Ocean Acidification Resulting from High Atmospheric CO2 Levels Via Volcanic Eruptions Implicated in the Early Cretaceous Aptian-Albian Boundary Extinction Event

     Calcium isotope ratios (44Ca/40Ca) in the calcium carbonate shells of planktic foraminifera, a single-celled microscopic organism common in coastal and deep-sea environments, were used to determine the amount of CO2 and carbonate saturation in seawater. It was determined that those levels were elevated during the early Cretaceous at the Aptian-Albian age boundary when there was a significant mass extinction event of planktic foraminifera.

     The results were reported in the journal Science. About three-quarters of these species went extinct during the event. This is thought to have occurred after volcanic eruptions emitted large quantities of CO2 into the atmosphere. Much of the CO2 was absorbed by the oceans, which led to a lowering of the pH of seawater, meaning it became more acidic.

     According to Science Advisor:

“The acidity spurs chemical reactions that erode the calcium carbonate shells of marine organisms and reduce the amount of calcium available for repair. To test whether this sparked the planktic foraminifera mass extinction, a team of paleontologists analyzed the chemistry and structure of fossils recovered from a rock core drilled off the coast of Argentina. They found that the shells of affected foraminifera contained high concentrations of heavy calcium isotopes that marine organisms usually avoid. In addition, species that evolved after the extinction event had small, simple shells lacking the structural diversity found in older fossils. Both results suggest that calcium was at a premium, a hallmark of a more acidic ocean.”

     The study may have implications for studying modern-day ocean acidification.

     

 

References:

 

Calcium isotopes link ocean acidification to Aptian–Albian foraminiferal extinctions. Jonathan Chen, Andrew D. Jacobson, Brian T. Huber, Kenneth G. MacLeod, Chuyan Wan, Anna R. Waldeck, Barbara Balestra, and Bradley B. Sageman. Science. 30 Jul 2026. Vol 393, Issue 6810. pp. 527-531. Calcium isotopes link ocean acidification to Aptian–Albian foraminiferal extinctions | Science

Witness to ocean acidification: Calcareous marine plankton can indicate past and present ocean acidification. R. Mark Leckie. Science. 30 Jul 2026. Vol 393, Issue 6810. Witness to ocean acidification | Science

Acidic oceans doomed shelled microorganisms. Science Advisor. July 31, 2026.