Scientists have found direct evidence that a comet struck the Earth more than 50 million years ago, coinciding with a warm period often compared with today's global warming, a report said on Thursday.
The findings in the journal Science do not prove that the impact unleashed an unusual amount of carbon dioxide.
Rather, they provide more support to the highly debated theory that a sudden impact, rather than volcanic eruptions or some other cause on Earth, may have led to the warming period known as the Paleocene-Eocene Thermal Maximum (PETM).
"This could very well be the ground zero" of the PETM, said study co-author Dennis Kent, a researcher at Columbia University's Lamont-Doherty Earth Observatory and Rutgers University. "It got warm in a hurry. This suggests where it came from." Digging near what is now New Jersey, Kent and colleagues found small, round droplets of glass called microtektites.
These sand-grain-sized spheres are thought to form when an extraterrestrial object hits the Earth, spraying out vaporized material that solidifies while flying through the air, said the study. "It's got to be more than coincidental that there's an impact right at the same time," said lead author, Morgan Schaller, a geochemist at Rensselaer Polytechnic Institute. "If the impact was related, it suggests the carbon release was fast." Scientists say the outpouring of carbon took place over a period of 5,000-20,000 years.
Other theories suggest that a period widespread volcanism led to the sudden release of greenhouse gases, such as frozen methane from the seafloor.
Some think changes in the earth's orbit or shifts in ocean circulation also played a part in the rise of 9-16 Fahrenheit (5-9 Celsius) for a period of about 200,000 years.
The changes meant that ice virtually disappeared from the Earth and sea levels were far higher than today.
Some creatures went extinct, others migrated toward the poles. Experts say carbon emissions "are now far outpacing anything that took place during the PETM," said a statement from Columbia University.
"The consequences might be more drastic, because many life forms will not have time to evolve or move."
A separate study earlier this year showed that fossil fuel burning and pollution is sending carbon into the atmosphere 10 times faster than the natural forces that unleashed the warming 55 million years ago.
Still, researchers have not found the crater that would have been caused by such a massive strike.
"It could have been next door, or it could have been on the other side of the planet," said Schaller. Since the small spheres are thinly spread, the impact could have been large but far away, or close but relatively small, he said.
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