Researchers have resolved long-standing uncertainty about what caused the last mass extinction around 66 million years ago, which killed three quarters of the planet’s flora and fauna and wiped out the dinosaurs. Putting to rest other causal hypotheses, particularly a prominent one about gradual volcanic action, they have confirmed that the impact of the Chicxulub asteroid caused sudden ocean acidification, impacting marine ecosystems and the carbon cycle.
They also find that this plunge in ocean pH, coinciding with the mass extinction known more technically as Cretaceous-Paleogene (K-Pg), explains how biodiversity and marine carbon recycling were so slow to recover.
“For years, people suggested there would have been a decrease in ocean pH because the meteor impact hit sulphur-rich rocks and caused the raining-out of sulphuric acid,” says lead author Michael Henehan from Yale University, US, “but until now no one had any direct evidence to show this happened.”
The clue, Henehan and colleagues report in the journal Proceedings of the National Academy of Sciences, came from foraminifera – tiny plankton that grow a shell from calcium carbonate and have a complete fossil record over hundreds of millions of years.
Records from a shallow marine cave at Geulhemmerberg, in the Netherlands, comprising part of the K-Pg boundary, gave insights into the first 100 to 1000 years after the asteroid’s impact, a timeline not available from deep marine records.
“In this cave, an especially thick layer of clay from the immediate aftermath of the impact accumulated, which is really quite rare,” says Henehan.
“Because so much sediment was laid down there at once, it meant we could extract enough fossils to analyse, and we were able to capture the transition.”
Using isotope analysis of the element boron in the fossilised shells, the researchers clarified that there was a sudden drop in the ocean’s pH that took marine life millions of years to recover from.
“Before the impact event, we could not detect any increasing acidification of the oceans,” Henehan says.
Combined with Earth system modelling derived from deep-ocean drill cores and rocks formed around that time, the cave records confirmed that the sudden acidification would have prevented marine calcifiers such as foraminifera from making their shells, explaining why they were disproportionately wiped out in the mass extinction.
This was important, the team notes, because the calcifiers are a key foundation of the ocean’s food chain and hence the rest of the ecosystem.
Because of this, as life forms in the upper ocean layers became extinct, carbon uptake by photosynthesis was reduced by half.
This state lasted several tens of thousands of years before calcareous algae spread again. But it took several million years until the marine plants and animals recovered and the carbon cycle was re-established.
“In sum,” the researchers write, “we provide insights into the drivers of the last mass extinction, the recovery of marine carbon cycling in a post-extinction world, and the way in which marine life imprints its isotopic signal onto the geological record.”
Related reading: Dinosaurs fell victim to a one-two punch