The asteroid that killed most of the dinosaurs may be the most well-known extinction event...
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...but it is not the most deadly extinction event.
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Nearly 200 million years before that fateful impact, 95 percent of Earth’s marine life and about 75 percent of land life disappeared within just a few thousand years.
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This turbulent time marked the end of the Permian period and beginning of the Triassic period.
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Debate has raged in the scientific community about whether this extinction is tied to massive amounts of volcanism or to methane released from frozen pockets of deep ocean ice.
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Now, evidence from the shells of a humble, clam-like sea creature may have solved the debate.
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In a paper published in Nature Geoscience, scientists reconstruct the sequence of geological and climate events that led to this massive extinction.
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The researchers looked at shells of fossil brachiopods, clam-like animals that lived in the shallow Tethys ocean just before the extinction event.
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Boron isotopes in the shells revealed how acidic the ocean was — a measure directly tied to the concentration of atmospheric carbon dioxide.
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Using that measurement and computer simulations of Earth’s past, the researchers discover that a period of violent volcanism released an enormous amount of carbon dioxide into the atmosphere, which acidified the oceans.
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A highly acidic ocean means that shelled animals can’t build their shells.
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The increased carbon dioxide in the atmosphere caused global warming, which led to increased chemical weathering on land.
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Nutrients from weathered rock washed into oceans, where they overloaded ecosystems and led to huge, oxygen-depleted, dead zones — like the one that forms annually in the Gulf of Mexico.
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The combination of a hot globe, acidic oceans, and oxygen-depletion caused nearly all life on Earth to rapidly die out over the next few thousand years.
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Now that scientists understand what happened 252 million years ago, they can use those lessons to forecast what might happen in the future.