In the late 1970s, a group of scientists at the University of California at Berkeley began to wonder about the clay at the Cretaceous boundary. The most prominent members of the team were Luis Alvarez, a Nobel Laureate nuclear physicist, and his son, Walter Alvarez, a geologist, both professors at Berkeley. The elder Alvarez proposed using a technique called neutron activation analysis, which could measure extremely small quantities of, among other things, iridium. The Alvarez team had the presence of mind to examine the iridium content in, above, and below the clay layer at Gubbio that marks the end of the Cretaceous. They measured the abundances of twenty-eight chemical elements through this portion of the sedimentary column, and found something astonishing. Twenty-seven of the elements showed no major changes in abundance in and out of the layer. But there was 30 times more iridium in the clay than in adjacent sediments. Similar results have now been obtained from all over the world. In Haiti, there is roughly 300 times more iridium at the Cretaceous boundary than in adjacent layers; in New Zealand 120 times more; on the shores of the Caspian Sea, 70 times; in Texas, 43 times; and in the deep ocean of the Northern Pacific, 330 times.
A worldwide iridium-rich layer marking the end of the Cretaceous looks very much like direct evidence that a large cosmic body struck the Earth 65 million years ago. You can even calculate how big the body had to be to distribute this much iridium over the Earth. The answer turns out to be about ten kilometers across, a fairly typical size for a cometary nucleus or an asteroid. Of the four comets whose diameters had been measured by radar techniques through the middle 1980s, two of them were of this magnitude. Since many Earth-crossing asteroids seem to be extinct comets (Chapter 14), the chances are apparently better than even that the Cretaceous catastrophe was triggered by a comet that hit the Earth.* If so, that layer of clay is rich in comet stuff—minerals mainly; the ices should long ago have been melted and evaporated.
From the thickness of the clay layer, it follows that the iridium was not deposited instantaneously, but over 10,000 or even 100,000 years—much longer than the timescale of a single impact. But (Chapter 16) many comets may have been involved; and particles excavated by a single impact and ejected into near-Earth orbit may have continued to fall back to Earth for long periods of time. Occasionally, volcanoes can produce anomalously high concentrations of iridium, but material from the Cretaceous boundary shows alteration of the form and chemistry of minerals that are consistent only with an enormous shock—which can be provided by a cometary impact, but not by a volcanic eruption.
This story sounds familiar. We have heard something like it before—catastrophes caused by a comet raining down a worldwide layer of clay, the very thesis propounded by Ignatius Donnelly in Ragnarok (Chapter 10). The catastrophes are different, the timescales are different, but the idea is unmistakably similar. The clay layers of Donnelly were not chiefly at the Cretaceous boundary, Donnelly knew nothing of iridium, and the Alvarezes were not inspired by Donnelly. If enough borderline science is written—and enough certainly is—there are bound to be some lucky guesses, of which Ragnarok contains a few. But unlike the work of the Alvarez group, Donnelly’s arguments are insufficient. Today the iridium speaks for itself.
The Wedding Ring Anomaly
Of the dozens of iridium concentrations that have been found at the Cretaceous boundary, one turns out to be spurious. In ordinary rock from the crust of the Earth, the iridium content is less than a tenth of a part per billion. Yet even this is enough to be measured by neutron activation analysis. The abundance at Gubbio was much more—six parts per billion—and elsewhere still larger. But the iridium in one laboratory specimen of Montana Cretaceous clay turned out to be
due to the platinum wedding or engagement ring worn by a technician who had prepared the samples for analysis. Platinum used for jewelry contains about 10 percent iridium … If a platinum ring loses 10 percent of its mass in 30 years, the average loss per minute, if it all deposits on a sample, is about [a hundred times] higher than our sensitivity of measurement.*
The Alvarezes and their colleagues conclude that a few seconds’ exposure to a platinum wedding ring is enough to produce a spurious signal in the analysis. The more sensitive the instrument, the more careful you must be. Technicians henceforth wore gloves.
If an object ten kilometers across hit the Earth at cometary velocities, it would carve out a huge crater, more than two hundred kilometers in diameter. This would be true whether the comet struck on land or ocean, because the depth of the ocean is considerably less than the size of the comet. The resulting debris—some mix of pulverized comet and pulverized earth—would be thrown high; indeed, much of it would be ejected well above the atmosphere, into space. The debris, therefore, should have been transported all over the Earth, as is observed. The clouds of fine particles, ejected to escape velocity, would travel around the Sun in orbits that repeatedly intersected the orbit of the Earth; it is possible therefore that a steady rain of fine debris would have fallen for tens of thousands of years, until the cleansing of the inner solar system by the Poynting-Robertson Effect (Chapter 14) had been completed.
A crater two hundred kilometers across is too prominent to miss. Where is it? After years of assiduous searching, the crater has been found in the vicinity of the Yucatan Peninsula and Guatemala. It is called Chicxulub Crater. It is the right size and the right age, and the shocked quartz in it, among many other telling signs, shows that it is an impact and not a volcanic crater.
But how can a cometary impact kill hundred-ton land-dwelling dinosaurs as well as microscopic algae in an ocean halfway around the world? A number of suggestions have been made: Perhaps cometary cyanides (Chapter 8) poisoned everybody, or toxic metals, or acid rain. But the probable prime mechanism was suggested by the Alvarezes themselves: If you lift a one-centimeter-thick layer of fine particles off the Earth and up into the stratosphere, the individual particles will take a year or more to fall out. And since a centimeter thickness of clay covering all the Earth is opaque, it must also be opaque when it is distributed through the upper atmosphere, slowly falling. Sunlight would not have penetrated this cloud of iridium-rich cometary clay. For months or even years, the Earth would have been darkened and cooled.
The average temperature of the Earth during the Cretaceous was some 10 degrees Centigrade warmer than it is today; by current Standards, the Earth was then a tropical planet, with many lifeforms unprepared for bitter cold. Plants and animals in the tropics today, where the temperatures never fall below freezing, have few defenses against a major freeze. Calculations show that immediately after the Cretaceous impact, it would have gotten very cold all over the Earth—perhaps tens of degrees below freezing. Also, for months, the amount of light that reached the Earth’s surface would have been too low for most plants to photosynthesize, and even too low for animals to see. Through seeds, spores, and the like, land plants might have survived many years of cold and dark. But microscopic plants in the oceans, with no food reserves, would have died quickly—and the entire oceanic food chain which depends on those plankton would have collapsed soon after. The plight of the dinosaurs, stumbling across a frozen, somber, devastated landscape, unable to find either food or warmth, can readily be imagined. But small, warm-blooded burrowing mammals had a much better chance of surviving.
Premonition
The consequences of a collision with [a small earth-crossing asteroid] are unimaginable; the repercussions would be felt the world over. In dissipating the energy equivalent of half-a-trillion tons of TNT, 100 million tons of the earth’s crust would be thrust into the atmosphere and would pollute the earth’s environment for years to come. A crater 15 miles in diameter and perhaps three to five miles deep would mark the impact point, while shock waves, pressure changes, and thermal disturbances would cause earthquakes, hurricanes, and heat waves of incalculable magnitude. Should [the asteroid] plunge into the ocean a thousand miles east of Bermuda, for example, the resulting tidal wave, propagating at 400 to 500 miles per ho
ur, would wash away the resort islands, swamp most of Florida, and lash Boston—1500 miles away, with a 200-foot wall of water … The energy involved is the equivalent of 500,000 megatons of TNT—two orders of magnitude above that involved in the largest recorded earthquake, and four or five orders of magnitude more than Krakatoa … If the strike occurred in midocean, tsunamis in the 100-foot category would cause worldwide damage. If the strike occurred on land, the blast wave would level trees and buildings within a radius of several hundred miles, and some 10 tons of soil and rock dust would be thrown into the stratosphere, where for several decades it would act to reduce the solar radiation ordinarily received at earth’s surface and threaten the triggering of an ice age.
—MIT STUDENT PROJECT IN SYSTEMS ENGINEERING,
PROJECT ICARUS, MIT PRESS, CAMBRIDGE, MASSACHUSETTS, 1968
This paragraph represents an important premonition of the Cretaceous impact, and of Nuclear Winter.
One of the main causes of the Cretaceous extinctions, thus, seems to have been very similar to that consequence of modern nuclear war known as Nuclear Winter. Through the dust excavated by nuclear groundbursts, and smoke from the burning of “strategic targets” in and around cities, we humans can generate our own climatic catastrophe, perhaps adequate to bring about massive extinctions in our age as in the Cretaceous. The principal difference is that the dinosaurs did not contrive their own extinction. Since then, scientists have been working hard on compiling the grisly catalog of Earth catastrophes liable to follow an impact of a comet or asteroid of various sizes with the Earth. Bigger comets of course strike more rarely. The Cretaceous Tertiary comet (or asteroid), we recall, was about 10 kilometers across, and hit 65 million years ago. Its equivalent energy can be measured in megatons (millions of tons) of TNT. How many millions of tons? About a million. A million megatons—far more than the total contents of all the world’s nuclear arsenals at the peak of the Cold War. It carved out Chicxulub Crater, about 200 kilometers across.
A much more common impacter might be a hundred meters across, the size of a football field. It has impact energy of about 1 megaton, a routine explosive yield for contemporary nuclear weapons. These mostly break up in the atmosphere and cause no damage. In the vast range between 1 megaton impact energy and a million megatons impact energy is, as you might expect, a world of difference. We present here the latest data of Owen B. Toon, Kevin Zahnle, David Morrison, Richard B. Turco, and Curt Covey:
Impact energies between 10 megatons and about 10,000 megatons correspond to comets or asteroids a few hundred meters across; the big ones should appear once every several tens of thousands of years. But not enough fine dust is injected into the stratosphere to produce major climate effects. There would be blast damage, earthquakes, and fires on the scale of a small state. If the impact occurred in the ocean, tidal waves (tsunamis) would be elicited over entire ocean basins.
If the impact were in the 10,000 to 100,000 megaton range (a comet roughly a kilometer across), the impact occurs on average once every few hundred thousand years. The climatic effects might just be triggered. Global ozone depletion would begin. Because even an intact oral tradition does not extend back tens of thousands, much less hundreds of thousands, of years, there is no reason to expect a clear record of such catastrophes in human myth, legend, or folklore, but it is, we suppose, not entirely out of the question.
When we consider impacts in the 100,000 to 1,000,000 megaton range, we are considering a comet of a few kilometers diameter hitting the Earth, which would occur roughly every million years. Here, the sky becomes so hot from the impact that wildfires are ignited all over the world; along with dust in the stratosphere and sulfates from the impactors, the sky worldwide is calculated first to be on fire and secondly to become utterly pitch dark. The shock-heated air not only depletes but almost entirely eliminates the protective ozone layer that shields us from deadly ultraviolet sunlight.
And as we consider even larger comets—striking much less frequently it is true—the risks served up concatenate. It is clear that by whatever route—scientific deduction, fear of the unknown, residual myth—the ancient pervasive and global fear of comets is not without foundation.
It seems likely that had not a comet or asteroid hit the Earth 65 million years ago (or subsequently), the dinosaurs would still be here, and we would not. We would be merely one of the countless unrealized possibilities in the genes and chromosomes of the other beings of Earth.
*This material must come from somewhere, and meteoric dust generally falls thousands of times too slowly to be the source. In fact, the ground grows a little in one place because it is eroded a little somewhere else on Earth.
*Nevertheless, the advance of scientific knowledge has overtaken many of Laplace’s arguments here. We know that humans were on Earth for at least a million years before we invented written history and “civilization.” The fossil evidence that some mountainous terrain was once beneath the oceans puzzled many others, including Petrarch. The explanation, first advanced by Leonardo da Vinci and now firmly established by modern geology, is not a universal deluge which covered the mountaintops, but rather the slow rise of the mountaintops from the ocean floor.
*Astronomy I. The Solar System by H. N. Russell, R. S. Dugan and J. Q. Stewart, rev. ed. (Ginn and Co., Boston, 1945).
*As for humans, we have been here for 1 percent the time the dinosaurs ruled. There is no trace of us anywhere in the sedimentary column, except at the very top.
*While, in the following pages we do not everywhere add the words “or asteroid” after “comet,” we stress that there is still a significant probability that the Cretaceous impact was caused by an asteroid not of cometary origin.
*Walter Alvarez, Frank Asaro, Helen V. Michel, Luis W. Alvarez, “Iridium Anomaly Approximately Synchronous with Terminal Eocene Extinctions,” Science, Volume 216, p. 886, 1982.
CHAPTER 16
The Wrath of Heaven:
2. A Modern Myth?
Believing that every object and every event in the universe is arranged and directed by an Omnipotent Contriver, we must admit that when the Almighty formed the wondrous plan of creation, “foreseeing the end from the beginning,” he arranged the periods and the velocities of comets in such a manner that, although occasionally crossing the planetary orbits, they should not pass these orbits at the time when the planets were in their immediate vicinity. And should such an event ever occur, we may rest assured that it is in perfect accordance with the plan and the will of Omnipotence, and that it is, on the whole, subservient to the happiness and order of the intelligent universe, and the ends intended by the Divine government.
—THOMAS DICK, THE SIDEREAL HEAVENS AND OTHER SUBJECTS
CONNECTED WITH ASTRONOMY, AS ILLUSTRATIVE OF THE CHARACTER
OF THE DEITY AND OF AN INFINITY OF WORLDS, PHILADELPHIA, 1850
I feel rather at a disadvantage in speaking of comets to you; comets nowadays are not what they used to be.
—ARTHUR STANLEY EDDINGTON, “SOME RECENT RESULTS
OF ASTRONOMICAL RESEARCH,” FRIDAY-EVENING DISCOURSE,
THE ROYAL INSTITUTION, LONDON, MARCH 26, 1909
Stated baldly, it still sounds like something from the early days of pulp science fiction: 65 million years ago, a comet came out of space, hit the Earth, and extinguished much of the life on this planet. The Alvarez discovery has created something like a scientific revolution, with consequences propagating through geology, astronomy, and evolutionary biology, and even making incursions into international politics and nuclear strategy. The connection between cosmic events and our own existence is stirring.
But the Cretaceous extinctions are not the only, or even the major, instance of mass extinctions in the history of life on Earth. And—as Laplace and others realized—there must have been many impacts of comets (and asteroids) with the Earth. Iridium concentrations have been found in another sedimentary layer, associated with another mass extinction (although Permian sediments—associated with
a major mass extinction on everybody’s list—show so far not a hint of an iridium anomaly; the Permian catastrophe seems connected with an entirely internal event, the production of a superplume from the Earth’s core up to the surface). The Alvarez discovery set many other scientists from a range of disciplines thinking about cometary impacts and mass extinctions, which has led to still more extraordinary claims. However, these recent developments are more speculative, and—since some of the ideas contradict each other, and for other reasons—we must here tread cautiously.
The new developments began at the University of Chicago, where the American paleontologist J. John Sepkoski, Jr., had been compiling a master list of all the families of marine life recorded in all the sedimentary rocks. By far the great preponderance of families of life identified in the history of the Earth are now extinct. Sepkoski tabulated the epoch in which each extinct family died out. He could see a background rate of extinction, more or less constant through geological time, due to many causes—mountain building, greenhouse effects, disease, Darwinian competition, and the like. Superimposed on this background were times when the extinctions all piled up, one on top of another, as at the end of the Cretaceous, and during the other Great Dyings (see table, Chapter 15, this page). So far nothing much new, although the family extinction data were more encyclopaedic than anything compiled before.
But when Sepkoski, and David Raup, also of the University of Chicago, analyzed the extinction dates, they were surprised to find what looked like a periodicity.
The implications of periodicity for evolutionary biology are profound. The most obvious is that the evolutionary system is not “alone.” … With kill rates for species estimated to have been as high as 77% and 96% for the largest extinctions, the biosphere is forced through narrow bottlenecks and the recovery from these events is usually accompanied by fundamental changes in biotic composition. Without these perturbations, the general course of macroevolution could have been very different.