The impact of the Chicxulub asteroid 66 million years ago triggered a worldwide mass extinction and a global cold spell. So far, however, it has been disputed how large the proportion of the sulfur gases released during the impact was. Now, analyzes of sulfur isotopes in deposits from the time of the impact provide more information. They suggest that at that time a lot of sulfur gases must have risen to the stratosphere and formed aerosols there. These aerosols, which come from vaporized rock from the impact area and from fires, then spread around the world and provided a cooling effect that lasted up to 30 years, the team reports.
Around 66 million years ago, the impact of the Chicxulub asteroid in the Yucatan triggered a global mass extinction – and the demise of the dinosaurs. The impact released the energy of ten million Hiroshima bombs in fractions of a second and could have destroyed all life in a wide area within the first few hours and days. There is evidence that the impact triggered widespread fires and tsunamis in large parts of the world, and the impact also released large quantities of carbon dioxide, dust, soot and sulphurous gases. The latter in particular can form sulfur aerosols, which reflect part of the sunlight and thus have a cooling effect on the Earth’s climate. “This sulfate-induced cooling, sulphurous acid rain, and dimming of light available for photosynthesis have been proposed as primary causes of the post-impact mass extinction,” explain Christopher Junium of Syracuse University in New York and his colleagues.
Searching for clues in impact deposits
So far, however, it has been disputed how much sulfur was released during the catastrophe at the end of the Cretaceous – estimates range from 30 to 540 gigatonnes. It is also unclear how high these sulfur emissions rose in the atmosphere. “Prolonged cooling only occurs when the aerosols form in the stratosphere, where they can remain for years to decades,” the team explains. If, on the other hand, the sulphur-containing gases only rise to the lower atmosphere, the troposphere, they are quickly washed out again by rain and other precipitation and therefore only last for days or weeks. While model simulations have already suggested that significant outgassing from the Chicxulub impact also reached the stratosphere, direct evidence was scarce.
Junium and his colleagues have therefore examined some deposits formed before, during and after the impact on the Brasos River in Texas for possible traces of the impact-related sulfur emissions. These rock formations lie about 1,300 kilometers from the Yucatan impact site in an area that was a shallow marine lagoon at the end of the Cretaceous period. “The Brasos River sediments represent an exceptionally well-preserved and continuous sequence of strata across the entire Cretaceous-Paleogene boundary,” the researchers explain. The rock glass beads that fell after the impact, but also traces of the tsunami can be clearly seen in the rock formations. For their study, the scientists subjected rock samples to isotope analysis, in which they determined the relative proportions of the sulfur isotopes 33S, 34S and 36S.
Sulfur signature of the impact
The analyzes showed that in the sediment layers deposited after the impact, the proportion of the heavier sulfur isotope 34S was higher than that of the lighter 33S: “The 34S enrichment of an average of +7.6 parts per thousand within the ejecta materials is clearly based on the values of the surrounding layers,” report Junium and his colleagues. As they explain, the isotope ratio suggests that this sulfur does not come from biological and biogeochemical processes, but primarily from the rocks around the Chicxulub crater that were vaporized during the impact. According to the researchers, sulfur gases from the extensive fires after the impact may have made a further contribution. From the presence of these sulfur signatures immediately after the impact, but also far beyond and from the distance of the Brasos River to the Chicxulub crater, they also conclude that these sulfur gases must have been transported through the stratosphere to this deposition site.
“The sulfur isotope signature provides us with the geochemical fingerprint of stratospheric sulfate aerosols that arose from sulfur dioxide released during impact and/or biomass combustion,” the scientists explain. This confirms the results of theoretical models and supports the assumption that the Chicxulub impact produced an aerosol veil in the stratosphere that dimmed sunlight and cooled the climate for years. “We estimate that the production of the sulfur aerosols and their precipitation may have continued for around 30 years and were distributed globally,” write Junium and his colleagues. The impact of the Chicxulub asteroid could have triggered a longer cold period than previously assumed. “Our data provide direct evidence for the long suspected primary role of sulfate aerosols in the post-impact winter and global mass extinction event,” the team said.
Source: Christopher Junium (Syracuse University, New York) et al., Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2119194119