Approximately 66 million years ago, life on Earth changed forever.
A massive asteroid hit the planet, leaving behind the 93-mile-wide Chicxulub crater along Mexico’s Yucatan Peninsula and sending up enormous quantities of dust and volcanic rock that blocked out the Sun. In the asteroid’s aftermath, more than 70 percent of all species on Earth at the time were wiped off out, but new mammals also emerged, giving rise to modern life as we know it.
Among the animals wiped out during the Cretaceous–Paleogene (K–Pg) mass extinction event were of course the dinosaurs. The dominant theory among paleontologists is that the asteroid was likely the primary cause of the dinosaur extinction. But in recent years, theories have emerged about other contributing factors, like climate change.
According to new research published Monday in the journal Proceedings of the National Academy of Sciences, we now have even further proof that climate change — specifically, climate cooling — likely played a significant role in the dinosaur’s demise.
What’s new — In the wake of the asteroid’s impact at Chicxulub, vaporized rocks were ejected into the atmosphere, forming plumes containing sulfuric gases — known as sulfate aerosols.
“The rocks of the Yucatan where the impact occurred were rich in sulfur, but it’s unclear what happened to all the sulfur that was thrown into the atmosphere, and how it might have affected the climate,” Junium and Zerkle say.
However, scientists long suspected these sulfate aerosols — which can cool the climate — resulted in a post-asteroid global “winter” or mass climate cooling event.
The initial asteroid impact almost certainly killed many dinosaurs through earthquakes, tsunamis, fires, and soot that blocked out the Sun. But it’s also possible that climate change from this freezing global winter caused the extinction of any dinosaurs that happened to survive the cold period, along with three-quarters of the remaining species on Earth. These sulfate aerosols could have cooled the planet for years after the asteroid impact event.
According to Junium and Zerkle “the longer-term effects on the environment would ultimately have determined the severity of the extinction.”
Yet, it’s been difficult to establish concrete proof linking the sulfate aerosols to the Chicxulub event.
“Our new data provide the first direct evidence that sulfate aerosols were abundant in the post-impact atmosphere where they remained for months to years after the impact,” Junium and Zerkle say.
In fact, it seems sulfur lingered in the atmosphere after the asteroid impact event for far longer than previously thought, providing support for the idea that a global winter may have dealt the final blow to dinosaurs.
“Our results extend that window of time over which the environmental upheaval would have occurred and provide an additional, longer-term climate driver for the extinction,” Junium and Zerkle say.
Why it matters — The findings are big news for the paleontological community, which has long debated the exact nature of the dinosaur’s extinction.
“The asteroid impact has been viewed as the primary cause of the extinction by most of the paleontology community for some time, but details of exactly how the environment was affected have remained elusive,” Junium and Zerkle say.
By providing hard evidence of the sulfuric gases in the atmosphere after Chicxulub, the scientists can better understand how climate change may have played a role in the dinosaur’s downfall. Their findings confirm that sulfate aerosols were an “integral component of the post-impact winter that was the proximate cause of the global mass extinction at the K-Pg boundary,” according to the paper.
But it’s also big news for anyone seeking to understand how the mass extinction event shaped the evolution of Earth’s mammals — which, ultimately, includes humans.
“The extensive global winter that resulted could have played a central role in the extinction of the dinosaurs, and allowed for the subsequent diversification of mammals, which had previously served mainly as lunch for large predators,” Junium and Zerkle say.
How they made the discovery — The scientists analyzed sediment samples from rocks along Texas’ Brazo River, which is roughly 800 miles from the Chicxulub impact site in Mexico. The rocks contained circular white remnants or “spherules” of the plumes from the vaporized asteroid rocks, which condensed in the atmosphere and fell to Earth after the asteroid’s impact.
The rock samples, which date back to the Upper Cretaceous and lower Paleogene era, would allow the researchers to measure sulfur isotopes — different atomic forms of the same element — that were deposited from the asteroid impact event.
Using statistical models to estimate the concentrations of sulfur with abnormal isotope distributions in the rocks, the researchers could rule out other potential sources for the sulfur, such as natural geochemical cycles moving sulfur through water and rocks.
The researchers were left with only one plausible explanation: atmospheric sulfur gases. Effectively, the scientists confirmed the sulfur in the rocks could only have come from the persistence of sulfate aerosols — which were flung into the atmosphere due to the asteroid impact — over months or years.
What’s next — Further study of sulfur in sediment samples from the K-Pg mass extinction event will surely reduce uncertainties and yield more details about the impact of sulfate aerosols on the environment during the tumultuous post-asteroid period.
“We are hopeful that similar results will be found elsewhere and can be incorporated into climate and atmospheric models of the post-impact conditions that will help us better understand how and why mass extinctions occur,” the lead authors conclude.