Ocean Acidification

Foraminifera inhabit coastal and deep-sea marine environments. These microscopic, single-celled organisms are widely used as proxies for relative age determination and reconstructing past climates. The ratio of calcium isotopes (⁴⁴Ca/⁴⁰Ca) in their calcium carbonate shells depends on the amount of carbon dioxide (CO₂) and carbonate saturation in seawater at the time of the shells’ formation. On page 527 of this issue, Chen et al. (1) report geochemical evidence that a rise in atmospheric CO₂ concentration and ocean acidification at the end of the Aptian Stage [~113 million years ago (Ma)] may have caused the largest extinction of planktic foraminifera of the Cretaceous Period (146 to 66 Ma) (2–4), which is second only to the extinction event at the end of the Cretaceous Period (66 Ma) (5–7). The findings could be important for understanding potential effects of present-day increases in atmospheric CO₂ levels on marine organisms.

Paraticinella rohri collected from the South Atlantic is a species of planktic foraminifera that went extinct at the Aptian–Albian boundary. IMAGE: HUBER AND LECKIE (3)

Seafloor-dwelling benthic foraminifera appeared more than 500 million years ago. It took at least 350 million years for them to evolve into planktic species that float in the upper ocean (8–10). The earliest species of planktic foraminifera lacked morphological diversity and were minor constituents of marine sediments. The first major morphological diversification of planktic foraminifera occurred (2, 11) during the Aptian Stage (125 to 113 Ma) of the mid-Cretaceous Period. New species of planktic foraminifera arose, with elongated chambers (Leupoldina) and larger, more heavily calcified shells (Globigerinelloides and Hedbergella), and the first species with a peripheral keel (Pseudoplanomalina cheniourensis) appeared by the late Aptian. Large increases in atmospheric CO₂ concentration caused by massive volcanic eruptions triggered substantial depletion of oxygen in the oceans, including Oceanic Anoxic Event 1a and other similar events. Atmospheric CO₂ likely drove warming of the atmosphere and water; warmer waters hold less dissolved oxygen compared to cooler water.

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Leckie R. M., 2026. Witness to ocean acidification. Science 393: 462-462. Article.