Document 162415

Charlgs L, Orake, Department01Earth Scrences
DartmouthCollege,Hanover,New Hampshre 03755
Yvonne Herman, Departmentof Geology,Washngton Stale Un versity,Pu lman, Washngton 99164.2A12
Did the DinosaursDie or Evolveinto Red Herrings?
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
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
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
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,
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
for it indicated
NorthwestScience,Vol. 62, No. 3, l9BB
that the Moon,aswell asthe Earth,lackedperfection, But perhaps this was too subtle for the
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
explodeand shatterthe crust;in otherwords,a
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
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
Drake and Herman
of the origin of meteoites.It concludedthat
stones to which a cosmic origin had been
assignedwere actuallyordinary terrestrial rocks
that had beensrruck by lightning and thar
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
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
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
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
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
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
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
of the meteoriticiron in the
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
waspresent(Chaoet dL t960).Bur
wherewasthe large meteor?It had remainedhidden despitegeophysical
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?
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
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
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
lmpactsand Extinctions
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
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
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
to be blownout of the atmospherein ballistictrajectories and be distributed around the Earth to
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
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
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
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).
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
Schrnitz (1985) suggesred that redoxcontrolledprecipitation
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
Did the DinosaursDie or Evolveinto Red Herrines?
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
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,
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
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 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
the 1960's,when the first Moon data were appearing,and comparisons
were made between
spherulesfrom explosiveand shieldvolcanoes
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
from Denmark,Nev Zealand,
and Spain were diageneticreplacementsof
prasinophytealgae.In New Zealandgoethite
spheruleswere found, probably derived from
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
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.
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
In the 1960'sit wasconcludedthat shocked
mineralscould not be producedby volcanoes
the modelsof volcanoes
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
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
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.
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
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?
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
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
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
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
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
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
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
rates.The lastofthe dinosaurs
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;
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
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
Sloan l9?7; etc). Extinctions were selective,affecting approxirnately90Vo of the warm-waler
calcareousphyto-and zooplanktongenerain the
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
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
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
rudists(e.g.Emiliani er aL 196l; Russelland
Rice,eds.,1982;Berggrenand van Couvering,
marine turtles and irregular echinoidsexhibit
only gradualextinctionacrossthe boundary(Tappan, 1982;Hermanet al, in press).Extinctions
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.
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
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
in the deepoceanwerewarm and
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
In the modernoceanthe surfacewatersare
with calciumcarbonate,but in the
oceandepththe watersare undersaturated
at the carbonatecompensationdepth [CCD],
which dependsupon temperatureand the partial pressureof COz,the CaCO3content
of the
sedimentgoesto zero;the CaCO.dissolves
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?
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
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
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
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
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
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
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
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
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
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
of time.
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
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
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
that the cumulativeeffectsof
COuemissionsover 500,000yearsfrom the Deccan traps of India at K/T time might reduce
oceanalkalinity,but this assumesthat COzwould
continueto accumulate.
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
biologicalactivity,or othersare difficult to assessand could seriouslyinhibit this
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
yearsthat wasconcentrated
in the mix,
ed layer of the surfaceoceancould havea significant effect.
The Deccantrapsof India representsuchan
unusualevent(Subbaraoand Sukheswala,
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?
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
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
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
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
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
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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
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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,
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
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
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