Toxic Oceans to Blame for Ancient Mass Extinction

It turns out that toxic oceans are to blame, at least in part, for an ancient mass extinction event that occurred over 200 million years ago, new research says.

The findings were published in the journal Geology.

Changes in the biochemical balance of the world's oceans were a crucial factor in the end-Triassic mass extinction, during which half of all plant, animal and marine life on Earth was wiped out, according to researchers from the University of Southampton.

That's due to a condition called "marine photic zone euxinia," which took place in the Panathalassic Ocean, the larger of the two oceans surrounding the supercontinent of Pangaea. This occurs when the Sun-lit surface waters of the ocean become deprived of oxygen and are poisoned by hydrogen sulphide - a toxic by-product of microorganisms that can live without oxygen.

To better understand the onset of this ancient mass extinction event, the research team analyzed fossilized organic molecules taken from sedimentary rocks that originally accumulated on the bottom of the northeastern Panthalassic Ocean, but are now exposed on the Queen Charlotte Islands, off the coast of British Columbia, Canada.

What they found were molecules resulting from photosynthesizing brown-pigmented green sulfur bacteria - microorganisms that only exist under severely anoxic (low oxygen) conditions. This indicates that severe oxygen depletion and hydrogen sulfide poisoning of the upper ocean occurred at the end of Triassic period, roughly 201 million years ago.

In addition, the researchers also observed significant changes in the nitrogen composition of organic matter, suggesting that these low oxygen conditions caused disruptions in marine nutrient cycles.

While previous studies have found evidence of photic zone euxinia during this time period, this study is the first to show that these changes may have occurred on a global scale. So what exactly triggered the oceans to become so toxic?

"As tectonic plates shifted to break up Pangaea, huge volcanic rifts would have spewed carbon dioxide [CO2] into the atmosphere, leading to rising temperatures from the greenhouse effect. The rapid rises in CO2 would have triggered changes in ocean circulation, acidification and deoxygenation," co-author Jessica Whiteside, from the University of Southampton, said in a statement.

"The same CO2 rise that led to the oxygen depleted oceans also led to a mass extinction on land," she added, "and ultimately to the ecological take-over by dinosaurs, although the mechanisms are still under study."

It should be noted that although the Earth was no doubt vastly different during the Triassic period compared to today, the rate of CO2 release from volcanic rifts were probably very similar as that caused by the current burning of fossil fuels. Therefore, this study has important implications for what we can expect in the future as we continue to burn fossil fuels and build up harmful greenhouse gases in our atmosphere.

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