Olmez et al. (1986\estimatedthat the iridium emissioncould be expectedto produce about 30,000tons,a little lessthan believedto be presentat the K/T boundary.They wereextruded at K/T time near the equatorwherevertical circulationofthe atmosphere is strongand were handily located near the mouth of the TethyanSea.The remnantsafter erosionhave a volume of at least 105krn3and the great bulk of them were extruded during one reversed magnetic interual, 29R, that spansabour half a Did the DinosaursDie or Evolveinto Red Herrinss? t4l million years and crossesthe K/T boundary. Courtillot e, al. 1986). lt they ryere extruded evenlyover this entire period, they would representan approximatedoubling of the presentrate volcanism(NakamuraI974).If of non-ocean-rift they wereextrudedin pulses,the short-termrate increasewould be ever.greater. Volcaniceruptionsare, of course,intermittent (Axelrodl98l; Kennett and Thunell 1975, t t a l . 1 9 7 7D ' o n na n d N i n k o v i c h 1 9 7 7K ; ennete 1980).On a historicaltime scalevariationsin volcanic energy release of two orders of magnitudeare found. The Cenozoicrecord of sedimentcoresfrom the seafloot showsvanationsof the sameorder.Swanson(1975)estimated that rhe ?00 km3 Roza flow in the Miocene Columbiabasaltsvas eruptedin abouta week.The idea of major volcanicpulsesdoesnot seemat all unreasonable. Data from Laki in Iceland and Kilauea in Hawaii indicate that fissure volcanoeslike the Deccantrapsemit more HrSOaper unit volume (Devineef of ejectathan do explosivevolcanoes al 1984;Sigurdsson1982,Rose et al. 1985\.A' back of the envelopecalculationsuggeststhat intensevolcanicpulsesin the time framefrom 102 to lOa years could produceenoughacidity to make life difficult for carbonate dependent organisms; any explosivevolcanismat K/T time would add to this effect. Geochemicaldata suggestthat the COzcontentof the Cretaceous atmospherewashigher than it is today and a further increasefrom volcanicCOzwouldaccentuate t h e e f f e c t sW . h e n o n e a d d st o t h e s ec m i s s i o n s the mountainbuilding,the major regressionof LiteratureCited Alvarez,L. W., W. Alvarez,F. Asato,and tl. V. Mich€I. 1980. exExrarerrestrial cause{or rhe C.etaceous-Tertiary - l0 8 . t i n c t i o n .S c i e n c e2 0 8 : 1 0 9 5 1 Alvarez, W., I986. To{ard a rheory of inpact crises. EOS 6?:649-658. Alvarez,L. V., 1987.Massexrinctionscausedby large bolide inpacts. PhysicsToday, July Pp. 24-33. Archibald, J. D. 1981.Th€ earliest known Paleocenenammal fauns and its implications {or |he CretaceousTertiary transition In Russell,D. A. and G. Rice, eds., Proceedinssof the KjTEC Worlshop on C.etaceo&s Tetttury Ertinctions and Poxible Tenestrinland Et traterre*rial Causes.National Museuns of Canada, Dnision, Ortawa,Canada,Pp. 650652. Paieobiolosical 142 Drake and Herman the sea, and the climatic effects to be expected frorn these, it is not surprising that we now live in a dinosaur-free environment, Conclusion? A novel causefor global eventsseemsto have much more popular appeal than a more pedestrianone. It is surprisinghow readilywe canacceptthat the spraycansthat we useto keep our bodiesinoffensiveand our blow-drycoiffures in placeare destroyingthe ozonelayeroverAntarctica, or that the effluent from the fossil fuel we burn is killing off the treesand changingthe clirnare,but at the sametime find it difficult ro believethat the extrusionof over a million cubic kilornetersof lavaswith their associatedvolatiles will havelittle effect on the environment.Surely if the CFCsin the spray canscan destroythe ozone layer, the volatiles(includingchlorine, fluorine, carbon dioxide and sulphur dioxide) frorn this enormouseruptiveeventmust havehad a significant effect. Can it be just coincidence that the other major Phanerozoiceruption of the sameorderof magnitudeoccurredin Siberiaat the transition betveen the Paleozoic and Mesozoiceras? Extinctionsare closeto the heart of geology. The geological time scalewasbasedon the comings and goingsof organismsthroughthe eons. Theoriesof evolutionare basedon the changes in organismsthrough time and how and why they changeis crucial to thesetheories.Regardless of whether the impact theory is right or wrong, we should be grateful to its developersbecause they have stirred up a great deal of research which will move us closer to the truth. Arthur, M. A., J. C. Zachos,and D. S. Jones. t98?. Prinarl productivity and the Cretaceous/Terriaryboundary event in the oceans.Cret. Res. 8:43-54. Axelrod, D. L l98l. Role ofvolcanisn in climaie and evolution. GeologicalSociewofAnerica, SpecialPaper185, 59 pp. Berger, W. H. 1979. Inpact of deep sea drilling on paleooceanography. Anerican GeopbysicalUnion, Maurice E w i n g S e r i e s 3, , 2 9 ? - 3 1 4 . Berggren,W. A., and C. D. Hollister. 1981.Paleogeography, paleobiogeographyand the history of circulation of the Adsntic Ocean. Soc. Econ. Paleont. and Min., Spec. Pap. 20:126. Berggren, W. A., and J. A. van Couv€.ins, eds. 1984. Catastrophesand f,arth Hisrory. Princeion Univer- Boersma,4., and M. J. Shaclleton. 1981.Oxygen and carbon isotope va.iarions and planktonic foraminifer deprh habitats, Lale Cr€lac€ousto Paleoc€ne,central Pacific. /tr, Rep. D€ep Sea Drilling Proj€ct 62:5t3-526. B o h o r , B . F . , E . E . F o o r d , P . J . M o d r e s k i ,a n d D . M . Triplehom. 1984.Min€ralogicevidencefor an impact event at th€ Cretaceous-Tertiaryboundary. Science 224t867-869. Bohor,B. F., P. J. Modreski,snd E. E. Foord. I985. A search for shock-rnetamorphosed qnartz at the Kn boundary. Abs. Lun. Plan. Cont 16:79-80. B o h o r ,B . F . , a n d G . A . I z € t t .1 9 8 6 W . o r l d v i d es i z ed i s h i b u tion ofshockedquarrzat the KiT boundary:Evidence for a Nonb Americanimpact site (abs.)Abs.lTth Lun. Plan. Sci. Conf. 17:68. Bohor, B.F., P.J. Modresli, and E. E. Foord. 1987.Shocked quartz in the Creraceous-Tertiaryboundary clays: f,vidence for a global distribution. Science 236:705.709. Bohor,B. F., and D. M. Triplehorn. 1987.Flyash:An Analogy for Spherulesin K-T BoundaryClays.(abs.)Abs. Lun. Plan. Sci. Cont lB:103. Boltovsky,E., and R. Wrighr 1976.R€centfo.aninifera. Junk, The Hauge. Branlette, M.N. 1965.Massiveextinctionsin biota at the end of Mesozoic tine. Science 148r1696,1699. Broecker, W. S. 1974. Chemical Oceanography,Harcourt Brace Jovanovich,New Yd!. Brooks,R. R., O. L. Ho€!, R. D. Reeres,R. V. C. Wallace, J. H. Johnson,D. E. Ryan, J. Holzbecher,and J. D. Collen. 1985. Veathered spheroids in a Cretaceous/Tertiaryboundary shale at Woodside Creek, New Zealand. Ceology 13:738.740. Brouwers,E. M., I9. A. Clemens,R. A. Spicer, T. A. Ager, L. D. Ca.ter, and W. V. Slirer. I98?. Dinosaurson the North Slope, Alaska: High larirude lstest C r e r a c e o uesn v i r o n m e n t sS. c i e n c e2 3 7 r I 6 0 8 ' 1 6 1 0 . Bucher,V. H. 1933.Cryptovolcanicstructuresin the United States-Proc. l6ih Int. Ceol. Cong., Unit€d Stares. 2:1055-I084. Bucher,W. H., I963: Cryptovolcanicstructurescausedfron without or fron within the earth?("astroblemes" or "eeoblemes?"). Amer. Sci. 261:597-64A. J. Budyko,M.I. l9?7. Clinate Changes.Ane.. Ceophyl Union. W a s h . ,D . C . C a n p s i e J, . , G . L . J o b n s o nJ, . E . J o n e sa, n dJ . E . R i c h . 1 9 8 4 . Episodic volcanisn and evolutionary crises. EOS 65:796-800. Caner, N. L., C. B. Officer, C. A. Chesner,and W. I. Rose. 1985.Dynamicdeformationat the Cretaceous/Tertiary boundary: interences fron Toba ejecu. Ceology 14r380-383. Caner, N. L., and C. B. officer. 1987.Reply on "Dynamic d€fomation ofvolcanic ejectafrom the Toba caldera: Possible relevance !o cretaceoushertiaryboundal]i phenornena." Ceology. l5:91-92. Carter, N. L., C. B. Officer and C. L. Drak€ (in preparation) Dynamic defornation of quartz and feldspar: Clues io causesof some natural crises. C h a o ,E . C . T . , E . M . S h o e m a k ear n d B . M . M a s d e n .1 9 6 0 . First natural occunence of Coesite. Science 132:220.222. Clancy,R. T. 1986.El Chichonand "Mlsterl Cloud" aerosols bet{een 30 and 55 lnr Global observations{ron the SMA visible spectrometer. Geophys. Res. Lett. 13:931.940. Clayton,R. N., T. K. Mayeda,and D. E. Browniee.1986.0xygen isotopesin deep-seaspherules.Earth. Plan. Sci. Le.r.79:235-240. C o u r t i l l o t ,V . , J . B e s s eD , . V a n d a n m e ,J . J . J a e g e r a , nd R. Montigny. 1986.Les epanchenentsvolcaniquesdu Deccan(lnde), causedes extinctionsbiolosiquesa la limit€ Cr6rac6-Tertiar6? Conptes Rendus,Acad, Sci. Paris. 303 (Ser. II): 863-868. Crockett,J. H., C. B. Officer,C. D. Johnson,and F. C. Vezel. 1986.Distribution ofnoble netals, ars€nic,antinony and tungstenacrossthe Cretaceous-Tertiary boundary at Gubbio,Italy: iridiun valuesas a possibleconstraint on ihe duration of Cretaceous-Tertiaryboundary events.Abs.and Progan,99th Ann. Me€t.,C€ol.Soc. Amer., San Antonio, p. 575. Crutzen, P. J. 1987.Acid rain ar the K/T boundary. Nature 330:r08,109. Darton,N. H. 1910.A reconnaisanceof parts of northwestern New Mexico and Norrh€rn Arizona. U. S. Geol. Sur. Bull.435:1-88. de Laubenfels,M. W. 1956.Dinosau. extinction: One more hrpothesis.J. Paleo. 30:20?'218. Davis, M., P. Hut, and R. A. Muller. 1984. Extinction of species by periodic conet showers. Nattrre 308:715-?l?. De Paolo, D. J., F. T. Krte, B. D. Marshall, J. R. O'Neill, and J. Smit. 1983. Rb-Sr,Sm-Nd,K, Ca, 0 and H isotope study of Cretaceous-T€rtiary boundary sediments,Caravaca,Spain: Evidencefor an oceanic impacr sire. Earlh PIan. Sci. Lett. &:356-373. Devine,J. D., H. Sigurdsson,A. N. Davis, and S. Selt 1984. Estimates of sulfur and chlorine yield to the at, nosphere from volcanic €ruptions and poteniial climatic e{fects.J. Geophy. Res. 8963094325. Drake, E., and P. D. Komar. 19B4.Origin of impact craters: Ideas and experiments of Hooke, Gilbert, and Wesener. Geolog) 121408-411. Donn, W. L., and D. Ninkovich. 1980. Rste of C€nozoic rolcanisn in the North Allanlic fron deep seacores. J. Ceophys.Res. 85:5455-5460. Duplessy,J. C. 1978.Isotopic studies.In, Climate Change, J. Gribbon, ed., Cambridge Univ. Press,Canb dse. Pp. 46-61. Ekdale, A. A., and R. G. Bronley. 1984.Sedimentologyand ichnologyofthe Cr€taceou+Tertiary boundaryin Dennarl: implications for the causes of terrninal C.€raceousextinction, J. Sedi. Pel.o. 54:681-703. Did the DinosaursDie or Evolveinto Red Herrines? 143 El.Baz, F. 1980.Gilbert and the Moon. /n, Yochelson,E. L., ed., The sci€ntific ideas o{ G. K. Gilbert. Geol. Soc. Aner. Spec. Pap. 183: 69.80. Emiliani, C., f,. B. Kraus, and E. M. Shoenak€.. 1981.Sudden death ai the end of the M€sozoic.f,arth Plan. Sci.Leu.55:317.334. Fish€r,A. G., and M. A. Ar$ur. I977. Secularvariarionsin the pelagic realm. Soc. Econ. Pet. MiD. Sp€c. P b. 25:19-50. Gensener, R. W., and S. S. Kilhan. 1984.Growlh rates of five fresh{at€r alsae in weli buffered acidic media, C a n a d i a nJ . F i s h . A q u . S c i . 4 l : 1 2 4 0 - 1 2 4 3 . Cilbert, C. K. 1983.The Moon's face, A srudy of rhe origin of its features. Phil. Soc. Vash. 12:241-292. fiallan, A. 1987.f,nd-Cr€taceous exiinciion evenr Argrment for t€rreslrial cslrsalion. Scienc€ 238:1237.1242. Hansen,H. J., R. Gwozdz,J. M. Hansen,R. G. Bronley, and K. L. Rasmussen. 1986. The Diachronous C/T plankton extinction in the Danish Basin (abs.)sth Alfred Wegener Conference,Cittiingen, 1986. Hansen,H. J., R. Cvozdz, R. C. Bromlel, K. L. Rasmussen, f,. W. Vosensen, and K. R. P€dersen. 1986. Cr€tac€ous-Terdarysphe les from Denmarl, New Z€alandand Spsin.Bull. Ceol.Soc.Dennark 35:?5€2. H a n s e nH , . J . , K . L . R a s m u s e n ,R . G r o z d z ,a n d H . K u n z e n , dod. 1988. Iridiun-bearing soot at th€ CrehceousTertiarl boundary.Bul] Ceol.Soc.Dennark (ir pr€ss). Hartmann,W. K. 1982.Aitonomy, The Cosnic Journey(2nd Ed.). Vadsworth, Inc., Belmont Cal. Heiken, G., and G. Lofgren. 1971.Terrestrial glasssphe.es. Geol. Soc. Aner. Bull. 82:I045-i050. Hermsn, Y., S. K. Bhattacharaya,K. Perch-Nielsen,L. F. Kopaevitch,D. P. Naidin, V. T. Frolor, J. D. Jeffers, G. E. Claypool,A. Sarker,and P. E. Rosenberg.1988, Cretaceous-Teiia./ marine extinctions:The Russian Platform .ecord. Proc. Int. Coni. Paieontologyand Evolution: Extinction Events,Leioa, Spain, l9B7 (in Hess,J., M. L. Bender, and J. G. Schilling. 1986.Evolution ofthe ratio ofstrontium€?lo strontium{6 in seawat€r from Cretaceousto present. Science231:979-984. Hickey, L. J. 1981. Land plant evidence conpstible {ith gradual, not catastrophic,change at rhe end of the Cretaceous.Nature 292:529.531. H i c k e y ,L . J . , R . M . W e s t ,M . R . D a $ s o n ,a n d D . K . C h o i . 1983.Arcdc rerrestrialbiola: paleomagn€tic€vidence of agedispariFJwithnidnonhern latitudesdudng the late Cretaceous and early Tertiary. Science 221:1153-l156. Hildebrand, A. R., and V. V. Boynton. I98?. The K/T er, cavBtedoceanic nantl€, f,vidence f.om REE abun. dances.Abs. from ISrh Lun. Plan. Sci. Conf., 103. Hooke, R. 1665.Micrographia: London, Jo. Martyn and Ja. Allestry, printers to the Royal Society.Reprinled bl Bruxelles, Culture et Civilisation, 1966,216 p. Hsu, K. J. 1980.Tenestrial catastrophecausedby com€hry irnpact at the €nd of Cretaceous.Nature 285:201-203. 144 Drake and Herman Hsu, K. J. t98t. Origin of geochenical snomalies at Cretaceous/Tertiaryboundary.Asteroid or comerary inpact? Ocean.Acta, No. Sp., 129-133,Paris. Hsu, K. J., Q. He, J. A. McKenzie, H. Weissert, K. PerchNi€lson, H. ob€rhansli, K. Kelts, J. LaBreque, L. Tauxe, U. Krahenbuhl, S. F. Percival,R. Vright, A. Karpof! N. Petersen,P. Tucker, R. Poore, A. Gombos, K. Pisciotto, M. F. Carman, and E. Schreiber. 1982.f, nvironmentaland EvolutionaryConsequences of Mass,Mo.tality at the f,nd of the Creraceous. Sclence 2t6:249-256. Hut, P., W. Wlvarez, W. Elder, T. Hansen,E. Kauffman, G. Keller, E. Shoemaker,and P. Weissman.1987.Con. et showers as a cause of na$ exrincrions. Narure 3 2 9 : lr B - t 2 6 . Izett, G. A. 1987.Aulhigenic "Spherules" in K/T boundary sedinents at Ca.avaca,Spain, and Raron Basin,Colorado and New Mexico, may not be impacr derived. Geol. Soc. Aner. Bull. 99:78-86. Izetr, G. A., and C. L. Pillnor€- 1985.Abrupl appearanceof shockedquartz at the Cretaceous-Tertiary boundary, Raton Basin, Coloradoand Ne{ Mexico, Ceological Society of America Annual M€eting Abstracts,617. Johnsson,M., and R. C. Reynolds.1985.Clay mineralogy of shalelimestone .hythmites in the Scaglia Rossa (Turonian-Eocene), Italian Apennines.J. Sedi. Petro. 56:501-509. Keirh, M. L. 1980. Cretaceousvolcanism and rhe disappearanceof dinosaurs.E0S 6l:400. Keith, M. L. I982. Viol€nt volcanisn, stagnant oceansand sone inferencesr€garding peiroleun, strata-bound oresand nass extinctions.Geochin.Cosrnochim. Acta 46:2621-2637. Kennett, J. P., and R. C. Thunell. 1975.Clobal inoease in Quaternary volcanish. Science t8?:497-503. Kennett,J. P., and R. C. Thune . 197?.0n e:<plosire Cenozoic volcanisn and clinatic implications. Sci€nc€ 196:1231-I234. K e n n e t t ,J . P . , A . R . M c B n n e y ,a n d R . C . T h u n e l l . l 9 ? ? . Episodeso{ Cenozoicvolcanisn in the circun-Pacific r e g i o n .J . V o l c a n .G e o i h e r .R e s . 2 : t 4 5 - 1 6 3 . Ken!, D. V. 1981.Asleroid extinction hypothesis.Science 2l l:648-650. Kilham, P., and S. S. Kilhan. 1980.The evolurionaryecology of phyroplankton./r, Moris,I., ed.,The physiological ecotogy of phytoplankton. Unir€rsity of California Press, Berkeley. Pp. 5?l-597. Koepnick, R. B., W. H. Burke, R. E. Denison,E. A. Hetherinston, H. F. Nelson,J. B. Oro, and L. E. Vaire. 1980. Constructionof the seawarer37Sr/sSrCurve Ior the Cenozoic and Cretaceous:Supporting data. Chen. Geol. (Isotope Geoscienc€Section).58:55-81. Kyte, F. T., and J. T. Wasson. 1986.Accretion rate o{ extraterrestrial mafler: Iridium deposiled 33 to 67 Million Years Ago. Sclence232,1225.1229. Lerbeckmo,J. F., M. E. Erans, and H. Baadsgaard.1979. Magnetostratigraphy, biosrratigraphyand geochronol, ogy o{ Cretaceous-Terdarybounda.y s€dinenrs. Nature 279:26,30. Lerbeckmo,J. F., and R. M. St Louis. 1986.The ierminal Cretsceousiridiun anomalyin the Red Deer Valley, Afberta, Canada.Can. J. f,arth. Sci. 231120.124. Luck, J. M. and K. K. Tur€kian. 1983.Osmiun 187/0smium 186 in manganese nodules and the CretaceousTertiary bourdsry. Sci€nce222:613. MacDougall,J. D. 1988.Seawaterstrontium isotopes,acid rain, and the CretaceousTertiary boundary.Science %9:445{87. Mclaren, D. J. 1970.Tine, life and boundaries.J. Paleont. 44:801-815. Mclaren, D. J. 1983.Bolidesand biostratigraphy.ceol. Soc. Amer. Buli. 94:313-324. M c l e a n ,D . M . 1 9 8 5 M . a n t l ed e g a s s i nisn d u c e dd e a do c e a n in the Cretaceous-Tertia.ttransition. Amer. Ceophy. Union, Geophy.Monosr. 32:493-503. Mclean, D. M. Deccan raps degassing in the terminal Cretac€ousmarine extinctions.Crer. Res.6:235,259. Michel, H. V., F. Asaro,V. Alvarez,and L. W. Aharez. 1985. Elementalprofile of iridiun and other elementsnear the Cretaceous-Terriary bounda.)' in hol€ 5778, Init i a l R e p o r t so f t h e D e e pS e aD r i l l i n g P r o j e c t , 8 6P . p. 847€49. Montanari,A., R. L. Hay, W. Alvarez,L. W. Alvarez,F. Asaro, H. V. Micheland, and J. Snir. 1983.Spheroidsat the Creraceous/TerriaryBoundary are alter€d impact droplersof basalticconposirion. Ceologyll:668671. Natamura, K. I974. Prelinina.y €srimateof global rolcanic produciion. In, Colp, J. and A. S. Furimoro, eds., Uiilization ofVolcanic En€rgy, Universiiy of Hawaii Pre$, Hilo. Pp. 273-286. Naslund, H. R., C. B. Officer, and G. D. Johnson. 1986. Microsphe.ulesin Upper Cretaceousand Lower Terriary clay laye.6at Gubbio,Iiab. Geology14:923-926. Nazarov,M. A., L. D. Barsukova,G. M. Kolesov,D. P. Haidin, and A. S. Alekseyer. t983. Origin of th€ iridium anomalyal the boundary betweenthe Maastrichtian a n d D a n i a n s t a g e s . C e o l h i m i a 8 : 1 I 6 0 - 1 1 7 8( i n Russian). Nininger, H. H. 1933.Our Stone,PeltedPlaner. Houghion Mifflin, Boston and Nen York. omcer, C. 8., and C. L. Drake. 1983.The Creraceous-Tertiary t . a n s i t i o n ,S c i e n c e2 t 9 : 1 3 8 3 , 1 3 9 0 . officer, C. 8., A. Haltam,C. L. Drake, and J. D. Devine.198?. Late Cretaceousand paroxysmalCretaceous/Terriary eatinctions. Nature 326t143.149. o l m e z ,L , D . L . F i n n e e a na , n d v / . H . Z o i l e r .1 9 8 6 .I r i d j u m enissions frorn Kiluaea rolcano. J. Geophys. Res. 91:653'663. Perch-NielsenK , ., J. McKenzie, and Q. He. 1982. Biosiratigraphy and isotope sirarigraphr and the catastrophicextincrion of calcareousnannoplankton at the Cr€taceous/Terdary bounda4. Geol.Soc.Amer. S p e c .P a p e r 1 9 0 : 3 5 3 - 3 7 1 . Phl€eet, F. 8., and J. S. Bradshaw. 1966. Sedimentaryenyironnenrsin a narine narsh. ScienceI54:1551-1553. P r i n n , R . G . , a n d B . I e g l e y , J r . 1 9 8 7 .B o l i d ei n p a c t s , a c i d rain and biospherictraunas at the C.etaceous-Tertiary Boundary. Eanh. Plan. Sci. Lex. 83:l-15. Preisinger,A., E. Zobetz, A. J. Gratz, R. Lahodynsly, M. Becle, H. J. Mauitsch, G. Eder, F. Cras, F. Rosl, H. Stradner, and R. Surenian. 1986. Th€ Cretaceous/Tertiaryboundary in the Gosau Basin, Austria. Nalure 322:794.799. Rampino, M. R., and R. C. Reynolds.1983.Clay nineralogy at the Cretaceous-Tertiary bou.dary. Science 2t9:495-498. Rast, U., and C. Craup. 1985. I.idium anomaly ar rhe C r e t a c e o u s l T e r t i a r yb o u n d a r y , L a t r e n g i b e r g e , Bavarian Alps. Terra Cognita 5,246. Rice,A. 1987.Shockninerals ar th€ K/T boundary:Eaplosive volcanism as a source.Phys. Plan. Int. 48:167-l?4. Rocchia,R., M. Renard,D. Boclet,and P. Bonte. 1984.Essai d'evaluation de la transition C,T par i'erolurion de l'anonalie en hidiun. Bull. Geol. Soc. France 26:1193-t202. Rose, V. I., R. L. Chaun, and P. R. Kyle. 1985. Rare of Sulphur Dioxide Enissio! f.om ErebusVolcano,Anr arctica, Decenber, 1983. Nature 316.710.712. RucHidge, J. C., S. de Caspa.is, and G. Norris. 1982. Stratig.aphic applicationsof acceleratornass speclrometer using Isot.ac€.3rd No. Amer. Paleo.Conv. Pro. 2:455-460. Russell,D. A. 1979.The enigma of the extinctions of ibe dinosaure. Ann. Rev. Eanh and Plan€ra.y Sci. 7:163-182. Russe, D. A., l9B2,The nass extinctionsof rhe late Mesozoic. Sci. Aner. 246:5865. Russell,D. A., snd C. Rice, f,ds. 1981.P.oce€dingsof the K/TEC V/orkshopon C.etaceous-Terliary f, xrinclions and PossibleT€rresrrialand Ertraterrestrial Causes, National Museuns of Canada,PaleobiologicalDivi s i o n , O t i a w a ,C a n a d a ,1 9 8 1 ,S l l l o g e o u s3 9 : 1 5 1 . Scholle,P. A., and M. A. Arihur. 1980.Csrbon isoiope fluciuations in Cretaceouspelagic lin€srones; porenrial sr.atigraphic and exploration tool. Aner. Asso.Pel. Geol. Bull. 64:67-87. Schmitz, B. 1985. Metal pr€cipitation in ihe Cretac€ousTertiary boundary clay at Stevns Klint, Dennark. Geochin. Cosmochin. Acta 49:2361-23?0. Schrnitz,8.,P. Andersson,and J. Dehl. 1988.Iridiun, sulphur isotop€ssnd rare earths in the Cretaceous-Terriary boundary clay st StevnsKlinr, Dennark. Geochim. Cosmochin. Acra. Gubnined). S e p h o s l ,J, . J . .J r . 1 q 8 4 .{ k i n F r i . m o d F o l t P h a n e r o z o irna r , onomicdiversir*.III. PosrPaleozoicfanilies and Mass E x t i n c t i o n s1 . 0 : 2 4 62 6 7 . Sha*, H. R., and D. A. Svanson. 1969. Eruption and floir raresofnood basalb.Proc.2ndColunbia Riyer Basair Sym.,Cheney,Wash.,E. Vash. SrateUniv. Press.Pp. 2?l-300. Shaw,H. F., and C. J. Vasserbere.1982.Ase and p.ovenance of the target materials for tektites and possible Did the DinosaursDie or Evolveinto Red Herrinss? 145 impacrires as inferred fron Sm/Nd and Rb/Sr sysrematics.Ea.lh. Plan. Sci. Letl. 60:1551?7. Shoemaler,E. M. l0. Pen€trationnechanicsof high velocity meteo.ites,illustrated by M€leor Crater, Arizona. Proc. Sec. 18, Stmcture of the Earth's Crust and Deformarionof Rocks,2lst Int. Ceol. Cong.,Norden, 4t8434. Sigurdsson,H. 1982.Volcanicpollution and climate:tbe l7B3 Lali eruption. EOS 63:601-602. Sikes,C. S., and K. M. Vilbur. 1980.Calcification by coccolithophorids: €ffects of pH and Sr. Phycology l6:433{36. Sikes,C. S., and K. M. Vilbur. 1982.Function of coccolith formation. Lirnn. Oceanog.27:18-26. Sikes,C. S., R. D. Roer, and K. M. Wilbur. 1980.Photosynth€sis and coccotilh formation; inoreanic catbor sources and ner inorganic r€action of d€position Limn. Oc€anogr.25:248-261. Simkin, T., and R. S. Fiske. t983. Kralatoa: a classic geophysicalevent. EOS 6?:513-514. Smit, J., 1982. Extinction and €volution of planktonic foraminifera after a major impact st the boundary.Geol Soc Aner. Spec. Cretaceous/Terriary Pap. 190:329-352. Smit, J., and A. J. T. Ronein. 1985 4 sequ€nceof events boundary,Earth Plan. acrossthe Cretaceous-Tertiary Sci.Lett.74:155'170. Smith, J., and G. Klav€r. 1981. Sanidine spherulesat the Cretaceous'Tertiaryboundary indicate large imPacl event. Nature 292:47-49. Subbarao,K. v., and R. N. Sukheswala.eds t98l. Deccan Volcanism. Geol. Soc. India Mem. 3:l'474. Sunley, S. M. 1984. Temperature and biotic crises in the msrine realn. Geology l2:205-208. Stothers,R. 8., J. A. Volfi S. Self,and M. R. Ranpino. 1986 Basaltic fissure eruptions, plune heighls, and atmosphericaerosols.Geophys.Res. Lett. l3:?25-728. Swanson,D. A., T. L. Wright, and R. T. Helz. l9?5. Linear vent systemsand estimatedrates of magna production and e piion for the Yakina basalt on the Columbia Plareau.Amet. J. Sci. 275:877-91)5. Takahashi,T. 19?5.Carbonarechemistry of sea warer and the calcite compensationdepth in the oceans.Iu, W. V. Slner, A. V. H. 86 and W. H. Berser, eds., Dissolution o{ deep sea csrbonate, Cushman Foundation for Foraminiferal Research,Spec. Pub. 13, 1t026. Tappan, H. 1968.Primary production, isoropes,extinctions Paleogeogr. Paleoclim.Paleoecol. and the atrnosphere. 4:185-210. Tappan, H. 1979.Protistan evolution and extinction at the boundarl. 12, Christensen,V. K. Cretaceous/Tertiary and T. Birleland, eds.,CretaceousTertiary Boundary Even|s,2C , o p e n h a e eU n n i v . P r o c .S y n p . P p . l 3 - 2 1 . Tappan, H. 1982. Extinoion or survival Selecrivity and canses.Geol. Soc. Aner. Spec. Pap. 190:265'275. Thierst€in, H. R. 1981.Lale Cretaceousnannoplanktonand boundary.Soc. rhe changeat th€ Cretaceous-Tertiary Econ. Paleo. Miner. Spec. Pub. 32:355-394. U.€y, H. C. l9?3. Cometa.ycollisionsand geolosic periods. Narure 242:32-33. Van Valen, L. M. 1984.Catsstrophies,expectationsand the e r i d e n c € .P a i e o b i o l o g ly0 : l 2 l - 1 3 7 . Van Valen, L., and R. E. Sloan. 1977.Ecolog] and exrinction of the dinoEaurs.Evol. Theory 2:37-64. Varelanp,J. C., and E. Thonas. 1982.Chalcophileelenents in Cr€laceous/Tertiarys€dinebts:Terrestrial or extralerreslrisl? Geol. Soc. Anet. Spec. Pap. lB0:461-468. Villians, D. F., M. A. Somner, and M. L. B€nd€r. 1977.Carbon isotope conposition of rec€nt plankionic fo.aninifera o{the lndian Oc€an.f,arth Plan Sci. L€ti. 36:391-403. Wolbach,V. S.,R. S. Lewis,and E. Arders. 1985.Cretaceous exrinctions: Evidence for *ildfires and search for net€oritic msterial. Science230:167-170. Worsley,T. 1974.The Cretaceous-Tertiarlboundary ev€nt in the ocean.Soc. Econ. Pal€ont. Min., Spec.Publ. 20.94-125. Vright, F. W., and P. W. Hodse. 1965.Studi€sof parricles for extrate..€strial origin, 4, Microscopic spherules from recenr volcanic eruption. J. Geophys. Res. 70:3889-3898. Zoller, W. H., J. R. Parringron,and J. M. PhelanKotra. 1983. Iridium enrichmentin airborn€pafticlesftom Kilauea r o l c a n o :J a n u a r y1 9 8 3 .S c i e n c e2 2 2 : l l l 8 ' l 1 2 l .