Oxygen levels fluctuated wildly long after the Cambrian explosion

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 in seawater. Some researchers even argue for near-modern levels of ocean oxygenation at this time.

But oxygen levels in Earth’s deepest marine environments fluctuated wildly long after the Cambrian, according to new research in Science Advances.

Using stable isotope ratios of thallium preserved in ancient marine mudrocks, the researchers reconstructed oxygen levels between about 485 million and 380 million years ago. This timeframe immediately follows the Cambrian rise of animals and intersects with the later rise of land plants. 

The findings, published Sept. 3, challenge some conventional views of ocean oxygenation, according to lead author Chadlin Ostrander, an assistant professor in the University of 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.”

Related: Revisiting the Cambrian explosion’s spark

An unusual record of Earth’s ancient oceans

To reach these findings, Ostrander and collaborators including Stanford University geobiologist Erik Sperling 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 along the Peel River in Yukon, Canada. Very few processes can strongly “fractionate” thallium isotopes – that is, partition them in ways that result in different ratios.

“When we sampled this section on the Peel River we could tell that it was special. It is very rare to have such a continuous and well-preserved record of Earth’s ancient oceans,” said Sperling, an associate professor of Earth and planetary sciences in the Stanford Doerr School of Sustainability. “What we needed, though, was a geochemical tool that could tell us about the entire world ocean at that time, not just at the Peel River site. Thallium isotopes are that tool."

Related: Longest known continuous record of Paleozoic discovered in Yukon wilderness

The strongest fractionations today occur in deep marine ferromanganese deposits. Oxygen 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 oxygen-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.”

Unstable oxygen levels

The team found that ocean oxygenation wasn’t a smooth or permanent shift. Instead, oxygen levels were dynamic and rose and fell over time. One particularly stable oxygenated period was identified between approximately 405 million 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.”

The new study adds to previous research from an international consortium of scientists led by Sperling, which found oxygen levels in the deep ocean did not approach those in modern seas until about 140 million years after the Cambrian explosion.

Sperling and study co-author Justin Strauss, who is a Dartmouth College geobiologist and a former postdoctoral scholar in Sperling’s lab, led expeditions to Canada’s Yukon Territory to recover the ancient seafloor deposits used in the study. 

In research published in 2021, Sperling and Strauss described this area and its potential for shedding light on Earth’s history following the 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.

Challenging conventional wisdom 

Earth’s surface ocean has been oxygenated for about the past 2.3 billion years, ever since the first rise of atmospheric oxygen after the Great Oxidation Event. This is because oxygen 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 oxygen-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.”

This story was adapted from a press release originally published by the University of Utah.