Approximately 183 million years ago, during the T-Oceanic anaerobic event (T-OAE), volcanic eruptions in what is now South Africa released around 20,500 gigatons of carbon dioxide (CO2) into the atmosphere and oceans over the course of 300-500 thousand years. This significant release of CO2 led to a drastic depletion of oxygen in the marine environment, resulting in a mass extinction of sea fauna.
Since the onset of the industrial revolution, human activities have led to CO2 emissions that account for 12% of the total CO2 released during the T-OAE, despite occurring in less than 0.1 percent of the time. Professor of Geochemistry and researcher Francois Tissso from the California Technological Institute, a co-author of a new study published in the journal “Proceedings of the National Academy of Sciences,” highlights that the disappearance of many fossils in sea sedimentation layers before T-OAE indicates the scale of oceanic anaerobiosis during that period.
A research team led by scientists from the University of George Mason collected thirty stratified limestone samples from the Merkato Severino region in southern Italy to evaluate the extent of ocean deoxygenation during T-OAE. The samples were analyzed for uranium content and isotopic composition, as different isotopes of an element can indicate levels of anaerobiosis. The relative abundance of uranium isotopes in the ocean is a key indicator of anaerobic conditions, helping scientists determine the degree of deoxygenation by measuring isotopic composition.
When oxygen levels decrease in the ocean, uranium starts to precipitate and accumulate on the seabed. By utilizing a modeling approach developed by the former postdoctoral researcher Michael Kippo, Tissso and colleagues were able to estimate the oxygen percentage in the ocean during the T-OAE.
The research findings reveal that during T-OAE, anaerobic conditions were 28-38 times more prevalent than in present-day oceans. Currently, only about 0.2 percent of the seafloor is covered with anaerobic precipitation, whereas 183 million years ago, approximately 6-8% of the sea was affected by such precipitation.
These results suggest that past OAE events can provide insights into the potential consequences of anthropogenic CO2 emissions on marine ecosystems. Failure to curb carbon emissions and the continued increase in CO2 levels could have severe repercussions for marine ecosystems.