Tracking the Course of Evolution


by Richard Cowen

NOTE: This is page 2 of a three-page document.

Did a Catastrophe Cause the Extinctions?

Almost all the scientists directly involved in trying to explain the K-T extinctions are emotionally committed to one catastrophic hypothesis or the other, or are emotionally against both. This has resulted in claims that seem to overinterpret the evidence. One must be prepared to make one's own decision, and certainly all claims must be subject to close scrutiny.

We think we understand impacts and explosions rather well, after direct study of the Moon's surface, photographic surveys of cratered surfaces on planets and satellites, and our experience with nuclear blasts. We also know that asteroids do strike the Earth. Meteor Crater in Arizona, Manicouagan Crater in Canada, and scores of others can be seen from air photographs; indeed, about 20% of the world's nickel is mined from the Sudbury impact site in Canada, where an asteroid struck about 2000 Ma. Over geological time scales, an asteroid impact is not an unusual event.
Some general predictions of the asteroid impact theory are clear and can be used as indirect tests of its plausibility. The impact of a 10 km asteroid would blow a mass of vaporized rock and steam high above the atmosphere, forming an immense dust cloud that would slowly settle out through the atmosphere over a period of weeks, perhaps several months, perhaps several years. The blast and the cloud would spread material worldwide (Figure 18.6). The scenario has been discussed extensively because similar consequences (nuclear winter or at least nuclear fall) could result from a thermonuclear war. But realistic models are still not available, and at least some of the discussion is biased one way or another because the topic is so important politically. Nuclear fall models and K-T impact models have been so intertwined in people's minds that results from one tend to be automatically applied to the other in spite of the differences between the two.
Here is one possible impact scenario. An impact at Chicxulub, where the target rocks contain high quantities of sulfur, produces enormous amounts of sulfate aerosols in the atmosphere that act as nucleation sites for acid rains much more intense and devastating than anything we have generated from industrial pollution. One model suggests rain with the strength of battery acid! The direct effect is enough to suffocate some air breathers, to destroy plant foliage, and to dissolve the shells of marine creatures living along shores and in the surface waters of the ocean. The balance of CO2 between air and ocean is upset, and a chain of climatic events makes ocean surface waters barren for perhaps 20 years. Among other effects of the impact, dust, smoke, and aerosols cut down the sun's rays for weeks or months, so that land plants and algal plankton in the ocean cannot photosynthesize. The dust also causes freezing air temperatures within days after the impact, and maintains them below freezing for weeks or even months. This may not be an unusual situation at a pole, and may not be a problem for an organism living deep in the ocean, but it is a catastrophe for organisms on continental land masses.
Later, once the dust and aerosols have settled out, the enormous amount of CO2 released into the atmosphere by the impact generates a greenhouse effect that elevates temperatures on Earth for a thousand years or more.
The most extreme impact scenario could be called the microwave summer because it contrasts so much with nuclear winter. It was put together by Jay Melosh and colleagues. In this scenario, some of the material produced in a very large asteroid impact was blasted upward at a velocity greater than Earth's escape velocity, although most of it eventually fell back into the atmosphere on ballistic trajectories after a travel time of about one hour. An asteroid of mass 1015-1016 kg would have supplied the observed iridium and spherules, in a depositional layer averaging 10 kg/sq m (about 20 pounds per square yard of Earth's surface).
One can calculate how much thermal radiation the mass of ballistic debris would have emitted as it re-entered the atmosphere. Data on nuclear weapons suggest that the radiation pulse from infalling dust would have been 1000 times more than enough to ignite dry forests.
Ejecta radiation arrives spread over time, however, not in the single radiant pulse generated by an H-bomb. Even so, when we calculate this effect, the rates of worldwide radiation were somewhere between 30 and 100 times that of full sunshine, predominantly in the form of heat.
Of course, half of the radiation was directed upward into space, and some was absorbed by atmospheric water vapor and CO2. Nevertheless, one-third reached the Earth's surface. It would have taken most of the radiation to evaporate dense cloud, which would therefore largely have protected the surface beneath. Light cloud or no cloud would of course have given little or no shielding. Therefore, Melosh and colleagues estimate surface heating of perhaps 10 kilowatts per square meter for several hours, comparable with the heating in a domestic oven set at Broil.
This radiant heat then generated global wildfires that allegedly left soot in the K-T boundary sections. In general a surface temperature of 545°C is needed for wood to ignite spontaneously, and the radiation could not have produced this on a worldwide basis. But the volatile gases given off by hot wood will burst into flame after 20 minutes at 380°C, which is attained in the scenario. Even local variations in received radiation would have been sufficient to begin fires.
In perhaps the most bizarre of the "What if?" scenarios, if the tropical ocean surface were to reach 50° C, hypercanes (gigantic hurricanes) might have sucked up ice and dust and blown them into the stratosphere, blocking sunlight even more and destroying the ozone layer!
What do we do with these impact scenarios? Naturally, we compare them with the evidence from the geological record. Birds, tortoises, and mammals live on land and breathe air: the evidence from the K-T boundary shows that they survived the K-T boundary event. Therefore they and the air they breathed weren't set on broil for several hours. To put it simply, these scenarios did not happen.

We also think we know rather a lot about volcanic eruptions. Gigantic eruptions could produce results similar to those of an impact. Volcanic eruptions produce ash, but, even more important, they produce vast amounts of aerosols in the form of sulfuric acid droplets, which stay suspended longer than ash and produce long-lasting effects on climate. Eruptions can sometimes blow material into the stratosphere, where it can be carried over great areas. The eruption of Tambora in 1815 blew out 30 cubic km (7 cubic miles) of ash and dust, which caused spectacular sunsets worldwide and inspired Turner's finest paintings. The darker side of the eruption was that the dust and ash blocked off enough sunlight to cause "The Year Without a Summer" in 1816. Crops failed all over the Northern Hemisphere, resulting in widespread hunger, and even starvation in some areas. The summer was so gloomy in Europe that it depressed Mary Shelley enough to write the famous novel Frankenstein. The eruption of Toba, 75,000 years ago, is the largest documented eruption on Earth, perhaps 100 times the scale of Tambora. Yet the Toba ash is nowhere near the scale of the Deccan Traps. The possible results of the Deccan Traps eruptions include acid rain, ozone depletion, a greenhouse effect, a cooling effect, or any combination of the above: in other words, many of the same effects cited for an asteroid impact.

The Ecology of a Catastrophe

It's easy to imagine that a giant eruption or impact might have caused some kind of catastrophe at the K-T boundary. But it is not certain that it would. The problem with discussing impacts, nuclear war, and eruptions is that we don't know how much dust, smoke, and aerosols would be produced, even though it's absolutely critical to calculations of darkening and temperature change that we know those factors rather precisely. We don't know how far aerosols and stratospheric dust would be carried over the Earth, or in detail what effects they would have. Dust in the air can help absorb solar heat rather than reflect it, for example, and some models of nuclear war suggest that parts of the Earth would warm, parts would cool, and parts would stay at about the same temperature.
In some ways, some volcanic and impact scenarios are similar. For example, some calculations suggest that a Chicxulub impact could have produced hundreds of times more sulfate aerosol than the Tambora eruption in 1815, with its dramatic climatic effect.
The most persuasive scenarios of catastrophic extinction are quickly summarized. Regionally, there is little doubt that the North American continent would have been absolutely devastated. Globally, even a short-lived catastrophe among land plants and surface plankton at sea would drastically affect normal food chains. Pterosaurs, dinosaurs, and large marine reptiles would have been vulnerable to food shortage, and their extinction after a catastrophe seems plausible. Lizards and primitive mammals, which survived, are small and often burrow and hibernate; they would have found plenty of nuts, seeds, insect larvae, and invertebrates buried or lying around in the dark. In the oceans, invertebrates living in shallow water would have suffered greatly from cold or frost, or perhaps from CO2-induced heating. But deeper-water forms are insulated from heat or cold shock and have low metabolic rates; they therefore would be able to survive even months of starvation. High-latitude faunas in particular were already adapted to winter darkness, though perhaps not to extreme cold. Thus, tropical reef communities could have been decimated, but deep-water and high-latitude communities could have survived much better. All these patterns are observed at the K-T boundary.

Doubts about Catastrophes

The problem with catastrophic hypotheses for the K-T extinctions is that the catastrophes must have been severe but not too severe, because so many creatures survived. Dust and soot must have fallen quickly (within a year) to satisfy some scenarios, but had to remain suspended longer in the atmosphere to produce other effects.
Some specific evidence shows that impacts and eruptions do not necessarily cause catastrophes. For example, a major impact formed the Ries crater in Germany at 15 Ma, throwing huge masses of boulders more than 100 km (60 miles) into Switzerland and the Czech Republic, and tektites several hundred kilometers. The Ries impact did not affect even the local mammal fauna. A major impact at 51 Ma formed the Montagnais crater in the North Atlantic, 45 km (28 miles) across, and an impact hit Chesapeake Bay at 35 Ma, causing a crater 90 km (56 miles) across, but neither of them caused an extinction.
One should beware, however, of dismissing catastrophic explanations because small events do not trigger catastrophes. There may be a threshold effect: if the event is not big enough it will do nothing, but if it is big enough it will do everything. Perhaps there has been only one asteroid impact in the last 500 m.y. large enough to cause a mass extinction (at the K-T boundary); perhaps there have been only two eruptions large enough, at the K-T boundary and/or the P-Tr boundary.
Despite the model predictions and despite reasonable evidence about the physical effects, we don't yet know whether an impact and/or an eruption would have catastrophic, severe, or only mild biological and ecological effects, or whether those effects would be local, regional, or global. In each scenario, however, the killing agent is transient: it would have operated for only a short time geologically. Clearly, if such events occur, they are rare. That does not make them impossible, only unlikely. And that means they have to be very persuasive before we accept them!

The paleontological evidence from the K-T boundary is ambiguous. While many phenomena are well explained by an impact or a volcanic hypothesis, others are not. The fossils do provide us with real evidence about the K-T extinction events, instead of inferences from analogy or from computer models.
The best-studied terrestrial sections across the K-T boundary are in North America. Immediately this is a problem, because we know that the effects of the asteroid impact were greater here than in most parts of the world. Perhaps this has given us a more catastrophic view of the boundary event that we would gather from, say, comparable careful research in New Zealand. Even so, it is obvious that life, even in North America, was not wiped out: many plants and animals survived the K-T event.

Land Plants
North American land plants were devastated from Alberta to New Mexico at the K-T boundary. The sediments below the boundary are dominated by angiosperm pollen, but the boundary itself has little or no angiosperm pollen and instead is dominated by fern spores in a spore spike analogous to the iridium spike (Figure 18.7). Normal pollen counts occur immediately after the boundary layer. The spore spike therefore coincides precisely with the iridium spike in time and is equally intense and short-lived.
The spore spike could be explained by a short but severe crisis for land plants, generated by an impact or an eruption, in which all adult leaves died off for lack of light, or in a prolonged frost, or in acid rain. Perhaps ferns were the first plants to recolonize the debris, and higher plants returned later. This happened after the eruption of Krakatau in 1883. Ferns quickly grew on the devastated island surfaces, presumably from windblown spores, but they in turn were replaced within a few decades by flowering plants as a full flora was reestablished.
Evidence from leaves confirms the data from spores and pollen. Land plants in North America recovered from the crisis, but many Late Cretaceous plant species were killed off. The survivors probably remained safe during the crisis as seeds and spores in the soil, or even as roots and rhizomes.
Angiosperms were in the middle of a great expansion in the Late Cretaceous, and the expansion continued into the Paleocene and Eocene. Yet there were important and abrupt changes in North American floras at the K-T boundary. In the Late Cretaceous, for example, an evergreen woodland grew from Montana to New Mexico in a seasonally dry, subtropical climate. Changing leaf patterns indicate that the climate was slowly warming during the latest Cretaceous. At the boundary the dominantly evergreen Late Cretaceous woodland changed to a largely deciduous Early Cenozoic swamp woodland growing in a wetter climate. The fern spike represents a period of swampy mire at the boundary itself. Deciduous trees survived the K-T boundary events much better than evergreens did; in particular, species that had been more northerly spread southward. These changes could be explained in two different catastrophic scenarios: a regional catastrophe that wiped out all vegetation locally, with recolonization from survivors from the north; or a catastrophe that selectively destroyed evergreen plants.
Plants in Japan were affected less than North American ones, and Southern Hemisphere plants were hardly affected at all. Most likely, this reflects the fact that North America was hit far harder than any other continent by the Chicxulub impact.

Freshwater Communities
Some ecological anomalies at the K-T boundary are not easily explained by a catastrophic scenario. Freshwater communities were less affected than terrestrial ones. For example, turtles and a more primitive group of aquatic reptiles, the champsosaurids, survived in North Dakota while dinosaurs were totally wiped out. Freshwater communities are fueled largely by stream detritus, which includes the nutrients running off from land vegetation. It has been suggested that animals in food chains that begin with detritus rather than with primary productivity would survive a catastrophe better than others. That may be true generally and seems to be true for freshwater communities at the K-T boundary, but such communities would survive any ecological crises better, catastrophic or not.

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