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| Jonathan Chen, the study's lead author, examines foraminifera fossils under a microscope. Photo Credit: Northwestern University |
Scientific Frontline: Extended "At a Glance" Summary: Cretaceous Planktic Foraminifera Extinction
The Core Concept: A 113-million-year-old mass extinction event of marine planktic foraminifera was driven by severe ocean acidification caused by massive volcanic carbon dioxide emissions. Microscopic fossil evidence confirms that acidic surface waters dramatically reduced the ability of these organisms to build calcium carbonate shells.
Key Distinction/Mechanism: By measuring calcium isotope ratios within individual microfossils, researchers differentiated between the severe calcification stress experienced by surface-dwelling plankton and the milder impact on deep-sea benthic organisms, proving that atmospheric carbon dioxide acidified surface waters before reaching the ocean floor.
Origin/History: During the Early Cretaceous period, specifically at the Aptian-Albian boundary, the massive Kerguelen Plateau volcanic province erupted in the southern Indian Ocean, spewing vast quantities of carbon dioxide into the atmosphere.
Major Frameworks/Components:
- Isotope Geochemistry: The use of calcium isotope ratios as a geochemical proxy to measure historical biocalcification rates and physiological stress in shell-building organisms.
- Carbon Sequestration: The role of foraminifera in the natural carbon cycle, locking away carbon within external, solid calcium carbonate shells.
- Volcanic Forcing: The mechanism by which large igneous provinces, such as the Kerguelen Plateau, emit atmospheric carbon dioxide that subsequently dissolves into surface seawater and lowers its pH.
- Biocalcification: The biological process by which marine organisms construct shells from carbonate ions, a process significantly hindered by increased seawater acidity.
Branch of Science: Isotope Geochemistry, Paleoclimatology, Evolutionary Biology, Marine Biology, Earth Science and Planetary Science.
Future Application: Establishing a precise historical baseline for ancient ocean acidification provides a natural experiment, allowing scientists to model the potential magnitude of future marine ecosystem collapses caused by contemporary, human-generated carbon dioxide emissions.
Why It Matters: Planktic foraminifera act as a fundamental stabilizing force in Earth's carbon cycle. Understanding their historical vulnerability to sudden chemical shifts provides a stark warning about the fragility of modern marine carbon sinks and the ecosystems that depend on them.
A 113-million-year-old marine murder mystery may finally be solved.
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| Scanning electron microscope images of planktic foraminifera from before (bottom) and after (top) the Aptian-Albian extinction event. Clockwise, from upper left: Microhedbergella renilaevis, Microhedbergella miniglobularis, Hedbergella infracretacea, Hedbergella occulta, Paraticinella rohri. Image Credit: Huber and Leckie (2011)/Smithsonian National Museum of Natural History |
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 (\(\mathrm{CO_2}\)) into the atmosphere. As the oceans absorbed that \(\mathrm{CO_2}\), seawater became more acidic, making it more difficult for marine organisms to build and maintain their shells.
The new study, published in Science, 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 \(\mathrm{CO_2}\) can alter ocean chemistry and harm marine life. As current-day oceans absorb human-generated \(\mathrm{CO_2}\), 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 coadvised by study coauthors 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, \(\mathrm{CO_2}\) first entered the atmosphere and then dissolved into surface seawater. As the \(\mathrm{CO_2}\) 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 \(\mathrm{CO_2}\) 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 \(\mathrm{CO_2}\), 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 \(\mathrm{CO_2}\) 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.”
Funding: The study was supported by the National Science Foundation and the Paula M. Trienens Institute for Sustainability and Energy.
Published in journal: Science
Title: Calcium isotopes link ocean acidification to Aptian–Albian foraminiferal extinctions
Authors: Jonathan Chen, Andrew D. Jacobson, Brian T. Huber, Kenneth G. Macleod, Chuyan Wan, Anna R. Waldeck, Barbara Balestra, and Bradley B. Sageman
Source/Credit: Northwestern University | Amanda Morris
Edited by: Scientific Frontline
Reference Number: es073026_01