Ocean acidification triggered devastating extinction, study finds

Carbon dioxide was a major player in the worst extinction event on Earth, scientists say

Ocean acidification triggered by massive volcanic eruptions helped cause the worst mass extinction in the history of life on Earth, according to a new study.

The findings, published in the journal Science, shed light on the dangers of rising ocean acidity – a phenomenon that is contributing to the deaths of coral and other marine life today.

If you thought the worst extinction event on Earth was the one that killed the dinosaurs some 66 million years ago, think again. A far worse event, the Permo-Triassic Boundary mass extinction event, happened some 252 million years ago, which over the course of about 60,000 years is thought to have wiped out more than two-thirds of land species and more than 90% of marine species on the planet.

Scientists have debated exactly what caused this devastating die-off. Now, in a new paper, a team led out of the University of Edinburgh has found that there were two major phases to this extinction that acted as a one-two-punch to wipe out most living things.

The researchers studied chemicals in rocks in cliffs in the United Arab Emirates that, some 250 million or so years ago, had been submerged in an ancient ocean. They looked at the ratios of boron isotopes, which are closely linked to levels of ocean acidification, as well as certain carbon isotopes, which are often used to study the rate that carbon dioxide is entering the ocean (and thus, how acidic the ocean is becoming).

The scientists think that massive amounts of carbon dioxide were released by what’s known as Siberian Trap volcanism, which was then absorbed into the oceans, causing them to grow more acidic far too fast for ocean life to adapt. But they also found that there appeared to be two phases to the carbon influx. The first one, signaled by the carbon isotopes, was a slower process that took place over 50,000 years; but the second phase was fast and furious, dumping massive amounts into the ocean in just 10,000 years.

“The first phase of extinction was coincident with a slow injection of carbon into the atmosphere, and ocean pH remained stable,” the authors wrote. “During the second extinction pulse, however, a rapid and large injection of carbon caused an abrupt acidification event that drove the preferential loss of heavily calcified marine biota.”

This double-whammy proved too much for life on Earth – particularly for the animals, such as oysters or coral, that need to pull minerals out of the ocean to build their shells and skeletons. Acidification reduces the amount of available calcium carbonate that these creatures can pull out of the water, and without these crucial building materials, their shells often start to look like they’re withering or being eaten away.

Today, the human-generated carbon dioxide emissions that are contributing to climate change are also contributing to ocean acidification. Today’s carbon influx isn’t nearly as massive as the one possibly triggered by Siberian Trap volcanism some 252 million years ago, the scientists pointed out – their model requires 24,000 petagrams of carbon, far more than the roughly 5,000 petagrams of carbon available in conventional fossil fuels today.

But it is being injected into the atmosphere today at a similar rate as it was back then. A high rate of carbon injection, not just the overall amount injected into the oceans, was probably a major part of the problem, because it left species with little time to adapt.

"Scientists have long suspected that an ocean acidification event occurred during the greatest mass extinction of all time, but direct evidence has been lacking until now,” lead author Matthew Clarkson of the University of Edinburgh said in a statement. “This is a worrying finding, considering that we can already see an increase in ocean acidity today that is the result of human carbon emissions."

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