Toxic oceans played a huge part in prehistoric mass extinction

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An oxygen-depleted ocean played a huge role in the prehistoric mass-extinction that occurred at the end of the Triassic Period, new research revealed. According to a study to be published in Geology, changes in the biochemical balance of the Panathalassic Ocean– one of two oceans that surrounded the supercontinent Pangea – were a critical factor in the extinction where half of the Earth’s animal, plant, and marine life died.

The Triassic period saw the emergence of dinosaurs, who became the dominant animal life form during the subsequent Jurassic period.

“This is significant because it is the first time an open ocean setting was investigated,” Study Co-author Jessica Whiteside of the University of Southampton in the U.K., told “Previous work focused on shallow coastal areas in what is now Europe, where regional effects could predominate. Thus, by studying the Panathalassic Ocean, we provided strong evidence that these environmental changes were global in nature.”

When Pangea broke apart 201 million years ago, volcanic rifts spewed massive amounts of carbon dioxide into the atmosphere. While this created a rise in temperatures from the greenhouse effect, the huge spike in carbon dioxide brought about a massive chemical bio-imbalance in the Earth’s oceans. When the oceans’ surface waters that were exposed to the sun (the photic zone) lost oxygen, they became toxic by way of hydrogen sulfide – an extremely poisonous chemical produced by microorganisms that don’t need oxygen to survive. This process is called photic zone euxinia (PZE).

To find rocks that lay at the bottom of the Panathalassic Ocean during this period, the researchers travelled to the Queen Charlotte Islands off the coast of British Columbia, Canada. “We of course did not know if the Canadian rocks would contain evidence of PZE, only that they were deposited in well-mixed and rather deep oceanic waters of the Panathalassic Ocean, in contrast to earlier studies that [focused on] terrestrial and very shallow marine sediments,” Whiteside told “We wanted to test the hypothesis that PZE was more widespread than just the shallow marine environment, and therefore of potential global, rather than local, significance.” The team knew that if there was evidence of PZE, the rocks were comprised of the right sediment to preserve the indicative molecules.

To find these molecules, which are so small they couldn’t even be seen under a light microscope, the team had to take the sediment samples to a lab. After chemically isolating the fossilized organic cells from the sediment in the lab, Whiteside and her colleagues measured the molecules (or biomarkers) from the cells’ fat membranes.

“In their structural formulae, these molecules record environmental conditions,” Whiteside explained. “Because we know how they affect the environment today, and how the environment affects them, we can infer what happened in the geologic past if we know their concentrations.” For example, the researchers knew that the presence of green sulfur bacteria today indicates an environment where there is light and hydrogen sulfide, but little oxygen (green sulfur bacteria is found in volcanic hot springs, salt marsh sediments, and in the depths of freshwater lakes). As noted earlier, hydrogen sulfide is a by-product of anaerobic organisms and is extremely toxic to most forms of life.

“Thus,” concluded Whiteside, “if we find biomarker evidence for green sulfur bacteria in the geologic record, we can infer that oxygen is depleted and levels of hydrogen sulfide also increased during that time. Our study demonstrates that for the mass extinction 201 million years ago, hydrogen sulfide poisoning disrupted the distribution of nutrients and altered food chains essential for the survival of marine ecosystems in the area. All this, from tiny bits of fat.”

She notes it took dozens of thousands of years for the rise in carbon dioxide and greenhouse gases to bring about the mass extinction, which on a geologic timescale is nearly instantaneous.

Though there were no polar ice caps at the end of the Triassic– and the carbon dioxide levels were higher– Whiteside warns that it bears a strong resemblance to the world we live in today, thanks to the burning of fossil fuels. “The amount of CO2 released during the extinction, and the speed at which it was released, is very similar to what is occurring today, and thus serves as a cautionary tale for what might be in store for us in the near future.”