When the sky rained beads of carbon
Some 65 million years ago, an extraterrestrial object smashed into Earth near Mexico’s Yucatan Peninsula. The impact, which produced the 180-kilometer-wide Chicxulub crater, is thought to have spawned a mass extinction — most famously killing off the dinosaurs — and may have sparked giant wildfires that raged across much of the globe. Researchers hypothesized the wildfires first cooled Earth by blocking the sun with soot and dust; subsequently, the sulfur oxides and carbon dioxide they injected into the atmosphere may have triggered acid rain and/or a period of greenhouse warming — any of which could have been the final nail in the dinosaurs’ coffin. But, a new study asks, what if the wildfires weren’t global?
Evidence for the wildfires set off by this event, which marked the transition from the Cretaceous to the Paleogene period (known as the K-P boundary), came from the discovery of abundant soot in sediment layers around the world dating to that time. But global wildfires don’t explain all of the available data, say Mark Harvey, a geologist at Sinclair Knight Merz in New Zealand, and his colleagues in a study in May in Geology.
Charcoal provides better evidence of the fires’ location than soot does: Small particles of soot can be blown far around the world, but the distribution of charcoal directly reflects the fires. An earlier study by one of the paper’s authors, geologist Claire Belcher of the Royal Holloway, University of London in Surrey, England, found very little charcoal in sediment layers at the K-P boundary in the United States, where researchers would expect to find an abundance of charcoal if fires indeed swept across the continent. That, she reported, suggests that there were no wildfires on a broad scale.
That finding prompted Harvey, Belcher and their co-authors to look for a different source for the soot. For clues, they focused on another anomaly found in many K-P boundary sediments in addition to soot: tiny beads of carbon called carbon cenospheres. Cenospheres are familiar to industry, where they form from the incomplete combustion of powdered carbon, but they don’t usually form in nature, Harvey says. In a coal-fired power station, for example, pulverized coal is burned in a furnace. Most of it is converted to carbon dioxide, but some particles survive total combustion, and merely melt. Still suspended in the air, they develop into little round blobs that cool and solidify into cenospheres.
But no obvious natural reason explains why these cenospheres would be floating around the K-P atmosphere, millions of years before coal-fired power plants, Harvey says. So their presence suggests an alternate theory: that the impact 65 million years ago pulverized and vaporized parts of Earth’s carbon-rich crust, sending the particles into the atmosphere, where they formed these unusual structures, cooled and rained back to Earth.
The impact could account not only for the cenospheres, but also for the global layers of soot, he says. Chicxulub crater is near a hydrocarbon-rich part of the crust, where the modern-day Cantarell oilfield now lies. Finely dispersed organic-rich crust, kicked up by the impact, could be the true source of the soot.
By suggesting that the soot layers were not the result of wildfires, Harvey says, the study “sets a limit on the total amount of heat energy released by re-entering ejecta from the impact itself. In other words, dinosaurs in China were probably not barbecued to death in giant global fires. It had to be something else that killed them.”
Few researchers still think that the impact spawned truly global wildfires, however, says David Kring, a geologist at the Lunar and Planetary Institute in Houston, Texas. “But to argue that there were no fires carries it to an extreme.” More recent models suggest that the fires were widespread — likely in North America and northern South America, as well as on the opposite side of the globe — but not actually global. “The real question,” he says, “is not whether there were fires, but to what extent their geographic distribution was.”
Whether or not there is evidence of charcoal at the K-P boundary within North America remains at issue, however. Several independent groups of researchers have found charcoal at the K-P boundary in the United States, Kring says, so for this team to indicate that there is no charcoal and thus no fires may be more of an issue of sampling within a complex, heterogeneous boundary layer than anything else.
“I’m happy and intrigued to see studies of this kind,” Kring adds, because it means the community is taking a closer look at the question. “But it doesn’t convince me. We’ve shown that the magnitude and distribution of these wildfires depend on a lot of details associated with the impact, some of which are still unknown,” such as the trajectory of the impacting object, he says. “It’s a complicated package of rocks.”
That the carbon-bearing rocks near the impact might have been a huge source of soot does make sense, says Wendy Wolbach, a chemist at DePaul University in Chicago, Ill. “When we proposed global wildfires back in 1985, no one had any idea where the meteorite hit,” she says. Therefore, there was no way of knowing whether the impact would have been near another source of carbon. “Since Chicxulub was determined to be the K-P crater, however, the fossil-carbon-containing rock makes sense as a contributor.”
However, Wolbach says, that does not mean wildfires were not also contributing to the global soot layer. “I think it is entirely possible that there were regions on Earth’s surface that did not burn, but it is probably wrong to conclude that wildfires did not contribute at least in part to the conflagration,” she says. “I suspect that there was a combination of fuel sources for the K-P fires.”