Sediments reveal an ancient ocean during a mass extinction event

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

Human activity since the Industrial Revolution already accounts for 12 percent of total CO2 emissions, with less than 0.1 percent during the overall T-OAE. T-OAE predicts what will happen to our oceans if greenhouse gas emissions continue to rise.

„You can find a lot of fossils in the ocean sediments before the T-OAE, and then they suddenly disappear,” says Caltex. Francois DeSot, professor of geochemistry and researcher at the Traditional Medicine Research Institute. Tissot is a co-author of the new study, which appears June 24 in the journal Proceedings of the National Academy of SciencesDescribes the degree of marine anoxia during T-OAE.

Led by researchers at George Mason University, the team collected thirty samples of layered limestone from the Mercado San Severino area in southern Italy to assess the intensity of ocean oxidation during the T-OAE.

The team analyzed the samples for their uranium content and isotopic composition. Isotopes are double versions of an element with different numbers of neutrons and thus slightly different masses. The relative abundance of isotopes of uranium in the ocean depends on the degree of anoxia. This means that by measuring the isotopic composition of uranium in the ocean, scientists can infer the extent of anoxia in the ocean. In the absence of actual seawater samples from the past, scientists can use proxies such as carbonate rocks, which provide a true record of seawater composition.

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When there is more oxygen in the ocean, uranium prefers to be in its soluble form, dissolving in seawater. But when oxygen in the water becomes too scarce, uranium begins to leach out of the seawater, and settles as sediments on the sea floor. Thus, through careful modeling developed by former Caltech postdoctoral scholar Michael Gibb, Tissot and collaborators, the amount of uranium in coastal samples can represent the percentage of oxygen in the ocean at T-OAE time.

„Using this model, we found that anoxia was 28 to 38 times greater in the modern ocean,” says Tissot. „Today, only about 0.2 percent of the sea floor is covered by anoxic sediments, as seen in the Black Sea. During the T-OAE period, 183 million years ago, it was 6 to 8 percent of the sea floor. It was covered in anoxic sediments.”

The results indicate that past OAE events can predict the effects of anthropogenic CO2 emissions on marine ecosystems.

„We can clearly see that if we do not control carbon emissions and continue on the path of increasing CO2, there will be serious negative impacts on the ocean’s ecosystem,” Tissot says.

The title of the article is „Carbonate uranium isotopes record global expansion of oceanic anoxia during Thorsian oceanic anoxic event..” The first authors are Mariano N. Remirez and Jeffrey J. Guilloteau of George Mason University. In addition to Tissot, other co-authors are Tian Khan of George Mason University and former Caltech postdoctoral scholar Michael A. Gibb of the University of Maryland. Alan J. Kaufman of Duke University, University of Maryland and Mariano Parente of the University of Naples, Italy, funded by George Mason University.

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