In a laboratory at Northwestern University, Jonathan Chen holds a brush no bigger than an eyelash. With painstaking precision, he sorts specks of sand under a microscope—each grain a fossil record of one of Earth's oldest cold cases.
These fossils, called foraminifera, are tiny organisms that build shells from calcium carbonate. Though smaller than a grain of sand, they played an enormous role in regulating Earth's climate. For more than 60 million years, they locked carbon away in their shells, acting as what scientists call a "fundamental stabilizing force in the carbon cycle."
Now, Chen and his team have solved a 113-million-year-old mystery: what killed roughly half of all foraminifera species during the Aptian-Albian extinction? Their answer, published in the journal Science, points to ocean acidification caused by a massive volcanic eruption.
During the Early Cretaceous, the Kerguelen Plateau—a volcanic province larger than Japan in the southern Indian Ocean—spewed enormous amounts of carbon dioxide into the atmosphere. As the oceans absorbed this CO2, the water became more acidic. For foraminifera, this was catastrophic. Acidic water makes it harder for organisms to build and maintain their calcium carbonate shells.
"By examining fossils, scientists already knew surface plankton were getting smaller and building thinner shells, which suggested they were under stress," Chen said. "But we didn't know that ocean acidification was responsible. 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."
The team spent months separating foraminifera fossils from ocean sediment samples—each fossil painstakingly sorted with a fine brush. By measuring calcium isotopes in these ancient shells, they found chemical evidence linking ocean acidification directly to the extinction. Chen calls it "the smoking gun."
This was no small event. The Aptian-Albian extinction ranks as the second-largest in foraminifera's evolutionary history, surpassed only by the asteroid impact that wiped out the dinosaurs. Surface-dwelling foraminifera nearly vanished, while their cousins living on the seafloor survived—something that puzzled scientists for decades.
"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."
The new findings explain that mystery: surface waters acidified faster than deep waters, sparing seafloor species while devastating surface populations.
The research carries urgent modern relevance. Today's oceans absorb about a quarter of human-generated CO2, and acidification is already affecting shellfish and coral worldwide. By studying ancient extinction events, scientists can better predict what lies ahead.
"If we didn't have this carbon sink anymore, our carbon cycle would be altered in unimaginable ways," Chen said. The question now is whether ocean life can adapt fast enough to survive our own chemical experiment.