Some 200 million years ago, the rocks that became the Palisades cliffs just across the Hudson River from Manhattan formed during volcanic activity that helped rip apart the ancient supercontinent Pangea. That volcanism helped lead to the birth of the Atlantic Ocean while it also contributed to killing off as much as a quarter of all life on Earth during the event known as the end-Triassic mass extinction.

Marine animals like ammonites, ichthyosaurs and corals took huge hits during the extinction, and scientists have long suspected that the Atlantic-forming volcanism had something to do with it because of its effects on the climate and oceans. But evidence of what exactly killed life has been scant, 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 Geology, though, is starting to fill in the gaps of this prehistoric murder mystery.

By studying rocks in the southwest of England, a team of scientists found evidence of two triggers. One is that as oceans absorbed carbon dioxide emissions from the volcanic activity, they became so acidic that animals with shells dissolved in the water and died. The other is that the oceans lost their oxygen and became toxic to all but the most hardy ocean creatures.

“The main question that we set out to address is: What are the specific kill mechanisms of marine life at the end-Triassic?” said Jessica Whiteside, a geochemist at the University of Southampton in England, and an author of the new research. “The answer to which helps provide context for, and perhaps helps predict the future ecological and biodiversity effects of current CO2.”

Dr. Whiteside described the discovery of clues in rocks of England’s Blue Lias Formation, which emerged in the wake of the volcanism.

“What I noticed early on were these weird ghost fossils,” she said. Ghost fossils are impressions of things like shells that remain in the rock, but without any remnant of the shells that made them — a sign that the shells 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 — on top of being dissolved alive — was all but doomed.

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

These kill mechanisms also reveal how mass extinctions aren’t always instant 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 work.

“We’re used to thinking of mass extinctions as these single catastrophic events, where there is a lone killer that we can put all of the blame on,” Dr. Brusatte said. “But this study shows that there is often nuance to these episodes of mass death.”

Less clear is what drove the extinctions on land. Until the end-Triassic extinction, relatives of today’s crocodiles dominated land ecosystems, while early dinosaurs were relatively minor players. But after the extinction, the crocodile relatives vanished, and dinosaurs started shifting into the limelight.

“This part of the story is still poorly known compared to what was happening in the oceans, and it’s intriguing to wonder whether there were multiple kill mechanisms on land, too,” Dr. Brusatte said. “If so, this could help explain why the dinosaurs were able to survive, and then disperse across the wasteland world in the aftermath.”