Microscopic Fossils Reveal the Culprit of an Ancient Extinction Is Still at Large

Chemical clues preserved in tiny shells reveal that ocean acidification doomed a group of planktonic organisms that have outsized impact on climate

The early Cretaceous was a bad time to be a floating foraminifera. These microscopic, shell-building organisms experienced a major extinction around 113 million years ago that wiped out most of the planktonic ‘forams’ that lived near the surface. This was bad news for the rest of the ocean—these miniscule organisms absorb carbon to build their shells, giving them an outsized impact on Earth’s climate.

Now scientists have discovered the “smoking gun” behind this sudden die-off. A team of researchers recently sifted through sediment samples in the National Museum of Natural History’s collection that contain fossilized foram shells that were deposited around the extinction event. In a paper published last month in the journal Science, the scientists analyzed the chemical composition of the shells and concluded that this ancient extinction was caused by ocean acidification, an environmental phenomenon that still plagues Earth’s oceans.

“The results reveal that ocean acidification caused planktonic forams to calcify their shells at a much slower rate,” said Jonathan Chen, the paper’s lead author who began the work as an undergraduate intern at the museum. “Because they are having a harder time building their shells, a lot of them ended up going extinct.”

The new paper is the culmination of years of work for Chen, who graduated from Northwestern University in Illinois this past spring. During his childhood in Iowa, Chen became interested in the state’s rich assemblage of ancient shells, corals and other invertebrate fossils. He was fascinated that these fossils preserved detailed clues to what Earth’s climate was like millions of years ago.

When he arrived at Northwestern, he made a beeline for the Earth Sciences department and asked about opportunities to pursue paleontology. There, geology professor Brad Sageman got him in touch with Brian Huber, the museum’s curator of foraminifera, who was happy to share more information on forams.

According to Huber, once a young scientist “drinks the foram Kool-Aid” they often end up specializing in the environmental-focused fields of paleontology like paleoclimatology or paleooceanography. “I’ve witnessed so many students express their surprise and excitement when they first see the tiny shells scattered among sediments and then find out that they are so valuable for reconstructing Earth’s history,” he said.

Chen was no different and focused on forams during a Natural History Research Experiences (NHRE) internship with Huber in the summer of 2024. He used a paintbrush with just a few hairs to pick the sand grain-size fossils out of sediment cores and examined them under the microscope. He found no shortage of fossils to study in the Smithsonian’s sprawling foraminifera collection. “These are organisms whose shells literally rain down onto the seafloor,” Chen said. “In any other branch of paleontology, your main problem is always sample size. But forams present the closest thing we have to a complete fossil record because there are so many of them.”

Some of the samples he investigated during his internship came from cores collected in 1980 in the southern Atlantic Ocean. These cores contain sediment that dates back around 113 million years, which marks the boundary between two ages of the early Cretaceous period. The transition between these ages, known as the Aptian and Albian, saw the second largest extinction event of planktonic forams in Earth’s history.

In the new paper, Chen, Huber and their colleagues aimed to determine what caused this extinction by measuring the chemical composition of fossilized foram shells across the Aptian/Albian boundary. Foraminifera build their shells with calcium carbonate, the same compound found in limestone and chalk. But they can use different isotopes, or variants, of the element calcium from seawater to craft these structures. For example, some shells contain more calcium-44, a heavier version of the element, while others contain a higher concentration of the lighter isotope calcium-40.

Thanks to past research by coauthors Sageman and Andrew Jacobson, two of Chen’s professors at Northwestern, the team could use the concentrations of these calcium isotopes to recreate what environmental conditions were like in the Cretaceous ocean some 113 million years ago. For example, higher concentrations of calcium-40 are markers of faster shell-building while shells with higher concentrations of calcium-44 point to slower calcification rates, which often occur as a consequence of ocean acidification.

The team measured the calcium isotopes in fossilized foram shells from both before and after the extinction event. They discovered that planktonic foram shells around the boundary contained higher concentrations of calcium-44 compared to shells from older sediments. These shells were also smaller and thinner than the older specimens of extinct forams. Collectively, the evidence confirms that Cretaceous oceans were acidifying at this time.

"Paleontologists like to say that ‘small fossils paint big pictures.' In the case of forams, that’s definitely true.” 
— Jonathan Chen, Former NHRE Intern at the NMNH

The researchers hypothesized that the ocean acidification event around the Aptian/Albian boundary was caused by eruptions across the Kerguelen Plateau, a massive stretch of bubbling magma located in the southern Indian Ocean. The peaks of some of the plateau’s volcanoes poked above the waves, spewing carbon dioxide (CO2) gas into the Earth’s atmosphere.

As the surrounding oceans absorbed this glut of CO2 clogging the atmosphere, seawater became more acidic. This, in turn, made it more difficult for foraminifera to build their shells. Benthic forams living near the seafloor were also impacted, but they did not experience a major extinction like planktonic forams. Chen posits that this is because the CO2 gas from the volcanic eruptions filtered through the surface waters first before descending to deeper depths. As their counterparts in shallower water died out, benthic forams utilized the leftover alkalinity to buffer themselves against acidity. “The planktonic forams were sacrificed in a sense, which allowed other forams to buffer themselves against the acidification,” he said. “This ocean acidification event reveals that there were different consequences for organisms living in the surface ocean and the deep ocean.”

And this was far from a one-off event. According to Huber, there have been multiple other episodes in the geologic record where massive volcanic eruptions caused global concentrations of atmospheric CO2 to spike. And he adds that scientists are observing similar trends in the shells of living forams compared to the fossils from the Aptian/Albian boundary. “The ocean has absorbed massive amounts of CO2 since the beginning of the industrial age and this increase in ocean acidification is beginning to affect foraminifera and other marine calcifying organisms,” he said.

Chen thinks that the best way to determine how forams today will fare is to examine how their ancestors responded to ocean acidification in the past. He will continue studying foram fossils this fall as he begins his PhD at Princeton University. He’s excited to uncover the insights preserved inside the chemistry of their shells.

“Paleontologists who study microfossils like to say that ‘small fossils paint big pictures,’” Chen said. “In the case of forams, that’s definitely true.”

Jack Tamisiea | | Read More

Jack Tamisiea is a Science Communications Specialist at the Smithsonian's National Museum of Natural History. In addition to covering all things natural history for the museum's blog, Smithsonian Voices, he tracks media coverage and coordinates filming activities for the museum's Office of Communications and Public Affairs. Jack recently completed his masters in science writing at Johns Hopkins University and his writing has appeared in the New York Times, Scientific American, National Geographic and other science-focused publications. In his free time, he loves exploring the outdoors with a sketchbook and camera. You can read more of Jack's work at https://jacktamisiea.com.