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The mighty dinosaurs may have been done in by dust, according to a new study in Nature Geoscience.
For decades, scientists have known that a giant asteroid smashed into what is now the Yucatán Peninsula roughly 66 million years ago. Most experts agree the event triggered a mass extinction that wiped out three-quarters of all species, including almost all the dinosaurs.
But precisely how the impact led to an apocalypse has remained unsettled, with much attention focused on the “impact winter” that occurred afterwards – a period of cold, global darkness.
An illustration of the Cretaceous Paleogene extinction event.Credit: Getty
In 1980, scientists posited that the asteroid kicked up a big cloud of pulverised rock dust that starved plants of sunlight. But more recent investigations focused on sun-blocking soot from the initial impact and subsequent global bushfires, or on long-lived sulfur aerosols released by the cataclysm.
The question of how the sun was blocked, and for how long, has been critical to tease out because it shaped the evolution of life on the planet in fundamental ways. A prolonged period of darkness that shut down plants’ ability to turn sunlight into energy could have led to the collapse of the entire food chain. Understanding how life responded and, in some cases, outlasted such an extreme climatic event may provide insight into future extinctions.
For the new study, researchers coupled computer simulation with an analysis of sediment layers at the Tanis paleontology site, which preserves the aftermath of the Chicxulub impact in extraordinary detail. The work reveals that a massive plume of fine-grained dust blanketed the planet and would have lingered in the atmosphere for 15 years, cooling Earth’s surface by 27 degrees and shutting down photosynthesis for two years.
Scientists think that a giant asteroid, which broke up long ago in the main asteroid belt between Mars and Jupiter, eventually made its way to Earth and led to the extinction of the dinosaurs. Credit: Artist impression/NASA
“Dust could shut down photosynthesis for such a long time that it could pose severe challenges,” said Cem Berk Senel, a planetary scientist at the Royal Observatory of Belgium who led the study. “It could result in a chain reaction of extinction to all species in the food chain.”
New clues from North Dakota
Many research teams have attempted to model the aftermath of the Chicxulub impact. The new study stands out because the team drew on data from the Tanis site in present-day North Dakota, which was unveiled to the public in 2019. Unique among fossil sites, Tanis captures the immediate aftermath of the impact event in such incredible detail that scientists can tell what season the dinosaurs died. (It was a spring day in the late Cretaceous.)
Senel’s team discovered fine dust grains from the Tanis site are about the same size as microscopic bacteria, a small but not too-tiny size range that would have allowed the dust to persist in the atmosphere for 15 years. That means dust would have made a greater contribution to blocking sunlight from reaching the surface than soot particles or sulfur aerosols, they report.
“It is a fascinating study,” said Clay Tabor, an assistant professor in the earth sciences department at the University of Connecticut. Tabor in 2020 published a paper in Geophysical Research Letters showing that soot from fires would have been a primary driver of photosynthetic shutdown.
Tabor said the new information on dust size will help improve simulations of the climate after the impact. The grains of dust the researchers discovered could have persisted in the atmosphere for a long time, according to the model the researchers used, leading to a larger decrease in surface sunlight than other sun-blocking particles.
But he questioned whether differences in climate models might explain the differences between various studies. For example, the new model predicted a similar amount of soot would have had a smaller effect on sunlight than some previous models.
The behaviour of tiny particles in the atmosphere is complicated, and “these processes can be difficult to accurately simulate, especially in the extreme case of the Chicxulub impact,” Tabor said.
Dinosaur tracks at the Red Fleet State Park east of Salt Lake City in the US.Credit: AP
Kunio Kaiho, a planetary scientist at Tohoku University, has published research showing that the asteroid smashed into Earth at just the right spot to cause a mass extinction, striking oil-rich rocks to generate sun-blocking soot.
Kaiho said the new study “holds significant importance in our understanding of the mechanisms responsible for global cooling and mass extinctions”.
A ‘potpourri’ of catastrophe
The giant impact didn’t just create a dust cloud. The nearly 10-kilometre-wide asteroid sent shudders through the earth, generating tsunamis. It ejected debris that then fell back down, heating the atmosphere upon reentry to cause global bushfires. It kicked up rock dust and other types of aerosols, and it released greenhouse gases that kicked off a later period of global warming that may have lasted for tens of thousands of years.
David Kring, a planetary scientist at the Lunar and Planetary Institute in Houston who was not involved in the study, said it affirms the original hypothesis behind the mass extinction, which attributed the shutdown of photosynthesis to dust.
“The duration of darkness is really important because if, in fact, photosynthesis was shut down and that drove extinctions, it has to be dark for a fairly substantial length of time,” Kring said.
But he added that the wide-ranging environmental consequences of the impact – from the global bushfires to sulfuric acid rain to major climatic shifts– make it hard to pinpoint a singular cause for the widespread deaths.
“Each of those environmental consequences affected different parts of the world and lasted for different amounts of time,” he said. “So one of the things that we understand in general, and not yet in detail, is that it’s really this potpourri of environmental effects that led to the extinctions. There’s no single silver bullet.”
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