Most geologists accept that a meteorite plunging into the Gulf of Mexico 65 million years ago spelled the end for the dinosaurs. Now a team claims to have found proof of another major impact responsible for a mass extinction. But others say the newcomers still haven't got the right kind of evidence to indict a second killer.
When geologists first proposed 20 years ago that an impact did in the dinosaurs, the scientific community was skeptical. Nowadays the idea is widely accepted, and impacts have been proposed for most other major extinctions. So far, however, the evidence for other impact-extinction links has been underwhelming. Part of the difficulty is recognizing markers of impacts in the geologic record.
In the 21 November issue of Science, petrologist and geochemist Asish Basu of the University of Rochester, New York, and four colleagues offer one such new marker: Tiny bits of a meteorite they found in Antarctica. The researchers say that the fragments were created in a geologic instant of the Permian-Triassic (P-T) mass extinction 251 million years ago, the biggest mass extinction of all time. Microscopic inspection of mineral textures and electron microprobe measurements of 20 tiny fragments show that they must be from a meteorite, says the group, which includes meteoriticist Michail Petaev of Harvard University. These meteoritic fragments join other nontraditional impact markers--gas-filled molecular carbon cages called “buckyballs” and odd metal grains--reported from the same P-T boundary rock. The new work “is clear evidence of an impact at the P-T boundary,” says Basu.
Impact geologists tend to disagree. Although they accept that the fragments came from outer space, they wonder if the fresh-looking, unaltered bits of meteorite could have survived unaltered for 251 million years. “It's astonishing, it's incredible, it's unbelievable,” says meteoriticist Jeffrey Grossman of the U.S. Geological Survey in Reston, Virginia.