K-T mass extinctions: abrupt or what?

K-T mass extinctions: Abrupt or what?

A small, barren island off the coast of Antarctica is revealing a surprising and remarkably detailed picture of the mass extinctions at the end of the Cretaceous period. These new findings, far different from the record found elsewhere in the world, were reported this week in Phoenix at the Geological Society of America's annual meeting. The preliminary results come from the first study of the Cretaceous-Tertiary (K-T) boundary in the Antarctic region, and they may help scientists decide whether a cataclysmic meteor impact killed off the dinosaurs and a significant proportion of life before 65 million years ago.

In the past, most paleontologists who have studied the K-T boundary in Europe, North America and other midlatitude sites have found that species started to die out abruptly and in record numbers at the close of the Cretaceous period. However, William Zinsmeister of Purdue University in West Lafayette, Ind., and his colleagues found a different extinction pattern on Seymour Island, near the Antarctic peninsula.

"In high latitudes,' says Zinsmeister, "you just don't see the marked extinctions at the K-T boundary. You see a gradual change, a gradual dropoff.'

In the last decade, scores of scientists from the fields of geology, paleontology and even astrophysics have jumped into an often-heated debate over the cause of the K-T extinction (SN: 2/1/86, p.75). Luis Alvarez, a Nobel prize-winning physicist from the University of California at Berkeley, launched the present controversy around 1979 when he proposed that the extinctions resulted from the impact of a comet or meteorite. Such a catastrophic crash would have thrown a cloud of debris into the atmosphere, blocking out sunlight and abruptly killing off much of Cretaceous life.

Recently, however, many paleontologists have argued that the extinctions may not have been so abrupt. And, says Zinsmeister, the Seymore Island data will add to the debate. "It's going to be a hot one,' he told SCIENCE NEWS before delivering his paper to the conference.

Zinsmeister adds, though, that a gradual decline of life near the South Pole would not preclude the possibility that species closer to the equator died off abruptly. If Antarctica was meteorologically isolated from the rest of the world, as it is today, "whatever mechanism caused the apparent abrupt mass extinctions at midlatitudes may have been damped by the time it got to the high latitudes,' he says.

But the Seymore Island K-T boundary might offer a truer picture of the mass extinction pattern than do the European K-T boundaries. In Europe, because comparatively less sediment accumulated over a given period of time, crucial time periods have been condensed into rock segments only a few centimeters thick, as compared with 30-meter-thick sections on Seymore Island. Says Zinsmeister, "These gradual changes that you see in the Antarctic may actually have occurred in Europe but you don't see them because the picture isn't as clear.'

However, Marilyn Kooser from the University of California at Riverside, who also worked on the Seymore Island fossils, cautions that the conclusions are preliminary and that the data await statistical analysis.

According to Kooser, the Seymore Island data also highlight a problem concerning the definition of the K-T boundary. Traditionally, scientists have used several different methods to define the boundary, sometimes relying on the last appearance of large, nautilus-like creatures called ammonites, sometimes relying on other benchmarks, such as a characteristic change in microfossils. However, when the team used the microfossil definition on Seymore, they found ammonites 20 meters above the boundary.