A massive series of volcanic eruptions in the distant past of the Earth, creatures of the sinister ocean that were panting to breathe. The greenhouse gases emitted by the volcanoes have drastically lowered the levels of oxygen in the oceans, a deadly scenario that may have been the main culprit of the Great Dying, the researchers report.
Earth scientist Justin Penn of the University of Washington in Seattle and colleagues analyzed how the oceans were warm at the time of the largest mass extinction on Earth, about 252 million years ago, at the end of the Permian. From those climate simulations, the team studied where hot water led to oceanic anoxia, dangerously low concentrations of dissolved oxygen.
Thus, the team combined these data with the oxygen requirements of modern ocean dwellers. Scientists determined that hypoxia – a lack of sufficient oxygen for the metabolic needs of the species – could have been the main cause of death. The research, published in 7 December Sciencemoreover, it foresees that the effects of hypoxia would have been worse at polar latitudes and the support of available fossil data would result.
"Anoxia has been invoked as a primary killing mechanism for marine extinctions for 20 years," says Lee Kump, a Penn State geochemist who wrote a commentary on the find in the same issue. Science. But what is unique in this study is the inclusion of how that anoxia affects organisms that live in different ecological niches within the oceans, he says.
In the Great Morire, as many as 90% of all marine species and 70% of terrestrial vertebrate species died. The massive volcanic eruptions, unloading the impulses that began about 300,000 years before the beginning of the extinction, were almost certainly the trigger of the Great Morire (SN: 19/9/15, p. 10).
But how, exactly, those eruptions led to death is not clear. There are many ways volcanoes could have made the Earth unsustainable. The volcanoes emitted great explosions of carbon dioxide and methane, powerful greenhouse gases that rapidly and dramatically increased temperatures on land and sea. Eruptions can also have holes punctured in the ozone layer, allowing ultraviolet radiation to explode the planet and perhaps sterilize plants on earth (SN Online: 2/12/18).
Oceans have taken the greatest success. Ocean temperatures have risen by at least 10 degrees Celsius in the tropics, and ocean acidification or hypoxia may have hit a fatal blow to many creatures.
To identify a principal culprit, Penn and his colleagues decided to take a look at the animals themselves. Or rather, to the modern stand-in for long-extinct species. The team determined where in the ocean the supply of oxygen would fall below the demand for oxygen – for nutrition, reproduction and defense – for various creatures.
The tropics have suffered, the researchers found, but many species have adaptations that allow them to survive the warming water and lower oxygen conditions. The worst of the death toll due to the lack of oxygen would have happened at high latitudes, where creatures do not have such adaptations and have nowhere to go.
The team also scoured a huge online database of fossils, the Paleobiology database, to look for endangered geographic models. To the surprise of the researchers, the fossils have suggested that the species have suffered more at the poles than in the tropics. Such a model had not been previously reported, says the biological oceanographer Curtis Deutsch, also from the University of Washington and co-author of the study. "No one had ever described a difference in latitude," he says. The similarities between fossil data and model data were "disturbing," he says.
The team also considered the role of ocean acidification. But acidification, it turns out, would have had the greatest impact in the tropics, not at the poles. "It's not a test, but a strong indication that the underlying mechanism was this loss of oxygen," says Deutsch.
If more creatures are actually dead at the poles at the end of the Permian it is not entirely clear. The fossil record can be clear, recognizes Deutsch and therefore presents an incomplete picture. But, he notes, the apparently higher risk of death at high latitudes has appeared in many different types of species, from vertebrates like fish to bombed creatures like mollusks.
One of the most striking results of the new study is the geographical pattern of the intensity of extinction, says Kump. Applauds the "new and sophisticated" approach that researchers have taken to examine hypoxia as the main culprit, although it notes that volcanic gases have probably made the oceans toxic to respiratory oxygen even in other ways, including adding hydrogen sulfide and carbon dioxide to water
Yet, he says, the new research is "the most complete analysis of a killing mechanism and its physiological impacts that has been done so far." It really is a step forward. "