Massive burps of carbon dioxide triggered widespread ocean anoxia 300 million years ago, sediment cores suggest

New research from the University of California, Davis, the Chinese Academy of Sciences and Texas A&M University reveals that massive emissions, or burps, of carbon dioxide from natural Earth systems led to significant decreases in ocean oxygen concentrations some 300 million years ago.

Combining geochemical analyses of sediment cores and advanced climate modeling, the study, published in Proceedings of the National Academy of Sciences, highlights five periods when significant decreases in ocean oxygen levels (by 4% to 12%) coincided with significant increases in levels of carbon dioxide in the atmosphere. Such oxygen-less, or anoxic, events are known for their detrimental effects on marine life and biodiversity.

Despite their roots in the deep past, the findings are relevant to the current global climate and its future. If events of a similar scale were to happen today, they would likely affect coastal areas that are important for fisheries and marine biodiversity.

“This is our only analog for big changes in carbon dioxide at levels comparable to what we’re living in today, where we see doublings and triplings of the levels,” said senior author Isabel P. Montañez, a Distinguished Professor in the Department of Earth and Planetary Sciences at UC Davis.

What’s different, though, is the source of the carbon dioxide. While carbon dioxide levels of long-past climates were influenced by natural systems like volcanic eruptions, human-produced and human-related carbon dioxide emissions strongly influence today’s levels.

“We’re creating a burp now and at a rate two, maybe three, orders of magnitude faster than in the past,” Montañez said.

Sediment cores and deep climate modeling

In the study, the team used sediment cores sourced from a geological formation in South China called the Naqing succession. By analyzing the geochemical makeup of these deep-water cores, specifically carbonate uranium isotopes, the team chronicled Earth’s environmental conditions from 310 to 290 million years ago.

“Through that analysis, we see these ‘burps’ not just in carbon dioxide but in the ocean’s uranium isotope signature too,” Montañez said. “They’re totally aligned, and the size of those uranium spikes tell us about the magnitude of the ocean anoxia.”

The team then used that information to inform leading-edge climate models, developed by the authors of this study, that are used to better understand ancient climates.

“It’s a mathematical framework in which we put in all our proxy information and we run it hundreds of thousands of times on a supercomputer,” Montañez said. “It basically best models what is most realistic given all the uncertainties, all the knowns, all the information that it’s given.”

Based on the modeling, the team found five instances of decreased oxygen in the global ocean by 4% to 12% from 290 to 310 million years ago. Each period lasted for roughly 100,000 to 200,000 years.

While the decrease in ocean oxygen doesn’t appear to correlate to any known mass extinctions, it does align with pauses in biodiversity that can be seen in the geological record.

“We do see these pauses in biodiversity each time these burps happen,” Montañez said. “It had an impact. Most likely, coastal regions were impacted the most.”

Records of the past, lessons for the future

The Earth of 300 million years ago was vastly different than the Earth of today. For one, oxygen in the atmosphere was 40% to 50% higher than it is today. Despite the differences between past and present, the magnitude of the rises in carbon dioxide levels are similar.

That could be interpreted as a warning, according to Montañez.

“This is a huge discovery, because how do you take an ocean sitting under an atmosphere with much more oxygen than today and permit this?” Montañez said. “The message for us is, “Don’t be so sure that we can’t do this again with our current human-driven release of carbon dioxide.'”