Dino-Killing Asteroid Also Wiped Out Mollusks?

The great Chicxulub impact is known for supposedly killing all of the dinosaurs on Earth, but new research suggests that this famous asteroid also wiped out unseen mollusks, making scientists question the role played by ocean acidification.

About 66 million years ago at the end of the Cretaceous period, ammonites, which were free-swimming mollusks of the ancient oceans, went extinct - as did more than 90 percent of species of calcium carbonate-shelled plankton (coccolithophores and foraminifera).

Interestingly, comparable groups lacking 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 extinctions.

Calcifiers such as mollusks, as well as starfish and corals, are particularly vulnerable to ocean acidification because they possess hard carbonate skeletons and shells, which are necessary for stability and protection. But as seen today, as atmospheric carbon dioxide (CO2) rises, the world's oceans are becoming warmer and more acidic. This threatens the survival of calcifying species because of the reduced saturation of the carbonate minerals required for calcification.

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.

A team of researchers led by the University of Southampton simulated several acidifying mechanisms in the first modeling study of ocean acidification which followed the asteroid impact. For example, they focused on wildfires that release CO2 emissions into the atmosphere, which in turn dissolve in seawater and lower the pH of the oceans, making them more acidic and more corrosive to shells. Also, the researchers considered vaporization of gypsum rocks, which causes sulfuric acid or "acid rain" to be deposited on the ocean surface.

Based on their results, the team concluded that the acidification levels produced were too weak to have caused the disappearance of mollusks and other calcifying organisms.

"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. Out of several factors we considered in our model simulation, only one (sulfuric acid) could have made the surface ocean severely corrosive to calcite, but even then the amounts of sulfur required are unfeasibly large," professor Toby Tyrrell, a co-author of the study, said in a statement.

"It throws up the question, if it wasn't ocean acidification what was it?"

The research team speculates that intense and prolonged darkness from soot and aerosols injected into the atmosphere could have been the cause, and they plan to further investigate these alternatives to find out.

The results were published in the journal Proceedings of the National Academy of Sciences (PNAS).

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