The magnetic reversal time scale offered one possibility for determining how much time the clay layer represented: The particular pattern of reversals above and below the clay might bracket its age of formation and allow an upper limit to be placed on how long it could have taken to deposit the layer. Alas, during this period of geologic history the reversals had not happened often enough: All that could be told is that the clay layer fell within a 6-m section of limestone deposited during a single period of magnetism, called 29 R (for reversed), that was known to have lasted for about 750,000 years. Six meters in 750,000 years is equivalent to 0.8 cm of sediment deposited every 1,000 years. Since the boundary clay is about 1 cm thick, at that rate it would have taken a little more than 1,000 years to form. This appeared to be an improvement over Walter's rough estimate, but since the clay is quite different from the limestone, there really was no basis for assuming the same sedimentation rate for both. The attempt to determine the time interval using the magnetic chronology thus failed, but in another way the effort succeeded, for the mind of Luis Alvarez was now locked in.
What was needed, he reasoned, was a geologic clock that had been operating at the time the clay layer formed but that could be read today. Because no one knew how much time the clay layer might represent, the clock might have to measure small differences. None of the standard geologic clocks—the ones based on radioactive parent-daughter pairs of atoms that are used to calculate exact ages—had enough sensitivity or would work on the chemical elements in the clay layer. Therefore, as he had done so many times in his career, Luis Alvarez invented a new technique. To do so, he looked not down to the earth but up to the heavens, postulating that the amount of a rare metallic element called iridium might provide the clock.
When the earth formed, iridium, like other elements of the platinum group (which includes osmium, palladium, rhodium, and ruthenium), accompanied iron into the molten core, leaving these elements so rare in the earth's crust that we call some of them precious. Their abundance in meteorites and in average material of the solar system is many times higher than in the earth's crust. The iridium found in sedimentary rocks (and often it is too scarce to be detected) appears to have settled from space in a steady rain of microscopic fragments—a kind of cosmic dust—worn from tiny meteorites that form the shooting stars that flame out high above the earth. Such meteorites are believed to reach the upper atmosphere at a constant rate, so that the metallic rain falls steadily to earth, where it joins with terrestrial material—dust eroded from the continents and the skeletons of microscopic marine animals—
to settle to the bottom of the sea. There it is absorbed into the muds that accumulate on the seafloor and that eventually harden into rock.
But over geologic time, the rate of accumulation varies greatly. Since one component, the meteoritic, is arriving at a constant rate from space and the other, the terrestrial, is accumulating at a varying rate, the percentage of meteoritic material in a deep-sea sedimentary rock provides a gauge of how fast the terrestrial component built up: The greater the percentage of meteoritic debris in a given thickness of rock, the slower the sediment accumulated, and vice versa. The rate at which meteorites fall on the earth is known, as is the amount of iridium in meteorites, so that the iridium content of sediments can be used as proxy for the total amount of meteoritic material they contain. Luis Alvarez later discovered that two scientists from the University of Chicago had tried to use iridium in this way to measure sedimentation rates but without success. "Fortunately, I hadn't heard of their work," Luis commented of the Chicago scientists, "If I had, I'm sure we wouldn't have bothered to look for iridium at the K-T boundary."8
The samples necessary for testing the iridium clock were readily available from the Gubbio clay layer, but measuring the expected low levels of iridium required a research nuclear reactor. Fortunately, the Alvarezes and their Berkeley colleagues Frank Asaro and Helen Michel had access to one. The reactor allows neutrons to bombard a rock sample and cause atoms of an isotope of iridium to become radioactive and to emit gamma rays of a distinctive energy. The number of such rays emitted per second is counted and is proportional to the amount of iridium in the original material. When analyzing at the parts per trillion level, however, it is extremely difficult to eliminate contamination (from the iridium that is always present in platinum jewelry, for example).
Walter Alvarez selected samples ranging over the Gubbio section—above, below, and at the K-T boundary—and brought them back to Berkeley for analysis. Samples from above and from below the boundary had the predicted amounts of iridium, about the same as had been measured by others in deep-sea clays—300 parts per trillion (ppt) or so. The samples from the boundary clay, however, revealed an earthshaking surprise—iridium levels 30 times higher than those in the limestones on either side] Back-of-the-envelope calculations showed that Walter's original idea—that the clay had built up when the limestone for some reason ceased to be deposited—could not be correct because then the clay would have taken an impossibly long time to form. Thus the attempt to use iridium as a clock failed, but, as often happens in science, no sooner does one idea fall by the wayside than another springs up.
To check that the startling result was not somehow a bizarre characteristic of the K-T boundary clay at Gubbio, the team analyzed two clay layers contained within the limestones above and below the boundary and found both to have low levels of iridium. Thus the iridium anomaly was associated with the thin K-T boundary clay, not generally with clays from the Gubbio region.
If not an anomaly of Gubbio clays, perhaps the iridium spike was merely a local aberration. To find out, the scientists needed to find another site where the K-T boundary is exposed, collect samples, and analyze them for iridium. As an indication of just how little was known about the K-T boundary in the late '970s, even a knowledgeable geologist like Walter had no idea where to look. As would any intelligent person in a similar quandary, he went to the library. There he discovered a reference to the sea cliffs south of Copenhagen, which contain a classical and thoroughly studied K-T rock section where, as in Italy, a clay layer marks the precise boundary. Measurements of the amount of iridium in the Danish clay by Frank Asaro showed an even greater enrichment—'60 times background. The Alvarez team was clearly onto something: The iridium anomaly was not restricted to Italian rocks and might even be a worldwide phenomenon.
What did the Alvarezes know at this point? That at two widely separated sites, abnormally high levels of one of the rarest metals in the earth's crust occur in the exact thin layer that marks the great K-T extinction and the demise of the dinosaurs. They concluded that this could hardly be due to coincidence—the high iridium level must somehow be linked to the extinctions. They knew that finding how this linkage had occurred was the 65-million-year question; if they could, they might solve the age-old riddle of dinosaur extinction.
Since iridium is many times more common in meteorites and in the solar system in general than in crustal rocks, the Alvarezes began to consider extraterrestrial sources for the Gubbio iridium. The first idea they pursued was the one paleontologist Dale Russell favored. Exploding stars, or supernovae, which generate and then blast cosmic material throughout the galaxy, might have implanted the K-T iridium, suffusing the earth with deadly cosmic rays and thus causing the extinction. Such nuclear furnaces give birth to a wide variety of chemical elements, including plutonium. One isotope of plutonium, Pu 244, is a diagnostic marker of supernovae explosions. A
diligent search for Pu 244 in the boundary clay came up empty however, so the scientists had to abandon the supernova theory.
A Berkeley colleague, Chris McKee, suggested that a large asteroid could have provided the iridium. This made sense, for iridium is present at high levels in lunar soils, where it has presumably been emplaced by impacting meteorites. For many months, however, the Alvarez team was unable to figure out how the impact of an asteroid at one spot on the earth's surface could have caused a mass extinction everywhere. How did the effects get spread around the globe? Luis later recalled that he had invented a new scheme a week and shot each down in turn.
The critical clue arose in a way that further illustrates the strong scientific ties within the Alvarez family. In 1883, the island volcano Krakatoa, in the Sunda Strait between Java and Sumatra, blasted itself to pieces in one of the most violent eruptions of modern times, scattering debris as far as Madagascar. People 5,000 km away heard the explosion. Walter Alvarez, Sr., the physician father of Luis, had given his son a volume describing the Krakatoa event published by the Royal Society of London in 1888; Luis in turn had passed it on to the younger Walter. Now Luis asked for the volume back so that he could study the consequences of a dust-laden atmosphere. The Royal Society volume estimated that the Krakatoa explosion had blasted 18 km3 of volcanic material into the atmosphere, of which about 4 km3 reached the stratosphere, where it stayed for more than two years, producing some of the most remarkable sunsets ever witnessed. (In comparison, the eruption of Mount St. Helens in 1980 is estimated to have released about 2.7 km3 of volcanic rock; the eruption that formed the giant Yellowstone crater about 3,000 km3.)
Krakatoa caught Luis's attention, and he proposed by analogy that 65 million years ago a large meteorite struck the earth and sent up such a dense cloud of mixed meteoritic and terrestrial debris that it blocked the sun. This successively caused world temperature to drop, halted photosynthesis, choked the food chain, and led to the great K-T mass extinction and the death of the dinosaurs. Luis and his Berkeley gang phoned Walter, then in Italy, to announce their exciting conclusion and to propose that the idea be presented at an upcoming meeting on the K-T boundary in Copenhagen, which both Alvarezes could attend. Although Luis was anxious to explain to paleontologists the cause of dinosaur extinction, Walter knew better and urged him to stay home.9 While the physicist and his chemist colleagues did remain in Berkeley, Walter journeyed to the Copenhagen meeting. There he met Dutch geologist Jan Smit, the only other person present to give any credence to the embryonic theory.
Luis, Walter, Frank Asaro, and Helen Michel spent the next months preparing a long paper describing their theory, which they then submitted to Science. Its editor, Philip Abelson, had been Luis's graduate student at Berkeley in 1939 and a longtime colleague. Perhaps having grown weary of dinosaur extinction theories, Abelson responded that the paper was too long and that furthermore, since Science had published many papers purporting to solve the mystery of the dinosaurs, "at least n-1 of them must be wrong"—a scientist's way of saying that only one could be right. The authors submitted a shorter version (still twice as long as the journal's typical lead article); it appeared in the issue of June 6, 1980.'"
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