Rapid fluctuations in oxygen levels coincided with Earth's first mass extinction

Rapid changes in marine oxygen levels may have played a significant role in driving Earth's first mass extinction, according to a new study led by Florida State University researchers.

About 443 million years ago, life on Earth was undergoing the Late Ordovician mass extinction, which eliminated about 85% of marine species. Scientists have long studied this extinction and continue to investigate its possible causes, such as habitat loss in a rapidly cooling world and low oxygen conditions in the oceans.

By measuring isotopes of the element thallium — which shows sensitivity to changes in oxygen in the ancient marine environment — the team found that previously documented patterns of this mass extinction coincided with an initial rapid decrease in marine oxygen levels followed by a rapid increase in oxygen. The U.S. National Science Foundation-supported work is published in the journal Science Advances.

"Paleontologists have noted that there were several groups of organisms, such as brachiopods, that started to decline very early in this mass extinction interval, but we didn't have good evidence of an environmental or climate signature to tie that early decline to a particular mechanism," said co-author Seth Young. "This research can directly link that early phase of extinction to changes in oxygen."

That decrease in oxygen was immediately followed by an increase. This rapid shift in oxygen coincided with a first die-off of mass extinction and major ice sheet growth over the ancient South Pole.

"Turbulence in oxygen levels in oceanic waters is what seems to have been problematic for organisms living in the Late Ordovician, which might have been adapted to cope with low oxygen conditions initially or vice versa," Young said. "Oxygen levels in the oceans next to the continents switching back and forth over short geologic time scales of a few hundred thousand years seemed to play havoc with marine ecosystems."

Added Alberto Perez-Huerta, a program director in NSF's Division of Earth Sciences, "This study shows the benefit of combining novel geochemical proxies with sedimentology and paleobiology to understand mass extinction events. It also highlights the importance of oxygen availability in modern marine benthic ecosystems in the context of climate change."