One of Earth’s most deadly mass extinctions came at the Cretaceous–Paleogene (K–Pg) boundary, 66 million years ago, when an asteroid hit what is now Mexico’s Yucatán peninsula. Researchers have substantial evidence to support the impact hypothesis—including extraterrestrial levels of iridium and sulfur-isotope anomalies of ejected materials. The findings aren’t the first of their kind, but even with the accumulating evidence, details remain uncertain regarding how the impact may have altered Earth’s climate to cause a mass extinction.
Aerosols sprayed into the atmosphere after the impact may have blocked enough sunlight to stunt plant growth and lead to a collapse of the entire food chain. On-the-ground evidence is limited to the crater or slim layers of ejecta material sandwiched between much thicker layers of rock elsewhere. Most modelers have focused on the lifetime and transport of a few light-blocking aerosols—that is, dust, soot, and sulfur—individually in different model runs.
Now Cem Berk Senel, from the Royal Observatory of Belgium in Brussels and the Free University of Brussels, and his colleagues have modeled dust, soot, and sulfur individually and together. According to their simulations, only microsized dust could have lingered in the atmosphere long enough to halt photosynthetic activity for an extended period of time.
The lifetime of solid particles in the atmosphere depends on, among other factors, their grain sizes. So Senel and his colleagues used grain-size measurements from the Tanis geological site in North Dakota, thousands of kilometers from the impact crater, to improve their model’s accuracy. Whereas the K–Pg boundary is often only centimeters thick, Tanis has about one meter of material. With real-world constraints, the researchers simulated the Sun’s photosynthetic active radiation reaching Earth’s surface. It’s a good proxy for biological activity because it’s the wavelength range (400–700 nm) of sunlight that plants and other organisms use for photosynthesis.
“It is a fascinating study,” says the University of Connecticut’s Clay Tabor. “This finding contrasts with some previous studies that found soot produced the greatest surface sunlight reduction after the Chicxulub impact.” Sorting out exactly what happened is useful for more than just the K–Pg mass extinction. Tabor says, “more generally, studying the end-Cretaceous mass extinction can help us understand biological response and resilience to extreme climate forcings, providing insight into future extinctions.” (C. B. Senel et al., Nat. Geosci., 2023, doi:10.1038/s41561-023-01290-4.)
