Calcium offers clues in mass extinction

Ocean acidification during Permian period may have caused the Great Dying

New clues in a mass murder that took place 252 million years ago points to a suspect: Ocean acidification may have driven the largest extinction of animals the world has ever seen.

Carbon dioxide belched out by volcanic eruptions during the Permian period could have caused the oceans’ chemistry to change. That’s worrisome because CO2 levels are rising today — thanks to the burning of fossil fuels — and pushing down seawater pH, researchers report online June 8 in Geology.

“The worst biodiversity catastrophe we’ve had in the history of animal life appears to have been associated with ocean acidification and other kinds of environmental changes we anticipate in the coming centuries,” says Jonathan Payne, a paleobiologist at Stanford University. “It’s a useful comparison point to have in mind as we think about the future of the modern oceans.”

The Great Dying at the end of the Permian period 252 million years ago wiped out most animal species, with marine critters hit hardest. Popular explanations include an upwelling of deep, oxygen-poor waters that suffocated life near the surface, or perhaps huge eruptions in Siberia that warmed and acidified oceans.  

To distinguish between those scenarios, Payne and colleagues examined minerals in marine sediments and in fossilized, toothlike parts from prehistoric creatures that looked like eels. The minerals, made from calcium that had once been dissolved in seawater, had higher proportions of a lighter form of the element after about 250 million years ago, the researchers found.

A world warmed by volcanic eruptions would have increased levels of this light calcium isotope, the researchers’ simulations suggest. More intense rains would have flushed calcium into the ocean by eroding rocks on land, which tend to be made of the light calcium. Seawater that soaked up carbon dioxide and became more acidic would have stunted the productivity of organisms that pull calcium out of the water, allowing it to build up in the ocean.  

Other lines of evidence corroborate this story. Previous studies have found that not only calcium but also carbon tends to get lighter in limestone that was formed after the extinction, a shift that could also be explained by more erosion on land. And acidified oceans would have made life particularly difficult for thick-shelled creatures, which died out in droves during the extinction.

But Paul Wignall, a paleontologist at the University of Leeds in England, says the records are too crude to reveal the timing of the calcium shift. The light calcium could have come from the shells of creatures wiped out by other means.

“I suspect what their data really record is that a lot of skeletal invertebrates died out,” says Wignall. “They’ve recorded an effect, not the cause.”

The new calcium records also don’t reveal how strongly the oceans acidified. One recent simulation (SN: 10/8/11, p. 10) claimed a huge, devastating drop in pH would have been possible. But an unpublished simulation from Lee Kump, a geoscientist at Penn State, suggests that any changes in water chemistry were too gradual and mild to do significant damage.

“The notion that ocean acidification led to the extinction is still a viable one and perhaps even the leading explanation,” says Kump. “But the calcium isotopes aren’t the nail in the coffin.”

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