Did the Cretaceous asteroid wipe out mollusks, too?

"It throws up the question, if it wasn't ocean acidification, what was it?" asks Toby Tyrrell. (Credit: Will Bryson/Flickr)

The asteroid impact that wiped out the dinosaurs 66 million years ago caused a spike in ocean acidification. But was it enough to cause free-swimming mollusks to go extinct at the same time?

Ammonites and other planktonic calcifiers that were common in ancient oceans and are common as fossils today disappeared at the end of the Cretaceous period, as did more than 90 percent of species of calcium carbonate-shelled plankton (coccolithophores and foraminifera).

Comparable groups not possessing calcium carbonate shells were less severely affected, raising the possibility that ocean acidification, as a side effect of the collision, might have been responsible for the apparent selectivity of the extinctions.

Acid rain

Previous CO2 rises on Earth happened so slowly that the accompanying ocean acidification was relatively minor, and ammonites and other planktonic calcifiers were able to cope with the changing ocean chemistry. The asteroid impact, in contrast, caused very sudden changes.

In the first modeling study of ocean acidification following the asteroid impact, researchers simulated several acidifying mechanisms, including wildfires emitting CO2 into the atmosphere (as carbon dioxide emissions dissolve in seawater they lower the pH of the oceans making them more acidic and more corrosive to shells) and vaporization of gypsum rocks leading to sulphuric acid or “acid rain” on the ocean surface.

The researchers concluded that the acidification levels produced were too weak to have caused the disappearance of the calcifying organisms.

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Their findings are published in the Proceedings of the National Academy of Sciences.

“While the consequences of the various impact mechanisms could have made the surface ocean more acidic, our results do not point to enough ocean acidification to cause global extinctions, says Toby Tyrrell, professor of ocean earth science at the University of Southampton.

“Out of several factors we considered in our model simulation, only one (sulphuric acid) could have made the surface ocean severely corrosive to calcite, but even then the amounts of sulphur required are unfeasibly large.

“It throws up the question, if it wasn’t ocean acidification, what was it?”

Possible alternative extinction mechanisms, such as intense and prolonged darkness from soot and aerosols injected into the atmosphere, should continue to be investigated.

Researchers from the Leibniz Center for Tropical Marine Ecology contributed to the study.

Source: University of Southampton