Asteroid impact acidified the prehistoric ocean

Sixty-six million years ago, an asteroid struck the earth, initiating mass species die-offs. Scientists have long believed that the impact’s marine extinctions were due to ocean acidification, but they never had physical evidence for that hypothesis—until now. A new study uses boron isotopes in tiny plankton shells to reconstruct the pH of the oceans around the time of the impact, providing evidence for a rapid acidification event that disrupted the marine ecosystem for hundreds of thousands of years (Proc. Natl. Acad. Sci. U.S.A. 2019, DOI: 10.1073/pnas.1905989116).

Within 1,000 years of the impact, the pH of the surface oceans dropped by 0.25 units, the new work shows. The researchers also investigated the ecological impact of the asteroid and found that the amount of photosynthesis by aquatic organisms was cut in half during the same time frame. Although the pH of the oceans returned to preimpact levels within about 40,000 years, the ecological ramifications persisted for much longer, says geochemist Michael Henehan of GFZ German Research Center for Geosciences, who led the new study.

Henehan and his postdoc adviser, Yale University geochemist Pincelli Hull, had been searching for years for the appropriate site to study this extinction. In a cave in the Netherlands, the duo finally found what they were looking for—a thick layer of clay studded with fossilized foraminifera, or forams—microscopic plankton with calcium carbonate shells. Foram shells also contain minuscule amounts of boron.

Boron exists in two forms in seawater: boric acid and borate. Each form has a distinct, pH-dependent isotopic ratio of boron-11 to boron-10. Because forams incorporate only borate into their shells, the isotopic ratio of the boron in a foram’s shell is analogous to oceanic pH. From the forams in the cave, the team could calculate the postimpact pH drop in the surface ocean.

The team also investigated changes in carbon isotope signatures after the impact. Plankton at the ocean surface preferentially take up carbon-12 from the surrounding seawater. When the plankton die and sink to the bottom, the surface waters end up with more isotopically heavy carbon compared with the deep ocean. However, after the asteroid impact, this surface-to-deep gradient disappeared.

Scientists have two possible explanations: a near-complete extinction of marine life or a selective extinction of the calcifying plankton that move carbon to the deep. Both scenarios would produce the observed lack of isotope gradient. But they have very different pH signatures, since in the second scenario, smaller plankton are still alive. By comparing their pH data with different model configurations, the researchers could reconcile these hypotheses. They found that an immediate 50% reduction in the ocean’s photosynthetic life postimpact was followed by a particularly slow recovery of calcifying phytoplankton. Thus the two scenarios are “both sort of right,” Henehan says.

Bärbel Hönisch, a paleoceanographer at Columbia University’s Lamont-Doherty Earth Observatory, says that the combination of the geochemical record and the large-scale modeling is “very powerful.” She notes that the lag between the recovery of the ocean’s pH and the marine ecosystem is concerning when considering the impacts of today’s human-driven ocean acidification.