Scientists have shed new light on an ancient apocalypse that affected the entire planet

About 183 million years ago, during the Toarcian Oceanic Anoxic Event (T-OAE), volcanic eruptions in what is now South Africa released approximately 20,500 gigatons of carbon dioxide (CO2) into the atmosphere and oceans over a period of 300,000 to 500,000 years. . This significant influx of CO2 led to severe oxygen depletion in the marine environment, resulting in widespread die-offs of marine life.

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

Scientific analysis of ocean deoxygenation

“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, published June 24 in the journal 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 thirty 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.

Impact of anoxia on the ocean floor

“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 percent 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, 6 to 8 percent of the ocean floor was covered by anoxic sediment.

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

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

Reference: “Uranium carbonate isotopes record global expansion of marine anoxia during the Toarcian Oceanic Anoxic Event” by Marian N. Remírez, Geoffrey J. Gilleaudeau, Tian Gan, Michael A. Kipp, François LH Tissot, Alan J. Kaufman, and Marian Parente, 24 June 2024, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2406032121