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When 96 per cent of life died

Bowring and his colleagues have now gotten an important break in the case.

Updated: March 2, 2014 12:56:01 am

Carl Zimmer

Sam Bowring is officially a geologist at MIT. Unofficially, he’s a homicide detective trying to solve the ultimate cold case. Dr Bowring wants to understand how an estimated 96 per cent of all species on Earth became extinct at the end of the Permian Period 252 million years ago. It was the biggest of the five mass extinctions recorded in the fossil record. But because this killing happened so long ago, the culprit has evaded discovery for decades.

Bowring and his colleagues have now gotten an important break in the case. They’ve made the most precise measurement yet of how long it took for all those species to become extinct. Writing in the Proceedings of the National Academy of Sciences, they report that the extinction took less than about 60,000 years. That’s a geological blink of an eye — a fact that will help scientists evaluate different hypotheses for what triggered the mass extinction.

For now, however, the new result has Bowring puzzled. “I think there’s probably something really fundamental that we don’t understand,” he said. Scientists have long known that something big happened 252 million years ago, at the end of the Permian Period and the beginning of the Triassic. By the mid-1800s, they had collected enough fossils to notice that this boundary marked a dramatic change in the diversity of life on Earth. Before the Triassic, for example, horseshoe-crab-like creatures called trilobites left scads of fossils on the floors of the world’s oceans. Afterward, they left none.

As paleontologists found more fossils, the mass extinction came further into focus. It was so big that it transformed entire ecosystems. Permian forests of tree ferns and cycads vanished, replaced by huge expanses of a few weedy plant species. In the ocean, coral reefs collapsed. And both on land and at sea, millions of years passed before the world’s ecosystems returned to their former levels of diversity.

Studies on the rocks that formed around the time of the mass extinction revealed that strange things were happening to the environment. Huge volcanoes in Siberia belched molten rock that covered millions of square miles. The oceans warmed dramatically, climbing 18 degrees Fahrenheit.

One of the most intriguing clues geologists have found comes from the carbon in the rocks that formed around the time of the mass extinction. Rocks are made of a mix of “light” and “heavy” carbon. (Heavy carbon atoms have an extra subatomic particle in them.) Rocks from around the time of the mass extinction have a drastically higher ratio of light-to-heavy carbon. One way to create such a planet-wide shift would be to deliver a huge surge of carbon dioxide into the ocean.

Did any of these factors kill off most of life on Earth? It’s hard to build a solid case without a good chronology of the crime. One of the best places on Earth to build such a chronology is a set of hills in eastern China made of rocks from the late Permian and early Triassic. Bowring and his colleagues have catalogued fossils from the rock layers to pinpoint the window in which species became extinct. To estimate the age of these fossils, the scientists gathered volcanic ash preserved in the rocks. The ash contains radioactive atoms that have been breaking down ever since it was released from a volcano. They can thus serve as geological clocks.
In a 2011 study, Bowring and his colleagues estimated that the mass extinction took less than about 200,000 years. But since that study, Bowring and other researchers have improved the precision of their methods for dating rocks.

“We can really nail the duration of the extinction now,” Bowring said. In their latest study, he and his colleagues narrowed the window down to less than 60,000 years (their estimate has a margin of error of 48,000 years). Efforts to improve dating methods may narrow the window even more in years to come, Bowring predicted. In addition to the mass extinction itself, Bowring and his colleagues are fitting other events into their chronology. The surge of light carbon, for example, occurred over a 10,000-year period just before the extinctions.

Bowring doesn’t think any current hypothesis does a good job of explaining the chronology of the mass extinction. Some researchers have suggested that the Siberian volcanoes triggered the Permian-Triassic mass extinctions, for example. They released carbon dioxide and methane, and these gases raised global temperatures and acidified the oceans, thus making life unsustainable for many species.

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