A 113-Million-Year-Old Ocean Mystery Points to Volcanic CO₂ and Acidification
A 113-million-year-old marine extinction may have been caused by a chemical crisis that has a striking modern parallel. By analyzing calcium isotopes preserved in microscopic fossils, scientists at Northwestern University have found compelling evidence that ocean acidification severely disrupted the ability of surface-dwelling plankton to build their shells. The ancient event, which occurred during the Early Cretaceous, was linked to enormous volcanic eruptions from the Kerguelen Plateau in the southern Indian Ocean. The findings offer one of the clearest geological records yet of how a rapid injection of carbon dioxide can destabilize ocean ecosystems.
The organisms at the center of the mystery were planktic foraminifera, single-celled marine organisms that drift near the ocean surface and construct intricate shells from calcium carbonate. Although each fossil is no larger than a grain of sand, foraminifera have an outsized role in Earth’s carbon cycle. When they die, their shells sink through the water column, transporting carbon into the deep ocean and sediments. This biological carbon pump helps regulate atmospheric carbon dioxide over geological timescales. The extinction at the boundary between the Aptian and Albian ages was the second most severe extinction in the evolutionary history of planktic foraminifera, surpassed only by the asteroid-associated catastrophe at the end of the Cretaceous.
The timing of the extinction had long suggested that ocean chemistry might have been involved, but a crucial piece of evidence was missing. Fossils showed that surviving foraminifera became smaller and developed thinner, weaker shells, signs that they had experienced severe physiological stress. Yet the apparent survival of benthic foraminifera on the seafloor complicated the picture. If acidification had affected the entire ocean equally, researchers asked, why did bottom-dwelling species fare better than their surface-dwelling relatives? The Northwestern team addressed this question by investigating the chemistry recorded inside the fossils themselves.
The researchers examined hundreds of fossilized foraminifera from sediment collected at a Deep Sea Drilling Project site on the Falkland Plateau in the South Atlantic. The location preserves an unusually complete sequence of sediment spanning the Aptian/Albian boundary. Jonathan Chen, the study’s lead author and a recent Northwestern graduate, painstakingly separated planktic and benthic specimens using a fine-tipped brush. He also removed secondary calcium carbonate that formed later in the sediment, isolating pristine shell material that could preserve the chemical conditions present when the organisms were alive.
The key evidence came from calcium isotopes, which are different forms of the same element distinguished by their atomic mass. As foraminifera calcify, the relative abundance of calcium isotopes incorporated into their shells depends partly on how rapidly shell formation occurs. Under favorable conditions, organisms build their shells efficiently and produce a characteristic isotopic signature. When acidification reduces the concentration of carbonate ions—the raw material required to form calcium carbonate—calcification becomes more difficult and slows down. That change alters the ratio of calcium isotopes preserved in the shell, allowing scientists to reconstruct biological stress millions of years after the organism disappeared.
Across the extinction interval, calcium isotope values in the planktic foraminifera rose sharply. The shift occurred at the same time as major declines in abundance, body size and shell robustness. Together, these signals indicate that the organisms were not simply being eliminated by a predator, temperature change or habitat loss; they were struggling to manufacture their shells. The isotope evidence provides what Chen described as the “smoking gun” connecting ocean acidification with the severe biocalcification stress that preceded the extinction. In contrast, benthic foraminifera displayed much smaller isotopic changes, suggesting that acidification was strongest in surface waters and less intense in the deeper ocean.
The researchers identify the Kerguelen Plateau as the likely source of the carbon dioxide. This enormous volcanic province covers approximately 1.2 million square kilometers, an area comparable to Western Europe. During the Early Cretaceous, repeated eruptions released vast quantities of CO₂ into the atmosphere. Because the eruptions broke through into the air rather than occurring solely beneath deep water, the gas first affected the atmosphere and then dissolved into surface seawater. Once absorbed by the ocean, CO₂ reacted with water to form carbonic acid, lowering pH and reducing the availability of carbonate ions. The result was an environment in which surface-dwelling shell builders could no longer calcify normally.
This sequence also helps explain the apparent refuge of benthic foraminifera. Atmospheric carbon dioxide entered the upper ocean first, creating a surface-focused chemical shock before the signal propagated into deeper waters. As surface-dwelling foraminifera declined, the dissolution and circulation of carbonate altered the ocean’s alkalinity. Some of that excess alkalinity moved through the water column, replenishing carbonate availability at depth and reducing the impact on benthic organisms. The contrast between the strong planktic isotope signal and the milder benthic response therefore records not only acidification, but also its vertical structure within the ancient ocean.
The study is the latest in a series of Northwestern investigations linking major volcanic episodes, ocean acidification and biological crises across deep time. Earlier work examined calcium isotopes in shells and plankton from the Cretaceous–Paleogene extinction, the Early Cretaceous, the Paleocene–Eocene Thermal Maximum and the Cenomanian–Turonian boundary. Across these very different intervals, the researchers have identified comparable chemical patterns. The new results strengthen the idea that large igneous provinces repeatedly injected enough carbon into the atmosphere to transform ocean chemistry and trigger ecological upheaval. They also demonstrate that calcium isotopes can serve as a geochemical proxy for extinction-related stress preserved in individual fossils and bulk sediments.
The ancient disaster carries an urgent warning for the present. Human activities are now increasing atmospheric CO₂ at a rate that is exceptionally rapid in the geological record, and the ocean is absorbing a substantial fraction of that carbon. Modern acidification is already measurable, with the greatest consequences expected for organisms that build shells or skeletons from calcium carbonate. The Cretaceous event does not predict an identical future extinction, because today’s ecosystems and rates of change differ. But it reveals how quickly a carbon-driven chemical disturbance can undermine shell formation, disrupt the marine carbon cycle and place surface ocean life at extraordinary risk.
Subject of Research: Ocean acidification, volcanic CO₂ emissions, calcium isotopes, marine extinction and foraminiferal shell formation.
Article Title: Calcium isotopes link ocean acidification to Aptian/Albian foraminiferal extinctions
News Publication Date: 30-Jul-2026
Web References: https://doi.org/10.1126/science.aed9359; https://news.northwestern.edu/
References: Science, “Calcium isotopes link ocean acidification to Aptian/Albian foraminiferal extinctions,” DOI: 10.1126/science.aed9359.
Image Credits: Jonathan Chen examines foraminifera fossils under a microscope. Credit: Northwestern University.
Keywords: Ocean acidification, climate change, carbon dioxide, volcanic eruptions, Kerguelen Plateau, calcium isotopes, foraminifera, marine extinction, Cretaceous period, ocean chemistry, paleoclimatology, geochemistry.
