Toxic, Oxygen-Depleted Oceans May Have Caused a Mass Extinction Event

Changes in the ocean may have pushed some species over the edge 200 million years ago. Scientists have discovered that oxygen-depleted toxic oceans had a key role in a mass extinction event during that time.

In order to learn a bit more about the conditions of the past, the researchers studied fossilized organic molecules that were extracted from sedimentary rocks that originally accumulated on the bottom of the northeastern Panthalassic Ocean. Today, the region can be off of the coast of British Columbia, Canada.

The scientists discovered molecules that were derived from photosynthesizing brown-pigmented green sulphur bacteria, which are microorganisms that only exist under severely anoxic conditions, which are conditions in which little oxygen can be found. This, in particular, reveals that it's likely that there was severe oxygen depletion and hydrogen sulphide poisoning of the upper ocean at the end of the Triassic period, about 201 million years ago.

"As tectonic plates shifted to break up Pangaea, huge volcanic rifts would have spewed carbon dioxide into the atmosphere, leading to rising temperatures from the greenhouse effect," said Jessica Whiteside, one of the researchers, in a news release. "The rapid rises in CO2 would have triggered changes in ocean circulation, acidification and deoxygenation."

So what does this mean? These changes would have had the potential to disrupt nutrient cycles and alter food chains essential for the survival of marine ecosystems. The oxygen-depleted conditions would have severely impacted food chains of the time. This means that it could have helped push the mass extinction event forward.

"The release of CO2 was probably at least as rapid as that caused by the burning of fossil fuels today, although the initial concentrations were much higher in the Triassic," said Whiteside. "The consequences of rapidly rising CO2 in ancient times inform us of the possible consequences of our own carbon dioxide crisis."

The findings are published in the journal Geology.

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