Volcanoes and Extinctions: Round Two

Abstract

In 1979, a group of researchers injected new life into an old study of death. Led by Alvarez at the University of California at Berkeley, they proposed that an asteroid had crashed into the earth at the end of the Cretaceous period, 65 million years ago. This impact, they suggested, threw a blanket of dust into the atmosphere, which blocked out the sunlight, causing famine and mass extinctions (SN: 6/2/79, p.356). most famous casualties were the dinosaurs. Now, after stimulating a whirlwind of research in paleontology, stratigraphy, geochemistry, astrophysics and statistics, Alvarez believes the impact theory of the Cretaceous-Tertiary (K-T) extinctions on solid enough ground for him to move on to other geologic pursuits. As he concluded last fall in Eos, the weekly newspaper of the American Geophysical Union, ... the unusual features of the K-T [stratigraphic] boundary layer are exactly compatible with a major impact. competing theory that intense volcanic eruptions caused the extinctions (SN: 3/16/85, p.172), he suggested, simply cannot account for all of the evidence. Alvarez may have hoped that his Eos article, by carefully outlining all the evidence and what he sees as the shortcomings of the volcanic hypothesis, would put the debate to rest. But for some researchers, it only sparked more arguments for the volcanic cause. At issue the resolution of one of the most intriguing puzzles in science. By understanding what killed off the dinosaurs and three-quarters of the species living on earth 65 million years ago, researchers not only would solve a longstanding mystery of the past, but might catch a glimpse of what could befall the planet in the future. Walter [Alvarez] says the volcanic road shut. I think, on the contrary, it very wide open, says Vincent E. Courtillot, who visiting the University of California at Santa Barbara (UCSB) from the Institut de Physique du Globe in Paris. In a letter in the April 7 Eos, Courtillot and Stanley Cisowski of UCSB respond to Alvarez's Eos paper. The purpose of this letter, they write, is to stress that answers have changed rapidly in recent years and may still evolve, and that the case for a volcanic cause may have been determined extinct by Alvarez somewhat prematurely Courtillot and Cisowski are not alone. Charles Officer at Dartmouth College in Hanover, N.H., and his colleagues published a paper in the March 12 NATURE that outlines the volcanic arguments and looks in detail at how a series of large volcanic eruptions could affect the environment and life. Among the recent lines of evidence and theory that have motivated Courtillot and Officer to write their papers are: *A date for the Deccan Traps. Deccan Traps in India are vast flood basalts, or lava flows, that today cover an area about the size of France and may once have contained 1 million cubic kilometers of volcanic material. Proponents of the volcanic theory have suggested that the Deccan eruptions caused the K-T extinctions by spewing out sulfur and other volcanic material that darkened the skies, cooled the planet and produced acid rain. But the correlation between the K-T extinctions and the Deccan Traps has been sketchy at best, says Courtillot, because the traps have not been very precisely dated. While the most frequently quoted age for the traps falls around 60 million to 65 million years old, he says, other estimates, determined 6 68 7 72; .--.76 80 B4 88 92 96 36! (' '. |