Fullerenes Provide a Clue to the Cause of the Earth’s Greatest Mass Extinction

A catastrophic event 250 million years ago wiped out most life on earth. Ninety percent of marine species and 70 percent of land species vanished in less than 100,000 years, a blink of an eye in the geological time scale. Scientists have long suspected that the cause of this mass extinction was a series of changes in the atmosphere and oceans that were set off when a large asteroid or comet struck the earth, but the evidence for such an impact was lacking. Now the "smoking gun" has been found and it is a fullerene.

Scientists know about the extinction from examining sedimentary rock, which accumulates over time with the deposit of sand, silt and the remains of plants and animals. Deeper layers are older than those above them. Layers deposited more than 250 million years ago contain fossils of many species that are not found in layers deposited more recently. The dividing line is rather sharp and is called the Permian-Triassic Boundary, marking the transition between those two periods in the earth’s history.

Researchers led by Dr. Luann Becker of the University of Washington examined formations at the Permian-Triassic Boundary in Japan, China and Hungary. They found an abundance of fullerenes that had atoms of Argon or Helium trapped inside.

A tremendous amount of heat and pressure is required to form such molecules. One theory is that they originated in carbon stars. The distribution of isotopes of Helium found in the fullerenes also indicates an extraterrestrial origin.

C60He molecule Click on molecule to see it from all sides. (Requires Chime plug-in)

Most Helium atoms have two protons and two neutrons (Helium-4). Some have only one neutron (Helium-3). The proportion of the lighter isotope was higher in the fullerenes found at the Permian-Triassic Boundary than in the earth’s atmosphere.

A similar mass extinction occurred 65 million years ago when the doomed species included the dinosaurs. The boundary layer from that time, known as the K/T or Cretaceous-Tertiary Boundary, was found to contain significant amounts of Iridium, an element that is rare on earth but found in some kinds of asteroids.

It is believed that the meteor that struck the earth 250 million years ago was about the same size as the more recent visitor - about 10 kilometers across - but its composition was different. Instead of leaving behind traces of Iridium as evidence, its calling card was an unusual variety of fullerene.

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