Earth Science

Research: ‘Earlier’ Impact Nearly Wiped Out Life, Too

Long before the dinosaurs ever lived, the planet experienced a mass extinction so severe it killed 90 percent of life on Earth, and researchers at the University think they’ve identified the unlikely culprit.

“An ancient meteorite body, one from the days when the solar system was still forming, struck the Earth 251 million years ago,” says Asish Basu, professor of earth and environmental sciences.

The research, led by Basu and published last November in Science, is the latest volley in a decades-long debate over what caused the “great dying,” a period in the planet’s early history when the fossil record nearly disappears, indicating most prehistoric plants and animals were wiped out.

Basu was joined in the new research by Robert Poreda, professor of earth and environmental sciences, along with colleagues at Harvard University and the University of California at Santa Barbara.

While scientists have been wrangling over whether a meteor caused the early extinction ever since a meteor was fingered for the demise of the dinosaurs 65 million years ago, the new findings add weight to the argument that a major meteorite also struck Earth nearly 190 million years earlier.

Such an impact likely triggered climate change and unprecedented volcanic activity, researchers say.

The one-two punch so affected the composition of the atmosphere that it took thousands of years to recover—leaving only a relative handful of plants and animals alive.

The new findings are the latest in Basu’s work to unravel the mystery of the great dying. In 1991, he also published a study in Science that showed a massive lava flow in Siberia dated precisely to 251 million years ago. The molten rock oozed for thousands of years—so much lava that if spread evenly, it would bury the surface of the planet under 10 feet of magma.

Further testing by Basu and Poreda showed that the lava had come from as deep as 1,800 miles beneath the surface.

“These were not just examples of local magma bubbling through the crust,” Poreda says. “Something brought this lava all the way up from near the Earth’s core.”

To find the cause of the Siberian flows meant finding rock samples 251 million years old—not an easy prospect because the oceanic tectonic plates that make up 70 percent of the Earth’s surface are younger than that. The oceanic plates slide underneath continental plates as they move, carrying any evidence far beyond the reach of humans.

From an area in Antarctica called Graphite Peak, Basu and Poreda took rock from a stratum that sat between a layer that contained many fossils and a layer nearly devoid of fossils called the Permian/ Triassic, or P/T, boundary. Previous tests by Poreda on the same layer found shocked quartz and fullerenes—cage-like molecules, containing atoms of extraterrestrial gases—which hinted at a meteorite or comet strike.

Coming at the problem from another angle, Basu and Poreda separated out magnetic particles from the Graphite Peak samples and from sources of P/T strata in China and Japan.

To their surprise they found that the grains that sorted out contained an iron alloy that does not occur on Earth. Some 40 pieces were tiny fragments of meteorite 4.56 billion years old, while other grains displayed metallic characteristics that were more indicative of being formed by extreme heat, such as that in a severe meteorite impact.

That the grains had not deteriorated from weathering indicates they must have been buried quickly under sedimentary deposits, another sign of a major impact.

“At the end of the Permian era, Antarctica was close to its present position as the southernmost part of the ancient supercontinent, Pangea, while south China was at the equator and Japan was to the north of the equator,” Basu says. “Such a wide, global distribution of these metal grains in the P/T boundary strongly suggests that these grains mark a major impact of a celestial body at that time.”

Basu and Poreda plan to continue searching for evidence of a catastrophic impact in the P/T layer in different sites around the world.

The two researchers hope that if enough samples from enough locations show evidence of a major impact, then scientists will be able to construct the exact scenarios of how the two largest mass extinctions in Earth’s history were caused by meteorite collisions.

—Jonathan Sherwood