Word: iridium
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Alvarez, his curiosity aroused, shipped samples of the sediment back to the U.S. and showed them to his father Luis, a Nobel-prizewinning physicist also at the University of California, who had the clay analyzed. To everybody's surprise, it turned out to be 30 times as rich in iridium as normal rocks. The Berkeley team knew of only a few places where such high concentrations of the rare element might occur: in the earth's core, perhaps 2,000 miles belowground; in extraterrestrial objects like asteroids (or their fragments, meteors) and comets; or in the cosmic dust drifting...
...revolution began with an unassuming element known as iridium, a rare and hard silvery-white metal related to platinum and gold. In the spring of 1977, Geologist Walter Alvarez of the University of California, Berkeley, was carefully chiseling through the rocks outside Gubbio, a medieval Italian town halfway between Florence and Rome, seeking clues to continental drift. Gubbio has long been an appealing site to geologists and paleontologists because its rocks provide a complete geological record of the critical boundary line between the end of the Cretaceous period, when the dinosaurs disappeared, and the Tertiary period, which followed...
...kinds of life would have been the most gravely affected by the climatic changes. They decided that plants, totally dependent on the sun for photosynthesis, and a variety of marine organisms would have died first, followed by the land animals highest on the food chain, the dinosaurs. Eventually the iridium-enriched debris, circulating around the world in the atmosphere, would have settled like a sprinkling of black snow over much of the globe. (It was the Alvarezes' theory that helped to shape the now familiar concept of nuclear winter...
Journeying to Denmark, another site where the Cretaceous geological record is complete, Walter Alvarez gathered corroborating samples of iridium and received still more from colleagues working at a third site on the other side of the world, in New Zealand. The evidence seemed overwhelming. In 1980 the Alvarez team finally published its results in the journal Science and stirred up some scientific debris of its own. Says Paleontologist Leo Hickey, director of the Peabody Museum of Natural History at Yale: "My first thought was this is one of Walter's practical jokes...
Several researchers rushed forth to deny any extraterrestrial origins for the iridium, attributing it first to a gradual process of sedimentation that concentrated the metal. Later an old favorite was proposed--volcanic eruptions, which might have forced iridium from the mantle to the surface. The most recent naysayers are Dartmouth Geologists Charles Officer and Charles Drake, who reported in Science on their studies of two other telltale elements in the clay boundary between the Cretaceous and Tertiary periods. They found that the levels of arsenic and antimony correspond to decidedly terrestrial, not cosmic, concentrations...