Heat wave at the K-T boundary?

Heat wave at the K-T boundary?

The widespread death of microscopic ocean plants 65 million years ago could have triggered an extreme global heat wave that helped kill off roughly half the existing species of plants and animals, including the dinosaurs, at the Cretaceous-Tertiary (K-T) boundary. This scenario, derived from new calculations by scientists at New York University, is helping to bring into focus the series of climatic plagues that were dramatically changing the living conditions on earth at that time.

Scientists have spent years debating what catastrophic event let to the events at the K-T boundary. According to the leading-candidate theory proposed in 1980, a comet or meteor collided with earth, creating a global dust cloud that blocked out sunlight and cooled the planet for a period of up to several months.

More recently, however, researchers have realized that the climatic troubles would not have ended when the lights came back on. "What we're seeing is that the K-T boundary was a pretty complex event," says NYU's Michael Rampino. He and Tyler Volk have examined how the elimination of one type of life would have affected the climate.

According to the researchers, a catastrophic impact could have triggered the death of floating one-celled ocean plants, called calcareous nannoplankton. This, in turn, would have weakened the earth's ability to reflect radiation from the sun, raising surface temperatures by 6[deg.]C for several hundred thousand years. Such a rise would finish off many species that had survived the earlier changes in climate.

It was only recently that scientists discovered a connection between nannoplankton and the earth's climate (SN: 12/5/87, p.362). These plants exert a strong cooling effect on the earth by producing a sulfur compound, which helps form water particles in marine clouds. The water particles reflect sunlight and prevent radiation from reaching the earth's surface. Rampino and Volk, reporting in the March 3 NATURE, are the first to use this relationship to explain events at the K-T boundary.

"What's interesting is that people have been talking about climatic changes that cause mass extinctions. Looking at it another way, here is a mass extinction that would have affected the climate," says Rampino.

A warming of 6[deg.]C (about 12[deg.]F) would have been a "major shift in climate," says James Coakley of the National Center for Atmospheric Research in Boulder, Colo. "You look at the kinds of changes that occured during the ice ages, for example. It was 6[deg.] but it was in the other direction."

The fossil record does indeed suggest that ocean temperatures rose by 5[deg.]C to 10[deg.]C for tens of thousands of years beyond the K-T boundary. As well, the record shows that more than 90 percent of the calcareous nannoplankton species went extinct at that time, and that most life disappeared from the upper portions of the ocean for almost a half million years, an effect geochemists call the "Strangelove Ocean."

The plankton may have been killed off by a lack of sunlight needed for photosynthesis or by the acid rain that would have followed a meteor impact. Recent studies have suggested that rain as corrosive as battery acid would have lowered the pH of the surface waters of the world's oceans. In acidic water, the plankton's calcareous shell would disolve.