Paleoweltschmerz
In 1980, a paper appeared in Science titled “Extraterrestrial Cause for the Cretaceous-Tertiary Extinction.”1 The authors were the father-son team of Luis and Walter Alvarez, together with their University of California colleagues Frank Asaro and Helen Michel. The Cretaceous-Tertiary boundary, known as the K-T, is the point in geological time at which the dinosaurs and 70 percent of all species became extinct, one of the five big mass extinctions in earth history.2
Though none of the four authors was a paleontologist, they claimed to have solved one of the most venerable puzzles of the fossil record: what killed the dinosaurs? There was no lack of proposed explanations. In a 1964 article titled “Riddle of the Terrible Lizards,” the paleontologist Glenn Jepsen tallied the scores of proffered theories, which ranged from “evolutionary drift into senescent overspecialization” to “paleoweltschmerz,” a prehistoric world-weariness.3 Whatever one might think of these proposed solutions individually, few if any could explain the simultaneous extinction of giant terrestrial dinosaurs and microscopic marine organisms like foraminifera.
If lunar scientists could joke that they had proved that the Moon cannot exist, paleontologists might have joked that they had proved that the great dinosaurs still do exist. What no one could have imagined is that the origin of the Moon and the demise of the dinosaurs over four billion years later would turn out to have the same fundamental cause.
Luis Alvarez (1911–1988) was one of the most distinguished physicists of the twentieth century. During World War II, he worked with Robert Oppenheimer on the Manhattan Project. In July 1945, Alvarez flew above the New Mexico desert to observe Trinity, the first atomic bomb test. On August 6 he was aboard the Great Artiste, the companion plane to the Enola Gay, on its fateful flight over Hiroshima. Thus Luis Alvarez was a rare witness to both of the first two atomic bomb explosions. On the return flight from Hiroshima, he wrote a poignant letter to his young son Walter expressing his hope that the “new destructive force,” so much more powerful than conventional weapons, would realize Alfred Nobel’s dream and prevent further wars.4 Thirty-five years later, father and son would uncover evidence of an explosion more than ten thousand times larger than the one Luis had witnessed above Hiroshima.
After the war, Luis returned to Berkeley to study subatomic particles produced in cloud chambers, work for which he won the Nobel Prize. His 1968 Nobel citation was one of the longest in the history of the prize at that time. But having ascended to the summit in his chosen field, what does a scientist do next? A move into geology seemed unattractive, for in spite of his son Walter having chosen the field, Luis confessed a pronounced lack of enthusiasm. Then one day in the mid-1970s, returning from a field trip abroad, Walter produced a specimen that sparked Luis’s imagination and, as he put it, “rejuvenated his career.”5
Walter Alvarez (b. 1940) earned his doctorate at Princeton under Harry Hess, then worked as a petroleum geologist in Libya. He joined the faculty at Berkeley, where he became interested in magnetic reversals. One section of the magnetic timescale that required more work was near the K-T boundary, 65 million years ago, where there are many short magnetic events whose detailed chronology scientists needed to sort out. An especially complete K-T section outcropped near the small Italian mountain town of Gubbio, where Walter journeyed in the summers to collect samples. The K-T boundary at Gubbio is easily visible to the naked eye. Below it, owing to the presence of white-shelled fossil foraminifera, the rocks are white. Above the boundary, almost all the “forams” have disappeared, and the rocks are dark. But the forams were not the only ones to disappear at the K-T boundary: so did the dinosaurs.
FIGURE 24. Luis and Walter Alvarez at Gubbio. Walter has his fingers on the K-T Boundary, when the dinosaurs disappeared. Source: Lawrence Berkeley National Laboratory.
Right at the boundary, in between the K and the T, Walter found a one-centimeter-thick layer of reddish clay with no fossils. On his return, he showed a hand specimen that included the boundary clay to his father and explained that it marked the time when the dinosaurs had gone extinct. Walter estimated that the boundary clay might have taken five thousand years to form, which if true suggested that dinosaur extinction had happened in a geologic eyeblink. Luis was fascinated and proposed that they try to measure the time span. Since no known method was suitable, Luis invented what he thought was a novel one for measuring the rate of accumulation of sedimentary rocks. The method depended on measuring the amount of the rare element iridium in the boundary clay.6
Samples of the limestones above and below the K-T boundary held about 300 parts per trillion of iridium, about the amount expected. But to the Alvarezes’ surprise, the boundary clay had more than 30 times that much. To check that the result was not an unusual characteristic of clays from this part of the geologic column, they analyzed clay layers in the limestones above and below the boundary but found that neither had detectable iridium. The next obvious question was whether the iridium spike was unique to the K-T boundary clay and Gubbio. They analyzed a K-T boundary clay from Denmark and found that it contained even more iridium, as did another from New Zealand. The team measured the abundances of other chemical elements in the boundary clay and found them also to be high in meteoritic elements. Evidently the boundary clay worldwide contains an anomalous amount of extraterrestrial material. But how had it gotten there? And what did it have to do with the K-T extinction?
Luis had once read a book published in 1888 by the Royal Society on the 1883 explosion of the Indonesian volcano Krakatoa. The book described how the eruption had ejected eighteen cubic kilometers of volcanic material, about one-quarter of which had reached the stratosphere. The volcanic dust circled the Earth, remaining aloft for more than two years and causing some of the most remarkable sunsets ever witnessed. The example of Krakatoa led Luis to propose by analogy that 65 million years ago, a large meteorite struck the Earth, throwing up a cloud of cosmic and terrestrial debris so thick that it blocked the Sun. Global temperatures plummeted, photosynthesis halted, the food chain collapsed, and the dinosaurs and 70 percent of all species became extinct.
Luis phoned Walter, then in Italy, to tell him of his exciting deduction and to propose that Walter present the finding at an upcoming meeting on the K-T boundary in Copenhagen, which both Alvarezes planned to attend. Writes Luis, “We thought the paleontologists would be delighted to learn what caused the extinctions. Walt knew better and correctly urged us to stay home” (257). Walter was right: paleontologists were not delighted. A knockdown, drag-out battle ensued between the two camps that turned the Alvarez theory into one of the most vituperative scientific controversies in history.
Crater of Doom
The first and hardest-working opponents of the Alvarez theory were two scientists from Dartmouth College, Charles Officer and Charles Drake. Both had been Bucher’s students at Columbia. They began with two papers in Science, one in 1983 and the other in 1985.7 Their intent was to show that
• The K-T event took place at different times around the world and therefore could not have resulted from a simultaneous global catastrophe.
• The dinosaurs did not go extinct all at once, as in the Alvarez model. They and other species were already dying out well before the K-T boundary. The impact, in the unlikely event one had occurred, did no more than administer the coup de grâce.
• The evidence that geologists had found at the K-T boundary, which turned out to include not only an iridium spike but also quartz and zircon that had suffered high-pressure shock, as well as glassy spherules, was far from diagnostic of impact and often not even consistent with it.
• Volcanism explains the K-T evidence as well or better than the Alvarez theory and does not rely on a nonuniformitarian bolt from the blue.
Though the Alvarez team was no doubt disappointed that geologists did not rush to accept their proposal, in the end the skeptics did them and science a favor by forcing the theory to confront and pass critical tests. The result was that scientists determined that the K-T boundary is the same age everywhere: it is a true global event. The dinosaurs and other species disappeared suddenly and had not started to decline earlier. Though high iridium, shocked minerals, and glassy spherules can each have other causes, taken together they are markers of extraterrestrial impact. A major volcanic eruption, the Deccan Traps in India, did occur near the end of the Cretaceous, but the detailed chronology of the eruption and the geochemistry of the boundary clay showed that the clay cannot be attributed to volcanism.
Of course, or so one would think, the clincher would be to find the “Crater of Doom,” as Walter Alvarez named the K-T impact site in his fine book.8 By 1990, ten years after the publication of the original paper, scientists still had not found the crater—or had they? Calculations showed that it should be about 150 kilometers in diameter, yet that did not mean it would be easy to spot. Any arriving meteorite has a two-thirds chance of landing in the ocean, where the crater could have disappeared down a subduction zone or lie hidden beneath younger sediments. Had the meteorite landed on a continent, 65 million years of erosion might have made the crater hard to recognize. Had it landed near one of the poles, thousands of feet of ice might now bury the crater. It was easier to think of reasons why the crater, if it ever existed, should not be found than reasons why it should. Clever detective work, and even more importantly, good luck, would surely be required.
Geologists had noted that the K-T boundary clay is thicker, and the particles that comprise it larger, in North America than in other places, suggesting that ground zero lay in that part of the world. Alan Hildebrand, then completing his Ph.D. at the University of Arizona, scoured the literature for reports of unusual K-T deposits and descriptions of the circular geophysical patterns that might reveal a buried impact crater. In May 1990, Hildebrand and William Boynton reported in Science that the only candidate their literature survey had turned up was a vaguely circular structure lying beneath two to three kilometers of younger sediment in the Caribbean Sea north of Colombia.9 They acknowledged that an impact at this ocean-floor site probably could not have provided the continental grains and rock fragments found in the boundary clay. In a throwaway final sentence, they noted that in 1981 the geologists Glen Penfield and Antonio Camargo had reported circular magnetic and gravity anomalies from the northern Yucatan Peninsula and had speculated that there, beneath younger sedimentary rocks, might lie a buried impact crater.
Penfield’s discovery of the K-T crater is another example of a young scientist being in the right place at the right time. But as we have seen, that may not be enough. The scientist’s mind must be prepared to think the previously unthinkable and recognize the previously unrecognizable. Even though still only in his twenties, Penfield, a 1975 graduate of Oberlin College, had already gained experience from magnetic surveys of volcanoes and volcanic features in Alaska and Mexico. In the spring of 1978, as he was transferring the results of magnetic surveys over the Yucatán to paper, he spotted an unusual set of concentric magnetic highs and lows. The deeply buried structure had been accepted as volcanic by authorities on the geology of the region, but Penfield knew that he was not looking at the magnetic signature of a volcano.10 Instead, he saw “symmetry on the scale of a Copernicus crater,” as he later described his revelation. He and Camargo presented the idea at the 1981 meeting of the Society of Exploration Geophysicists, but until the mention by Hildebrand and Boynton, no one seems to have noticed.
In a 1991 article in Natural History titled “Cretaceous Ground Zero,” Hildebrand and Boynton took partial credit for the discovery: “In 1990, we, together with geophysicist Glen Penfield and other coworkers, identified a second candidate for the crater. It lies on the northern coast of Mexico’s Yucatán Peninsula, north of the town of Merida. The structure, which we named Chicxulub for the small village at its center, is buried by a half mile of sediments.”11
In a reply a few months later, Penfield took exception, noting that he had “identified” the structure in 1978 and reminding readers of the words with which he and Camargo had closed their 1981 presentation: “We would like to note the proximity of this feature in time to the hypothetical Cretaceous-Tertiary boundary event responsible for the emplacement of iridium-enriched clays on a global scale and invite investigation of this feature in the light of the meteorite impact–climatic alteration hypothesis for the late Cretaceous extinctions.”12
The science reporter for the Houston Chronicle interviewed Penfield and Camargo in late 1981 and wrote a front-page article about their report.13 In March 1982, the magazine Sky and Telescope published an account of the Penfield-Camargo finding entitled “Possible Yucatán Impact Basin.” It included this statement: “Penfield . . . believes the feature, which lies within rocks dating to Late Cretaceous times, may be the scar from a collision with an asteroid roughly 10 km across.”14 But in spite of these prominent articles, neither the proponents nor the opponents of the Alvarez theory followed up, and the Chicxulub crater had to be rediscovered a decade later. As we have seen and will see, this is not the only time that scientists have overlooked a report that they should have taken to heart. It seems likely that most continued to accept the Chicxulub structure as volcanic; after all, it was buried beneath half a mile of younger rocks and thus hard to be sure about.
To test whether Chicxulub was indeed an impact structure, in 1981 William Phinney of the Johnson Space Center urged Penfield to find samples from the deep exploration wells that PEMEX, the Mexican oil giant, had drilled in the early 1950s. Despite searching at PEMEX and revisiting the Chicxulub well site, Penfield was unable to locate the samples until a decade later, when he finally found them in storage at the University of New Orleans.
At a 1991 conference, David Kring, Hildebrand, and Boynton presented the results of a microscopic study of the Chicxulub drill cores. They found quartz that had undergone high-pressure shock and a rock that they interpreted as an impact melt, similar to the suevite that Shoemaker had identified at the Rieskessel. “The Chicxulub structure is probably an impact crater,” they concluded.15 In a paper in Geology later that year, the three, together with Penfield and others, followed up with detailed evidence to back up their conclusion.16 Surely the discovery of the Crater of Doom would silence the critics of the Alvarez theory—or would it?