Isotope evidence weakens the role of volcanism in extinction event

One of Earth’s largest extinction events occurred 66 million years ago. It’s known as the Cretaceous–Paleogene (K–Pg) boundary, and 75% of the planet’s species perished, including dinosaurs.

Convincing evidence for an asteroid impact at that time has been found at the Chicxulub Crater on the Yucatán peninsula in Mexico (see Physics Today, April 2021, page 64 ). The asteroid impact spewed sulfur gas into the atmosphere. Once aloft, the gas rapidly formed sulfate aerosols that reflected sunlight and consequently cooled global surface air temperature by 2–8 °C.

Another possible cause of or contributor to the mass extinction is the contemporaneous eruption of what’s known as the Deccan Traps—the large igneous province of volcanic rock on the west-central Indian subcontinent. In addition to emitting significant volumes of carbon dioxide, the eruption also added sulfur to the atmosphere. Most flora and fauna weren’t able to survive such a significant change to Earth’s climate.

To determine whether the climate-altering sulfur came from Chicxulub or the Deccan Traps, Christopher Junium of Syracuse University, Aubrey Zerkle of the University of St Andrews, and their colleagues carefully analyzed the isotopic composition of sulfur in ejecta materials that were deposited near the Chicxulub Crater. They found that the sulfur-isotope anomalies are most consistent with sulfur forced high into the stratosphere from a meteorite impact rather than from volcanic activity.

The samples that the researchers analyzed are from the US Gulf Coastal Plain, some 1300 km from the Chicxulub Crater. They were dated to the K–Pg boundary and come from well-preserved marine mud rocks filled with bits of meteorite debris.

In isotope geochemistry, Earth processes often separate one isotope from another according to their mass differences. (Evaporation, for example, preferentially removes the lower-mass oxygen-16 atoms from seawater over oxygen-18.) In contrast, Junium, Zerkle, and their colleagues examined the mass-independent fractionation of sulfur isotopes, which is controlled by biological, photochemical, and thermochemical processes. Sulfur mass-independent fractionation (S-MIF) has been used for years to learn more about when Earth’s atmosphere acquired oxygen (see “New and old ideas about Earth’s oxygen history,” Physics Today online, 24 April 2014 ).

In the samples at the K–Pg boundary, the researchers found significant negative S-MIF anomalies relative to normal marine sediments, including those deposited before and after the impact. No telltale signs of biological or thermochemical reactions were present in the samples. That left the researchers to conclude that the S-MIF anomalies must have been caused by gas-phase photochemical reactions. In the stratosphere, photolysis of sulfur dioxide produces S-MIF anomalies at a magnitude consistent with the samples.

According to the researchers, S-MIF is unlikely to have been the product of the Deccan Traps eruption. That type of volcano typically spews material no higher than the troposphere, not quite high enough in the atmosphere to produce the photochemical reactions necessary for S-MIF. (C. K. Junium et al., Proc. Natl. Acad. Sci. USA 119, e2119194119, 2022 .)