Ocean acidification causes biocalcification stress. This study synthesizes morphometric observations of planktic foraminifera with multi-archive calcium isotope records from Gubbio, Italy and the Western Interior Seaway spanning Cretaceous Ocean Anoxic Event 2 (-94 million years ago). Calcium isotope ratios increased ~60 thousand years prior to the event, coinciding with foraminiferal abnormalities and correlating with existing proxy records for carbon dioxide release during large igneous province volcanism. Decreasing calcium isotope ratios during the event indicate ocean alkalinization, potentially shifting air-sea carbon dioxide partitioning and explaining the Plenus Cold Event. These findings offer implications for refining ocean alkalinity enhancement strategies.

Gabriella D. Kitch, Andrew D. Jacobson, Bradley B. Sageman, Rodolfo Coccioni, Tia Chung-Swanson, Meagan E. Ankney, Matthew T. Hurtgen