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About 200 million years ago, the rocks that became the Palisades cliffs just across the Hudson River from Manhattan were formed during volcanic activity that helped break up the ancient supercontinent Pangea. While this volcanism led to the birth of the Atlantic Ocean, it also contributed to the death of a quarter of all life on earth during the event known as the end-Triassic mass extinction.
Marine animals such as ammonites, ichthyosaurs, and corals were hit hard during the extinction, and scientists have long suspected that Atlantic-forming volcanism had something to do with it, due to its effects on climate and oceans. But the scant evidence of what exactly killed life makes it one of the least understood of the so-called Big Five mass extinctions that punctuated the history of life on Earth.
Research published in January in the journal geologynevertheless, this prehistoric murder mystery is beginning to fill in the blanks.
A team of scientists examining rocks in southwest England has found evidence of two triggers. First, as the oceans absorbed carbon dioxide emissions from volcanic activity, they became so acidic that crustaceans dissolved in the water and died. The other is that the oceans lose their oxygen and become toxic to all but the hardiest ocean creatures.
The main question we set out to address is this: What are the specific killing mechanisms of marine life at the end of the Triassic? said Jessica Whiteside, a geochemist at the University of Southampton in England and author of the new study. “The answer that helps predict future ecological and biodiversity impacts of current CO2, and perhaps helps provide context.”
Dr. Whiteside described the discovery of clues in the rocks of the Blue Lias Formation in England, which emerged after volcanism.
“What I noticed early on were these strange ghost fossils,” he said. Ghost fossils are impressions that, like shells, remain in the rock but do not contain any remains from the shells that formed them – a sign that the shells were dissolved in acidic waters.
Other clues were chemical traces or “biomarkers” of a type of bacteria known to thrive in anaerobic waters and waters with dangerously high levels of a toxin called hydrogen sulfide.
Sea life, bathed in poisonous waters with no oxygen to breathe, were doomed to die as well as to dissolve alive.
Noah Planavsky, a biogeochemist at Yale University who was not involved in the research, said the discovery of the biomarkers provides strong evidence for toxic, oxygen-deficient waters. “This is something we can expect in the future,” he said in our contemporary oceans.
These killing mechanisms also reveal that mass extinctions aren’t always instantaneous events, like an asteroid hitting the planet, said vertebrate paleontologist Stephen Brusatte of the University of Edinburgh, who was not involved in the new study.
Dr. “We’re used to thinking of mass extinctions as these singular catastrophic events where there’s only one killer on whom we can blame it all,” Brusatte said. “But this study shows that there are often nuances in these mass death events.”
It is less clear what triggered extinctions on land. Until the late Triassic, relatives of modern-day crocodiles dominated land ecosystems, while early dinosaurs were relatively minor players. But after the extinction, crocodile relatives disappeared and dinosaurs began to be in the limelight.
Dr. “This part of the story is still little known compared to what happens in the oceans, and it is intriguing to wonder if there are multiple killing mechanisms on land as well,” Brusatte said. “If so, it might help explain why these dinosaurs were able to survive and then disperse across the wasteland.”
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