Thomas: Carbon Impact—Not Volcanism—Key in Driving the Cretaceous Mass Extinction

(By Kayleigh Schweiker ’22)

As scientific study regarding the mass extinction of marine life during the Cretaceous era has progressed, theories including extraterrestrial impact and intense volcanism have surfaced. However, a recent study co-authored by Ellen Thomas, Harold T. Stearns Professor of Integrative Sciences, suggests that carbon impact—not volcanism—was key in driving the Cretaceous mass extinction.

In a paper titled “Rapid ocean acidification and protracted Earth system recovery followed the end-Cretaceous Chicxulub impact,” which was published in the Oct. 21 issue of the Proceedings of the National Academy of Sciences (PNAS), Thomas and her colleagues discuss how increases in ocean acidity played a driving force in the mass extinction of marine organisms. This mass extinction, labeled the “Crustaceous-Palogene die-off,” or the K-Pg event, led approximately 75% of plant and animal life on Earth to extinction. Though scientists have suggested that the presence of sulphuric acid proceeding the crash may have caused ocean pH levels to drop, Thomas and her team’s research on this topic reveals a different possibility.

The scientists on Thomas’s team examined foraminifera, a small plankton with a hard shell, to offer more answers as to how the mass extinction of marine life occurred post-meteorite. The team examined how the planktons’ chemical composition had evolved before, during, and after the Crustaceous-Palogene die-off.

Despite older studies suggesting that organisms containing a calcium carbonate shell were most susceptible to extinction during the die-off, Thomas’s study presents the idea that increased acidity levels in the ocean may have prevented these calcifiers from forming their shells after the meteor crash. As these calcifiers were vital in forming the foundational layer of the food chain during this era, the entirety of marine life suffered.

As Thomas writes, this meteorite’s crash into Earth caused an increase in the atmosphere’s carbon levels. As these carbon levels escalated, oceanic pH levels gradually dropped, causing an intensification of ocean acidity. This effect then presented complications for marine organisms. As acidification intensifies, Thomas states, “it takes more energy for organisms to make a calcium carbonate shell.” Thus, marine life that required a sturdy shell to sustain the ability to thrive underwater perished in an ocean with increased acidity levels.

This phenomenon is not unique to the Cretaceous era. During the Industrial Revolution, ocean pH levels dropped significantly, leading to a 30% increase in acidity levels. Though this acidification is reversible, Thomas notes that “humans aren’t really interested in that,” as the timeline for notable change spans from 10,000 to 100,000 years.

The paper has received attention from a number of media outlets, including the New York Times, The Guardian, alternet, and Yale News.