Sediments reveal ancient ocean during a mass extinction

About 183 million years ago, volcanic activity in modern South Africa unleashed an estimated 20,500 gigatons of carbon dioxide (CO₂) in the ocean-atmosphere system over a period of 300 to 500 thousand years. Known as the Toarcian Oceanic Anoxic Event (T-OAE), the lack of oxygen, or anoxia, in the water during this time caused a mass extinction of marine species.

Human activity since the Industrial Revolution has already resulted in cumulative CO₂ emissions that represent 12% of total CO₂ released during the entire T-OAE, less than 0.1% of the time. The T-OAE predicts what could happen to our oceans if greenhouse gas emissions continue to increase.

“You can see a lot of fossils in ocean sediments before the T-OAE, and then suddenly they disappear,” says Francois Tissot of Caltech, professor of geochemistry and researcher at the Heritage Medical Research Institute.

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

Led by researchers from George Mason University, the team collected 30 samples of stratified 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 a double version of an element with a different number of neutrons and therefore a very slightly different mass.

The relative abundance of isotopes of uranium in the ocean depends on the amount of anoxia. This means that by measuring the isotopic composition of uranium in the ocean, scientists can infer the amount of anoxia in the ocean.

In the absence of actual seawater samples from the past, scientists can use a proxy for this, such as carbonate rocks, which faithfully record the composition of seawater.

When there is enough oxygen in the ocean, uranium likes to remain in its soluble form, dissolved in the seawater. But when oxygen in the water becomes scarce, uranium begins to precipitate from the seawater and settle in sediments on the ocean floor.

Thus, through careful modeling developed by former Caltech postdoctoral researcher Michael Kipp, Tissot and collaborators, the amount of uranium in seafloor samples can indicate the percentage of oxygen in the ocean at the time of the T-OAE.

“Using this model, we found that anoxia peaked at 28 to 38 times that of the modern ocean,” says Tissot. “Today, only about 0.2% of the ocean floor is covered by anoxic sediments, similar to those found in the Black Sea. At the time of the T-OAE, 183 million years ago, this was 6% to 8% of the ocean. floor that was covered with anoxic sediment.”

The results indicate that past OAE events can predict the effects of anthropogenic CO2₂ emissions to marine ecosystems.

“If we do not reduce CO2 emissions and continue to increase CO2 emissions₂ We can clearly see that there will be serious negative consequences for the ocean ecosystem,” says Tissot.

The article is titled “Carbonate uranium isotopes record global expansion of marine anoxia during the Toarcian Oceanic Anoxic Event.”