Tiny fossils crack a Cretaceous cold case

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Tiny fossils crack a Cretaceous cold case

Ancient ocean acidification triggered one of the ocean's most severe extinctions

EMBARGOED UNTIL 2 P.M. EDT (U.S.) ON THURSDAY, JULY 30, 2026

• Calcium isotopes in fossilized shells show ocean acidification caused the extinction event
• Massive volcanic eruptions released CO2 into the atmosphere, acidifying seawater
• Fifth Northwestern study to link volcanic eruptions to ocean acidification and extinction
• Ancient events help scientists understand current CO2 increases and potential consequences

EVANSTON, Ill. - A 113-million-year-old marine murder mystery may finally be solved.

Using chemical clues locked inside microscopic fossils, Northwestern University scientists found evidence that ocean acidification drove one of the largest extinction events in the history of planktic foraminifera - tiny shell-building organisms that help regulate Earth's carbon cycle.

The scientists attribute the acidification to the eruption of the Kerguelen Plateau, a massive volcanic province in the southern Indian Ocean. During the Early Cretaceous period, the volcanic province spewed vast quantities of carbon dioxide (CO2) into the atmosphere. As the oceans absorbed that CO2, seawater became more acidic, making it more difficult for marine organisms to build and maintain their shells.

The study will be published on Thursday (July 30) in the journal Science.

The new study marks the fifth Northwestern-led investigation to link widespread volcanic eruptions to ocean acidification and extinction, strengthening evidence for a recurring pattern that played out across more than 60 million years.

The findings also provide one of the clearest examples yet of how rising CO2 can alter ocean chemistry and harm marine life. As current-day oceans absorb human-generated CO2, the ancient events offer a natural experiment for understanding how modern ocean acidification might affect shell-building organisms and the ecosystems that depend on them.

"By examining fossils, scientists already knew surface plankton were getting smaller and building thinner shells, which suggested they were under stress," said Northwestern's Jonathan Chen, who led the study. "But we didn't know that ocean acidification was responsible. By measuring the fossils' calcium isotopes, we finally provided that missing evidence. We found a giant increase in calcium isotope ratios right as the extinction unfolded, indicating the organisms' shells were calcifying at a much slower rate. That was the smoking gun linking ocean acidification to the severe biocalcification stress that ultimately led to their extinction."

Chen is a recent graduate from Northwestern, where he was co-advised by study co-authors Brad Sageman and Andrew D. Jacobson, who are both professors of Earth, Environmental and Planetary Sciences at Northwestern's Weinberg College of Arts and Sciences.

A Cretaceous crisis

At the boundary between the Aptian and Albian ages within the Early Cretaceous period, the second largest extinction event in the history of planktic foraminifera swept through the oceans. Foraminifera are microscopic organisms that build external shells from calcium carbonate, a solid form of carbon. By locking away carbon inside their shells, foraminifera play a key role in natural carbon sequestration.

"They act as a fundamental stabilizing force in the carbon cycle," Chen said. "If we didn't have this carbon sink anymore, our carbon cycle would be altered in unimaginable ways."

Across foraminifera's evolutionary history, only the asteroid-triggered extinction at the end of the Cretaceous surpassed the devastation of the Aptian/Albian boundary event. The surviving species became smaller and developed thinner, less robust shells. But, while the extinction event largely wiped out planktic foraminifera living on the oceans' surface, it spared benthic foraminifera living on the seafloor.

"For decades, scientists proposed that maybe ocean acidification caused the extinction," Chen said. "But ultimately, many scientists thought that wasn't possible because organisms on the seafloor weren't affected."

Calcium clues

To explore what caused the Aptian/Albian event, the Northwestern team examined hundreds of fossilized specimens collected from sediments spanning the extinction interval. Previous researchers collected the sediment samples in the 1980s from a Deep Sea Drilling Project site on the Falkland Plateau in the South Atlantic. Located off the southern coast of Argentina, the site preserves an unusually complete record of the Aptian/Albian boundary.

After obtaining drilled samples from the Smithsonian Institution, Chen used a fine-tipped brush to gently sort planktic and benthic foraminifera from the sediment. He also separated pristine shell material from secondary calcium carbonate. Because each fossil is the size of one grain of sand, the process was painstaking and time consuming, Chen said.

From there, the team analyzed the samples' calcium isotopes, or different forms of calcium preserved in the carbonate shells. When foraminifera build shells quickly under favorable conditions, their shells preserve a specific calcium isotope signature. But when ocean chemistry makes the calcification process more difficult - such as during acidification - shell-building slows, and the isotope signature shifts.

Death by chemistry

The results were striking. Across the extinction interval, calcium isotope values in planktic foraminifera increased dramatically, indicating a major reduction in calcification rates. That shift aligned with known declines in planktic foraminifera populations, shell size and shell structure - clear signs of stress before and during the extinction event.

"Not only did planktic foraminifera decrease in abundance but also in size," said Jacobson, who is an expert in isotope geochemistry. "They get smaller, and the most logical reason for that is because they grew less quickly. Calcium isotopes are sensitive to that. As the shells get smaller, their calcium isotopes change in a way that's consistent with slower formation under conditions that are less favorable for growth."

Benthic foraminifera, on the other hand, experienced much milder shifts in calcium isotopes. That suggests the deeper ocean did experience acidification but not as severely as the surface. According to previous studies, the Kerguelen Plateau erupted multiple times before the mass extinction. At 1.2 million square kilometers, the plateau is enormous - roughly the size of Western Europe.

When the massive volcanic province erupted, CO2 first entered the atmosphere and then dissolved into surface seawater. As the CO2 accumulated, it lowered the water's pH level, reducing the availability of carbonate ions - the chemical building blocks foraminifera use to make their shells.

"Many large igneous provinces are submarine," Sageman said. "When they erupt, they are beneath kilometers of water. But this one spewed into the air."

"Because the CO2 entered the atmosphere first, it hit the surface waters and created ocean acidification before it could reach the deeper ocean," Chen added. "That wiped out the planktic foraminifera, so they couldn't make their calcite shells anymore. The excess alkalinity circulated through the water column, making it available for benthic foraminifera living on the seafloor. That saved the benthics from being affected too badly."

A recurring pattern through deep time

The study marks the latest discovery in a series of investigations led by Jacobson and Sageman. In 2019, the team studied the calcium isotope composition of fossilized clam and snail shells from the Cretaceous/Paleogene mass extinction event. In 2020, they analyzed calcium and strontium isotope abundances in nannoplankton fossils from the Early Cretaceous. In 2021, the team studied calcium isotopes in shells from the Paleocene-Eocene Thermal Maximum, a period of abrupt global warming and ocean acidification that occurred 56 million years ago. And, in 2023, they analyzed malformed fossils from the Cenomanian/Turonian boundary during the Late Cretaceous. This work was initiated with support from the Paula M. Trienens Institute for Sustainability and Energy.

Although the Northwestern team collected samples from various time periods and vastly different locations around the globe, they consistently pieced together parallel stories based on the similar calcium isotope signals. Massive volcanic eruptions caused ocean acidification, which led to extinctions.

"We've applied these tools at a different number of places where there was reason to suspect ocean acidification occurred," Sageman said. "We're seeing a pretty consistent pattern in the data. Now this is the first study to very conclusively show that the signal is consistent from the chemistry recorded inside the individual shells all the way to the bulk sediment. That really puts the nail in the coffin of any doubt that we're seeing a primary signal - a record of an event that happened at the time and was directly linked to a major extinction."

"We've taken calcium isotopes to the level where we can use them as a geochemical proxy for death in the rock record," Jacobson said. "That's a really big advancement that our field has been waiting for."

While these events occurred in an ancient greenhouse world, they are still relevant for today's world. As human activities rapidly increase atmospheric CO2, the ocean absorbs it and acidifies. The new study suggests that surface-dwelling, shelled organisms may be especially vulnerable to changes in ocean chemistry.

"We know ocean acidification is already happening as a consequence of human-made CO2 increases," Sageman said. "It's measurable in our oceans. Studying these past events gives us a sense of the range of variance. In this case, we see that a significant extinction event occurred as a consequence of this process, so that allows us to evaluate the potential magnitude of what could happen in the future."

The study, "Calcium isotopes link ocean acidification to Aptian/Albian foraminiferal extinctions," was supported by the National Science Foundation and the Paula M. Trienens Institute for Sustainability and Energy.

Study photos

Jonathan Chen

Lead author

Recent graduate

• [email protected]

Andrew D. Jacobson

Senior author

Professor of Earth and planetary science
Director of Graduate Studies
Faculty Affiliate, Paula M. Trienens Institute for Sustainability and Energy

• [email protected]

Brad Sageman

Senior author

Professor of Earth and Planetary Sciences

• [email protected]