Some 430 million years ago, sea levels began to rise and oxygen levels dropped precipitously. According to new research, the sudden changes triggered a massive marine die-off known as the Ireviken extinction event.

As scientists detailed in a new paper, published this month in the journal Earth and Planetary Science Letters, the oceans of the so-called Silurian Period looked a lot like the oceans today.

As previous surveys of the fossil record have revealed, 80 percent of all conodonts, primitive eel-like animals, and 50 percent of all trilobites, distant relatives of the horseshoe crab, disappeared from the ocean around 430 million years ago.

Until now, the exact causes of the Ireviken extinction event remained unconfirmed.

"The connection between these changes in the carbon cycle and the marine extinction event had always been a mystery," Seth Young, an assistant professor of earth, ocean and atmospheric sciences at Florida State University, said in a news release.

Young and his colleagues used a new isotopic analysis technique to analyze ancient ocean sediments and precisely estimate the changing oxygen levels in the Silurian Period oceans. The team of researchers used stable carbon isotopes, stable sulfur isotopes and iodine geochemical signatures to reveal fluctuations in ancient oxygen levels.

"Those are three separate, independent geochemical proxies, but when you combine them together you have a very powerful data set to unravel phenomena from local to global scales," Young said. "That's the utility and uniqueness of combining these proxies."

The new analysis revealed a close correlation between oxygen depletion and the disappearance of large numbers of marine species.

Young and his colleagues were surprised to find low oxygen levels affected just 8 percent of the planet's oceans -- proof, the scientists claim, that a small but sudden change can have a global impact on living organisms.

"Our study finds that you don't necessarily need the entire ocean to be reducing to generate these kind of geochemical signatures and to provide a kill mechanism for this significant extinction event," Young said.

Several recent studies have shown ocean oxygen levels are declining today. Global warming will continue to push them lower, scientists predict. Some suggest the ecological impacts of these declines will begin to surface within the next few decades.

Authors of the latest study suggest further analysis of Silurian Period deoxygenation and the Ireviken extinction event will help scientists better predict how marine life will respond to today's changing ocean conditions.

"I think it's important to see how these events played out all the way from extinction interval through recovery period, how severe they were and their connections to the ancient environment along the way," said Young. "That could help us figure out what's in store for our future and how we can potentially mitigate some of the negative outcomes."

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