A massive series of volcanic eruptions in the distant past of Earth have left ocean creatures bursting. The greenhouse gases emitted by volcanoes have drastically reduced oxygen levels in the oceans, a deadly landscape that may have been the main culprit of the Great Death, researchers report.
Earth scientist Justin Penn of the University of Washington in Seattle and his colleagues mapped 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 weather simulations, the team investigated where hot water led to oceanic anoxia, dangerously low concentrations of dissolved oxygen.
The team then combined these data with the oxygen requirements of the modern oceans. Scientists determined that hypoxia – lack of enough oxygen for the metabolic needs of species – could have been the main culprit behind death. The research, published on December 7 Science, also predicts that the effects of hypoxia would have been worse in polar latitudes and support for available fossil data would result.
"Anoxia has been invoked as a primary mechanism for extinction of marine extinctions for 20 years," says Lee Kump, a geochemist at Pennsylvania State University who wrote a commentary on the finding in the same issue of the report. Science. But what's unique about this study is the inclusion of how anoxia affects organisms living in different ecological niches within the oceans, he says.
At Great Death, 90% of all marine species and 70% of terrestrial vertebrate species died. Massive volcanic eruptions, discharging into pulses that began about 300,000 years before the onset of the extinction event, were almost certainly the trigger for the Great Dying One (SN: 19/9/15, p. 10).
But how exactly these eruptions led to death is unclear. There are many ways in which volcanoes could have made the Earth unsustainable. The volcanoes emitted large explosions of carbon dioxide and methane, powerful greenhouse gases that have dramatically increased temperatures on land and at sea. The eruptions may also have drilled holes in the ozone layer, allowing ultraviolet radiation to blow up the planet and perhaps sterilize plants on land (SN Online: 12/2/18).
Oceans led the greatest success. Ocean temperatures have risen by at least 10 degrees Celsius in the tropics, and acidification or oceanic hypoxia may have caused a fatal blow to many creatures.
To identify one of the main culprits, Penn and his colleagues decided to take a look at the animals themselves. Or rather, in modern substitutes for extinct species long ago. The team determined where in the ocean the oxygen supply would have fallen below the oxygen demand – for food, breeding and defense – for various creatures.
The tropics have suffered, researchers have discovered, but many species have adaptations that allow them to survive by heating the waters and reducing oxygen conditions. The worst of the deaths from lack of oxygen would have happened in high latitudes, where creatures do not have such adaptations and have nowhere to go.
The team also rummaged through a huge online fossil database, the Paleobiology Database, to look for endangered geographical patterns. To the researchers' surprise, the fossils suggested that species suffered more at the poles than in the tropics as well. Such a pattern had not been reported previously, says biological oceanographer Curtis Deutsch, also of the University of Washington and co-author of the study. "No one has ever described a difference in latitude," he says. The similarities between the fossil record and the model data were "strange," he says.
The team also considered the role of ocean acidification. It turns out that acidification would have the greatest impact on the tropics, not the poles. "It's not a proof but a strong indication that the underlying mechanism was this loss of oxygen," says Deutsch.
If more creatures actually died at the poles at the end of the Permian is not entirely clear. Fossil records may be irregular, recognizes Deutsch, and therefore presents an incomplete picture. But, he notes, the apparently higher risk of death in high latitudes has appeared in many different types of species, from vertebrates like fish to bark-like creatures like mollusks.
One of the most surprising findings of the new study is the geographic pattern of extinction intensity, says Kump. He applauds the "new and sophisticated" approach that researchers have considered hypoxia as the main culprit, although he notes that volcanic gases have probably made the oceans toxic to oxygen respirators as well, including the addition of hydrogen sulfide and carbon dioxide to the water.
Still, he says, the new research is "the most comprehensive analysis of a mechanism of death and its physiological impacts that have been made so far. It's really a step up. "