CHAPTER 3: TARGET EARTH
If the other planets in our solar system are heavily cratered, why do we see so few craters on the Earth? What most distinguishes our planet from the Moon, Mercury, Mars, and Venus is the Earth's higher levels of geologic activity, including volcanism and plate tectonics. These effects, together with water and ice erosion from the continents and sedimentation in the oceans, effectively fill in, erode, and erase impact craters almost as quickly as they are made. Nevertheless, a few craters can be found. We discuss Meteor Crater in Arizona is considerable detail, including the early 20th century excavations by D.M. Barringer and the space-age research by Gene Shoemaker, and explain the reasoning that led to the crater's identification as the product of explosive impact by an iron asteroid the size of a small office building. The scene then moves to Australia, where we accompany Shoemaker through the Outback in his continuing search for terrestrial impact craters. At Sudbury, Ontario, the world's largest nickel mine is the product of an impact 1.7 billion years ago. And as we note at the end of the chapter, the water and carbon in our own bodies are directly related to impacts, brought to our planet by comets striking Earth more than 4 billion years ago.
CHAPTER 4: DEATH OF THE DINOSAURS
"To suggest that the dinosaurs were not wiped out by the comet impact is something like saying that the 1500 victims of the sinking of the Titanic in 1912 did not die in the collision with the iceberg. Technically, that is correct. No one died in the collision. Everyone who died did so later, as the result of drowning, hypothermia, heart attacks, falling to their death, or being crushed by swinging lifeboats. In the same way, virtually no species went extinct at the moment of impact. It is what happened afterward that took its toll. That was when habitat was destroyed and entire species disappeared because they burned, drowned, froze, or starved to death." -- Astronomer Gerrit Verschuur in Impact: The Threat of Comets and Asteroids (1966)
More than 99 percent of the species that have lived on the Earth are extinct. An extinction occurs when all organisms comprising a species die out so that the species ceases to exist. Extinction is a part of biological evolution, just as death is a part of an individual's life. Roughly speaking, for every new species that appears, one disappears from the world. However, as University of Chicago paleontologist David Raup has noted, naturalists have usually concentrated on the origin of species rather than on their extinction. Darwin's famous book is The Origin of Species, not The Origin and Extinction of Species. Only in the past few years has the study of extinctions moved into the mainstream of natural science.
We have abundant fossils only from the most recent 15% of Earth's history, the past 600 million years. Throughout this span, pollens, shells, bones, and so on have been deposited on the bottoms of lakes and oceans. The muds became rocks, and the layers of embedded fossils -- when exposed by a geologist's rock hammer -- provide windows onto the evolving diversity of life. Occasionally a period is well documented by a particularly fine set of fossils, while the biota of other periods is less well understood. We see general patterns in the origin, modification, and extinction of species, but it is not easy to pinpoint the exact time that a particular species first appears on the scene or when its last members die out.
Despite such uncertainties, the fossil record shows that the formation of new species and extinction of old ones are discontinuous processes. Many paleontologists used to think that the abrupt changes in observed fossils reflected gaps in the geological record rather than sudden changes in the evolution of life, but we now know that this is not generally true. Many and probably most extinctions have happened during a few brief intervals of time. These great killings, when a large fraction of the normally well-preserved marine species suddenly disappear from the face of the planet, are called mass extinctions. They are among the most provocative features of the history of life on Earth.
What sort of global catastrophe could cause such dramatic dying? There had been hypotheses of volcanic cataclysms, sudden ice ages, even supernovas exploding near the solar system. Still, no consensus had been reached as the 1970s drew to a close. Meanwhile, several Berkeley scientists were collecting rock samples in the foothills of the Umbrian Apennines, from a road-cut in a gorge about a mile up the highway from Gubbio, Italy. Their work had a revolutionary effect on scientific thinking about the history of our planet and its relationship to the cosmos.
The largest recent mass extinction occurred 65 million years ago, at the end of the Cretaceous era of geological history. Although the Cretaceous extinction is better documented than earlier mass extinctions, rocks formed from marine sediments of the Cretaceous are still difficult to find. Many remain beneath the oceans; others lie buried far beneath the ground. Some of the most accessible exposures are near the medieval, walled town of Gubbio, Italy. You can board a small aerial tram in Gubbio and ascend to hill-top overlooks of the agricultural valley below and the Bottaccione Gorge to the north, through which the road to the east ascends. Near Gubbio, ocean-floor rocks spanning 150 million years are exposed in tilted strata. One can walk out through an ancient gate from the town and up the road, passing rocks that are progressively older. Soon, you reach a site where the rocks are 65 million years old. For decades, geologists have used their rock hammers to chip out hand samples near a dark layer of lithified or petrified clay, only about as thick as a pencil's width.
To the trained eye, a magnifying glass reveals -- below the dark band -- fossilized calcite shells of tiny, single-celled animals that floated in the surface waters of the ancient oceans. Above the dark band, the rock looks lifeless at low magnification. However, microscopes reveal a suite of entirely different, tinier fossils that are absent from the part below. Similar abrupt changes are found in 65-million-year-old rock samples collected from around the world. Nineteenth-century geologists identified this band as the boundary between the Cretaceous (abbreviated K) and the Tertiary (abbreviated T), the name given to most of geologic time since the changeover. The intervening band -- obscure to a layperson's eye until a geologist points it out -- marks the K-T boundary. This is an especially interesting boundary because dinosaur bones are found in Cretaceous rock beds but are absent in Tertiary rocks. Evidently something profound happened 65 million years ago to end the age of the dinosaurs.
The Berkeley father/son team of Luis and Walter Alvarez and their coworkers realized that the enigmatic, intervening layer of lithified clay might hold clues to what happened at the time of the K-T boundary. Their idea was to measure how fast sediments had accumulated before, during, and after the clay layer was deposited. They reasoned that meteoritic dust -- microscopic remnants of "shooting stars" -- falls to Earth at a steady rate. This makes sense, since the rate at which Earth intercepts cometary and asteroidal dust doesn't vary much, no matter what is happening to our planet's climate or geology. By measuring the meteoritic contamination in sedimentary rocks, they hoped to infer how fast the silt had been accumulating in the ocean bottoms where these sediments formed. When silting was very slow, the ocean floor clays would be relatively rich in meteoritic dust; rapid sedimentation would swamp the slowly accumulating cosmic dust. They hoped that sedimentation rates, measured this way, might provide clues about how the Earth's climate changed at the end of the Cretaceous period.
There isn't much cosmic dust; we hardly need to sweep it away from our driveways. How does one detect minute quantities of this extraterrestrial material? Nobel-Prize-winning physicist Luis Alvarez had a way. A sensitive modern chemical technique called neutron activation analysis permitted the Alvarez team to search for certain elements in the rocks at concentrations of less than 1 part in a billion. Cosmic dust contains several metals that are almost absent from the crust of the Earth. Metals are quite common among the raw materials of which planets are made, including the Earth -- but not in the crust. Early in our planet's history, iron, nickel, and much rarer metals like platinum, iridium, and osmium, sank to form the Earth's core. Less than a thousandth of the Earth's complement of such metals remains in the crust, providing a challenge to miners. So the amount of iridium in a rock provides a sensitive measure of extraterrestrial dust. If other rare "cosmic" elements can be measured in addition to iridium, and all show similar concentrations, the chemist has virtually a fingerprint for meteoritic material.
What the Alvarezes found was startling. First from K-T boundary rocks at Gubbio, and then from another K-T site in Denmark, the research team measured more than a hundred times as much iridium in the boundary layer as in the rocks immediately above and below. Could it mean that oceanic sedimentation rates dropped by a factor of 100? That was inconceivable. The alternative hypothesis was a sudden influx of iridium-bearing cosmic dust at the end of the Cretaceous period. Moreover, since the amounts of other elements that contaminated the boundary layer were in proportions similar to what's found in meteorites, extraterrestrial material was clearly implicated in the K-T mass extinction.
The Alvarez research team had sampled only two sites, both in Europe, but they made a bold extrapolation. Suppose the K-T boundary layer were enriched in iridium and cosmic elements around the entire Earth? The layer is only 1 cm thick, and the enriched concentration of iridium is a millionth of a percent. But the Earth is big, so the total extraterrestrial iridium in the K-T boundary would be 100,000 tons. In meteorites and other cosmic material, iridium makes up only one atom in a million, so the presence of 100,000 tons of iridium implies the presence of a million times as much total extraterrestrial material, or 100 billion tons. A global iridium-enriched boundary layer would require a rocky impacting object fully 10 kilometers across. The Alvarezes stopped thinking of micrometeoritic dust and began to read up on asteroids. They soon learned that Gene Shoemaker and other scientists had estimated that 10-km sized asteroids strike Earth every 50 to 100 million years. The 65-million-year-age of the K-T boundary fit perfectly.
Based on this chain of reasoning, Luis and Walter Alvarez offered a radical, even revolutionary hypothesis in a paper submitted to Science magazine in autumn 1979: the Cretaceous-Tertiary extinctions were caused by the impact of a 10 km-asteroid. Dust ejected into the stratosphere from this impact would spread around the globe, dimming sunlight worldwide and triggering an ecological catastrophe. Having concluded that an impact explained the K-T mass extinctions, the Alvarezes boldly went on to suggest that the other major mass extinctions in the fossil record could be impact catastrophes, as well.
That simple suggestion began the biggest change to our view of the evolution of life since Darwin. If the Alvarezes were right, most species became extinct not because they lost out struggling with competitors within a relatively static environment, but because asteroids from outer space capriciously struck our planet. The opening of ecological niches encouraged new species to arise and multiply. The fittest were not necessarily the biggest or fastest or smartest species, but rather those that happened to be able to survive a global impact catastrophe. The Alvarez paper, after review and revision, was published in the June 6, 1980, issue of Science, by which time a K-T iridium anomaly had been found in New Zealand, as well. They acknowledged that their data did not yet totally prove their theory, that it deserved to be tested further. How true. In fact, their theory provoked a storm of debate that continues today.
For astronomers and planetary scientists, much of the Alvarez hypothesis seemed logical and almost self-evident. Of course the Earth is struck by asteroids from time to time. Of course they would create enormous damage and distribute ejecta far and wide -- just look at the Moon! It was reasonable that the stratosphere might carry the dust around the globe, block sunlight, and cool the climate. They recalled reading about the "year without summer" in 1816 following volcanic contamination of the stratosphere, when crops failed across New England and there was famine in Asia. The Alvarez hypothesis seemed so natural, it was immediately embraced by planetary scientists, many of whom probably wondered why they hadn't thought of such a theory themselves. In a way, actually, the idea had been published before: Nobel Prize-winning cosmochemist Harold Urey had said as much in a short article in Nature magazine published in 1973. But Urey had lacked the measurements of iridium in the K-T boundary layer that made the Alvarez paper so compelling.
The Alvarez hypothesis also sounded good to Stephen Jay Gould, the Harvard evolutionist and popular columnist for Natural History magazine. He had promoted the "punctuated equilibrium" concept for evolution, a variation of Darwin's scheme in which long periods of comparative stability within an ecosystem were interrupted by times of crisis, during which most new species arose. Mass extinctions are basically the punctuational style of evolution extrapolated to a global scale. The causes of global ecological crises had been cloudy. Now Gould and the evolutionary geologists and biologists who agreed with him had a natural -- indeed inevitable -- mechanism for creating such crises.
The Alvarez hypothesis was less warmly received in other quarters. Indeed, among the geologists and paleontologists who had spent their careers studying the fossil record, trying to reconstruct the environments of the Cretaceous era, it was so much poppy-cock. Everyone knew, they asserted, that the dinosaurs and other species that became extinct at the end of the Cretaceous had declined gradually, over millions of years. A catastrophic killing based on darkness, devastation, and extinction within just a year or so was ridiculous.
The proponents of gradual hypotheses for mass extinctions each thought that their own theories remained valid. Those who had previously concluded that the extinctions were due to gradual climate change during an era of increased volcanism that peaked 60 to 70 million years ago still liked their own idea. Why look (they said) for ad hoc, out-of-this-world causes for the extinctions, when we had the answer right here in our back yard? To astronomers the idea of cosmic impacts may have seemed natural, but to most geologists it was an alien idea, inconsistent with their education and professional experience.
Resolution of the debate, it became clear, would require interdisciplinary discussions spanning a broad array of sciences. Rarely has there been any reason for astrophysicists to debate volcanologists, for cosmochemists to confront paleontologists, or for zoologists and botanists even to meet with planetary scientists. In this case, however, the issues were too profound, too close to the fundamental assumptions of these fields of science, for the issue to be swept under the rug. Traditional geologists were unwilling to let the radical new hypothesis go unchallenged, while the physicists and planetary scientists were equally unhappy to hear one of the fundamental discoveries of the Space Age -- the impact history of the solar system -- characterized by opponents as "ad hoc" (not a nice thing to say to a scientist!).
In October 1981, the first interdisciplinary meeting on the K-T boundary took place at the Snowbird ski resort, near Salt Lake City. More than a hundred scientists crowded into the small meeting room to listen to talks by leading experts on topics in diverse fields. They mingled during lunch breaks and got to size each other up. The organizers from the National Academy of Sciences and the Lunar and Planetary Institute had encouraged many of the major players in the controversy to attend, including the Alvarezes and Eugene Shoemaker, plus paleontologists like David Raup from the University of Chicago and Dale Russell of the Canadian National Museum.
By the time this meeting convened, evidence for iridium-enriched layers at the K-T boundary had been extended beyond Europe. Six research groups, in addition to the Alvarez team, had found iridium enrichment in K-T boundary-layer rock extracted from drill cores in the Atlantic and the Pacific, in North America, and elsewhere. There was no question that the Alvarez's intuitive extrapolation -- that the iridium layer had been deposited world-wide -- was by now established beyond dispute. Furthermore, microscopic glassy spherules (called microtektites), thought to be droplets from molten ejecta, were found embedded in the boundary layer rocks. And evidence accumulated that major species of microscopic plankton had died out at precisely the time the iridium and microtektites had been deposited.
However, some contradictory evidence had emerged, as well. In northern New Mexico, at least one dinosaur bone had been found well above the iridium-enhanced boundary layer. If the dinosaurs had survived into the Tertiary era, then whatever had caused the iridium enrichment and affected oceanic plankton had perhaps not been so catastrophic after all. The argument was made that even one dinosaur bone found above the K-T boundary disproved the impact hypothesis.
The question of the death of the dinosaurs, although not essential to the Alvarez impact hypothesis, is a fascinating topic to paleontologists and laypersons alike. However, it is difficult empirically to locate the exact stratum in the geological record where the last dinosaurs appear. There are relatively few surviving dinosaur bones, and the skeletons themselves are large with respect to many of the layers dated by geologists. The impact proponents replied that a few bones have been displaced by subsequent soil motion or even by being dug up and moved long after the animal's death, so we expect a scattering of the recorded locations where the bones are found. There have never been any complete skeletons of dinosaurs found above the K-T boundary, only the odd isolated bone.
It is worth noting just how rare fossils really are. Imagine a single farmer's field, which is the home to hundreds of field mice and dozens of rabbits that are born and die each year. Over the past million years, hundreds of millions of these animals would have died on this plot of land, yet it is very unlikely that the farmer will ever dig up a mouse or rabbit fossil. Each fossil found on Earth represents untold millions of creatures who lived and died, dust to dust. Indeed, it is estimated that far more species have existed on the planet than are identified by fossils, even for such large animals as dinosaurs.
One of the outcomes of the Snowbird Conference was to challenge paleontologists to test the hypothesis of abrupt extinction at the K-T boundary. Careful statistical tests were applied to define the stratigraphic location of the last occurrence of many common Cretaceous species, including some of the dinosaurs. The result of these studies is that the data are statistically consistent with a sharp extinction for many species, taking place precisely at the K-T boundary. Thus, while we cannot prove that no dinosaurs survived beyond the impact, we also cannot prove from existing data that the decline was gradual. In spite of an occasional misplaced bone, the reign of the dinosaurs appears to have ended abruptly.
The Snowbird Conference probably changed few minds at the time, but the 120 attendees (who included one of us: Chapman) departed Utah aware that the Alvarez hypothesis would be no passing fad. The interdisciplinary clash was a profound challenge to paleobiology. Indeed, it heralded another revolution in the Earth sciences as profound as the 1960's establishment of continental drift and plate tectonics, when century-old dogma of continental stability was swept away by irrefutable geophysical data. That was the view expressed by Snowbird Conference organizer Lee Silver, who said that the planetary lessons of the Space Age were finally coming home to roost on planet Earth.
Not long after the Snowbird Conference, public and scientific interest were raised even higher by the Nemesis Hypothesis. The Alvarezes had speculated that impacts might have caused other mass extinctions, but searches for tell-tale iridium at other extinctions had yielded equivocal results. Now, however, two paleontologists presented fossil (not chemical) evidence that virtually all mass extinctions had an extraterrestrial cause.
While still a graduate student at Harvard, studying under Stephen Jay Gould, Jack Sepkoski had begun assembling a huge compendium of information about marine fossils. In November 1982, after a decade of combing through the paleontological literature and databases, he published the stratigraphic ages of the first and last occurrences for more than 3,500 different fossil types. By then, Sepkoski was at the University of Chicago, working with the eminent theoretical paleontologist David Raup. Raup, a pioneer in applying mathematics to paleontology, helped Sepkoski subject the compendium to statistical tests. Almost immediately, an extraordinary pattern leapt from their graphs: major extinctions happened every 26 million years! Such a periodicity could hardly result from the complex interplay of climate and evolutionary processes, Sepkoski reasoned. It must have an extraterrestrial cause, because astronomical orbits have long-term, cyclical periodicities.
Sepkoski announced his results at a 1983 geological meeting on mass extinctions. Ordinarily, results presented at such a specialized conference would never be heard by astronomers, but the K-T debates had risen to such a pitch that many science journalists attended. Their reports, published in general science magazines and newspapers, alerted astronomers, who quickly came up with a theory to explain periodic extinctions: periodic comet showers. The idea that the history of life on Earth was shaped dominantly by the regular impacts of comets was too much for paleontologists to take. A New York Times editorialist said it for them: "Terrestrial events, like volcanic activity or changes in climate or sea levels, are the most immediate possible causes of mass extinctions. Astronomers should leave to astrologers the task of seeking the cause of earthly events in the stars."
At another Snowbird meeting in 1988, Jack Sepkoski heaped more data onto his paleontological colleagues. He had subdivided his 1983 compendium of 3,500 fossil families to trace the durations for 30,600 genera, a finer taxonomic division. The 26-million-year periodicity was even stronger. Rubbing salt in paleontologists' wounds, Sepkoski's colleague Raup formally suggested that impacts might be the sole cause of extinctions, even of minor extinctions. Few scientists would be so daring as to undercut the philosophical foundation of their own field, but as Raup wryly told the Snowbird audience, "I have tenure!"
Throughout the 1980s, a few diehards held out against any large K-T impact at all, and they raised a debater's point: Where was the crater? Failure to find it was hardly a fatal flaw to the impact theory, for if the crater had formed on the ocean floor, it might well have been destroyed by movement of the Earth's crust during the intervening 65 million years. But the Alvarez supporters sorely longed for a crater to point to. In the next chapter we recount the story of how geologists homed in on the K-T crater called Chicxulub.
3650 words (3/28/97)
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