Sediments reveal the primordial ocean during a mass extinction

About 183 million years ago, volcanic activity in what is now South Africa released an estimated 20,500 gigatons of carbon dioxide (CO₂) into the ocean-atmosphere system over a period of 300,000 to 500,000 years. The lack of oxygen, or anoxia, in the water during this time, known as the Toarcian Oceanic Anoxic Event (T-OAE), led to a mass extinction of marine species.

Human activity has already led to cumulative CO₂ emissions, which account for 12% of total CO₂ released less than 0.1% of the time during the entire T-OAE. The T-OAE provides a glimpse of what could happen to our oceans if greenhouse gas emissions continue to increase.

“Before the T-OAE, you could see a lot of fossils in the marine deposits, and then suddenly they disappear,” says Francois Tissot, professor of geochemistry and researcher at the Heritage Medical Research Institute at Caltech.

Tissot is co-author of a new study published on June 24 in Proceedings of the National Academy of Sciencesdescribes the extent of ocean anoxia during the T-OAE.

Led by researchers at George Mason University, the team collected 30 samples of layered limestone from the Mercato San Severino region of southern Italy to assess the severity of ocean deoxygenation during the T-OAE.

The team analyzed the samples for their uranium content and isotopic composition. Isotopes are twin versions of an element with different numbers of neutrons and therefore very slightly different masses.

The relative abundance of uranium isotopes in the ocean depends on the extent of anoxia. This means that by measuring the isotopic composition of uranium in the ocean, scientists can draw conclusions about the extent of anoxia in the ocean.

Since there are no real seawater samples from the past, scientists can use substitutes such as carbonate rocks that faithfully reproduce the composition of seawater.

When there is plenty of oxygen in the ocean, uranium likes to stay in its soluble form, dissolved in seawater. But when oxygen in the water becomes scarce, uranium begins to precipitate out of the seawater and settle in sediments on the seafloor.

Using careful modeling developed by former Caltech postdoctoral fellow Michael Kipp, Tissot and colleagues, the amount of uranium in seafloor samples can provide insight into the oxygen content in the ocean at the time of the T-OAE.

“Using this model, we found that anoxia was 28 to 38 times higher than today’s ocean temperature,” says Tissot. “Today, only about 0.2 percent of the ocean floor is covered with anoxic sediments, similar to those in the Black Sea. At the time of the T-OAE 183 million years ago, 6 to 8 percent of the ocean floor was covered with anoxic sediments.”

The results suggest that past OAE events may have influenced the impact of anthropogenic CO₂ -Emissions on marine ecosystems.

“If we do not curb CO2 emissions and continue to see rising CO2 emissions,₂ We can clearly see that this development will have serious negative impacts on the ocean ecosystem,” says Tissot.

The title of the paper is “Carbonate uranium isotopes record global spread of marine anoxia during the Toarcian oceanic anoxia event.”