Researchers: Meteor Likely Cause of 'The Great Dying'

By GAIL GALLESSICH

"The Great Dying," a time of Earth's greatest number of extinctions, appears to have been caused by the impact of a large meteor, according to a research team that includes Luann Becker, a scientist with the Institute for Crustal Studies and UCSB's Department of Geology.

The theory explains that this extinction event, which occurred approximately 250 million years ago, is much earlier than the demise of the dinosaurs, which is estimated at approximately 65 million years ago and is also believed to have been caused by a large meteor.

In fact, scientists increasingly believe that meteors have played major roles in changing Earth's climate and ecosystems. It is estimated that about 60 meteorites five or more kilometers across have hit the Earth in the past 600 million years. The smallest ones would have carved craters some 95 kilometers wide.

The evidence amassed by the team is the most convincing yet for an impact at the end-Permian, a time commonly referred to as "The Great Dying," when life was nearly erased, explained Becker. Her Antarctica research at the Graphite Peak in the Central Transantarctic Mountains, described in a recent Science article, has revealed several meteoritic fragments, metallic grains in a thin claystone breccia layer. Becker and the research team believe this to be strong evidence for a large impact that appears to have triggered the die-off.

Breccia is ejected debris that resettled in a layer of sediment. The metallic grains also appear in the same layer (end-Permian) in Meishan, southern China. They also resemble grains found in the same strata in Sasayama, Japan. (The Earth was a single continent at the time of the impact.)

The team also found "shocked quartz" in this same layer in Graphite Peak. Unlike extreme volcanic activity, which fractures quartz in only one direction, shocked quartz is fractured in several directions and is believed to be a good tracer for the impact of a meteor.

Becker is currently in Antarctica searching for more impact tracers, the geological markers that show evidence of large meteors hitting the Earth. She has made several research trips to Antarctica, and in 2001 she received the National Science Foundation Antarctic Service Medal.

The researchers are somewhat surprised that they have not found the strong presence of the mineral iridium in the Graphite Peak work. In e-mail from Antarctica Becker stated, "Interestingly, we do not see a strong iridium anomaly (the impact tracer that marks the dinosaurs' extinction at the Cretaceous-Tertiary boundary)."

Other impact tracers, including extraterrestrial fullerenes, have been found in the Graphite Peak boundary layer. These soccer ball-shaped carbon molecules, called fullerenes, trap extraterrestrial gases in space and travel to the earth in the meteor.

The team concludes the Science article by saying, "In light of the new evidence presented here, this is a reasonable interpretation of the global extinction event at the Permian-Triassic boundary."