Was it sudden death for the Permian period?

Paleontologists have long looked for the cause of the massive extinctions at the end of the predinosaur Permian period. A little more than 250 million years ago, these die-offs swept the Earth clean of most life, including almost 95 percent of species in the oceans (SN: 2/1/97, p. 74). Favorite candidate causes include severe changes in ocean chemistry, huge volcanic eruptions in Siberia, and the impact of a comet or asteroid.

Whatever the cause, a report in the July GEOLOGY indicates that the end of the Permian could have unfolded in less than 8,000 years. This strongly suggests a "catastrophic" cause for the large-scale die-off, says Michael R. Rampino, associate professor of biology at New York University.

Rampino and his colleagues at New York and the University of Ottawa studied core samples of limestone collected from the Austrian Alps. These sediments formed in shallow seas during a period that included the so-called P-T boundary. That's the borderline that separates the last portion of the Permian from the first portion of the Triassic, the period during which the dinosaurs rose.

Regular variations in the concentration of carbon-13 and the density of the limestone throughout the Alpine sample indicate that sediments accumulated during this period at the relatively constant rate of 1 centimeter per century. The researchers say these fluctuations were most likely caused by changes in sea level and climate that correspond nicely with known cycles of Earth's orbit.

The steady rate of sedimentation across the P-T boundary isn't surprising, says Rampino. Late-Permian sources of limestone, such as clams and corals, likely were replaced by calcium-depositing stromatolite algae that began to flourish after Permian predators perished, he explains.

In one sample of sediments, the last fossils of typical Permian organisms appeared about 6 meters below the first examples of Triassic invertebrate fossils, with no fossils in between. Rampino contends this indicates that the P-T boundary occurred within a 60,000-year period. In a nearby sample, the fossils change across just 0.8 m of sediment, indicating a mere 8,000 years or less.

Rampino says he isn't suggesting that new organisms evolved within a few thousand years. Rather, a sudden lack of predators may have allowed species that weren't readily apparent in the late Permian fossil record to flourish.

Scientists can now zero in on specific sedimentary layers in their search for clues to the cause of the Permian extinctions, including layers that could point to a deathblow from space, Rampino says.

"We don't know enough about the geochemical markers from certain types of extraterrestrial impacts, but now we know where [in time] to look for them, if they're there," Rampino says. "We now know that something really nasty happened at the end of the Permian."

Not all scientists agree that periods of just a few thousand years can be discerned by comparisons against orbital cycles of Earth that last between 20,000 and 400,000 years. Douglas H. Erwin, a paleobiologist at the Smithsonian Institution's National Museum of Natural History in Washington, D.C., says trying to do this is like trying to measure extremely small objects with a ruler marked off only in inches.

"It's clear that the extinction at the end of the Permian is a very abrupt event," Erwin says. "However, I'm not sure that it's possible to resolve events that happened that long ago to this narrow of a timescale."