Oxygen came late to ocean depths during Paleozoic

September 4, 2025

Thallium isotopes show O2 levels rose and fell at the ocean floor long after marine animals appeared and diversified half billion years ago, according to study of ancient marine sediments exposed by river cuts in Canada's Yukon

Chadlin Ostrander

The explosion of animal life in Earth’s oceans half a billion years ago during and after the Cambrian Period is commonly attributed to a substantial and sustained rise of free oxygen (O2) in seawater. Some researchers even argue for near-modern levels of ocean oxygenation at this time.

But O2 levels in Earth’s deepest marine environments fluctuated wildly long after the Cambrian, according to new research published by a University of Utah geologist with colleagues from other institutions.

Using stable isotope ratios of thallium (Tl) preserved in ancient marine mudrocks, the researchers reconstructed O2 levels between about 485 and 380 million years ago. This timeframe immediately follows the Cambrian rise of animals and even intersects the later rise of land plants. The findings, published this week in Science Advances, challenge some conventional views of ocean oxygenation, according to lead author Chadlin Ostrander, an assistant professor in Utah’s Department of Geology & Geophysics.

“It wasn’t like someone flipped a switch and the deep ocean became forever oxygenated,” Ostrander said. “Just a decade ago, it was thought that a deep ocean oxygenation switch was flipped around 540 million years ago. Our new dataset pushes that forward in time by at least ~160 million years.”

To reach these findings, Ostrander and his collaborators analyzed the stable isotopes of thallium—a heavy metallic element that occurs in trace amounts in Earth’s crust—contained in ancient marine sediments they recovered from Yukon, Canada. Very few processes can strongly fractionate Tl isotopes, that is, partition them in ways that result in different ratios.

The strongest fractionations today occur in deep marine ferromanganese deposits. O2 must accumulate in deep marine waters to stabilize these deposits, according to Ostrander. Thallium isotope ratios in the new study were rarely strongly fractionated, meaning these O2-dependent deepwater deposits were also rare.

“We do find some evidence of O2 building up in the deep ocean, but only for very brief periods of time,” Ostrander said. “Even at the youngest end of our dataset, the ocean seems to plunge back into an episode of widespread anoxia.”

 

Read the full story by Brian Maffly in @ The U.