Those cataclysmic events appear to have caused abrupt inversion of ocean waters, triggering one of the most dramatic climatic changes ever, according to a geologist on the team that discovered the eruptions.

The inversion caused release of massive amounts of sea floor methane into the atmosphere, leading to global warming and possibly speeding evolution of countless new plant and animal species, including many primates and carnivores, the scientist believes. At the same time, close to half of all deep-sea animals went extinct, asphyxiated in the suddenly warmer and stagnant deep waters.

"We found multiple very distinct blue, green and occasionally red volcanic ash layers that were very different from whitish-gray sediments above and below them in the core samples," said Dr. Timothy Bralower, associate professor of geology at the University of North Carolina at Chapel Hill. "The time those ash layers -- direct evidence of volcanic eruption -- were deposited corresponds to the beginning of a known period of rapid warming of the Earth. This is the first evidence for a major volcanic episode at that time in the Caribbean."

A report on the discovery appears in the November issue of the journal Geology.

Core sampling to about 3,000 feet took place aboard the 470-foot JOIDES Resolution, the world's largest scientific drill ship, in late 1995 and early 1996.

"Many geologists are interested in this interval 55 million years ago because we don't know of any other geologic period when warming occurred so precipitously, and warmth covered the globe like this," Bralower said. "The million-dollar question is what triggered the sudden burp of methane. A dramatic climatic change had to have occurred first."

The new work shows a huge Caribbean volcanic eruption occurred just as the warming began, Bralower said.

"That's a microsecond of geologic time, and the chances that the events are just coincidence is minute," he said.

The geologists speculate that like most explosive volcanism, the Caribbean eruptions first led to minor cooling of the atmosphere and that the cooling was most severe near volcanoes in the tropics. They believe that temperature drop caused the tropical ocean waters to become denser and to sink, replacing the usual cold polar waters in the deep ocean and fomenting an oceanographic change of titanic proportion.

"Sudden warming of the deep ocean would cause methane trapped in sediments under the sea floor to become unstable and rapidly rise into the ocean and atmosphere," Bralower said. "That would fuel further warming and instigate the dramatic environmental and evolutionary changes."

Scientists already know that comparable volcanic activity occurred in the North Atlantic Ocean beginning some 61 million years ago, the geologist said. The new discovery suggests the Caribbean volcanism may have been even more important for the Earth and somehow acted in tandem with the earlier known eruptions farther north.

Understanding major warming events in the past is one of the best ways of determining the possible effects of global warming, he said. "The giant Caribbean eruption provides a worst-case scenario of what could happen to Earth in the future."

Co-authors of Bralower's paper are UNC-CH geology graduate student Debbie Thomas and faculty members at the universities of California at Santa Cruz, Rhode Island and Minnesota, Bremen University in Germany and Wesleyan University in Connecticut. The National Science Foundation and the Texas A&M University-based Ocean Drilling Program supported the research.

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