Earth has experienced several mass extinctions over the course of its existence—the Toarcian Oceanic Anoxic Event (T-OAE), for example, decimated the planet’s marine ecosystems. But according to a new study by international researchers at Caltech, George Mason University, the University of Naples and elsewhere, the T-OAE’s devastating impact over 300,000 to 500,000 years pales in comparison to what humanity can accomplish in a fraction of that time.
About 183 million years ago, devastating volcanic eruptions in what is now South Africa released about 20,500 gigatons of carbon dioxide into the atmosphere, causing sea level rise, water temperature, and acidification. The resulting lack of oxygen (called anoxia) triggered a mass extinction of marine life that would take up to half a million years to recover. Although researchers have long known about the T-OAE, its true extent is not yet fully understood. This is particularly a problem when it comes to predicting how future scenarios involving anoxic oceans might affect the planet.
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“Despite the recognition of T-OAE as a potential analogue for future ocean anoxia, current knowledge of the severity of global ocean anoxia is largely limited to studies of the trace element and isotopic composition of black shales, which are often affected by local processes,” the team explained in the abstract of their paper published on June 24 in Proceedings of the National Academy of SciencesTo better understand the dynamics of the T-OAE, the researchers therefore resorted to uranium isotopes.
As explained in an accompanying Caltech announcement, the amount of uranium isotopes in the ocean is directly related to oxygen depletion. If one can measure the isotopic composition of uranium samples, one can estimate the oxygen content (or lack of oxygen) of seawater. Although it’s impossible to take water samples from the T-OAE directly, rocks like limestone provide witnesses thanks to their uranium content. Uranium generally remains water-soluble when the oceans are oxygen-rich, but settles during periods of oxygen depletion and settles on the seafloor. By studying how much uranium is contained in marine deposits from the T-OAE, experts can estimate how bad things really were during the extinction event.
After collecting 30 sections of layered limestone from the Mercato San Severino region of southern Italy, the researchers analyzed them for both their uranium content and isotopic variations. Using a model developed by former Caltech postdoctoral fellow and current Duke University lecturer Michael Kipp, the team then determined the anoxic levels for this period.
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“Using this model, we found that the anoxia lasted 28 to 38 times longer than today’s ocean temperature,” Francois Tissot, a professor of geochemistry at Caltech and co-author of the study, said in a statement. “Today, only about 0.2 percent of the ocean floor is covered with anoxic sediments similar to those 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 with anoxic sediments.”
By better understanding how much greenhouse gas is needed to trigger such dramatic oxygen depletion in the oceans, the team can extrapolate this to the impact of humans on the environment. And as unfortunate as it may be, modern society is giving the T-OAE a run for its money. According to the researchers’ calculations, human emissions since the Industrial Revolution already represent 12 percent of all the CO2 produced during the entire T-OAE – and in less than 0.1 percent of the time.
“If we do not curb carbon dioxide emissions and CO2 levels continue to rise, we can clearly see that this will have serious negative impacts on the ocean ecosystem,” Tissot said on Monday.
The worrying figures show how important it is to reduce the catastrophic pollution of our society through truly sustainable practices. If we fail to do so, it is clear that the T-OAE could be small compared to our impact.