The end-Cretaceous mass extinction 66 million years ago eradicated roughly 75% of the animal and plant species on Earth, including whole groups like non-avian dinosaurs and ammonites. Debate lingers over what caused this extinction event, with intense volcanic activity in India’s Deccan Traps and the asteroid strike off Mexico’s Yucatan Peninsula the most widely supported hypotheses. Now, a team of researchers has found the first direct evidence that the asteroid impact coincided with a sharp drop in the pH levels of the oceans and was key in driving the mass extinction.
This painting depicts an asteroid slamming into tropical, shallow seas of the sulfur-rich Yucatan Peninsula in what is today southeast Mexico. Image credit: Donald E. Davis / NASA.
“For years, people suggested there would have been a decrease in ocean pH because the meteor impact hit sulfur-rich rocks and caused the raining-out of sulfuric acid, but until now no one had any direct evidence to show this happened,” said Dr. Michael Henehan, a scientist at GFZ German Research Centre for Geosciences.
In the study, Dr. Henehan and colleagues analyzed the chemical composition of foraminifera — tiny plankton that grow a calcite shell — from before, during, and after the end-Cretaceous mass extinction event.
Specifically, measurements of boron isotopes in foraminifera shells allowed the team to detect changes in the ocean’s acidity.
A species of foraminifera called Heterohelix globulosa that were picked and isolated from the end-Cretaceous clay at Geulhemmerberg in the Netherlands. Each fossil measures between 150 and 212 microns. Image credit: Yale University.
“We found decisive data for this during an excursion to the Netherlands, where a particularly thick layer of end-Cretaceous rocks is preserved in a cave,” the scientists explained.
“In this cave, an especially thick layer of clay from the immediate aftermath of the impact accumulated, which is really quite rare,” Dr. Henehan added.
“In most settings, sediment accumulates so slowly that such a rapid event such as an asteroid impact is hard to resolve in the rock record.”
“Because so much sediment was laid down there at once, it meant we could extract enough fossils to analyze, and we were able to capture the transition.”
“The ocean acidification we observe could easily have been the trigger for mass extinction in the marine realm,” said Dr. Pincelli Hull, a researcher at Yale University.
The Chicxulub impact 66 million years ago generated a tsunami-like wave in an inland sea that killed and buried fish, mammals, insects and a Triceratops, the first victims of a cataclysm that led to Earth’s last mass extinction. Image credit: Robert DePalma.
Meanwhile, the team’s boron isotope analysis and modeling techniques may have reconciled some competing theories and puzzling facts relating to ocean life after the mass extinction event.
Why, for example, are carbon isotope signatures immediately after the asteroid impact identical in fossil material from the sea floor and the surface waters, when normal carbon cycling in oceans should lead to different signatures?
One theory, the ‘Strangelove Ocean’ theory, argued that for a time after the event, the ocean was essentially dead and the normal carbon cycle just stopped.
The problem with this theory, according to some researchers, is that many organisms on the sea floor that rely on food sinking from the ocean’s surface continued unharmed across the event — an unlikely occurrence in a dead ocean.
Another popular theory, called the Living Ocean, suggested that the event killed off larger plankton species, disrupting the carbon cycle by making it harder for organic matter to sink to the deep sea, but allowed for some marine life to survive.
This new study splits the difference. It says the oceans had a major, initial loss of species productivity — by as much as 50% — followed by a transitional period in which marine life began to recover.
“In a way, we reconciled both of these ‘Strangelove’ and ‘Living Ocean’ scenarios. Both of them were partially right; they just happened in sequence,” Dr. Henehan said.
The study also may have settled a question regarding ocean pH levels leading up to the mass extinction event.
“What we can show is that there is no real signal of gradual pH decline in the ocean in the lead-up to the end-Cretaceous mass extinction,” Dr. Henehan said.
“Our results do not support any major role for volcanic activity in priming the world for extinction.”
The findings were published in the Proceedings of the National Academy of Sciences.
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Michael J. Henehan et al. Rapid ocean acidification and protracted Earth system recovery followed the end-Cretaceous Chicxulub impact. PNAS, published online October 21, 2019; doi: 10.1073/pnas.1905989116
