For the first time, researchers have been able to show hard data that a large injection of sulfur impacted Earth's atmosphere after an asteroid strike caused the planet's mass dinosaur extinction 66 million years ago.
The study, published March 21 in the journal The Proceedings of the National Academy of Sciences, claims that sulfur isotopes "confirm a key role for atmospheric sulfur gases" in regards to climatic cooling, mass extinction and the demise of dinosaurs.
It was also suggested that sulfur produced during the earliest stages of the asteroid's impact and fallout was not limited to sunlight-blocking dust and soot—or a reduction of surface light for photosynthesis—but also led to sulfuric acid rain.
James Witts, a lecturer in paleontology in the School of Earth Sciences at the University of Bristol, is one of the study's co-authors. Witts told Newsweek that he and his fellow researchers originally set out to study the geochemistry of fossil shells—to obtain data like temperature and productivity—from sedimentary rocks deposited during the latest Cretaceous and early Paleogene time periods.
It led them to a site along the Brazos River in Texas, which at the time was underwater due to higher sea levels in the Cretaceous periods. Witts estimated the water level was probably 100 meters deep and 60 kilometers from the nearest coastline.
It's a site where he has personally conducted field work since 2017 "and
contains a really amazing record of the effects of the asteroid impact at Chicxulub," located about 1,500 kilometers from the Gulf of Mexico and the Yucatan Peninsula—where the mass extinction event occurred.
"At Brazos we can see evidence in the different rock units for processes that were caused by the impact itself, mass movement of sediment due to huge earthquakes, potentially tsunami waves, and then the fallout of debris from the atmosphere," Witts said.
He said his study colleague, Christopher Junium of Syracuse University, took samples of rocks at the site and later had them analyzed for sulfur isotopes.
Junium, an associate professor of Earth and Environmental Sciences, told Newsweek that climate change, or cooling, "has long been recognized as a central component of the extinction, and sulfur was suspected to play a central role" due to sulfur scattering incoming sunlight and causing cooling.
"The rocks in Texas have layers of the material that were blasted out of the impact crater that is on the north side of the Yucatan Peninsula," Junium said. "The particular spot along the Brazos River in Rosebud, Texas, is very well known amongst geologists for having the deposits associated with the impact."
He and another Syracuse professor, Linda Ivany, along with a few graduate students went to the site in June 2019 as part of another project. Witts joined them and provided insight due to his past excursions.
Junium said the team "had several goals, but doing sulfur analyses was not initially one of them."
Later that summer Junium visited the University of Saint Andrews in Scotland as a part of another collaboration with professors Aubrey Zerkle and Mark Claire—both co-authors of the study.
"We have worked together for a long time and the visit was kind of open ended," Junium said. "Dr. Zerkle is one of the world's experts on Earth's sulfur chemistry and had just set up some new instrumentation that allowed us to make these specific kinds of sulfur isotope analyses."
Junium brought different rocks on his journey and wanted Zerkle to study them, even though he did not expect to find anything unusual. COVID-19 lab lockdowns delayed testing and research.
Ultimately, the trip to Scotland was well worth it.
"To our great surprise, the sulfur in the Brazos River rocks had the unique isotope signatures that confirm that the sulfur from the impact spent a considerable amount of time in the stratosphere," he said.
Witts said that the hypothesis that sulfur played a big role in dinosaur extinction is not new; however, researchers are privy due to geological surveys of the bedrock underneath the Chicxulub crater that the asteroid "collided in the worst possible place," where rocks are marine limestones full of sulfur-rich minerals like gypsum.
He said the asteroid "vaporized" those rocks, "which would have liberated the sulfur into the atmosphere" and could have formed sulfur aerosols leading to "very severe cooling and the deposition of acid rain."
"But this was all theoretical.... We were lucky because the site at Brazos captures this unique time-window after the impact," Witts said. "The isotopic signature we found can only be generated by sulfur that has risen above the ozone layer and been exposed to UV radiation. This creates a really diagnostic signature in the stable isotopes of the sulfur gasses. This sulfur then rains back down onto the surface (or ocean in this case) in huge quantities, and was seemingly preserved in the rock record."
Both Witts and Junium said similar cases have occurred during large modern-day volcanic eruptions, such as Mount Pinatubo or Mount Tambora. But Junium noted that the difference in their research is that sulfur chemistry anomalies are typically not seen in rocks.
"For this to have happened there had to be an amazing amount of sulfur that was up in the stratosphere and the effect on climate would have been extreme and certainly played a central role in the extinction of the dinosaurs and the many other animals that didn't make it," Junium said.
He said the study provides "a clear understanding of the range of factors that affect climate," adding that beyond forecasting how Earth systems respond to excess carbon dioxide or asteroid impacts, geologists and climate scientists "have to look back into the deep geologic past during periods of past environmental upheaval to understand what we may experience in the future."
Witts said that since researchers know that sulfate aerosols are a really powerful climate-forcing agent, it has become more apparent how sulfur became a "kill mechanism" for the mass extinction event that killed off the dinosaurs and upwards of 75 percent of Earth's species at that time.
He and Junium said more analyses of other similar sites are necessary to properly understand the immediate impact of global cooling in situations such as these.
"This event is a sobering lesson because the rate of environmental change (decades to centuries) during the Cretaceous-Paleogene event is obviously very similar to what we are seeing today, and it was clearly very difficult for life to adapt to this leading to a mass extinction event," Witts said.
Newsweek reached out to Zerkle for comment.
