A massive series of volcanic eruptions in the earth's distant past made the sea creatures gasp. The greenhouse gases emitted by the volcanoes have drastically reduced the oxygen content in the oceans, a deadly scenario that may have been the main cause of the Great Dying, researchers report.

Earth scientist Justin Penn of the University of Washington at Seattle and his colleagues showed how hot the oceans were at the time of the largest mass extinction on Earth about 252 million years ago at the end of the Permian period. From these climate simulations, the team investigated where hot water led to anoxia in the ocean – dangerously low levels of dissolved oxygen.

Then the team combined this data with the oxygen demand of modern marine life. The scientists found that hypoxia – a lack of oxygen for the metabolic needs of species – could have been the main cause of death. The research, which was published on December 7 sciencealso predicts that the effects of hypoxia at polar latitudes would have been worst, and available fossil data supports this finding.

"Anoxia has been the primary killing mechanism for marine extinction for 20 years," said Lee Kump, a geochemist at Penn State, in the same issue of Science. The unique feature of this study is the inclusion of the effects of anoxia on organisms that live in different ecological niches in the oceans, he says.

In the dying died 90 percent of all marine animals and 70 percent of the terrestrial vertebrates. Massive volcanic eruptions, which triggered impulses that began about 300,000 years before the extinction of events, were most likely the catalyst for the great dying (SN: 19.09.15, p. 10).

But how exactly these outbreaks led to dying is not clear. There are many ways in which the volcanoes could have made the Earth unsustainable. The volcanoes emitted large outbreaks of carbon dioxide and methane, powerful greenhouse gases that rapidly and dramatically increased the temperatures on land and at sea. The outbreaks could also have punched holes in the ozone layer, causing ultraviolet radiation to blow up the planet and sterilize plants on land (SN Online: 12.02.18).

The seas had the biggest blow. Sea temperatures rose at least 10 degrees Celsius in the tropics, and ocean acidification or hypoxia could have killed many creatures.

To identify a leading culprit, Penn and his colleagues decided to look at the animals themselves. Or rather, modern representatives of long extinct species. The team noted where the oxygen supply to various marine organisms would have fallen under the oxygen demand – for nutrition, reproduction and defense.

The tropics suffered, the researchers found, but many species there have adaptations that allow them to survive warming waters and oxygen depleted conditions. The worst fatalities due to lack of oxygen would have happened at high latitudes where creatures have no such adjustments and nowhere to go.

The team also searched a huge online database of fossils, the Paleobiology Database, to search for geographic patterns that were threatened with extinction. To the surprise of the researchers, the fossils indicated that the species are more affected by the poles than in the tropics. Such a pattern has not yet been reported, says the biological oceanographer Curtis Deutsch, also from the University of Washington and a co-author of the study. "No one has ever described a difference between the latitudes," he says. The similarities between fossil record and model data are "scary," he says.

The team also investigated the role of ocean acidification. The hyperacidity would have had the biggest impact in the tropics, not at the poles. "It's not proof, but a clear indication that the underlying mechanism was this oxygen loss," says Deutsch.

It is not very clear whether more creatures died at the end of the Permian at the poles. Fossil recordings can be inconsistent, speak German and therefore show an incomplete picture. However, the seemingly higher risk of death in high latitudes has occurred in many different species of species, from vertebrates such as fish to shelled creatures such as molluscs.

One of the most surprising results of the new study is the geographic pattern of extinction intensity, says Kump. He welcomes the "novel and sophisticated" approach used by researchers to study hypoxia as the major culprit, although it notes that volcanic gases have likely also rendered oceans toxic to oxygen breathing equipment in other ways, for example by adding hydrogen sulphide and carbon dioxide to the water

However, the new research is "the most comprehensive analysis of a killing mechanism and its physiological impact that has been done so far. It is really a progress. "