Corals May Have Defense Against Global Warming

Helen Scales
for National Geographic News
October 4, 2007

Ancient corals may have been more adaptable to changing ocean chemistry than previously thought, a new study shows.

The findings may offer hope that if the diversity of modern corals is preserved, they may be able to adapt as global warming causes seas to become more acidic.

These fossil corals in diverse reef communities adjusted to an acidic environment by altering the way they built their chalky skeletons.

Modern hard corals—known as scleractinians—form reefs of thousands of tiny skeletons made from a calcium carbonate called aragonite.

Aragonite is susceptible to the corrosive effects of acidic oceans, which today has become a byproduct of a build-up of carbon dioxide in the atmosphere.

"We now have many different arguments to prove that these corals were actually made originally out of calcite—and not just aragonite that was transformed after the coral died and become fossilized," said study co-author Jaroslaw Stolarski, a paleontologist from the Institute of Paleobiology at the Polish Academy of Sciences.

Curious Coral

The calcite skeletons were identified using several tests. Scientists inspected the microscopic structure of the skeleton and detected a high ratio of magnesium to calcium—a telltale fingerprint of calcite, not aragonite.

The fossil corals look like two-inch-long (five-centimeter-long) sea anemones nestled in porcelain egg cups. They belonged to the genus Coelosmilia, which existed during the Cretaceous period about 70 million years ago.

The Cretaceous oceans had a chemistry different than today, with much less of the metal ion magnesium and higher acidity.

"In the same environment there were coral neighbors, some with aragonitic skeletons, some with calcitic skeletons," Stolarski said.

"There was great biological variability among the corals, and some of them adjusted perfectly to the prevailing geochemical situation," he said.

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