Sediments reveal ancient ocean during mass extinction

About 183 million years ago, volcanic activity in modern South Africa released an estimated 20,500 gigatons of carbon dioxide (CO₂) into the ocean-atmosphere system for 300 to 500 thousand years. A lack of oxygen or anoxia in the water, known as the Toarcian Oceanic Anoxic Event (T-OAE), caused mass extinctions of marine species.

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

“You can see a lot of fossils in the ocean sediments before the T-OAE, and then suddenly they disappear,” says Caltech’s Francois Tissot, professor of geochemistry and researcher at the Heritage Medical Research Institute.

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

A team led by scientists from George Mason University collected 30 samples of stratified limestone from the Mercato San Severino area in southern Italy to assess the severity of ocean deoxygenation during the T-OAE.

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

The relative abundance of uranium isotopes 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 samples of seawater from the past, scientists are able to use proxies for it, such as carbonate rocks, which faithfully record the composition of seawater.

When there is enough oxygen in the ocean, uranium likes to stay in its soluble form, dissolved in seawater. But when oxygen in the water becomes rarer, uranium begins to precipitate out of the seawater and settles in sediments on the ocean floor.

Thanks to careful modeling developed by former Caltech postdoctoral fellow Michael Kipp, Tissot and colleagues, 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, it was 6 to 8% of the ocean floor that was covered by anoxic sediment .”

The results suggest that past OAE events may foreshadow the effects of anthropogenic CO₂ emissions to marine ecosystems.

“If we don’t reduce carbon emissions and continue to increase CO₂ trajectory, we can clearly see that it will have serious negative impacts on the ocean ecosystem,” says Tissot.

The paper is titled “Uranium carbonate isotopes record global expansion of marine anoxia during the Toarcian Oceanic Anoxic Event.”