Researchers studying an ancient episode of high ocean acidity have discovered that a group of marine creatures living at the time adapted to the change in water chemistry. But the findings may provide little comfort for scientists worried about ocean conditions today, which are changing much more quickly.
About 120 million years ago, during the early part of the Cretaceous period, a series of massive volcanic eruptions pumped huge amounts of carbon dioxide (CO2)into Earth's atmosphere. The air's CO2 content rose to about twice today's level. Eventually, the oceans absorbed much of that CO2, which significantly increased the water's acidity. The change reduced the amount of calcium carbonate (CaCO3) in the water, making it more difficult for creatures such as some kinds of plankton to form shells. Ocean pH returned to normal after about 160,000 years.
Micropaleontologist Elisabetta Erba of the University of Milan in Italy, and geochemist Helmut Weissert of the Swiss Federal Institute of Technology in Zürich, Switzerland, wanted to gauge how the high acidity affected the ancient marine ecosystem. They examined fossils from ancient ocean sediments at two drill sites—one from a now-above-ground formation in northern Italy and the other from deep water in the mid-Pacific Ocean. "The Pacific Ocean was the only big ocean at that time," Erba says. In particular, they studied the numbers and condition of fossilized specimens of calcareous nannoplankton, the microscopic ancestors of modern plankton. The creatures' shells consist mostly of CaCO3 and therefore could reveal their overall health and the state of the ocean's chemistry.
Reporting in tomorrow's issue of Science, Erba, Weissert, and colleagues reveal that the effects on the nannoplankton varied. As acidity increased, for example, the tiny skeletons of some species became malformed, other species shrank, and some died out altogether. Most striking, most nannoplankton seemed to adapt to acidification.
The study produced a second surprise. The acidification occurred progressively, from the surface down to deep waters. That's something unseen today, Weissert says, "perhaps or probably because it will take a long time to happen."
It's a "very important paper [that] provides state-of-the-art understanding of the effects of massive amounts of CO2 in the oceans," says marine geologist Timothy Bralower of Pennsylvania State University, University Park. The difference today is that the rate of CO2 increase "is far faster than anything we see in the ancient geologic record," he says. "The big question is whether modern species will be able to adapt to what I expect will be much more rapid pH reduction in coming centuries."
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