Geotimes - April 2001: News Notes aprilheader.html
News Notes
Geochemistry

P/T extinction explained

The search might be over to explain Earth’s most severe mass extinction — the Permian-Triassic event that wiped out almost all life on Earth 250 million years ago. A group of scientists has pointed to noble gases trapped within a complex carbon structure as evidence that a giant extraterrestrial body collided with the supercontinent Pangaea and triggered a series of catastrophic events that obliterated nearly 90 percent of Earth’s organisms.

Last spring, Luann Becker of the University of Washington and her colleagues found that fullerene molecules, which often form in space, could trap noble gases. Spurred by this discovery, they investigated outcrops of Permian-Triassic stratigraphic layers for extraterrestrial materials that could link an asteroid impact to the Permian-Triassic Boundary extinction (Geotimes, April 2000).

Because so few 251-million-year-old rocks are exposed, Becker’s investigations took two years to complete, but the scientists analyzed soil and rock from Permian-Triassic outcrops in Japan, Turkey and China and have correlated peculiar characteristics that may provide some plausible scenarios after all. They published their findings in the Feb. 23 Science.

In the absence of an impact crater — such as the Chixulub Crater, believed to be linked to the extinction of dinosaurs and to the Cretacous-Tertiary Boundary extinction event — or other distinctive impact effects, such as shock metamorphism to study, the scientists looked at the chemistry of the stratigraphic layers bounding the 251.4-million-year mark.

The key discovery was the rare type of fullerene, which encases argon and helium atoms that have isotopic signatures such that they could not have formed on Earth: The isotope found in these particular fullerenes is mostly the extraterrestrial helium-3, while terrestrial helium is primarily helium-4. What’s more, similar fullerenes have been found in two well-known carbonaceous chondrites, the Murchison and Allende chondrites that are of undisputed extraterrestrial origin.

If an asteroid hit Pangaea at the end of the Permian, it could have started a chain of events that, compounded, could have blanketed the Earth with a laundry list of inhospitable conditions and caused the Permian-Triassic extinction.

The theory proposed by Becker and her colleagues pulls together different theories that other scientists independently proposed might have caused the extinction: massive flood basalt volcanism, ocean anoxia, climate change and sea level change.

Laura Wright



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