In the Umbrian Apennines, near Gubbio, above the ancient cities of Perugia and Assisi, tather the streams that form the Tiber River. Here layers of limesones provice a geological records that spans the entire period from the Early Jurassic, 185 million years ago, to the Oligocene, 30 million years ago.
The Berkeley group has looked for other unusally high iridium abundances, which might be found at or near the geological layers that mark the time of other mass extinctions, such as those at the boundaries between the Paleozoic and Mesozoic eras (the Permian-triassic boundary, about 248 million years ago) and between the Eocene and Oligocene periods, about 38 million years ago. In samples from China and the United States, the Berkeley scientists found a thin layer of clay laid down at the Permian-Triassic Bounday that is chemically quite distinct from the clay layers above and below it. They conclude that this clay probably formed from ash that rained down onto the seas and the land, but they did not find any excess iridium within the clay layer. . .
At the Eocene-Oligocene boundary, in a deep-sea drilling core taken from the bottom of the Caribbean Sea, Asaro and his colleagues did find an excess of iridium at just the position at which the extinctions of five species of radiolaria (microscopic sea creatures something like foraminifera, but with silicon-rich rather than calcium-rich shells) had previously been detected.
Goldsmith: 1985, p33.
The saber-toothed cat became extinct 14,000 years ago; the woolly mammoth 10,000 years ago; the giant ground sloth 8,500 years ago; the giant armadillo 7,800 years ago; and the colossal mastodon some 6,000 years ago. Another dozen mammalian species died out within this interval, which covers 8,000 years- a mere moment in geological history, but much longer than the theories based on nuclear-winter effect would predict.
Goldsmith: 1985, p 41-42.
Officer and Drake are properly impressed by a recent measurement of the iridium abundance in material ejected from the Kilauea volcano, on the island of Hawaii. When the geologists William Zoller, Josef Parrington, and Janet Phelan Kotra, of the University of Maryland, measured the element abundances in tiny particles collected in air filters during the volcano's eruption in January 1983, they found an abundance of iridium ten to twenty thosand times greater than normal.
Goldsmith: 1985, p 43.
Paleontologists have long known of several such episodes, the most drastic of which occurred 248 million years ago, at the end of the Permian era. This "Permian-Triassic extinction" eliminated more than 96 percent of the species then in existence: . . . "
Goldsmith: 1985, p 47.
In thinking aout the extinction of species of life on Earth, it is important to bear in mind one key fact: By far the majority of all species that have ever existed on this planet are now extinct. The usual fate for species is total disappearance.
Goldsmith: 1985, p 48.
(A useful mnemonic device, known to generations of biology students, gives the initial letters of classification scheme in order from kingdom to species: "King Philip Came Over From Germany Singing.") Human beings, for example, belong to the animal kingdom, the phylum of chordata (and the subphylum of vertebrates), the class of mammals, the order of primates, the family of hominids (of which humans are currently the only representatives), the genus Homo (large-brained hominids capable of speech), and the species Homo sapiens.
Goldsmith: 1985, p 49.
Although we cannot predict when any one particular carbon-14 nucleus will decay into a nitrogen-14 nucleus, we do know that after 5,560 years one-half of any group of caron-14 nuclei will have become nitrogen-14 nuclei; after another 5,560 years, one-half of the remaining carbon-14 nuclei will have become nitrogen-14 nuclei; and so on, indefinitely. This means that carbon-14 has a half-life of 5,560 years.
Goldsmith: 1985, p 52.
Unfortunately, after a hundred thousand years or so, too little carbon-14 remains to be measurable, so the method loses its usefulness.
Luckily, though, other radioactive nuclei decay more slowly. These nuclei can be used to date fossils whose ages range in the millions, rather than in the thousands, of years. The most important of these nuclei are those of potassium-40, which has a half-life of 1.3 billion years, and those of rubidium-87, which has a half-life of 47 billion years. In an approach analogeous to carbon-14 dating, geologists measure the amount of the "daughter" nuclei produced by the radioactive decay-argon-40 and strontium-87, respectively. They also estimate how much of these two types of nuclei existed the the rocks when the rocks formed, typically estimating zero for the amount of the daughter isotope. Then, just as in the case of carbon-14 dating, our knowledge of the half-life of the radioactive (naturally decaying) isotopes tells us how much time must have elapsed to produce the parent-daughter ratio measured today.
Dating rocks by radioactive decay works well for those rocks that formed from a molten state--igneous and, in some cases, metamorphic rocks. The heating of such rocks released as gases any argon-40 or strontium-87 within them, so the rocks "began life" with a zero abundance of these "daughter" isotopes. Unfortunately, the method does not work for sedimentary rocks--those that contain the best fossil beds.
Goldsmith: 1985, p 53.
When you look at the graph made by Raup and Sepkoski, it is clear that the fit between peaks in the actual data and a hypothetical recurring cycle of extinction with a period of 26 million years is good but certainly not perfect. The four largest of these peaks occur at the Permian-Triassic Boundary, 248 million years ago; at the end of the Norian stage in the Triassic period, 219 million years ago; at the Cretaceous-Tertiary boundary, 65 million years ago; and at the Eocene-Oligocene boundary at the end of the Tertiary period, 38 million years ago.
Goldsmith: 1985, p 58.
About thirty Apollo asteroids have been discovered; they range in size from Hephaistos, which is 9 kilometers across, down to objects only half a kilometer, or even less, in diameter. One of the Apollo asteroids, Icarus, 1.4 kilometers in diameter has an orbit that carries it far inside the orbit of Mercury, to within 0.2 A.U. of the sun, and well beyond the orbit of Mars, to a maximum distance of 2 A.U.
Goldsmith: 1985, p 86.