By examining under microscopes the skeletons of tiny marine organisms—foraminifera, radiolarians, and such—in the cores, Dr. Hays and his colleagues can read the temperature of the sea surface when the creatures lived, then died and drifted down in a slow, constant rain to the seabed. They can tell as well the amount of ice in the world—how much fresh water, fallen as snow, remained locked up in ice caps and glaciers in those dim past ages.
They have known, for example, that about 65 million years ago, at the end of the Cretaceous geologic period, a sudden catastrophic extinction of surface-dwelling ocean life occurred. It happened very close to the time that the dinosaurs, equally mysteriously, died out on land.
In May 1980 Glomar Challenger brought up 655 feet of core from a 70-million-yearold ridge in the Atlantic off South Africa. In it the shipboard scientists found a clear record of that “boundary event.” A layer only 23 feet thick, datable by fossils, showed that the mass extinction took place in less than 100,000 years, within 500,000 years of the time the dinosaurs were gone. Sea-surface creatures that previously had thrived disappeared, leaving only a few species of minute plants and animals. It took several million years for the diversity of ocean life to reappear.
Studies of sedimentary layers on land in Italy, Denmark, and Spain, show the same sudden kill-off. Coincidentally in 1980, chemical studies of those layers revealed abnormally high amounts of exotic elements such as iridium, arsenic, and antimony.
The conclusion reached by some scientists, among them Nobel laureate Luis Alvarez and his son Walter, of the University of California at Berkeley, is that the planet was struck by an asteroid or comet as big as six miles across. Such a collision would have thrown vast quantities of dust into the atmosphere and scattered traces of heavy metals worldwide. The screening of sunlight from the land and sea for several years could have led to a massive kill of plant and animal species. “The lights went out, and that stopped the food chain,” says Luis Alvarez.
Marine geologist Cesare Emiliani of the University of Miami in Florida, however, believes that the asteroid fell into the ocean and caused a sudden temperature rise. “This,” he says, “could have brought the selective extinction of the dinosaurs and other animal and plant groups.”
Other researchers speculate instead that continental drift may have suddenly changed ocean circulation and killed off the sensitive Cretaceous life forms. The Arctic basin, blocked from the infant North Atlantic, may have suddenly released cold, relatively fresh water into the highly salty young sea to the south, and the shock to life there could have wiped out much of it. Climate around the Northern Hemisphere could have cooled enough to undo plant life on land, and thus the dinosaurs.