Jansa 1993 Marine Impacts

8/2/2019 Jansa 1993 Marine Impacts

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Palaeogeography, Palaeoclimatology, Palaeoecology, 104 (1993): 271-2 86 271Elsev ier Science Publ i shers B.V., Amsterdam

Com e t ar y im p ac t s in t o oc e an : t h e i r r e c ogn i t i on an d t h et h r e s h o ld c on s t r a in t f or b io log i c a l e x t in c t ion sL u b o m i r F . J a n s a

Geological Survey o f Canada, Atlan tic Geoseience Centre, P.O. Box 1006, Dartmouth, Nova Scotia, B 2 Y 4A2, C anada( R e c e i v e d D e c e m b e r 1 9, 1 9 91 ; r e v i s e d a n d a c c e p t e d M a y 1 5, 1 9 92 )

A B S T R A C TJa n sa , L .F . , 1 9 9 3 . C o m e t a ry i mp a c t s i n t o o c e a n : t h e i r r e c o g n i ti o n a n d t h e t h re sh o l d c o n s t ra i n t fo r b i o l o g i c a l e x t i n ct i o n s .

Palaeog eogr. , Pa laeo cl imato l . , Pa laeoecol . , 10 4 : 271-286 .Th e M o n t a g n a i s i mp a c t c ra t e r i s p re se n tl y t h e o n l y s it e i n t h e o c e a n wh e re t h e e f fec t o f a me t e o r i t e f a ll o n m a r i n e o rg a n i smsh a s b e e n s tu d ie d . Th e i mp a c t c ra t e r i s 4 5 k m i n d i a me t e r a n d w a s fo rme d a t 5 0 .8 M a b y a f a l l o f p ro b a b l y a n o l d c o m e t a ryn u c l e u s 3 .4 k m i n d i a me t e r , i n t o sh a l l o w ( < 6 0 0 m) o c e a n . C o m p a r i so n o f t h e i mp a c t s t ru c t u re a n d re l a te d d e p o s i t s wi t h t h o seon lan d sho ws severa l majo r d i fferences of which the m ost s ign i f ican t is the absence of an e levated cra ter r im. Instead , thec ra t e r p e r i me t e r is b e v e l le d a n d e ro d e d a s a c o n se q u e n c e o f i mp a c t i n d u c e d b o t t o m c u r re n t s a n d t u rb u l e n t , r e t u rn wa t e r f l o win to the excavat ing cav i ty . By th is p rocess most o f the fa l l -ou t b reccia i s reworked back in to the cra ter cav i ty where i taccum ula tes in mu ch larger th icknesses than in impac t c ra ters on land . A t a microsco pic sca le , the shock me tamo rphism

fe a tu re s a re a b o u t t h e s a m e a s t h o se fo r l a n d i mp a c t s .Ge o c h e m i c a l ly , i mp a c t s o f c o me t n u c l ei i, ma y n o t l e a v e a r e c o g n i z a b l e si g n a t u re a t t h e i mp a c t h o r i z o n e x c e p t fo r a mi n o ri n cre ase i n i r i d iu m. T h u s s t r a t ig ra p h i c h o r i z o n s a s so c i a t ed w i t h e x t in c t i o n s a n d / o r ma j o r c h a n g e s i n b i o t a h a v e t o b e c l o se lyexam ined for o ther im pact ind ica to rs , l ike the presence of tec ti tes , g lass spheru les, and qua rtz g ra ins wi th shoc k fea tures .Oc c u r re n c e o f m e g a t su n a mi w a v e d e p o s i ts , e x t e n s i v e e ro s i o n o n c o n t i n e n t a l ma rg i n s , ma rg i n fa i l u re s a n d fa u n a l m i x i n g a b o v ee ro s i o n a l u n c o n fo rm i t i e s a re o t h e r p o t e n t i a l i mp a c t i n d i c a to r s . Th e re i s n o s in g le i n d i c a t o r t h a t c a n p ro v i d e su ff ic ie n t p ro o fo f a n i mp a c t e v e n t . Su c h i n t e rp re t a ti o n s h a v e t o b e b a se d o n m u l t i p a ra me t e r s t u d ie s o f g l o b a l e x te n t , s in c e ma n y o f t h e i mp a c ti n d i c a to r s a re o n l y o f r e g i o n a l e x t e n t.Th e l a c k o f e x ti n c t i o n o f a n y ma r i n e p l a n k t o n g e n e ra , o r o f b o t t o m d we ll er s a t t h e M o n t a g n a i s i mp a c t s i te a l lo ws u s t op l a c e a l o we r l i mi t fo r b i o l o g i c a l ex t i n c t io n s c a u se d b y c o m e t a ry i mp a c t s o n t h o se wi t h n u c l e u s > 4 k m i n d i a me t e r . Th ec a l c u l a te d f re q u e n c y fo r a c o m e t a ry i mp a c t w h i c h c o u l d re su l t i n a 1 0 % e x t i n c ti o n o f ma r i n e g e n e ra i s a b o u t 6 1 0 -v y r - ia n d fo r t h e K/ T b o u n d a ry t y p e o f e x t i n c ti o n s a b o u t 2 10 -8 y r -1 . Ev e n a l l o wi n g fo r a l a rg e d e g re e o f u n c e r t a in t y i n t h e seest imates , i t i s un l ike ly tha t the b io log ica l ex t inc t ion events fo r the las t 250 Ma iden t i f ied by Sepkosk i (1990) could have beena l l c a u se d b y m e t e o r i t e i mp a c ts .

Introduct ion

G e o l o g i c a l s t u d i es d u r i n g t h e l a st d e c a d e h a v el ed t o t h e r e c o g n i t i o n o f e v e n t s in E a r t h h i s t o r yt h a t a r e a s s o c i a t e d w i t h d i s t in c t g l o b a l c h a n g e s ,e i t h e r i n b i o t a , s e d i m e n t c o m p o s i t i o n , b i o m a s sr e d u c t i o n , o r g e o c h e m i c a l c h a n g e s a t s p e ci f ic s tr a ti -g r a p h i c l ev e ls . T h e e v i d e n c e i s n o w a c c u m u l a t i n gt h a t s o m e o f t h e r a re g e o l o g i c e v e n t s o r i g i n a t e df r o m c o m e t a r y o r a s t e r o i d a l I i m p a c t s , a t h e o r ya d v o c a t e d f o r l o n g t i m e b y M c L a r e n ( 1 97 0 ) . S o m eo f t h e s e ev e n t s w e r e d e b a t e d a t th e s e c o n d c o n f e r -e n c e " G l o b a l C a t a s t r o p h e s i n E a r t h H i s t o r y : A n

i n t e r d is c i p l in a r y c o n f e r e n c e o n i m p a c t s , v o l c a n i s ma n d m a s s m o r t a l i t y " , h e l d a t S n o w b i r d , U . S . A . i nO c t o b e r 1 9 8 8 ( S h a r p t o n a n d W a r d , 1 9 90 ). T h ec o n f e r e n c e d e m o n s t r a t e d p e r s i s t i n g d i f f e re n c e s i nv i e w s o n c a u s e s o f b i o l o g i c a l e x t i n c t io n s , r a n g i n gf r o m m e t e o r i t e i m p a c t t h e o r i e s t o a v a r i e ty o f1 As te ro i ds a re a mu l t i t u d e o f "m i n o r p l a n e t s " , p re c u rso rs o fwh i c h we re p l a n e t is i ma l s o rb i t i n g t h e Su n . T h e a s t e ro i d p o p u l a -t ion inc ludes o ld com etary nuclei i, w i th dep le ted comet vo la t i lesand so l id core . Meteori tes are fragments o f p lanetary bodies ,a n d h a v e a b ro a d ra n g e o f c h e mi c al c o mp o s i t i o n a n d mi n e ra l -ogy . However, t rad i t ional ly , a l l ex t ra terres t r ia l ob jec ts impact -i n g Ea r t h ' s a t m o sp h e re a re c a l le d me t e o r i t e s

0031-0182/93/$06 .00 199 3 - - Elsev ier Science Publ i shers B.V. Al l r igh ts reserved .


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c l ima t i c , t e c ton i c and oceanograph ic p rocesse s .Thus t he ma jo r t a sk conf ron t ing t he geo log i ccom mu ni ty i s t o impro ve r ecogn i t i on o f c r i te r i afo r impac t even t s and t he ab i l i t y t o app ly t hem inthe geo log i c r ecord .

Geoc hemica l s t ud i e s o f ma jo r s t r a t i g raph i cb o u n d a r y l a y er s h a v e s h o w n t h e p r e se n c e o f h o r i-zons en r i ched i n i r i d ium and o the r p l a t i num groupe l emen t s , such a s Os , Au , P t and Ru (Or th e t a l . ,1990; W ang an d C hai , 1991; X u an d 5(an, 1991).The co inc idence o f such occur rences o f en r i chedl aye r s wi th ma jo r changes i n b io t a have beenexp la ined by some re sea rche r s a s an i nd i ca t i on o fmeteor i te col l i s ions wi th the Ear th . But , Orth e t a l .(1990) f rom ana lyse s o f more t han 8000 sample sf rom Phane rozo i c geo log i c bounda r i e s , conc ludedtha t excep t fo r t he t e rmina l Cre t aceous , t he re i sno com pe l l i ng ev idence fo r o the r impac t s . C l ea r ly ,w e h a v e a p a r a d o x b e t w e e n p a l e o t o l o g i c a l a n dgeochemica l i n t e rp re t a t i on o f s t ra t i g raph i c ho r i -z o n s m a r k e d b y b i o m a s s e x t i n ct io n . S o m e o f t hepo ten t i a l c ause s o f b io log i ca l ex t inc t i ons w e rer e v i e w e d b y M c L a r e n a n d G o o d f e l l o w ( 1 9 9 0 ) .These au thor s conc luded t ha t nea r ly a l l ma jo rex t inc t i on even t s i n t he Phane rozo i c a re due t osome fo rm o f ongo ing s t r e ss wh ich cu lmina t e s i nd e v e l o p m e n t o f a b i o m a s s " k i ll in g h o r i z o n , "exp la inab l e by an impac t o f an a s t e ro id o r a come t .

Bu t , t o r e so lve whe the r an ex t ra t e r re s t r i a l bodywas t he con t r i bu t ing f ac to r i n a b io t i c ex t i nc t i onrequ i re s c lea r and u nb ia sed r ecogn i t i on o f such aneven t i n geo log i c r ecord . Wi th t h is g oa l we b r i e f lyd i scuss some o f t he mos t cha rac t e r i s t i c f e a tu re s o fa come ta ry impac t i n to t he ocean , a s i n t e rp re t edf rom the Montagna i s impac t c ra t e r , l oca t ed o f f -s h o r e N o v a S c o ti a , e a s t er n C a n a d a ( J a n s a a n d P e -Pipe r , 1987) . Howeve r , s i nce t he Montagna i si m p a c t c r a t e r w a s f o r m e d i n a n o c e a n i c d e p t h o fl e ss t han 600 m , a l l t he compar i sons and conc lu -s ions p re sen t ed he re a re va l i d on ly fo r impac t sin to a s imi l a r ocean i c env i ronment . TheM o n t a g n a i s i m p a c t i s t h e n u s e d t o p o i n t t o s o m eof t he f ea tu re s wh ich cou ld be he lp fu l fo r r e cogn i -t i on o f ocean i c impac t s . O cean i c impac t s a reemphas i zed be cause compi l a t i ons o f b io log i ca lex t i nc t i ons by Sepkosk i (1990) po in t t o t he mar ineecosys t ems be ing t he mos t heav i ly a f fec t ed . Thel ack o f b io log i ca l changes a t t he M ontagn a i s s t ruc -

tu re a s r epor t ed by Aubry e t a l . (1990) a l l owspos tu l a t i on o f t he lower t h re sho ld fo r t he s iz e o fthe com e ta ry nuc l eus nece ssa ry t o cause b io log i ca lext inc t ions.

Oceanic m eteorite impactsDesp i t e b ro ad recogn i t i on o f impac t s o f ex t ra t e r -

r e s t r i a l ob j ec t s on t he Ea r th ' s l and su r face (Denceet a l. , 19 77 ; Sh arp ton and Grieve , 1990) l i tt lea t t en t i on ha s been devo ted t o s t ud i es o f impac t sin to t he ocean . Th i s undoub ted ly s t ems f rom thefac t , t ha t on ly two mar ine impac t s i t e s( M o n t a g n a i s a n d K a r a / U s t a - k a r a ) w e r e s t u d ie d insu f fi c ien t de t a i l ( Jansa and Pe -Pipe r , 1987 ; Ko ebe r le t a l . , 1990) . Only pre l iminary da ta a re avai lablef r o m a n o t h e r s h a l l o w m a r i n e ( ? ) i m p a c t - -Ch icxu lub (Hi ldebrand e t a l . , 1991) . For seve ra lo the r impac t s a sha l l ow wa te r env i ronment havebeen pos tu l a t ed , bu t such cond i t i ons r ema in t o beproven . The emphas i s i s p l aced he re on ocean i cimpac t s because : (1 ) p robab i l i t y a rgument s i nd i -ca t e tha t ab ou t 70% of the me teor i t e fl ux wou ldland in the ocean; (2) the di f ference in the impactp rocess and i ts env i ronm enta l e f fec t s i f wa t e r , o rl and a rea w as t he ta rge t ; and (3 ) mo s t o f t he ma jo rb io log i ca l ex t i nc t i ons r ecorded i n t he Phane rozo i ca f fec ted ma in ly m ar ine an ima l s.

The gene ra l f e a tu re s and e f fec t s o f a m e teo r i t eimpac t i n to t he ocean we re mode l l ed by seve ra lau thor s spec if i c al l y a f t e r pub l i c a t i on o f t he impa c ttheory by Alva rez e t a l. (1980). Som e o f t he semode l l i ng r e su l t s a re summar i zed be low and a recompared wi th obse rved fea tu re s a t t heMontagna i s impac t s i t e .

Cons ide r ing t he excava t ion p rocesse s , t he mos ts ign if i can t expe r imen t s fo r deep and sha l l ow oceanimpac t s a re t hose by Gau l t an d S one t t (1982) . Fo rthe sha l l ow wa te r wi th b< 1 .5 (b=h/dm,w h e r e hi s the me an wa te r (ocean) de p th and dm i s t hemax imum dep th o f a tr ans i en t c av i ty ; fo r t he deepwa te r se t t i ng (b > 4) , t he above au thor s found tha twa te r i s r emarkab ly e f fec t ive i n sh i e ld ing t he f l oo ragainst s igni f icant excavat ion. The cent ra l peakwhich i s a p rominen t s t ruc tu ra l f e a tu re o f complexc ra t e r s fo r sha l l ow ocean impac t ha s a cy l i nd r i ca lshape and ex t ends above t he wa te r su r face . In t hedeep wa te r se t t i ng i t i s a rounded cone - shape .


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COMETARY IMPACTS INTO OCEANS AND BIOLOGICAL EXTINCTIONS 273

Gault and Sonett (1982) further suggested thatshallow-water impact structures would be muchless obvious topographically in comparison to landimpacts. Strelitz (1979), from modelling of anocean impact reached a similar conclusion, butsuggested that the penetration of the meteorite andhence depth of the burst would be about 50%greater in the ocean. Roddy et al. (1987) computedthat the fall of a 10 km in diameter meteorite into5 km deep ocean would result in formation of alarger rim uplift than in land impacts and that70% of the ejecta would be finally resting withinan area three times the crater diameter.

The effect of energy transmission during the fallof a l0 km meteorite into 5 km deep ocean hasbeen studied by O'Keefe and Ahrens (1982). Theseauthors concluded that 5-15% of the energy isused up in passage of the meteorite through theatmosphere and the hydrosphere and that, uponimpact, 10 to 100 times the meteorite mass ofwater and rock can be ejected into the stratosphere.Since the ejecta velocity for the Shocked extrater-restrial material in continental impacts is higherthan in oceanic impacts, the fraction of extraterres-trial material should be lower in ocean impacts.McKin non (1982) estimated kinetic energy transferfor a 10 km diameter meteorite impact to result ina 10.1 magnitude earthquake. When such a meteor-ite impacts an ocean, the seawater at the impactsite could be hea ted up to 100,000C (Melosh,1982). By such a process, much of the meteoriteand a large volume of water would be convertedinto vapor and steam which would explode upwardinto the atmosphere above the impact site. Thegenerated vapor plume may reach a diameter of700 km. This plume would be abou t f ifty timeslarger than the atmospheric thickness, and asconsequence it would not only cause global distri-bution of meteoroid particles, but also result intheir partial escape from Earth. Such mechanismof extraterrestrial particles dilution when consid-ered together with the fact that half of the cometnucleus could be ice, indicate that a high velocitycomet impact may not leave strong geochemicalevidence at the Earth's surface. Even though suchlimitations were mentioned by Orth et al. (1990)in their geochemical analysis of extinction bound-aries, these were not incorporated into their final

analysis of the origin of geochemically distinctgeologic horizons.

Not enough attention has been given to theeffect of tsunami waves resulting from meteoriteimpact into the ocean. Some theoretical calcula-tions have been provided by Strelitz (1979), Gaultand Sonett (1982), Van der Bergh (1988) andSonett et al. (1991). O'Keefe and Ahrens (1982)calculated a tsunami wave initially 5 km in height,from a 10 km in diameter meteorite impacting into5 km o f water, to decay in elevation to 150 mhalfway around the world. Such a wave wouldpropagate in the deep ocean at a maximum speedof 0.2 km/s and, according to the above authors,would inundate all low lying continental areas onEarth within 27 hours. This will be discussed inmore detail further in the paper.S h a l lo w- o c e a n imp a c t f e a t u r e s a s o b s er v e d a t t h eM o n t a g n a i s c r a te r

The Montagnais impact crater probably repre-sents the most studied marine impact event. Thecrater was formed in an outer shelf/upper continen-tal slope environment about 50.5 Ma ago (Jansaet al., 1989). The crater is nearly circular, at least45 km wide, about 2.7 km deep and has a centraluplift from 1250 m to 2900 m in height (Fig. 1).The central uplift has a flat-cone shape; it is11.5 km wide, with the upper surface protrud ingslightly above the level of the crater rim. The crateris partially filled by polymictic breccia made up ofthe target rocks (Fig. 2A). The breccia layer ismore than 550 m thick over the central uplift andencloses two horizons, 72 and 34 m thick, ofrecrystallized melt rock. The crater is covered bya 40.5 m thick layer of accretionary lapilli andvesicular glass bombs of melt rock which fell backinto the crater (Fig. 2D). This layer correspondsto the suevite layer of St6ffler et al. (1977).

The crystalline melt at the Montagnais impactsite is enriched in iridium (up to 0.32 ppb) whencompared to the low level in the target rocks (0.016ppb, Jansa et al., 1989). No enrichment in othersiderophile or platinum group elements has beendetected (C. Orth, pers. commun., 1988), whichled Jansa et al. (1990) to suggest that the impactorwas most probably a comet nucleus. The 3.4 km


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27 4 L.F. JANSA

A " "o . . . . . . . . . . . . . . . j o .o

o

A I" I M P A C T C R A T E R 4 5 K m ,'] BSW MONTA GNAIS 1-94 NE

SEA LEVEL -00 ooO- ~

~ M ,.'., ^.t,~ ...L~,.9 BR .~~ ~ _ . ~F I Oc. ~E -_ ,o ~

[ ] [ ] BO ...o Krn 5 SUEVITE METAMORPHIC BASEMENT E UNCONFORMITY M CALLOVIAN MISAINESHALE)i i [MEGUMA)HORIZONTAL SCALE ~ MEL T ROCK HORIZON C CONIA CIAN MARKER B BASEME NT MEGUMA GROUP)

I N T E R P R E T E D S E I S M I C S E C T I O N T H R O U G H M O N T A G N A I S 1 -9 4 W E L L .Fig. 1. (A) Uninterpreted, 60 fold, time variant, scaled migration, reflection seismic section across the Montagnais impact crater,located a t the edge of the Scoti an Shelf, off eastern C ana da ( Petro -Ca nada seismic line 3203-82). (B) Interpre ted seismic line asabove; shown is the crater geometry, central uplift, breccia fill and location of melt rocks as verified by oil exploratory wellMontagn ais 1-94, dri l led a t the center o f the central uplif t. Note that the r im of the crater is bevelled (BR)and not ra ised as in landcraters and that i t lacks accumulation of a fa l l-out breccia , which was reworked in to the f inal crater cavity.

diameter of the nucleus has been calculated fromthe equations given by Shoemaker and Wolfe(1982), using a 28.9 km s- 1 velocity and 0.6 g cm - 3bulk density (Wiessman, 1990) and a crater diame-ter of 45 km. But, both the velocity and densityparameters are poorly constrained due to insuffi-cient knowledge of cometary bodies and thus thevelocity may vary by a factor of 5 and the densityby a factor o f 7. This is reflected in calculations ofcraters similar in size to Montagnais by Shoemakeret al. (1990). The latter authors concluded that theimpact of a long-period comet about 2.5 km indiameter at an impact speed of 57.7 km s- 1 andtypical angle of impact 45 , will produce craters40-50 km in diameter, i f cometary densities are inthe range of 0.5 to 1.2 g cm -3. Craters of this sizeare comparable with those produced by averageS-type (stony-iron composition) asteroids of 3.4 to4.6 km diameter) impacting at a speed of 17.9 kms-1. A different set of parameters was selected by

Vickery and Melosh (1990) who, for a comet witha nucleus 6.1 km in diameter, selected an impactingspeed o f 25 km s -1 and 1.5 gc m -3 density toproduce an impact crater 55 km in diameter; andfor a nucleus of about 20 km in diameter a craterof about 185 km in diameter. The 3.4 km diameterfor the nucleus of the Montagnais comet corres-ponds reasonably well to the above calculations,even when different sets of parameters are used.

The fall-back breccia at the Montagnais impactsite has all the diagnostic shock features, includingplanar shock lamellae in quartz and feldspars(Fig. 2B), variable stages o f development o fdiaplectic glass and extensive microfracturing(Fig. 2A,C) and frequent occurrence of quartz andfeldspar grains with highly decreased birefringence.Such features are revealed under cross polars inthin sections as darker, cloudy, areas (Fig. 2A,B).Locally, some of the ejecta clasts show completetransition from shocked mineral structure with


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Fig. 2. Photomicrographs of characteristic petrographic im pact features from Mon tagnais structure (Mon tagnais 1-94 well samples).(A) Isotropization (i) of the minerals in metagraywacke clast of the impact breccia, a nd extensive m icrofracturing; 698 m, crossedpolars, scale bar = 0.2 mm. (B) Magnified Fig. 2A to show und ecorated shock lamellae (i) in quartz grains and partial isotropizationof grains recognizable by their cloudy appearance; 698 m, crossed polars, scale bar =0 .1 m m. (C) Partial isotropization of a largequartz grain (i), the gra in is cut by a thin glassy vein (g) developed proba bly along a microfracture. Note other glassy veins cross-cutting other grains of the graywacke clast; impact breccia, 689 m, crossed polars with gypsum plate, scale bar= 0.2 mm. (D) Meltglass in the suevite zone. Th in section shows fresh glass (g) with rare needle-shaped, highly elonga ted microlith s (m), vesicules (v),some rimme d by br ownish rim of a chlorite a nd zeolite (z); black round ish aggregates consist of unidentified impurities mixed withiron oxide; 652 m, p lane-p olariz ed light, scale bar = 0.08 mm.

p r e s s u r e l a m e l l a e t o p a r t i a l l y o r e v e n f u l l y m e l t e ds i l i c a t e g r a i n s w i t h i n a s i n g l e b r e c c i a c l a s t . T h ep r e s en c e o f a m e t a s t a b l e p o l y m o r p h o f q u a rt z -c o e s it e is s u s p e c t e d f r o m p e t r o g r a p h i c s t u d i es , b u th a s n o t b e e n v e r i f i e d y e t .

Comparison o f predicted and observed parametersfor shallow-ocean impact

T h e M o n t a g n a i s c o m e t ' s f al l i n t o s h a l lo w( 2 0 0 - 6 0 0 m d e e p) o c e a n p r o d u c e d a n i m p a c t s t r uc -


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t u re which c lose ly , bu t no t comple t e ly r e semblesimpac t s t ruc tu re s on l and . The gene ra l geome t ryof t he s t ruc tu re , such a s i t s roun ded bow l shapeand heavi ly faul ted f lanks, i s s imi lar to tha tobse rved in l and impac t c ra t e r s , and a s p red i c t edby sha l low ocean impac t mode l l i ng expe r iment s .The centra l upl i f t i s cyl indrica l in shape as pre-d i c t ed by Gaul t and Sone t t (1982) , howeve r , t hera i sed r im cons i s t i ng pa r t l y o f rock f ragm ent se j ec ted f rom the c ra t e r and pa r t l y o f bedro ck tha thas been l i f ted up an d m ove d rad i a l ly ( a s p red i c t edf r o m c o m p u t a t io n s b y M c K i n n o n , 1 9 8 2 ; R o d d yet a l. , 198 7; St re l i tz , 1979; and Ga ul t a nd Son et t ,1982) is lacking (Fig. l ) . In contra st , theM o n t a g n a i s i m p a c t c r a t e r e d g e i s e r o d e d a n dbeve l l ed . Th i s i s p robab ly one o f t he mos t s ign i f i -can t d i f f e rences be tween subae r i a l and submar ineimpac t c ra t e r mo rpho log ie s . The l ack o f a r a isedr im a t t he Mon tagna i s s t ruc tu re l ed some invest iga-to r s t o sugges t a t ec tono-vo lcan ic , o r c ryp tovo l -can ic p rocess fo r i t s o r ig in (Edwards , 1989 ; Wadeand M acLe an , 1990). How eve r , when the ef fec t o fwa te r i s cons ide red in t he c ra t e r ing p rocess , t hel ack o f a r a i sed r im i s expec t ed . As ca l cu l a t ed byO ' K e e f e a n d A h r e n s 0 9 8 2 ) l 0 t o 1 00 t im e s o f t h eme teor i t e mas s o f wa te r i s e j ec t ed dur ing thec ra t e r ing p rocess . Dur ing th i s p rocess t he wa te ren t e r ing the excava ted cav i ty r ap id ly evapora t e sbecause t he t emp era tu re i n t he cen te r o f the cav i tyc o u l d r e a c h t e m p e r a t u r e s o f s e v e r a l t h o u s a n ddegrees Ce l s ius (Me losh , 1982) . Combined wi thth is p rocess i s t he e f fec t o f w a te r which was in it i al lyd i s p l a c e d b y t h e s h o c k w a v e a n d w h i c h t h a nre tu rns a s a t u rbu len t f l ow inc reas ing the onru shof wa te r i n to t he excava t ing cav i ty and scour ingthe ocean f loor i n i t s wake . The re fo re , no t on lywi l l t he f a l l -ou t b recc i a no t come to r e s t a t t hec ra t e r pe r ime te r , bu t t he c ra t e r r im i s fu r the re roded by the i n f lowing wa te r . Such a mechan i smexpla ins t he g rea t e r t h i ckness o f t he e j ec t a b recc i ain t he Mon tagna i s c ra t e r t han obse rved a t landimpac t s i te s o f com parab le c ra t e r s ize. E j ec t a d i s t r i-b u t i o n i n t h e M o n t a g n a i s i m p a c t c r a t e r f u r t h e rd i ff e rs f r o m c o m p u t e r m o d e l l in g o f o c e a n i m p a c t ssuch a s by R od dy e t a l. (1987) wh o sugges t ed tha t70% of e j ect a shou ld co me to r e s t on the seabo t tom wi th in an a rea o f th ree t imes the c ra t e rd i ame te r . We have no t found conc lus ive ev idence

for sol id e jec ta in dr i ll ing ma ter ia ls f r om oi l explor-a to ry we l l s a t a d i s t ance o f twice t he c ra t e r d i ame-ter on the shel f . This indicates more rest r ic ted sol ide j ec t a d i s t r i bu t ion in submar ine impac t s i t e s whencompared wi th l and impac t s t ruc tu re s . The shockimpac t f ea tu re s such a s qua r t z l ame l l ae , deve lop-ment o f d i ap l ec t i c g l a ss and p re sence o f r ec rys t a l -l i zed me l t rock a re s imi l a r t o bo th mar ine andsubae r i a l impac t s .C o m e t a r y i m p a c t s a s e v e n t m a r k e r s i n E a r t h h i s t o r y

The a im he re i s no t t o r e so lve i f me teor i t eimpac t s on the Ea r th ' s su r face re su l t ed in anypa r t i cu l a r b io log ica l ex t inc tion , bu t t o r ev i ew someo f t h e c o n s t r a in t s i m p o s e d b y t h e c r a t er i n g m e c h a -n i sm o n the causes o f b io log ica l ex t inc t ions an don fo rm a t ion o f r a re , geochemica l hor i zons occa -s iona l ly a ssoc i a t ed wi th such even t s (McLaren andGoodfe l low, 1990) . I t cou ld be sugges t ed tha t i twas t he i nadequ a te co ns ide ra t ion o f f ie ld obse rva -t i ons on a g loba l sca l e , combined wi th conce rnsf o r t a x o n o m i c r a t h e r t h a n b i o m a s s c h a ng e , w h i c hhave re su l t ed in fo rc ing the ev idence toward anevo lu t iona ry mode l fo r l i f e , r a the r t han accep tsom e of t he ca t a s t roph i sm theor ie s . The causes o fbiological ext inct ions are examined here in a viewof t he recen t s t a t ement b y Sho ema ker e t a l . (1990),t ha t t he num ber o f ac t ive Ea r th c ross ing com e t sd i scove red i s four t imes g rea t e r t han the numberof Ea r th c ross ing a s te ro ids . Th i s wou ld imply upto four t imes l a rge r p robab i l i t y fo r a come ta ryimpac t i n compar i son wi th an a s t e ro id impac twhen on ly c ra t e r s > 60 km in d i ame te r a re cons id -ered (Shoemaker e t a l . , 1990) . The fol lowing dis-cussio n is limi ted to three them es: (1) ident i f ica t ionof impac t even t s and the i r geo log ic r ecogn i t i on(which inc ludes impac to r compos i t i on and second-ary effec ts of the impact) ; (2) imp acto r size andbiological ext inct ions; (3) impact per iodic i ty .1. Im pa ct even t identification an d their geologicrecognition

The recogn i t i on and iden t i f ica t i on o f an impac tin to t he ocean ic env i ronme nt b ecome s inc reasing lyi m p o r t a n t i n o u r e n d e a v o u r t o u n d e r s t a n d b i o lo g i -ca l evo lu t ion . S ince l a rge impac t c ra t e r s a re fo rm ed


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m o r e b y c o m e t a r y i m p a c t s t h a n b y a s t e r o i d s , a simpl ied by Shoemaker e t a l . (1990) , the fol lowingd i scuss ion concen t ra t e s on i den t if i c a t ion o f a com-e t a r y i m p a c t e v e n t h o u g h m o s t o f t h e d i sc u s s e dc r it e ri a a r e c o m m o n t o b o t h c o m e t a r y a n d a s t e r o idimpacts . Six cr i te r ia for impact ident i f ica t ion areb r i e fl y cons ide red be low:

1.1 Presence of impact craterThis i s the most convincing ident i f ica t ion fea-

tu re , bu t o ne o f t he mo s t d i f fi cu lt t o l oca t e . Fro mt h e s t u d y o f th e M o n t a g n a i s i m p a c t s t r u c t u re i tc an be shown tha t i n a sha l l ow sea t he s t ruc tu rei s rounded , bowl shaped , wi th a h igh ly f au l t edcra ter per imeter , and i s ident i f iable on ref lec t ionse i smic da t a even when sha l l owly bur i ed . Thes t ruc tu re d i f f e r s f rom l and impac t s by t he absenceof a r a i sed r im and by t he l a ck o f f a l l -ou t e j e c t aaccumula t i on a t t he c ra t e r pe r ime te r . No impac ts t ruc tu re ha s been recogn ized i n t he deep ocean ,bu t i t i s specu l a t ed f rom the M ontag na i s s t ruc tu re ,t ha t such a s t ruc tu re wi l l be sha l l ow and subduedin shape , a s sugges t ed by t he expe r imen t s o f Gau l tand Sonet t (1982) .

1.2 Solid ejectaThi s r ep re sen t s t he b u lk o f ma te r ia l e j e c t ed f rom

the c ra t e r a t ve loc it i es e s t ima ted by M e losh (1989)to be no t h ighe r t han abou t one - th i rd t o one - f i f t ho f t he immedia t e pos t - shock pa r t i c l e ve loc i t y .Ev iden t ly , fo r a 20 km/s impac to r o n Ea r th t hee j ec t a b l anke t wou ld be on ly l oca l . S tudy o f t heM ontagn a i s im pac t s i te shows the ma jo r i t y o f t hesol id e jec ta a t a sha l low ocean si te to be rest r ic tedto t he immedia t e v i c in i t y o f t he c ra t e r .Iden t i f i c a t i on o f so l i d e j ec t a wi th shock-me tamorph i sm fea tu re s i s a conv inc ing c r i t e r i onfo r an impac t (Sha rp ton and Gr i eve , 1990) . Bu t ,cons ide r ing t he l im i t ed a real d i s t r i bu t i on o f coa r see j ec t a , ex t i nc t i on hor i zon s tud i e s have t o be pur -sued g loba l ly . An example i s a mass ive b iomassr e d u c t io n e v e n t a t th e F r a s n i a n / F a m e n n i a n b o u n d -a ry , r e cogn ized i n Nor th Amer i ca , Europe , Ch ina ,Aus t ra l i a and nor th Af r i ca (McLaren , 1988) . Bu t ,i nc rea se i n i r i d ium to 0 .35 ppb in t he bounda ryl a y er in c o m p a r i s o n t o 0 .0 1 6 p p b a b o v e a n d b e l o wt h e b o u n d a r y a n d p r e s e n c e o f m i c r o c l a s t s w i t hdec rea sed c rys t a l b i r e f r i gency i s documented on ly

fo r Sou th Ch ina (Wang e t a l. , 1991) . I so t rop i sa t i onof g rains i n mic roc l a s t s a t the Fra s n i an /F ame nn ianbounda ry i n sou th Ch ina , a ssoc i a t ed wi th l owi r id ium enr i chment a t t he bounda ry i s s im i l a r t ofea tu re s obse rved a t t he Montagna i s impac t s t ruc -tu re , sugges t i ng t ha t t he Fra sn i an /Famenn ianb o u n d a r y e v e n t c o u l d h a v e r e s u l t e d f r o m a nimpac t o f a la rge come ta ry nuc l eus i n to sha l l owocean c lose t o sou the rn Ch ina . Thus , even t houghthe biomass reduct ion event i s g lobal in i t s e ffec ts ,the dist r ibut ion of sol id e jec ta i s a rea l ly rest r ic ted.Th i s i s fu r the r documented by d i s t r i bu t i on o fshocked qua r t z g ra ins f rom the K/T even t , wh ichwere i den t i f i ed a t t he bounda ry i n Europe , Nor thA m e r i c a , t h e C a r i b b e a n a n d N e w Z e a l a n d , o c c u r -r i ng i n a b road , e a s t - sou theas t t r end ing be l t ( I z et t ,1987 ; I z e t t e t a l ., 1991 ; Hi ld ebran d and Boy n ton ,1990 ; S igurdsson e t a l ., 1991 ; and o the r s ). Bu timpor t an t l y , t hey a re no t p re sen t eve rywhe re onthe g lobe .

1.3 TektitesThese are mi l l imeter-size sol idi f ied l iquid drop-

l e t s l o f t ed i n t he vapor p lume and d i s t r i bu t ed ove rwide areas (Melosh, 1982) . Thei r charac ter is t icsand geochemis t ry we re r ev i ewed in de ta i l by Shawand Wasse rburg (1982) , Koebe r l (1986) , Koebe r land Glass (1988) , Glass and Burns (1988) andSigurdsson e t a l . (1991) . These authors showedtha t t ek t i t e s and mic ro t ek t i t e s c an be d i spe r sedcon t inen t -wide , and geochemica l l y t r a ced back t othe impac t c ra t e r . In add i t i on , chemica l compos i -t i on o f t ek t i t e s c an p rov ide ev idence abou t t het a rge t rocks . Th e p re sence o f m ic ro t ek ti t e s , spec i -f ica l ly when associa ted wi th the co-occurr ing i r id-ium anoma ly , i s p robab ly t he mos t use fu l geo log i cmarke r fo r impac t r e cogn i t i on .

N ot comple t e ly i s r e so lved o r ig in o f m ic rosphe r -u l e s, t yp i ca ll y 1 mm in d i ame te r , wh ich co mm onlyoccur i n a ssoc i a t i on w i th t he t ek t it e s . Som e o f t hemic rosphe ru l e s a re ho l low, o the r s f i ll ed o r r ep l acedby a sco re o f m ine ra ls such a s san id ine , c l ays(smect i te , kaol ini te , ja rosi te ) , pyr i te , goethi te , ske l -e t a l sp inel s and c l i nopyroxenes ( I zet t , 1987 ; Gla sse t a l ., 1985 ; Ky te , 1990 ; Sigurd sson e t a l ., 1991).Recogn i t i on o f sphe ru l e o r ig in i s no t e a sy , s i ncemorp ho log i ca l l y s imi l a r f e a tu re s i n mar ine depo s i t sm a y r e p r es e n t p s e u d o m o r p h o s e d t e st s o f p ra s i n o -


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phy t i c a lgae (Hansen e t a l . , 1986) . Bohor andBe t t e r t on (1988) advanced an a rgument t ha t s i ncesuch mic rosphe ru l e s occur i n bo th mar ine andnonmar ine s i t e s and a re ub iqu i tous a t t he K/Tb o u n d a r y l a y e r , t h e y a r e a n a l o g o u s t o m i c r o t e k -t i tes . I t i s th is author ' s exper ience , tha t hol low,g l a ssy - l ike mic rosphe ru l e s occur i n t r a ce s i n o the rC r e t a c e o u s a n d T e r t ia r y s e d i m e n t s o f t h e C a n a d i a ncon t inen t a l ma rg in . S ome o f t hem have a sma l ldepre ss ion , wh ich l e ads t he au thor t o conc ludetha t som e o f t hem a re b iogen i c . Occur rence o fmic rosphe ru l e s i n mar ine depos i t s t hus can no t becons ide red a s p roof o f impac t -gene ra t ed e j ec t a ,un l e ss a ccompan ied by o the r impac t i nd i ca to r s a sd i scussed i n t h i s pape r .

1.4 VaporplumeA v a p o r p l u m e i s p r o d u c e d b y a t r a n s f e r o f

m o m e n t u m f r o m a n i m p a c t o r t o t h e a t m o s p h e r eand then t o t he t a rge t . At h igh impac to r ve loc i t ypa r t o f t h i s p lume can e scape f rom the Ea r th(Zahn le , 1990), bu t mo s t o f it conde nsa t e s and cangive a r i se to a geochemical ly dist inc t hor izon. Theconcen t ra t i ons o f i r id ium, p l a t i num grou p e le -men t s and t he s i de roph i l e e l emen t s a re d i f fe ren t ina s t e ro ids and com e t s (Maso n , 1971 ; Bas i l evsk iye t a l. , 1984) . Since the dens i ty of a com etar ynuc l eus cou ld be up t o 35 t imes l e ss t han t hedens i t y o f an i ron m e teor i t e (We i ssman , 1990), andp o t e n ti a l ly u p t o 8 0 % o f th e c o m e t v o l u m e c o u l dbe wa te r (Brand t , 1990), t he l a ck o f concen t ra t i onof s ide roph i l e o r p l a t i num group e l emen t s ini m p a c t h o r i z o n s s h o u l d n o t b e s u r p ri s in g a n d m a yno t nece ssa r i l y i nd i ca t e t ha t an impac t d id no toccur , as impl ied by Orth e t a l . (1990) . Vickeryand M e losh (1990) ca l cu l a t ed t ha t fo r shor t -pe r iodc o m e t t h e m i n i m u m m a s s r e q u i r e d t o d e p o s i tsu f fi c ien t am oun t s o f i ri d ium cor re s pond s t o acome ta ry nuc l eus o f 5 -23 km in d i ame te r , o r a l t e r -na t i ve ly fo r a l ong-pe r iod com e t t he d i ame te r m us tbe a t l e a s t 16 km. These ca l cu l a t i ons p rov id e sup-p o r t f o r t h e a b o v e s t a t e m e n t t h a t t h e i m p a c t o f al a rge com e t ma y no t p ro duc e an ea s i ly recogn izab l egeochemica l l y d i s t i nc t ho r i zon . T he p rob l em o fi r i d ium a s an impac t i nd i ca to r i s compounded byi r i d ium mobi l i t y du r ing d i agenes i s . The p re sen tconcen t ra t i ons o f i r i d ium may re f l e c t second a rymigra t i on r a the r t han t he p r imary depos i t i ona l -

impac t o r ig ina t ed even t . Bu t , de sp i t e t he se unce r -t a in t i e s even min or en r i chm ent o f i ri d ium a t b io -log i ca l ex t i nc t i on bounda r i e s shou ld be cons ide reds ign i f i c an t enough to i n i t i a t e a t ho rough sea rchfo r o the r impac t i nd i ca to r s , be fo re t he impac thypothesis i s re jec ted.

1.5 Megatsunami effectsT h e s e p r o b a b l y r e p r e s e n t o n e o f t h e m o s t u n d e r -

va lued and ove r lo oked c r i te r i a fo r ocean impac trecogn i t i on . The on ly ev idence p re sen t ed t o da t eis f o r th e m e g a t s u n a m i d e p o s i t s a t t h e K / T b o u n d -a ry i n t he mid-con t inen t a l she l f o f Texas(Bourgeo i s e t a l. , 1988), Ca r ibbe an (Hi ldebra ndand Boyn ton , 1990) , no r thea s t e rn Mexico (Smi te t a l . , 1992) , Hai t i (Florent in e t a l . , 1991) andf rom the Gul f o f Mex ico (Alva rez e t a l . , 1992) .Ahren s and O 'Kee fe (1983) impl i ed t ha t t sunamisgene ra t ed by an impac t wi th ene rg i e s o f 1028 1029and 1030 J i n an ocean 5 km deep wi l l p roducebo t tom e ros ion t o 360 , 2500 and 12 ,000km,re spec t i ve ly , f rom the impac t s i t e . The l a ck o fp rec i s ion o f such e s t ima te s i s i nd i ca t ed by ene rgycalcula t ions of Sleep e t a l . (1989) , who suggestedtha t a lo wer energ y of 2 x 1028 J wi l l resul t inevap ora t i on o f an en t i re ocean .

A n a p p r e c i at i o n o f t h e d es t ru c t iv e p o w e r o fm e g a t s u n a m i w a v e s c a n b e o b t a i n e d f r o m c a l c u la -t i ons o f t sunami w ave he igh ts a t va r ious d i s t ancesby Van de r Be rgh (1989) wh o sugges t ed t ha t ame teor i t e p rodu c ing a 150 km c ra t e r i n 5 km deepocean wou ld c rea t e a mega t sunami wave he igh t o f1300 m wi th in a 300km rad ius and o f 100 mheight a t a 10,000 km radius. Such a ca lcula t ionf i n d s s u p p o r t i n t h e G u l f o f M e x i c o s t u d y b yAlvarez e t a l . (1992) . In cont rast , an impact pro-duc ing a 50 km c ra t e r ( such a s t he M ontagna i s )wo u ld r e su l t i n a 200 m h igh wave wi th in a 300 kmrad ius and a 40 m w ave ove r a 3000 km rad ius .A n i m p a c t o f a m e t e o r i te o f 1 0 k m r a d i u s a n y w h e r ein the Pac i fi c Ocean sho u ld p ro duce a meg a t sunam iwave a long the ent i re Paci f ic r im. In this contexti t i s su rp r i s i ng t ha t t he on ly mega t sunami depos i t sfo r K/T impac t a re t hose i den t i f i ed i n t he Gul f o fMexico and i t s su r round ings (Bourgeo i s e t a l . ,1 98 8; H i l d e b r a n d a n d B o y n t o n , 1 9 9 1 ; F l o r e n t ine t a l ., 1991; Alvarez e t a l ., 1992; Sm ite t a l . , 1992).Does t h i s r e f l e c t t he complex i ty i n c a l cu l a t i ng


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nonlinear waves and in determining the conditionsfor breaking and bore formation and evolution assuggested by Strelitz (1979), or the effects of topog-raphy, or our mis interpretation o f geologic record?As deep ocean waves approach a shore, they firsttraverse the edge of the continental margin. Asshown by Sonett et al. (1991) from the conservationof momentum as the waves approach the shelfedge the waves' amplitude increases in dependenceon bottom topography. For an impact of a bolide0.2 km in diameter somewhere into ocean, at dis-tance of 2500 km when the wave strikes the marginits height would increase by a factor of two ormore if the margin is steep (Sonett et al., 1991).Thus, the impact of a body several kilometers indiameter into ocean has to result in significanterosion of the outer shelf, with the erosional effectgradually decreasing shoreward. This is consistentwith an observation that ninety percent of theworld-wide K/T boundary is characterised by ahiatus, which could be the result of erosion fromtsunami waves. On the same note, hiatuses iden-tified near the early/middle Eocene boundary inmuch of the U.S. Atlantic coastal plain (Olsenet al., 1988) could be related to the Montagnaisimpact. Miller et al. (1990) identified the strati-graphic position of one of these hiatuses in NewJersey as being within foraminiferal Zone P9 andthe Montagnais impact has been placed by Aubryet al. (1990) within the same foraminiferal zone.The New Jersey coast is located about 1200 kmfrom the Montagnais impact site within reach amegatsunami wave of about 90 m height, if weapply Van der Bergh's (1988) wave height calcula-tion for the Montagnais impactor. Action of suchwaves would cause bottom erosion on the shelfand could result in a stratigraphic gap, paleonto-logically recognizable as a hiatus.

An impac t o f 10 km in diameter asteroid willproduce enough energy to cause an earthquake of10.1 magnitude (McKinnon, 1982) to 12.4 magni-tude on the Richter scale (McLaren andGoodfellow, 1990). These magnitudes are largerthan any historically recorded earthquake. Such arelease of energy would be sufficient to generatemegaslumps and slides on continental margins andextensive turbidite deposition in the oceanic basin.There is no direct criterion to distinguish mass

wasting resulting from an impact f rom that causedby tectonically generated earthquakes. Only indi-rectly, if such deposits are synchronous with otherimpact indicators such as mega tsunami deposits incoastal areas, the presence of mixed pelagic andbathyal microfauna over shelves and/or oceanicplateaus and confined to a stratigraphically narrowhorizon, or with the development of a widespreadunconformity, then it would be reasonable toconsider an impact origin for such an event. Ifcombined with even a slight increase in iridium atthe top o f a such layer, or above the unconfo rmity,as observed at K/T boundary in northeast Mexicoby Smit et al. (1992), than this would strengthenthe possibility that the above features could resultfrom an asteroid or cometary nucleus impact intoan ocean.

1.6 Indu ced environmental changesThe produc tion of acid rain from nitrogen com-

plexes, a sudden increase in CO2, heating of theocean surface layer, and forest fires, have all beensuggested as potential by-products of a meteoriteimpact on Earth surface (Wolbach et al., 1990;Gilmour et al., 1990; Zahnle , 1990). The combinedeffects of these processes could have an environ-mentally devastating effect on marine organismsand culminate in the extinction of more environ-mentally sensitive genera as discussed by Hsii andMckenzie (1985) and Hsfi et al. (1985) in their"Strangelove Ocean" theory.2. Imp actor s ize an d biological ext inct ions

The near-Earth asteroid population containsthousands of objects ranging in diameter from afew meters to 40 kin, which cross or approach theEarth' s orbit ( McFadden et al., 1991). The impacton life of a collision with an asteroid or comet hasbeen investigated by several authors (e.g. Raup,1988, 1990; Gerst and Zardecki, 1982; Aubry et al.,1990; Jansa et al., 1990). According to Raup (1988)an impact of an asteroid as small as 1 km indiameter could result in biological extinction, butlarger 1 to 3 km diameter asteroids were suggestedby Gerst and Zardecki (1982). Aubry et al. (1990)pointed out that since the Montagnais impactordid not result in any biological extinction, the size


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280 L.F. JANSAo f a li f e t h r e a t e n i n g i m p a c t o r s h o u l d b e l a r g e rt h a n 3 k m . M c L a r e n a n d G o o d f e l l o w ( 19 9 0) a r b i-t r a r i l y s e l e c te d a 5 k m s i ze a s h a v i n g d r a s t i c e f f e c t so n p h y s i c a l a n d b i o l o g i c a l f e a t u r e s o n E a r t h , b u td i d n o t p r o v i d e a n y s u p p o r t i n g e v i d e n c e .

U s i n g t h e s i ze o f a n i m p a c t o r a s o n e o f th ev a r i a b l e s t o a s s e s s b i o l o g i c a l e x t i n c t i o n t h e o r i e s i sf l a w e d b y i n h e r e n t u n c e r t a i n t i e s i n i m p a c t o r v e l o c -i ty ( w h ic h m a y v a r y b y a f a c t o r f iv e o r m o r e f o r as l o w a s t e r o i d t o a r e t r o g r a d e l o n g - p e r i o d c o m e t )a n d b u l k d e n s i ty ( t hi s m a y v a r y f o r a c o m e t b y af a c t o r o f 6 ; 0 . 2 5 g c m - 3 t o 1 .5 g c m - 3 w i t h a v e r a g e0 . 6 g e m - 3 ; W e i s s m a n , 1 9 90 ). D e n s i t y f o r i r o na s t e r o i d s c o u l d b e e v e n a s h i g h a s 7 . 0 g c m - 1 ,w h i c h c o u l d i n c r e a s e t h e u n c e r t a i n t y b y a f a c t o ro f 28 . W h e n r e v is e d c o m e t a r y d e n s i ty o f 0 . 6 g c m - 3i s a p p l i e d t o t h e M o n t a g n a i s i m p a c t o r c a l c u l a ti o n ,i t s u g g e s ts a n i m p a c t o f a c o m e t w i t h n u c l e u s o fa b o u t 3 .4 k m i n d i a m e t e r , i f a n i m p a c t s p e e d o f2 8 .9 k m s - ~ i s s e le c t e d . A b e t t e r c o n s t r a i n t r e l a -t i o n s hi p b e t w e e n c o m e t a r y i m p a c t s a n d e x t i n c ti o nc a n b e o b t a i n e d b y u s in g t h e i m p a c t o r ' s e n e r g y ,a s r e f l e c te d b y t h e e x c a v a t e d c r a t e r d i a m e t e r . F i v ep o i n t s o n F i g . 3 h a v e b e e n u s e d t o c o n s t r u c t ac u r v e , w h i c h s h o w s t h e p o t e n t i a l r e l a t i o n s h i pb e t w e e n t h e s i ze o f t h e i m p a c t c r a t e r a n d t h ep e r c e n t o f g e n e r a b e i n g e x ti n c t . T h e p o i n t s u s e df o r c o n s t r u c t i o n o f t h e c u r v e c o n s t r a in o n l y t h el o w e r h a l f o f i t. T h e i n i t i a t io n p o i n t o n t h e " k i l l i n gc u r v e " a c c o r d i n g t o t h e M o n t a g n a i s i m p a c t c r a t e rs t u d y ( A u b r y e t a l . , 1 9 9 0 ) i s p l a c e d a t t h e c r a t e rs iz e > 4 5 k m i n d i a m e t e r . C r a t e r s s m a l l e r t h a nt h i s s i z e h a v e n o a s s o c i a t e d b i o l o g i c a l e x t i n c t i o n ,a s s u p p o r t e d b y s t u d y o f t h e 2 4 k m d i a m e t e r R i e sc r a t e r ( H e i s s i ng , 1 9 8 6) . T h e 1 8 0 k m w i d eC h i c x u l u b c r a t e r i n M e x i c o ( H i l d e b r a n d e t a l . ,1 9 9 1 ) i s a n o t h e r p o i n t u s e d i n t h e c u r v e c o n s t r u c -t i o n . I t i s h y p o t h e s i z e d t h a t t h e f o r m a t i o n o f t h i sc r a t e r p r o b a b l y c a u s e d e x t i n c t i o n o f 3 5 % o f g e n e r aa t t h e K / T b o u n d a r y ( S e p k o s k i, 1 9 9 0 ). T h e s y n -c h r o n e i t y o f t h e 3 5 k m M a n s o n c r a t e r , d a t e d b y3 9 Ar /4 Ar as o f 6 5 .7 +__ 1 M a (H ar t u n g e t a l . , 1 9 9 0 ) ,w i t h t h e C h i c x u l u b c r a t e r s u g g e s t s t h a t b o t h c r a t e r sc o u l d h a v e o r i g i n a t e d f r o m s p l it t in g o f a la r g e rd i a m e t e r a s t e r o i d o r a c o m e t ( I z e t t , 19 91 ; M c H o n ea n d D i e t z , 1 9 9 1 ) . T h e r e l a t i o n s h i p b e t w e e n l a t eE o c e n e e x t in c t io n s , w h e n 1 0 % o f g e n e ra b e c o m ee x t i n c t a n d t h e 1 0 0 k m w i d e P o p i g a i c r a t e r , f o r m e d

8o ~ / /~ / / / / ~

20 Z

2 4 3 2 ,1 ~ ~ 5 1 8 0 1 1 0 0CRATER DIAMETER Kin)

Fig. 3. Postulated extinctionscurves. The c urvesare constructedby plotting impact crater diameter versus percent of marinegenera killed. The threshold of ze ro extinction is placed atimpact crater o f 45 km diameter, defined by the Montagnaisstructure. The other p oints for curves construction are 35%extinction at the K /T boun dary and 10% extinction for thelate Eoce ne (Sepkoski, 1990). The relation between he Popigaistructure and la te Eoc ene extinctions s conjectural. Curve .4 isbased on Raup's (1990) suggestion hat such a curve should beof sigmoidal shape; curve B, preferred by the author is hyper-bolic and docume nts that additional impacts contributed toenvironmental stresses at the K /T boundary. The total lifeextinction is indicated for an impact of com et 70 km in diame-ter, which would result in excavation of ab out 1400km widecrater.a b o u t 3 9 to 3 5 M a a g o ( G r i e v e , 1 98 2; R . B o t t o m l e yp e r s . c o m m u n . , 1 9 8 9 ) i s s u s p e c t e d , b u t u n p r o v e n .

T h e s e e x t i n c t i o n p e r i o d s a r e u s e d f o r c o n s t r u c -t i o n o f " k i l l c u r v e s " A a n d B o n F i g . 3 . A s i g m o i d a lc u r v e s h a p e a s p r e f e r r e d b y R a u p ( 1 9 9 0 ) , i n d i c a t e st h a t t h e t o t a l e x t i n c t i o n o f l if e m a y o c c u r b y t h ei m p a c t o f a m e t e o r i t e a b o u t 7 0 k m i n d i a m e t e r,e x c a v a t i n g a c r a t e r a b o u t 1 4 0 0 k m i n d i a m e t e r( a s s u m i n g D/d=20; D = c r a t e r d i a m e t e r ; d =i m p a c t o r d i a m e t e r ; c u r v e A , F i g . 3) . T h e e s t i m a t e ds i z e o f t h e i m p a c t o r c l o s e l y c o r r e s p o n d s t o c a l c u -l a t e d 6 5 k m i n d i a m e t e r b o l i d e , w h i c h a c c o r d i n gt o M a h e r a n d S t e v e n s o n ( 1 9 8 8 ) c o u l d h e a t u p t h ea t m o s p h e r e a n d t h e s u r f a c e b y 1 0 0 C . T h e t o t a la n n i h i la t i o n o f e c o s y s te m s w a s e s t i m a t e d b y S l e epe t a l. ( 1 9 89 ) t o r e s u l t f r o m a n i m p a c t o f a n o b j e c t4 4 0 k m i n d i a m e t e r h i t t i n g t h e E a r t h a t 1 7 k m s - 1a n d c r e a t i n g a c r a t e r 1 5 00 k m i n d ia m e t e r . S i n c et h e c r a t e r e x c a v a t i o n i s b e t t e r r e p r e s e n t a t i o n o fk i n e ti c e n e r g y o f a n i m p a c t t h a n t h e b o l id e d i a m e -t e r , t h e n g o o d a g r e e m e n t e x i s t s b e t w e e n t h e e s t i -


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mate of a 1500 km crater and life annihilation onEar th by Sleep et al. (1989) and the 1400 km cra terestimated for life extinction in this paper (Fig. 3).

The curve B on Fig. 3 is hyperbolic and intu-itively favored by the author. If correct, then itsuggests that additional environmental distur-bances such as impact of other, smaller bodies(multiple impacts) may be responsible for increasedcumulative stress on the biosphere and extinctionsat K/ T boundary, as suggested by Izett (1991) andMcHone and Dietz (1991). That no vertebratesheavier than 25kg survived the terminalCretaceous event (Russell, 1976), indicates tha t thefall of comet or asteroid resulting in formation ofa 180 km diameter impact crater could be fatal tohuman society.

3. Per iodici tyThe difficulty in estimating the density of a

comet nucleus results in various theories about theproduction of craters by comets on Earth's surface(e.g., Shoemaker and Wolfe, 1982; Shoemakeret al., 1988; Weissman, 1982, 1990; Bailey andStagg, 1988). The estimates by Shoemaker and hiscolleagues that the number of Earth crossingcomets is four times greater than the number ofEarth crossing asteroids could be interpreted toimply that cometary impacts are 4 times morelikely to cause biological extinc tions. Grieve (1984)showed that during the last 250 Ma at least 5craters larger than 45 km have been produced. Thenewly discovered Chicxulub crater in Mexico(Hildebrand et al., 1991) is the sixth crater in thispopulation. Of these six impacts three are knownto be located in the ocean. If we consider a randomdistribution of impacts on the Earth's surface(taking the current ocean area of 72% of the Earthsurface) then the number of such large impactsshould be 12. This would translate into a rate of21 Ma for impact craters bigger than 45 km. But,if we limit impacts only to the oceanic areas, therate would decrease to 31 Ma.It is very probable that above estimates basedon curren t evidence for impacts do no t reflect trueimpact rates (Weissman, 1990). A location map ofknown impact sites compiled by Grieve (1987)shows the maximum concentration of impacts

within areas geologically well studied, such asNorth America and Europe. Since a random distri-bution can be expected, Grieve's tabulation givesonly lower limits to the number of impacts. Thisnumber should be increased at least by a factor of2 to account for those areas such as Africa, Asiaand the polar regions where geologic explorationis low. A further increase by a factor of 3 wouldaccount for the ocean covered area. Such adjustedestimates would increase an average time betweenimpacts to 10.5 Ma for format ion of craters> 45 km in diameter. Revised impact "fr equen cy"finds support in calculations of cratering rates forthe last 100 Ma by Shoemaker et al. (1990). Theseauthors concluded that for the above period thereshould be eight craters > 60 km in diameter, f romwhich five should have resulted from cometaryimpacts. Such estimates translate into a 12.5 Maaverage time between impacts not observed inbiological extinctions record. But, this "fr equen cy"agrees with the observations of Schultz (1987) whosuggested that youn g lunar craters appear to clus-ter in a nonrandom fashion at intervals of 10_+5Ma and 30+ 10 Ma. Similar time interval also hasbeen obtained by Perlmutter and Muller (1988),who noted that the cosmic ray exposure age oftype H chondrites (high metal ordinary chondrite)recovered on the Earth tended to cluster at valuesaround 7+_3 Ma and 30+_6 Ma and suggestedthat this may be proof of a flux of periodicimpactors through the planetary system at thosetimes, a conclusion disputed by Wiessman (1990).

Revised mean probabil ity o f collision with Ear thby the estimated population of Earth crossers byShoemaker et al. (1990) indicates that comet nucleiiwith diameters > 2.5 km could strike the Earth atthe rate of one every 10 Ma; comets with nucleii> 10 km in diameter probably at a rate of oneevery 100 Ma. Such data agree with asteroid fluxpublished by Alvarez (1987; Fig. 4 herein).Assuming that 10% of marine genera could bekilled by the impact o f a comet with an 8 kmdiameter nucleus as derived from Fig. 3, thenapplication of the Alvarez flux curve (Fig. 4) sug-gests a 6x 10 -~ yr -1 waiting time for such animpact and 2 x 10-8 yr-1 for K/T types of extinc-tions. Such a "frequency" of impacts that couldhave catastrophic effect on marine biota is two to


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282 L.F. JANSA

10_12,~ IMPACTORSON THE SURFACEOF THE10 1 " ~ E SH UTTLE1pro,30psec.

~ 10-6 ~ I N G STARS1ram,30sec.~10 4I.- 2, o - I--~ 1 k.METEOR ITES m , lyr~ o \U510 2 k METEOR RATER,AZZ = ~ 1 0 0 m , 104yr~ 104 ~ MONTAG NAISt ~ 3.5 kin, 8xl07yr

1 0 6 X / C f r B O L ID _ E- ,s= * 10 km 2x10 r1 0 e L LIFE EXTINCTION " ~ . / '/ 70 kin, 4xl0Syr ~ . . ~ %I I I I I 1 % I I I10 6 10 4 10 2 1 102 10 4 10 e 10a 101DIAMETER m)

F ig . 4 . I m p ac t f r eq u en cy cu r v e r ev i s ed a f t e r A lv a r ez ( 1 9 87 ) .M e a n t i m e b e t w e e n c o l l i s i o n s o n t h e E a r t h a n d v a r i o u s s i z eme teo r i t e s i s p lo t t ed . Th e cu r v e s h o w s th a t t h e KIT t y p e o fe x t i n c t io n o c c u r a b o u t e v e r y 2 x 1 0 a y r ; a n d t h e l if e-ex te r m in a t in g imp a c t 4 x 1 09 y r (4 b . y . ) a s t ak en f r o m F ig . 3 .

t h r e e t i m e s l o w e r t h a n t h a t o b s e r v e d f r o m t h epa leon to log ica l r eco rd (Sepkosk i , 1990 ) .Conc lus ions

T h e c r i t e ri a e m p l o y e d f o r r e c o g n i t i o n o f i m p a c t so f ex t r a te r r es t r ia l bo d ies in to sha l low ocean shou ldinc lude severa l f ea tu res d i f f e r en t f rom landimpac ts , such as: cy l ind r ica l shape o f cen t r a l up l i f t ,l a c k o f a c c u m u l a t i o n o f a f a l l - o u t b re c c i a o n t h ecra te r per imeter and a beve l led , r a ther than ar a i sed c r a te r r im. However , microscop ic sca le f ea -t u re s , s u c h a s c o m m o n o c c u r r e nc e o f gr a i ns w i t hdecreased an iso t rop iza t ion ; the p resence o f p r es -su re lamel lae in quar tz and f e ld spar g r a in s and thev a r y i n g d e g r e e o f d i a p l e c t i c g l a s s d e v e l o p m e n teven wi th in a s ing le e jec ta c las t a r e shock m etam or -p h i s m f e a t u r e s s i m i l a r t o t h o s e o b s e r v e d i n m o s tl a n d i m p a c t s . G e o c h e m i c a l l y , b a s e d o n t h e i n t e r -p r e t e d M o n t a g n a i s c o m e t a r y i m p a c t i n t o s h a l l o w

ocean ( Jansa e t a l . , 1989 ) , such an impac t cou ldb e a c c o m p a n i e d b y o n l y l o w i r i d i u m e n r i c h m e n ta n d m a y l a c k e n r ic h m e n t i n o t h e r s i d e ro p h i le a n dp l a t i n u m g r o u p e l e m e n t s . T h is o b s e r v a t i o n c o n t r a -d ic t s the in te rp re ta t ion o f s imi la r low i r id iuma n o m a l i e s a t o t h e r P h a n e r o z o i c b o u n d a r i e s b yOr th e t a l . ( 1990 ) , who conc luded tha t they a r e anu n l i k el y e v i d e n c e o f m e t e o r i t e i m p a c t s . T h e l a c ko f g e o c h e m i c a l e v id e n c e f o r a n i m p a c t e v e n t a t t h eimpac t ho r izon i s expec ted fo r la rge meteo r i tes ,s i nc e p a r t o f th e c o n d e n s a t e v a p o u r p l u m e e x p a n d sb e y o n d E a r t h ' s a t m o s p h e r e ( V i c k e ry a n d M e l o s h ,1990 ) . Th is wou ld r esu l t in h igh ly d i lu ted f a l lou to n E a r t h ' s s u r f a c e , a n d t h u s w o u l d n o t g e n e r a t ean eas i ly iden t i f iab le geochemica l ho r izon . Fo r th i sr e a s o n v e r i f ic a t io n o f im p a c t t h e o r ie s c a n n o t b ebased so le ly on geochemica l ev idence and a fu l lsearch fo r add i t iona l ind ica to r s mus t be r igo rous lypu r sued . Th is search shou ld inc lude ev idence fo rs o li d e je c t a w i t h s h o c k - m e t a m o r p h i s m f e a t u re s a n dtek t i tes . An impor tan t cons idera t ion i s the r eg ion -a l ly r es t r ic ted d is t r ibu t ion o f so l id e jec ta as ac o n s e q u e n c e o f a t m o s p h e r i c c o n d i t i o n s a t t h e t i m eof the impac t . T h is p laces cons t r a in t on in te rp re ta -t ion o f the impa c t even t in tha t the lack o f so l ide j e c t a o r o f g eo c h e m i c a l e n r i c h m e n t d o e s n o tcons t i tu te su f fic ien t ev idence to d i sc la im the im pac to r ig in fo r the r a r e geo log ic even t ho r izon , un lesss tud ied g loba l ly .

O n e o f th e m o s t o v e r l o o k e d i n d i c a t o r s o f o c e -a n i c i m p a c t s i s t h e d e v e l o p m e n t o f m e g a t s u n a m id e p o s i t s o n c o n t i n e n t a l m a r g i n s a n d i n n e a r s h o r ea r e a s , w h i c h m a y b e a c c o m p a n i e d b y t h e d e v e l o p -m e n t o f w i d e s p r e a d u n c o n f o r m i t i e s , h i a t u s e s a n deros ion su r f aces . I n con t r as t to tec ton ica l ly o reus ta t icaUy induced uncon fo rmi t ies , the impac tg e n e r a t e d u n c o n f o r m i t y w o u l d s h o w t h e l a r g e s tt ime gap (de epes t e ro s ion ) near the she l f edge . Thep resence o f ho r izons w i th mixed pe lag ic , ba thy a la n d b e n t h i c m i c r o f a u n a a b o v e t h e u n c o n f o rm i t i e ss h o u l d b e a n o t h e r i n d i c a t o r t o b e e x a m i n e d f o re v i d e n c e o f w a v e e r o s i o n a n d s e d i m e n t r e d e p o s -i t ion . Whi le s imi la r f ea tu res may a l so r esu l t f romupwel l ings on con t inen ta l marg in s the la t te r even tss h o u l d e n c o m p a s s a m u c h l o n g e r ti m e t h a n e f f e c tsp r o d u c e d b y a n o c e a n i m p a c t e v e n t . E x t r i n s i c a l l yr e la ted mass was t ing on con t inen ta l ma rg in s , spec i-f ic a ll y i n t h e u p p e r c o n t i n e n t a l s l o p e - o u t e r s h e l f


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COMETARY IMPACTS NTO OCEANS AND BIOLOGICAL EXTINCTIONS 283

area, could result from the combined effect ofshock wave propagation, generating major earth-quakes and megatsunami waves. Thus, the causesfor development of megaslumps on the continen talmargins and extensive turbidite deposits in thedeep ocean basins should include not only earth-quake or mass overloading triggering mechanismconsideration, but also large meteorite impactsinto oceans.

The Montagnais impact provided the criteria toscale the size of an impacting cometary body andthe resulting biological extinctions (Fig. 3). Whenthe known occurrence of impact craters >45 kmin diameter and formed during the last 250 Ma iscorrected to allow for the uneven geologic explora-tion of continental areas and for the ocean surfacearea, a 10 Ma average time between impacts isobtained. This "frequency" is in good agreementwith impact rate observed for young lunar cratersby Schultz (1987), the periodicity obtained forcosmic ray exposure time for H type chondritesby Perlmutter and Muller (1988), and the "fre-quency" of impacts from Earth orbit crossingcomets with nucleus > 2.5 km, by Shoemaker et al.(1990). If, 10% and 35% of all marine generawould become extinct as a consequence of animpact of comet which would excavate craters 100and 180 km in diameter respectively, such eventsmay recur at a rate of about 6x 10 -7 yr -1 to2 x 10 -8 yr 1 (Figs. 3 and 4). Such an impact ratesuggests that only three minor and one majorbiological extinction events, during the last 250Ma could be a consequence of an impact of acomet into an ocean. The remaining biologicextinctions as identified by Sepkoski (1990) mostprobably reflect tectonic, climatic, or oceano-graphic changes.

The final analysis of the causes of biologicalextinctions and other rare geologic events requiresgathering of ma ny local geologic observations intoa united global pattern, since even though theextinction event is globally recognizable, most ofthe physical indices of an ocean impact are onlyregional in extent. In pursuing the impact theoryno single criterion is sufficient to discriminatebetween potential causes of biological extinctions.It is only through a multiparameter study asemphasized in this paper, that sufficient evidence

may be provided for sudden and violent impactevents during Earth history, which may haveplayed significant role in biological evolution.

AcknowledgementsThanks are due to Digby McLaren and Ken

Hsii for stimulating discussions of the subject andto Richard Grieve for help with some impactcrater/impactor calculations. The author isindebted to Kevin Coffin and Graham Williamsand to unknown reviewers for critical commentsand helpful suggestions. Acknowledged is the help-ful assistance of K. Coffin, N. Koziel and G. Gran tduring manuscript preparation.

This is Geological Survey of CanadaContribution No. 41391.

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Jansa 1993 Marine Impacts