Dartmouth profs propose new theory on dinosaur extinction

Acid rain, not asteroids, caused the extinction of dinosaurs 65 million years ago, according to two Dartmouth geologists. Charles Officer and Charles Drake, both professors in the College's earth sciences department, have become the leading opponents of the now-popular theory that the dinosaurs' demise was caused by a giant meteorite or comet hitting the earth. They contend, instead, that dinosaurs disappeared gradually during a period of intense volcanic eruptions. They believe volcanoes emitted such concentrations of sulfur dioxide and chlorine that there was significant global cooling, causing acid rain 40 times more severe than that known today and badly depleting the ozone layer.

Scientists agree, Officer has explained, that 65 million years ago something extraordinary happened on the earth which caused dinosaurs to disappear and the age of mammals to begin. Just what the nature of that geologic event was, though, is much disputed.

The extraterrestrial impact theory claims that dinosaurs died out suddenly when a collision with a large object from space caused huge dust storms that blotted out the sun, thus killing various species of plants and animals. Recently, scientists from the U.S. Geological Survey identified an lowa site as the likely impact crater. Another significant clue was the discovery in Italy several years ago of an element called iridium in a clay layer formed at the time of the great change. Proponents of the impact theory interpreted the discovery as evidence of an extraterrestrial cause fo extinctions, since iridium comes only from beyond the earth or deep within the earth.

Officer and Drake refute these conclusions. They say the lowa crater is too small to have caused worldwide extinctions and is four million years too old. They also contend that the impact theory would have caused all species to die off at once. However, the geologic record shows selective patterns of extinction- that dinosaurs died but birds and mammals did not, more in keeping with a volcanic occurrence.

In addition, studies of additional samples from Italy indicate a gradual buildup and reduction of iridium during the Cretaceous/Tertiary period, not the sudden spike that should result from a meteoric impact. "That anomaly needs to be explained," says Drake.

He and Officer believe they know the location of the massive volcanic eruptions they feel doomed the dinosaurs. The geologic record makes it clear, they say, that volcanoes were active during the Cretaceous/Tertiary period near Bombay, India, in the South Atlantic, in Greenland, and in the British Isles.

Though the new theory is controversial, they say there is a good deal of support for it. They have published their research in Science magazine, and another article is coming out soon in Paleo-oceanography.

Dartmouth geologists Charles Drake, left, and Charles Officer are gaining attention for theirtheory that acid rain doomed the dinosaurs 65 million years ago.