A large meteorite, likely a comet, slammed into Earth 56 million years ago, spraying the landscape with bits of molten glass that have been found hundreds of feet below ground at two sites in southern New Jersey, scientists reported Thursday.
The apparent impact coincided with — and possibly triggered — a sharp rise in the amount of carbon dioxide in the air, which in turn led to a dramatic increase in global temperatures, according to the team from Rensselaer Polytechnic Institute, in Troy, N.Y., and Rutgers University.
Some other researchers are skeptical that a comet acted as a trigger, but they generally agree that the warming event occurred, with average temperatures rising as much as 10 degrees. The prehistoric episode has been the subject of intense scrutiny for years, because scientists think it will shed light on the current period of climate change, which also is blamed on a large increase in atmospheric carbon dioxide.
The new findings, to be published in Friday's issue of the journal Science, mainly concern the characteristics of the bits of glass, which hardened into "spherules" less than a millimeter across. They were found in clay deposits in Clayton's Scotland Run Park, in Gloucester County, and in Millville, in Cumberland County, which were beneath the ocean 56 million years ago.
It is unclear where the apparent comet struck land, but South Jersey was a good spot to detect the aftereffects because of its coastal geology, said Rutgers geophysicist Dennis V. Kent, one of the study's authors.
Gloucester and Cumberland Counties lay in fairly shallow water toward the end of the Paleocene epoch, so sediment was deposited faster there than in the deep sea, providing a detailed snapshot. In the cores of sediment that the scientists extracted from deep underground, the section containing the spherules spanned more than half a vertical foot.
"It turns out New Jersey is a primo spot to look at detailed records of this interval," said Kent, who also is affiliated with Lamont-Doherty Earth Observatory at Columbia University.
The scientists reported they also had identified spherules in cores of ocean sediment from a third site, located off the coast of Florida.
Other researchers not involved with the study said it provided good evidence of an impact from a comet or other extraterrestrial body, but they questioned the sequence of events proposed by the Rensselaer-Rutgers team.
Ellen Thomas, a Wesleyan University professor of earth and environmental sciences, said she had looked at sediment from the same time period and had not found any spherules. She said it was possible that the spherules were instead the result of another meteorite that hit the planet 10 million years earlier, the one thought to have triggered the extinction of dinosaurs.
"I think we need more data," said Thomas, who also is a senior researcher at Yale University. "I'm skeptical, but I can be convinced."
The authors of the new paper say they are convinced that their timing is right, for several reasons. Among them: Spherules associated with the earlier meteorite have higher levels of silica and sodium than what was found in New Jersey, said Rensselaer's Morgan F. Schaller, the lead author. He identified the spherules along with institute colleagues Megan K. Fung and Miriam E. Katz.
Next on the agenda is to look for spherules from more sites. That could guide scientists to where the comet may have struck, Schaller said. Higher amounts of spherules would be found closer to a point of impact, a bit like what would happen if a child spilled a bag of flour in the kitchen, Schaller joked.
"You'll find a big mess in the kitchen," Schaller said. "You'll find flour in smaller amounts in the living room."
Comet or no, another point of disagreement among scientists is how long it took for levels of carbon dioxide to rise. The Rensselaer-Rutgers team contends it may have taken place over little more than a decade, whereas others argue it took thousands of years.
The answer makes a big difference, said Lee R. Kump, head of the department of geosciences at Pennsylvania State University, who counts himself in the thousand-year camp.
If the prehistoric increase in carbon dioxide took place over more than a thousand years, then humans are currently pumping the gas into the air at a much faster rate, with potentially destabilizing effects on the world climate.
But if the prehistoric event took place over a decade, then today's rate of increase is less dramatic by comparison. The rate is important, because a slower change in climate allows time for living things to adjust, Kump said.
"Life can adapt if it's a slow hug," Kump said. "But if it's a punch in the face ..."
Carbon dioxide is a greenhouse gas, meaning it absorbs infrared radiation and warms the planet. The burning of coal, gas, and other fossil fuels has increased the atmospheric concentration of carbon dioxide from 280 parts per million in preindustrial times to 400 parts per million today, and average temperatures have risen about 1.8 degrees.
But 56 million years ago, the concentration rose from about 800 parts per million to at least twice that much, he said.
How could a comet have led to such an increase?
Several ways, Schaller said. One possibility is that an impact caused some initial warming that in turn triggered the melting of frozen deposits of methane, another greenhouse gas. Also, a comet itself is about 20 percent carbon.
Though he acknowledged there is debate, Penn State's Kump praised the new study for not overreaching in its conclusions. One thing is clear, he said:
"They're in the midst of a very interesting interval in earth history."