After the Dinosaur-Killing Impact, Soot Played a Remarkable Role in Extinction

The famous impact 66 million years ago kicked up soot into the atmosphere that played an even bigger role in blocking sunlight than experts had realized

Artist's impression of the Chicxulub impact. (Donald E Davis)
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The interstellar object (alternatively a comet or an asteroid) that killed the dinosaurs when it slammed into Earth didn't work alone. Researchers have shown previously that its after-effects, such as tidal waves and earthquakes, played an important role in the mass extinctions of three-fourths of plant and animal life. Now, new research suggests that one of the most important factors was the soot-rich smoke from fires sparked by the collision.

Clay Tabor, a geoscientist at the University of Connecticut, and his colleagues studied soot, sulfates and dust to see how each type of particle may have contributed to the cataclysm. They found that while all three particles blocked heat from the sun, soot played a dominant role. The results were published earlier this year in the journal Geophysical Research Letters.

When the impactor plowed into the Earth and created the Chicxulub crater in Mexico, it vaporized the crust and created a planet-wide plume of debris that emitted radiation at a rate about 20 times stronger than the sun. It ignited plants and animals in its path. Later, lightning from impact-generated storms ignited more fires, maintaining an atmosphere rich in soot.

"Soot is very good at absorbing sunlight," Tabor says. "As soot gets into the stratosphere, some of it heats the atmosphere and self-lofts higher, increasing its atmospheric residence time."

The soot was the death knell for many marine creatures. While oceans protected them from blazes, the soot remained in the atmosphere to block most sunlight for nearly two years, darkening the skies and preventing photosynthesis. The new calculations suggest that it took almost six years for sunlight levels at the surface to return to normal, hindering recovery and leading to mass marine extinctions.

"Soot blocked sunlight, greatly reducing if not shutting down photosynthesis on both the land and in the sea," says Chicxulub expert David Kring of the Lunar and Planetary Institute in Texas. "Without photosynthesis, the base of the food chain would have collapsed. While fires may have demolished vegetation on land in large areas of the world, globally distributed soot may have ravaged vegetation elsewhere."

A dark death

The colossal chunk of rock that carved out a circular fault in the Gulf of Mexico 66 million years ago exploded with the equivalent of 100 trillion tons of TNT. The years that followed would have been like the apocalyptic nuclear winter that scientists say would follow a nuclear war, complete with raging fires and black-out skies.

A challenge has been figuring out just how widespread fires were. Scientists have discovered microscopic particles of soot matching forest fire smoke in the ground, revealing that nearly 70 billion tons of ash from the Cretaceous world is embedded in Earth's crust. But wind could easily blow soot across the landscape, potentially depositing it far from its source, making it difficult to pin down the locations of fires.

KT-impact
KT-impact (Wiki Commons)

Tabor and his colleagues hoped to sort out the soot by modeling its impact separate from that of sulfates and dust. The new study started by modeling the topography, vegetation and greenhouse gases of the Cretaceous Period. The team also simulated the thermosphere and allowed the sizes of impact aerosols to change over time. Previous models had struggled to quantify these effects. "The impact and fire-generated pollutants were so voluminous that they caused previous computer models to crash," Kring says. "The current study seems to have succeeded where past attempts failed."

Eventually, the soot settled out of the atmosphere. The largest particles fell out in only a few months, but the fine particles left behind did plenty of damage. For nearly two years, the Earth received about one percent of pre-impact sunlight, with midlatitudes suffering the longest. Over the next four years, the dust slowly washed away with the rain.

Sulfates also played an important role. The longest-lasting particles reduced the light to about 77 percent of normal, blocking light for nearly six years. Dust had the most damaging effect, blocking all sunlight, but most of it fell out of the atmosphere in a handful of months.

Tabor and his colleagues simulated each particle individually because they wanted to understand how each one alone affected the environment. However, Joanna Morgan, professor of geophysics at Imperial College London, argues that it would be better to include all these factors together because they interact. "For example, soot and dust could coagulate, removing the soot more quickly than in the models," says Morgan, who wasn’t involved in this study but whose own work challenges assumptions about how ejecta traveled from the crater to its final destination.

The team is updating its model to include all three particles interacting at once but remains confident that a combined version will not change their current results. Still, Morgan isn't quite convinced that the new finding proves that soot is the most effective of the climate factors in this case.

"That seems to be quite a bold statement,” she says. “I am not sure that we can be that confident about how life reacted 66 million years ago in such extreme cold and dark conditions."

Chills and rain

While sooty darkness played a significant role in killing off life after the impact, it also had other environmental effects. Soot, sulfates and dust also would have stopped the sun's heat from reaching the surface. Global temperatures at the time of impact were relatively warm, which could have left life "particularly vulnerable to a deep freeze from the impact winter," the authors wrote.

In terms of rapid cooling, soot was once again the worst culprit, dropping temperatures over the land by nearly 30 degrees Celsius and over the oceans by 13 degrees in just three years. Because the particles lingered in the atmosphere, temperatures in some areas did not return to their pre-impact levels for more than a decade. Sulfates contributed to the temperature drop to a lesser degree, but their effects were more long-lasting, hanging around for a dozen years. Dust had the smallest effect.

The loss of sunlight also affected rainfall, as typical large-scale atmospheric circulation patterns would have broken down. The subsequent loss of precipitation could have had an impact on Cretaceous wildlife and meant that there was less rain to put out wildfires. However, the changes in circulation patterns meant that some areas likely received more rain than they had in the past.

Once again, soot produced the biggest change, dampening rainfall by over half in the short-term. Sulfates quickly followed. Dust significantly blocked global rainfall, but its quick exit from the atmosphere had a lesser effect.

Although these drivers likely contributed to the extinction of much of Earth's flora and fauna, soot-inspired darkness most likely led the way. In the long cold days that followed the impact, the lack of light likely played the most important role in the loss of life.

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