Massive Volcanic Eruptions Millions of Years Ago Caused Oxygen Shortage in Oceans, Prompting Mass Extinction

Scientists are already seeing areas of the ocean run out of oxygen, threatening fish in the regions who rely on that oxygen to breathe. They think the problem may get worse as the climate continues to change, but it's difficult to know exactly what consequences a massive oxygen shortage could have on the ocean. Because of this, a team of scientists recently looked to pin down exactly what happened to ocean oxygen levels during a volcano-induced period of climate change that took place about 183 million years ago.

The time period, called Toarcian, is also associated with a wave of extinction in marine animals, which raised scientists' suspicions about oxygen levels. Now they've found chemical evidence for the shortage, according to a new study published in the journal Proceedings of the National Academy of Sciences.

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The study's results suggest oxygen shortages are just as big a problem as the other issues facing the oceans. "We used to think of ocean temperature and acidification as a one-two punch," co-author Theodore Them, a geochemist at Florida State University, said in a press release. "But more recently we've learned this third variable, oxygen change, is equally important."

In the case of the Toarcian period, the collection of challenges arose from a massive spree of eruptions in what scientists call the Karoo and Ferrar Large Igneous provinces, which now stretch over the southern part of Africa and Antarctica. Those eruptions were powerful enough to cause a serious increase in carbon in the atmosphere.

To look at the marine impacts, the team gathered rock samples found in Canada and Germany, which at the time would have been covered by a shallow ocean. Then they isolated the chemical fingerprint of an element called thallium, which let them calculate the oxygen levels at the time.

Two of the study's co-authors hold a fossil sample and a rock core sample used in the research. Stephen Bilenky

They saw a steep dip in oxygen starting even a little bit before where scientists have traditionally marked the beginning of the extinctions. That finding fills in the gap between volcanic eruptions and marine extinctions that had been bugging scientists.

But it also raises concerns that our modern situation will get worse before it gets better. "It's extremely important to study these past events," Them said in the press release. "It seems that no matter what event we observe in Earth's history, when we see carbon dioxide concentrations increasing rapidly, the result tends to be very similar: a major or mass extinction event."