Hard rain's a-gonna fall?

Increasing acidity in oceans may have caused mass extinctions 65 million years ago.

The last mass extinction event occurred 66 million years ago, after an asteroid hit the earth. Now a Yale study has found that soon thereafter—in geologic time, at least—the acidity of the oceans increased. That change may have led to extinctions of ocean life. It may even have had a part in the eradication of the dinosaurs. The study raises new questions in an era when the oceans are acidifying once more.

The scientists examined fossilized foraminifera, ocean-dwelling microorganisms that lived around the time of the impact. Their calcium carbonate shells recorded the acidity of the waters they lived in. The team took samples from the deep ocean and from a Dutch cave whose sediments were laid down within a thousand years of the meteor’s impact.

The conclusion: “You can precipitate ecological collapse from an ocean acidification event,” says lead author Michael Henehan, a former Yale geology postdoc (now at the GFZ-Potsdam research center). This suggests that global darkness after the asteroid’s impact didn’t necessarily drive the extinctions; nor did volcanic activity, as some have argued.

Henehan and Pincelli Hull, an assistant professor of geology and geophysics at Yale, found that within centuries of the asteroid’s impact, the ocean’s pH fell by at least 0.25 units. That’s a rise in acidity, and it lines up with the hypothesis that the impact vaporized massive amounts of minerals that fell out as acid rain. The pH change was enough to dissolve the foraminifera’s calcium shells and potentially to devastate the ocean life they supported. (The results are in Proceedings of the National Academy of Sciences.)

The acidity normalized, but for a million years the seas stored far less carbon. This disturbed global carbon cycle could have altered the climate. Henehan hopes next to pinpoint how fast the acidification occurred. He also wants to find out whether the 0.25 drop in pH was the maximum ocean acidity—or an improvement. By this century’s end, the oceans will again experience a pH drop of 0.25 units or more. What if he finds that a drop of 0.25 units was the maximum acidity after that long-ago asteroid impact? “That would be a big worry,” he says.

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