The Permian-Triassic extinction event that happened some 252 million years ago is the worst extinction event our planet has ever seen. It wiped out around 90 percent of marine species and some 70 percent of vertebrate species on land, and was so severe that it's often called the Great Dying.

There are still lots of unanswered questions about the event, from its overall timescale to its causes, but a new study offers some intriguing extra detail on the calamity: a sudden spike in oxygen levels in the world's oceans at the same time as this widespread extinction was happening.

The researchers behind the study think that the sudden burst of oceanic oxygenation occurred around the start of the Great Dying, and was spread across tens of thousands of years, before oxygen levels then began to steadily drop again.

"For the geological record, that's practically instantaneous," says Earth scientist Sean Newby from Florida State University (FSU).

"And then you can of course compare that to modern, human-induced climate change, where we're having huge, rapid changes in fractions of the time compared to this mass extinction."

Without the aid of a time machine, measuring oxygen levels in the ocean eons ago isn't easy, but the team analyzed thallium isotopes buried in ocean sediments as a way of estimating the chemical mix of seawater stretching back millions of years.

Scientists have previously observed a slow reduction in ocean oxygen levels – technically known as ocean anoxia – across the course of the Permian–Triassic extinction event, but this earlier spike in oxygenation hasn't been seen before.

The next question is what caused it and what it means. It could be possible that a rise and then sudden fall in ocean oxygen levels is more dangerous for marine species than a more gradual decline, the researchers suggest.

"There's previous work that's been done that shows the environment becoming less oxygenated leading into the extinction event, but it has been hypothesized as a gradual change," says Newby.

"We were surprised to see this really rapid oxygenation event coinciding with the start of the extinction and then a return to reducing conditions."

Next, the researchers want to carry out further studies using the same techniques, looking at other mass extinctions to see if similar large-scale shifts in ocean oxygenation might have occurred.

An intense injection of carbon dioxide into the atmosphere is likely to have brought about the Great Dying, scientists think, quite possibly originating from Siberian volcanic activity across a vast scale.

If we can understand more about how this oxygen spike came about – and how it might have contributed to the extinction event – then that's another piece of useful information we can use in assessing the ongoing impacts of the climate crisis today.

"The loss of oxygen is important because the organisms living now are adapted for high oxygen, but if you have low oxygen there's also many organisms that may be able to adapt," says marine biochemist Jeremy Owens, from FSU.

"Any rapid fluctuation in either direction will have an impact."

The research has been published in Nature Geoscience.