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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 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 study, titled "Dynamic deep marine oxygenation during the early and middle Paleozoic" and this week in Science Advances, challenges 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."

The team found that ocean oxygenation wasn't a smooth or permanent shift. Instead, O₂ levels were dynamic and rose and fell over time. One particularly stable oxygenated period was identified between ~405 and 386 million years ago. But even this seems to have been short-lived, ending in the team's youngest mudrock samples.

"The more we actually look at this, the more complex it is. It's really unstable. Even where our data set ends, we don't find stability. We never found evidence of a sustained rise to near-modern levels of ocean oxygenation. This must happen sometime after 380 million years ago," Ostrander said. "That's the one disappointment of our findings: that we still don't know when the deep oceans became forever oxygenated."

Key collaborators Erik Sperling of Stanford and Justin Strauss from Dartmouth led expeditions to Canada's Yukon Territory to recover the ancient seafloor deposits, now exposed in river cuts in mountainous areas, used in the study. "This is a truly special sample set," Ostrander said. "It's very hard to find continuous seafloor deposits spanning so much geologic time."

In a 2021 study, Sperling and Strauss described this area and its potential for shedding light on Earth's history following evolutionary bursts known as the Cambrian Explosion and the Ordovician Biodiversification. This was when oceans teemed with strange creatures, like trilobites, tentacled graptolites and tiny tooth-like conodonts.

Earth's surface ocean has been oxygenated for about the past 2.3 billion years, ever since the first rise of atmospheric O₂ after the Great Oxidation Event. This is because O₂ from the atmosphere can diffuse into the surface ocean.

"Getting O₂ into the deep ocean is more difficult," Ostrander said. "This requires the sinking of cold and dense O₂-rich surface waters in polar regions, and also low rates of respiration in the water column. Many factors are implicated in these processes. We should perhaps not be surprised, then, to find that oxygenating Earth's deep oceans was a long and complicated process."

If deep oceans remained partly or episodically anoxic during key biological milestones, then the rise of animals may have happened under less O₂-rich conditions than previously assumed—challenging conventional wisdom about the relationship between oxygen and evolution.

"From a biological angle, this is well after the Cambrian 'explosion.' This is when marine life is already quite large and performing energy-expensive tasks," Ostrander said. "You have some phenomenal changes in evolutionary biology happening during this time. And none of it seems to require substantial and sustained deep ocean oxygenation."