Ocean acidification wiped out surface plankton 113 million years ago

One hundred and thirteen million years ago, the ocean's smallest architects stopped building. Planktic foraminifera—single-celled drifters no wider than a grain of sand—began constructing shells that were thinner, smaller, and fundamentally different from those their ancestors had made. Within a geological blink, many species vanished from the fossil record entirely. Yet in the mud of the seafloor below, their relatives carried on almost as if nothing had happened.

This story emerges from a single drill hole punched into the Falkland Plateau in the southern South Atlantic, where researchers from Northwestern University, the National Museum of Natural History, and the University of Missouri extracted and analyzed shells spanning the boundary between two geological ages: the Aptian and the Albian. What they found was a crisis written in calcium.

Planktic foraminifera are not obscure creatures. They still drift through every ocean today, and they matter far more than their size suggests. Marine life produces about 5.8 billion tons of calcium carbonate annually, and these tiny organisms may account for half of it. Their shells are engines of the carbon cycle, pulling carbon dioxide from the atmosphere and locking it into the seafloor. When they stopped building shells at full strength, something fundamental shifted in how the ocean worked.

The evidence lies in isotopes—different weights of the same element. Calcium comes in heavier and lighter forms, and the ratio an organism incorporates into its shell depends on how quickly it builds. Slow construction favors the heavier isotope. When Jonathan Chen's team measured the calcium in shells from before and after the boundary, they found something extraordinary: the shift in isotope ratios was six to seven times larger than any other ocean acidification event previously documented in the geological record. The shells themselves told the same story. They shrank. Fewer species survived. What had been a diverse community of large, thick-walled builders became a sparse population of small, thin-walled survivors.

The culprit was volcanic. The Kerguelen Plateau, a submarine mountain range the size of a small continent in the southern Indian Ocean, erupted massively during this period. Much of that carbon dioxide vented into the air rather than directly into the water—a crucial detail. When CO2 enters the atmosphere, it acidifies the ocean from the surface downward, not from below. The upper waters turned hostile to shell-building first. The deep ocean, initially spared, remained relatively stable.

But the plankton's collapse created an unexpected buffer. Building calcium carbonate shells requires alkalinity—the ocean's natural acid-neutralizer. When surface plankton sharply reduced their shell production, alkalinity that would normally have been stripped from the water remained in place. Over time, this alkalinity circulated downward, providing additional buffering capacity for the deep ocean and allowing benthic foraminifera to weather the crisis with minimal change. The seafloor was not entirely spared; some bottom-dwelling species shifted toward building shells from sediment grains rather than calcite. But the contrast was stark: the surface world transformed; the deep world endured.

Today's ocean is acidifying again, and it is following the same surface-first pattern. Andrew Jacobson, one of the study's senior authors, noted that acidity has risen about 30 percent in the past two centuries. The surface has already crossed a threshold that researchers had designated as a limit. The ancient record offers no comfort. When Chen's team examined calcium isotope changes at other extinction intervals—including the moment when an asteroid struck 66 million years ago—those records appeared strikingly similar to what they found at the Aptian-Albian boundary. If that pattern holds, the asteroid that killed the dinosaurs hit an ocean that was already in crisis, already weakened by acidification. The question now is whether modern life will prove as resilient as the foraminifera of the deep seafloor, or as vulnerable as those that drifted in the light.