Earth's largest mass extinction to date is sometimes called the Great Dying—and for good reason: it wiped out about 70 percent of life on land and 95 percent in the oceans. Researchers have long cited intense volcanism in modern-day Siberia as the main culprit behind the cataclysm, also known as the Permian-Triassic mass extinction, 252 million years ago. A recent study pins down crucial details of the killing mechanism, at least for marine life: oceans worldwide became oxygen-starved, suffocating entire ecosystems.

Scientists had previously suspected that anoxia, or a lack of oxygen, was responsible for destroying aquatic life. Supporting data came from marine rocks that formed in the ancient Tethys Ocean—but that body of water comprised only about 15 percent of Earth's seas. That is hardly enough to say anything definitive about the entire marine realm, says Feifei Zhang, a geochemist at Arizona State University, who led the new research.

In contrast, Zhang says, “our data point to a rapid, global intensification of marine anoxia during the Permian-Triassic mass extinction.” The key to the team's finding, which was published in April in Geology, is a novel method that uses uranium measurements in rocks to infer ancient oceanic oxygen levels. This approach enabled the researchers to spot clues in rocks from Japan that formed around the time of the extinction in the middle of the Panthalassic Ocean, which then spanned most of the planet and held the majority of its seawater. “The most exciting thing is the ... global signature they're seeing,” says Gregory Brennecka, a geochemist at the University of Munster in Germany, who was not involved in the study.

The findings may have special relevance in modern times because the trigger for this ancient anoxia was most likely climate change caused by Siberian volcanoes pumping carbon dioxide into the atmosphere. And today, as human activity warms the planet, the oceans hold less oxygen than they did many decades ago. Brennecka cautions against speculating about the future but adds: “I think it's pretty clear that when large-scale changes happen in the oceans, things die.”