Oxygen-starved oceans delayed life's recovery after 'Great Dying'

Scientists have found that chronically low levels of oxygen throughout the oceans hampered the recovery of life after the Permian-Triassic extinction, the most catastrophic die-off in our planet's history.

Also known as the "Great Dying," global ecosystems collapsed as some 90 percent of species perished in this extinction event 250 million years ago.

"Explaining the five million year delay in the Earth system's recovery to pre-extinction conditions after the Permian extinction has been a challenge," said one of the researchers, Kimberly Lau from Stanford University in California, US.

"Our results suggest a unified explanation for biological and biogeochemical observations stemming from the most severe biotic crisis in Earth's history," Lau noted.

Key to the new study was identifying an anoxic signal that could be traced independently of regional circumstances.

For that, the Stanford researchers turned to a new technique using uranium, preserved in limestone, that had once been dissolved and mixed evenly throughout the oceans.

"Because uranium is slowly cycled through the ocean, these records are thought to represent global changes in oxygenation," Lau said.

With this technique in hand, Lau and colleagues obtained rock samples from two widely separated sites, now located in modern-day China and Turkey.

The new findings, published in the journal for the Proceedings of the National Academy of Sciences, showed that ocean anoxia, or oxygen deficiency, was a global rather than an isolated phenomenon.

The study paints a dire portrait of how anoxic conditions reduced seawater oxygen levels by 100-fold at the onset of the mass extinction.

Oxygen levels then slowly rose, only returning to pre-extinction levels after five million years, corresponding to when the climate became more stable and life regained its former diversity.

The new findings also have implications for our modern world.

"These findings highlight the fact that ocean deoxygenation during the 21st century and beyond may lead not only to the loss of marine animal populations and species but also to unexpected feedbacks in the Earth system," said study co-author Jonathan Payne, associate professor and chair of geological sciences at Stanford University.

"The timescales of these feedbacks are long, meaning that the consequences of profound and extensive deoxygenation today could reverberate for many centuries, millennia, or longer," Payne noted.