A new study suggests that a series of big “burps” of carbon dioxide could have changed our oceans around 300 million years ago. Published in PNAS, the study identifies five instances in which increases in carbon dioxide in the atmosphere could contribute to decreases in oxygen in the oceans.
According to the study authors, these changes can be tied to stagnation and decline in the ocean’s biodiversity, particularly among benthic animals, or animals associated with the bottom of the sea, such as corals, crustaceans, and brachiopods. The study authors also stress that these changes can be used to predict what could occur if carbon dioxide increased at a similar scale in the atmosphere today — a situation that they warn is not that far off.
“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 level,” said paleoclimatologist Isabel P. Montañez, a study author and professor at the University of California, Davis, according to a press release. “We’re creating a burp now and at a rate two, maybe three, orders of magnitude faster than in the past.”
Read More: The 5 Mass Extinctions That Have Swept Our Planet
Carbon Dioxide and Anoxia
It’s well established that burps, or emissions, of carbon dioxide are bad for the oceans. In addition to causing ocean acidification, for instance, these burps can also contribute to ocean anoxia, in which the oceans’ oxygen concentrations decline.
One of the reasons why these anoxic events occur relates to temperature. The release of carbon dioxide into the atmosphere raises the temperature of the air, which, in turn, increases that of the water at the ocean’s surface, where most of the ocean’s oxygen is absorbed. Because warmer water is worse at taking in oxygen, the warmer waters at the ocean’s surface mean that less oxygen is absorbed into the ocean overall, as more carbon dioxide is burped into the atmosphere.
Moreover, because warmer water is also more buoyant, the water at the ocean’s surface tends to stay there, cutting off ocean circulation and stopping the most oxygenated layers of the ocean from mixing with the least oxygenated ones.
Setting out to learn more about these occurrences and their effects on ocean ecosystems, the study authors turned to sea sediment cores from the Naqing Formation in South China. Studying the chemical composition of these cores, the team pieced together the conditions of the atmosphere and the oceans around 310 million 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 in the release. “They’re totally aligned, and the size of those uranium spikes tell us about the magnitude of the ocean anoxia.”
Read More: Earth's First Stable Dose of Oxygen Nearly Ended All Life
A Warning For the Future
Plugging all of their information into a mathematical model, the team concluded that doubled or tripled carbon dioxide levels could’ve contributed to decreased oxygen levels in the oceans at least five times around 300 million years ago. Each lasting around 200,000 years to 100,000 years, these events also occurred alongside interruptions in ocean biodiversity.
“We do see these pauses in biodiversity each time these burps happen,” Montañez said in the release. “It had an impact.”
Certainly, these carbon dioxide burps were distinct from the human-caused emissions seen on Earth today. Not only did they emerge from different sources, such as volcanic and tectonic activity, but they also affected different atmospheres, being emitted into the air with 40 to 50 percent more oxygen than the air on Earth today.
That the effects of these ancient emissions were so severe seems to forecast trouble for the future, as human activities are spewing carbon dioxide at a similar scale into an atmosphere with substantially less oxygen.
“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 in the release. “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.’”
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
PNAS. Repeated Occurrences of Marine Anoxia Under High Atmospheric O2 and Icehouse Conditions
Scripps Institution of Oceanography. FAQ: Ocean Acidification
Scripps Institution of Oceanography. FAQ: Ocean Deoxygenation
Sam Walters is a journalist covering archaeology, paleontology, ecology, and evolution for Discover, along with an assortment of other topics. Before joining the Discover team as an assistant editor in 2022, Sam studied journalism at Northwestern University in Evanston, Illinois.
