Dissolving in Toxic Oceans: How an Ancient Extinction Happened

About 200 million years ago, the rocks that became the Palisades Cliffs just across the Hudson River from Manhattan were formed in volca...


About 200 million years ago, the rocks that became the Palisades Cliffs just across the Hudson River from Manhattan were formed in volcanic activity that helped tear apart the ancient supercontinent Pangea . This volcanism contributed to the birth of the Atlantic Ocean while helping to kill up to a quarter of all life on Earth in the event known as the late Triassic mass extinction.

Marine animals like ammonites, ichthyosaurs and corals suffered huge blows during the extinction, and scientists have long suspected that Atlantic formation volcanism had something to do with it due to its effects on climate and the oceans. But evidence of exactly what killed life is sparse, making it one of the least understood of the so-called Big Five mass extinctions that punctuate the history of life on Earth.

Research published in January in the journal Geologyhowever, begins to fill in the gaps in this prehistoric murder mystery.

While studying rocks in South West England, a team of scientists found evidence of two triggers. The first is that when the oceans absorbed carbon dioxide emissions from volcanic activity, they became so acidic that shelled animals dissolved in the water and died. The other is that the oceans have lost their oxygen and become toxic to all but the hardiest ocean creatures.

“The main question we decided to answer is: what are the specific mechanisms of destruction 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 research. “The answer to which helps provide context and perhaps predict future ecological and biodiversity effects of current CO2.”

Dr Whiteside described finding clues in rocks of the Blue Lias Formation in England, which emerged as a result of volcanism.

“What I noticed early on were these weird ghost fossils,” she said. Ghost fossils are impressions of things like seashells that remain in rock, but without any remnants of the shells that made them – a sign that the seashells have dissolved in acidic waters.

Other clues were chemical traces, or “biomarkers”, of a kind of bacteria known to thrive in waters without oxygen, and where there are dangerously high levels of a toxin called hydrogen sulfide.

Bathed in toxic waters with no oxygen to breathe, marine life – apart from being dissolved alive – was virtually doomed.

Noah Planavsky, a biogeochemist at Yale University who was not involved in the research, said the discovery of the biomarkers provided strong evidence of toxic, oxygen-poor waters. He added that “this is something we can expect in the future”, in our contemporary oceans.

These destruction mechanisms also reveal how mass extinctions are not always instantaneous events like an asteroid hitting the planet, said Stephen Brusatte, a vertebrate paleontologist at the University of Edinburgh who was not involved in the new works.

“We used to think of mass extinctions as those one-time catastrophic events, where there’s a lone killer that we can put all the blame on,” Dr Brusatte said. “But this study shows that there are often nuances to these mass death episodes.”

What led to the terrestrial extinctions is less clear. Until the end of the Triassic extinction, relatives of today’s crocodiles dominated terrestrial ecosystems, while early dinosaurs were relatively minor players. But after the extinction, the relatives of the crocodiles disappeared and the dinosaurs began to come into the limelight.

“This part of the story is still poorly understood compared to what was happening in the oceans, and it’s fascinating to wonder if there were multiple mechanisms of destruction on land as well,” Dr Brusatte said. “If so, this could help explain why the dinosaurs were able to survive and then disperse across the wasteland world afterward.”

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