A quarry illustrating bands of limestone layered from the ancient seabed in what is now Mercato San Severino in Italy. Credit: Mariano Remírez, George Mason University
About 183 million years ago, during the Toarcian Ocean Anoxic Event (T-OAE), volcanic eruptions in what is now South Africa released about 20,500 gigatons of carbon dioxide (CO2) into the atmosphere and oceans over a period of 300 to 500 thousand years. . This significant influx of CO2 led to severe oxygen depletion in marine environments, resulting in widespread extinction of marine life.
Human activity since the industrial revolution has already resulted in cumulative CO2 emissions representing 12 percent of the total CO2 released throughout the T-OAE, in less than 0.1 percent of the cases. T-OAE predicts 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 within the ocean sediments before the T-OAE, and then suddenly they disappear,” says Francois Tissot of Caltech, Professor of Geochemistry and Heritage Medical Research Institute Investigator. Tissot is a co-author of a new study, published June 24 in the journal Proceedings of the National Academy of Sciencesdescribing the extent of ocean anoxia during T-OAE.
Led by researchers from George Mason University, the team collected thirty samples of layered limestone from the Mercato San Severino region in 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 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 seawater samples from the past, scientists are able to use a proxy for it, such as carbonate rocks, which faithfully record the composition of seawater.
When there is a lot of oxygen in the ocean, uranium likes to stay in its soluble form, dissolved in seawater. But when the oxygen in the water becomes less, then the uranium begins to precipitate from the seawater and settles in sediments on the bottom of the ocean. Thus, through careful modeling developed by former Caltech postdoctoral researcher Michael Kipp, Tissot and colleagues, the amount of uranium in sea 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 with 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 sediments.
The results show that past OAE events can predict the effects of anthropogenic CO2 emissions on marine ecosystems.
“If we do not curb carbon emissions and continue on a trajectory of increasing CO2, we can clearly see that there will be severe negative impacts on the ocean ecosystem,” says Tissot.
Reference: “Uranium carbonate isotopes record global expansion of marine anoxia during the Toarcian ocean anoxic event” by Mariano N. Remírez, Geoffrey J. Gilleaudeau, Tian Gan, Michael A. Kipp, François LH Tissot, Alan J. Kaufman, and Mariano Parente, June 24, 2024, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2406032121