"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 sciences in today's issue of Science. The research is the latest volley in a decades-long debate over what caused "The Great Dying," the greatest elimination of life in the planet's history.
While scientists have been wrangling over whether a meteor caused this great extinction ever since a meteor was fingered with the blame for the later dinosaur extinction, these new findings add weight to the argument that a major meteorite did strike the Earth 251 million years ago, likely triggering climate change and unprecedented volcanic activity. That 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.
Two decades ago, Nobel laureate Luis Alvarez and his son, Walter, detected huge concentrations of iridium throughout the world in rock dated to the end of the dinosaur era. Iridium is only found in such concentrations in asteroids, so they concluded that a giant asteroid had struck the Earth at that time, likely leading to the downfall of the dinosaurs. The Alvarez claims were at first largely dismissed, but the evidence grew and today it is accepted that their interpretation was largely correct.
Basu added weight to the Alvarez claims in 1988 when he announced the discovery of "shocked quartz"--special crystals that have split along certain planes indicative of a large impact--immediately beneath the Deccan Traps of India. The Deccan Traps are areas of huge volcanic deposits that have been dated to 65 million years ago, the time of the dinosaur extinction, so finding shocked quartz immediately beneath them suggests that a giant impact preceded these giant lava flows.
While a meteorite has been largely accepted as the source of the dinosaurs' demise, the root of The Great Dying has been a mystery. In 1991, however, Basu published a study in Science that showed a massive and ancient lava flow in Siberia dated precisely to that greatest of extinctions 251 million years ago. The lava did not shoot out of the Earth like a giant volcano, but oozed molten rock for thousands of years--so much lava, in fact, that if spread evenly, it would bury the surface of the Earth under 10 feet of magma.
Further testing by Basu and Robert Poreda, professor of earth and environmental sciences at the University, and also co-author of the current Science research, showed that both the Siberian and Indian 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," explains Poreda. "Something brought this lava all the way up from near the Earth's core."
To find what might have caused the Siberian flows meant finding rock samples 251 million years old--not an easy prospect since oceanic tectonic plates that make up 70 percent of the Earth's surface are younger than that. Oceanic plates slide underneath continental plates as they move, thus 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. One of the fossils that had gone from prominence to sudden disappearance was Glossopteris flora, a plant that was widely known to have been wiped out in The Great Dying. This reassured the team that they had the right rock from the right period. Previous tests by Poreda on this same layer found shocked quartz and fullerenes, cage-like molecules, containing atoms of extraterrestrial gases, which again hinted at a meteorite or comet strike. These results, however, were disputed by some researchers.
Coming at the problem from another angle, Basu and Poreda separated out the magnetic particles from the samples from Graphite Peak and from a source of P/T strata in Meishan, 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. The very fact that these grains had not deteriorated from weathering means they must have been buried quickly under sedimentary deposits, again, indicative 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," explains Basu. "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." Critics of the P/T impact theory may point to the lack of iridium, the element that is so rare on Earth but common in asteroids and which alerted Alvarez to the possibility of a meteorite as the death knell for the dinosaurs. The Rochester team's work shows strong evidence that not all collisions with extraterrestrial bodies will leave an iridium footprint. Basu suggests that a collision with a comet, which may have a meteoric core, would be low in iridium. Thus the culprit that wiped out nine of every 10 creatures on the Earth and nearly ended life when it was just taking hold may have been created before the Earth itself was fully formed.
Basu and Poreda plan to continue searching for evidence of a catastrophic impact in the P/T layer in different sites around the world. They 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 history were caused by meteorite collisions.
Along with Basu and Poreda, the co-authors of the paper are Michail I. Petaev and Stein B. Jacobsen of Harvard University, and Luann Becker of the University of California, Santa Barbara.
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