K/T Boundary Impact Hypothesis

Ralph E. Taggart, Professor

Department of Plant Biology

Department of Geological Sciences

Michigan State University

As noted in our class discussions, none of the conventional hypotheses for dinosaur extinction were particularly persuasive, either singly or in combination. All that was to change in 1980, when L.W. Alvarez and colleagues published a paper proposing that, approximately 65 million years ago, the earth was struck by an asteroid-sized object:

An image of Phobos, one of the two moons of Mars, reconstructed from three Viking I images obtained at a distance of approximately 620 km. (National Space Science Data Center).

There can be no doubt that the earth and other planets and moons of the solar system have been heavily bombarded by large bodies earlier in their history. Most traces of these impacts have been obscured on the Earth, as a result of weathering and active geological processes. However, the Moon and Mars show extensive cratering, most of which dates from earlier in the history of the solar system. The photo of Phobos, one of the Martian satellites (shown above) documents what is actually a modest-size asteroid (about 20 km in diameter) that was captured by the gravitational field of Mars and which has achieved a stable orbit. What is interesting is that even Phobos, an asteroid, shows evidence of significant cratering, including the large impact crater in the upper-left quadrant. Innumerable smaller impact features are evident, including several small structures resulting from glancing impacts.

Although the frequency of large-body impacts has declined, such impact-events must be considered common on the scale of geological time:

The Boundary Clay

Lawrence Laboratory, University of California

The characteristic "boundary clay" layer as exposed near Gubbio in Italy. The rock layers are tilted as a result of later geological events, but the thin layer of dark clay is positioned precisely at the K/T boundary. Fossils in the layers below the clay are typical of Cretaceous-age sediments, while the fossils from the sediments above the clay are indicative of an early Tertiary age. This boundary clay layer appears to occur around the world, wherever sediments were accumulating (and have been preserved) across the K/T boundary interval:

Alvarez, et al., 1990.Geol. Soc. Amer. Spec. Pub. 190

Where found, the K/T boundary clay generally has three distinctive characteristics:

Image by J.S. Alexopoulos

So Where is the Crater?

Image by W.B. Hamilton

The size of the "impactor" has been variously estimated, but most projections suggest a diameter of 6-10 km, perhaps half the size of Phobos (shown above). The impact of such a large body should certainly have created a very large crater. Meteor Crater, near Winslow, Arizona, represents the impact of a modest-sized meteor perhaps 50,000 years ago. The K/T boundary impactor was much larger and should have left an impressive crater. Is so, where is it?

One problem with the K/T boundary crater, assuming there was one, was that it might very well have been buried by later sediments, particularly if the body landed in the oceans, which comprise over 70% of the Earth's surface area. One crater was identified in the sub-surface, near Manson, Iowa. That crater turns out to be the correct age, but it is far smaller than that projected for the primary K/T impactor. In fact, the Manson structure may represent the impact site of a fragment of the main K/T body.

Levin, 1999. The Earth Through Time

Attention is now focused on a 180-km crater-like structure located in the sub-surface and centered on the town of Chicxulub on the NW coast of Yucatan. The Chicxulub structure was apparently formed by a blast-like event that occurred 65.2 (+/- 0.4) million years ago. Virtually all the available geological evidence suggests that the Chicxulub structure was created by the impact of a body up to 10 km in diameter, striking the earth at a velocity of up to 30 km/sec.

Interestingly, the search for the K/T boundary impact site has led to the recognition of a large number of other presumed impact sites of varying size, covering a time span in excess of 500 million years.

Biological Effects

The list of catastrophic mechanisms that have been proposed as a result of the K/T impact event seems to grow by the day. Here is a short list:

Many of these proposed mechanisms would have such horrific ecological effects that explaining the K/T boundary extinctions might seem pretty obvious. It is, however, necessary, to separate the Impact hypothesis from hypotheses linking the impact with the K/T extinction events. It is even useful to differentiate between marine and terrestrial extinctions.

Marine Extinctions

Copyright J. Sibbick

Even slight acidification of the normally slightly alkaline waters of the ocean would have had disastrous effects on calcareous nannoplankton, a key element at the base of Cretaceous marine food chains. Given CO2 production as a consequence of impact, additional CO2 as a result of the Deccan Traps volcanics in India, and possible sulfate aerosols, it is reasonable to expect the collapse of marine food chains. This would leave animals at the top of these food chains (such as these Ichthyosaurs, other marine reptiles, and ammonites) highly vulnerable, especially as many of these groups may not have been doing particularly well toward the end of the Cretaceous. In short, it is relatively easy to build a link between K/T boundary impact and the extinctions in the marine realm.

Terrestrial Extinctions

Levin, 1999. The Earth Through Time

With respect to K/T extinctions, the terrestrial record is highly variable. Dinosaurs, of course, suffer 100% extinction. In fact, dinosaurs, despite their diversity, show a remarkable sensitivity to K/T boundary events - a subject we will explore in class.

Copyright J. Sibbick

Other terrestrial groups (amphibians, reptiles, mammals, and birds) demonstrate much more modest extinction levels, typically ranging from 25 to 50%. One group of reptiles that entirely disappeared were the flying forms (pterosaurs). In fact, pterosaur diversity had been on the decline throughout the Cretaceous, probably as a result of competition wit birds. By the close of the Cretaceous, birds dominated the flying niche for tetrapods, except in the very largest size classes, represented by these Pteranodon-like pterosaurs. Many of these animals were associated with coastal areas and were probably fish-eaters. As such, they would have been subject to the same extinction pressures as marine animals high in the Upper Cretaceous food chain.

The situation with respect to terrestrial plants is uncertain, with some workers suggesting high levels of extinction while others assert that extinction levels were lower and not tightly coupled to any K/T boundary event(s).

Given the scale of disturbance implied by the various catastrophic K/T boundary mechanisms, an alternative approach (Taggart and Cross, in press) to assessing the biological impact of the impact would be to look at the ecological consequences. Given the scale proposed for most impact-related mechanisms, one would expect massive disruption of Cretaceous terrestrial ecosystems and a period of recovery during which surviving organisms would gradually form new communities. Such processes are easily observed in modern ecosystems following any major disturbance and represent ecological succession.

In effect, we would expect to see sudden changes in the nature of terrestrial plant communities, followed by the reconstitution of stable ecological systems. One of the best ways to document such changes is the use of fossil pollen grains and spores recovered from very closely spaced rock samples across the K/T boundary. In a classic 1984 paper, Robert Tschudy and others were able to document a very pronounced ecological event in the stratigraphic record from the San Juan Basin in New Mexico:

Normally, pollen and spore samples from the Upper Cretaceous are made up of pollen of Gymnosperms and Angiosperms and the spores of ferns and related plants. In most such samples, fern spores make up between 10 and 30% of the pollen/spore total. What Tschudy noted was that in rock samples immediately above the boundary clay, fern spores suddenly peaked to 70-100% of the total pollen/spore count and that they gradually dropped to pre-impact levels through several samples above the boundary.

This sudden increase in the number of fern spores right above the K/T boundary has been given several names. The K/T boundary "fern spike" is perhaps most descriptive, but it has also been called the "Tschudy Effect" and, more recently, the K/T "Fern Spore Abundance Anomaly" or FSAA. Tschudy and others interpreted the FSAA as a succession marker, documenting the influx of large numbers of ferns into the vast area devastated by the impact. The study of smaller disturbance events (volcanism and fire, for example) show similar "fern spikes", confirming that ferns were the primary "weeds" on Cretaceous landscapes and that the gradual decline in fern spore abundance following disturbance represents the successional influx of later, more complex vegetation types.

In fact, something like the FSAA is to be expected as a result of the large-scale disturbance cause by the impact event. What remains to be seen, is the scale of the impact-related disturbance. Most of the disturbance mechanisms that have been proposed are global in scope, suggesting that we should find FSAA-like spore signatures associated with the boundary clay wherever it occurs. In fact, the only place we see the FSAA is in sites within the western Interior of North America:

 While the FSAA would seem to provide unambiguous evidence for major ecological disruption, sites showing the "fern spike" are all relatively close to the Chicxulub crater. This map (Taggart and Cross, in press), shows a selection of K/T boundary sites from western North America, all of which have the basic boundary clay layer. Note however that all the sites that have the FSAA (red) are located within 3400 Km of the impact crater. More distant sites in southern and central western Canada (yellow) don’t show the FSAA in the fossil pollen and spore record. In fact, only one other site outside of western North America, a marine core in the Far East, seems to show the FSAA. In the case of this core however, there is no associated record of the boundary clay. Since "fern spikes" have shown to be associated with other types of disturbance in the Cretaceous (volcanism and fire, for example), there is no reason to suppose that this FSAA event was associated with the boundary impact.

The geographic limitations of FSAA occurrences would suggest that disruption of communities as a result of the impact did not extend beyond 3500 Km from the impact site. If the ecological disruption associated with the impact were regional rather than global, it would suggest that many of the scenarios of global K/T catastrophe may need to be reassessed.

Selected References

The literature on the K/T boundary problem is immense. Here are just a few starting points if you are interested in doing some reading on the subject.

ALVAREZ, L.W., W. ALVAREZ, F. ASARO, and H.V. MICHEL. 1980. Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science 208:1095-1108.

BOURGEOIS, J., T.A. HANSEN, P.L. WIBERT, and E.G. KAUFFMAN. 1988. A tsunami deposit at the Cretaceous-Tertiary boundary in Texas. Science 241:567-570.

FLEMING, R.F. and D.J. NICHOLS. 1988. The Fern Spore Abundance Anomaly at the Cretaceous-Tertiary boundary: a regional bioevent in western North America, p.347-349, in KAUFFMAN, E.G. and O.H. WALLISER (eds.), Extinction Events in the History of Life, Lecture Notes in Earth Sciences 30, Springer-Verlag, New York.

HOOKER, J.S., 1987. Late Cretaceous ashfall and the demise of a hadrosaurian "herd". Geological Society of America Abstracts with Programs 19:284.

MCLEAN, D.M. 1985. Deccan traps mantle degassing in the terminal Cretaceous marine extinctions. Cretaceous Research 6:235-259.

O'KEEFE, J.D. and T.J. AHRENS. 1982. Impact mechanism of large bolides interacting with Earth and their implications to extinction mechanisms, p.103-120, in SILVER, L.T. and P.H. SCHULTZ (eds.), Geological Implications of Impacts of Large Asteroids and Comets on the Earth. Geological Society of America Special Paper 190.

O'KEEFE, J.D. and T.J. AHRENS. 1989. Impact production of CO2 by the Cretaceous/Tertiary extinction bolide and the resultant heating of the Earth. Nature 338 (16 Mar):247-249.

POPE, K.O., K.H. BAINES, A.C. OCAMPO, and B.A. IVANOV. 1994. Impact winter and Cretaceous/Tertiary extinctions. Results of a Chicxulub asteroid impact model. Earth and Planetary Science Letters 128:719-725.

RETALLACK, G.J. 1996. Acid trauma at the Cretaceous-Tertiary boundary in eastern Montana. GSA Today 6(5):1-7.

SHOEMAKER, E.M., R.F. WOLFE, and C.S. SHOEMAKER. 1990. Asteroid and comet flux in the neighborhood of Earth, p. 155-170, in V. SHARPTON and P. WARD (eds.), Global Catastrophes in Earth History. Geological Society of America Special Paper 247.

SWEET, A.R., D.R. BRAMAN, and J.F. LERBEKMO. 1990. Palynofloral response to K/T boundary events; a transitory interruption within a dynamic system, p.457-469, in SHARPTON, V.L. and P.D. WARD (eds.), Global Catastrophes in Earth History. Geological Society of America, Special Paper 247.

TAGGART, R.E.and A.T. CROSS. 1997. The relationship between land plant diversity and productivity and patterns of dinosaur herbivory. p.403-416, in WOLBERG, D.L., E. STUMP, and G.D. ROSENBERG (eds.), Proceedings of the Dinofest International Symposium, Arizona State University (Tempe). Academy of Natural Sciences, Philadelphia. 587pp.

THOMPSON, S.L. 1988. Multi-year global climatic effects of atmospheric dust from large bolide impacts (abstract). Conference on Global Catastrophes in Earth History, Snowbird, Utah, October 20-23, 1988, Lunar and Planetary Institute Contribution 673, Houston, Texas, p.194

TOON, O.B., K. ZAHNLE, D. MORRISON, R.P. TUREG, and C. COVEY. 1997. Environmental perturbations caused by the impacts of asteroids and comets. Reviews of Geophysics 35:41-78.

TSCHUDY, R.H., C.L. PILLMORE, C.J. ORTH, J.S. GILMORE, and J.D. KNIGHT. 1984. Extinction and survival of plant life following the Cretaceous-Tertiary boundary event, Western Interior, North America. Science 225:1030-1032.

WOLBACH, W.S., I. GILMOUR, and E. ANDERS. 1990. Major wildfires at the Cretaceous/Tertiary boundary, p. 391-400, in SHARPTON, V.L. and P.D. WARD (eds.). Global catastrophes in Earth history. Geological Society of America, Special Paper 247.

Ralph E. Taggart (taggart@msu.edu)