How could the worst extinction wave in history occur 250 million years ago? A new piece has been found in China to complete the puzzle: supervolcanism.
Life on Earth has taken its toll, but one disaster wiped out nearly everything that grew and crawled. Complete forests disappeared, no coral reef survived. It was over for the armored, woodlice-like trilobites and sea scorpions, abundant in the sea for hundreds of millions of years. An estimated 80 to 90 percent of all animal species: gone.
It happened at the end of the geological period called the Permian, 250 million years ago. The Great Dying, the English don’t call this event for nothing. Scientists speak of the Permian-Triassic mass extinction.
What went wrong here? As violent as the mass extinction was, its cause is still shrouded in mystery.
Chinese researchers are now putting a new puzzle piece on the table: supervolcanism in what is now China. In addition, such large amounts of sulfur must have been released that a harsh volcanic winter began worldwide, they write in the trade journal Science Advances.
There’s a simple reason that such a large eruption has been able to stay under the radar until now: 250 million years is a long time ago even for the seasoned geologist. Clues must come from rock layers that were formed at the time and there are not many of them left. And if you’re already finding rock from that time, demonstrating a major volcanic eruption takes better detective work.
Mercury and copper
The Chinese researchers found their evidence in layers of petrified clay and sand, deposited in ancient lakes, rivers and swamps. Those layers are at first full of remnants of a flourishing tropical rainforest – and then, suddenly, no more.
The sudden increase in concentrations of mercury and copper at the same time is striking. The chemical signature of these metals suggests a volcanic origin, the kind that releases a lot of sulfur into the air, the researchers write. The fact that ash deposits and volcanic rocks from the same period have been found in the region strengthens their conclusion.
Based on the large amounts of copper found, a measure of the intensity of the volcanism, the researchers estimate that so many sunlight-blocking sulfur particles ended up in the air that it became several degrees colder on Earth. This is a very rough estimate and it is impossible to say whether the cooling lasted a few years or hundreds of years earlier.
After the sulfur particles had rained out, the temperature quickly rose again, with the extra CO2 pumped into the atmosphere by the volcanism giving an extra push. Life took a double blow.
A plausible and well-founded story, according to paleoecologist Cindy Looy (University of California, Berkeley) and earth scientist Bas van de Schootbrugge (Utrecht University), who both study mass extinctions. But the case is by no means closed. Because as Looy puts it: ‘You try to make a puzzle when you have ten out of a hundred pieces.’
In Siberia, for example, there is still a smoking gun so large that it could cover the whole of Australia: kilometers thick slabs of solidified lava, the so-called Siberian Steps. Over hundreds of thousands of years, this lava has been spewed out by volcanoes, around the time of the mass extinction event. That can hardly be a coincidence.
Only that volcanism in Siberia had been going on for hundreds of thousands of years when plants and animals started to die, explains Cindy Looy. ‘Ecosystems may have already been under pressure from the effects of the Siberian Traps. And gave the newly discovered volcanism in China a final push, at least in the vicinity of southern China.’
Climate on hold
There are still more questions open about this period. On land, for example, why was there a repeated collapse of forests during the crisis, over many thousands of years, while ocean life apparently suffered one death blow? Looy: ‘Every time you think you understand it a bit, something new pops up that changes things.’
One thing is clear: the earth was not a pleasant place at the end of the Permian. According to a study published this year in Nature the climate went completely crazy. The researchers base this on the significant shift in the ratio of light and heavy carbon atoms during the crisis, which can be explained by the release of a great deal of light carbon.
From this they deduce that so much CO2 went into the air that the concentration appears to have multiplied several times over a time scale of tens of thousands of years, with all the warming that entails. Volcanism may have first caused emissions and warming, then the strong greenhouse gas methane was released from melting permafrost or the warmed ocean floor, they suggest.
Or take the remains of green sulfur bacteria, which have a rather unique combination of properties: they live in anoxic water and need sunlight. Usually, lack of oxygen is limited to greater depths, says Bas van de Schootbrugge. So at the end of the Permian the suffocating water may have reached just below the surface, where the light still penetrated.
If we then try to put the puzzle together, what picture emerges? Van de Schootbrugge thinks that problems first arose on land from climate change, pollution and other misery caused by volcanism to which the newly discovered eruption in southern China may have contributed. ‘As a result, forests died and large areas became more or less fallow.’
algae
Desolate enough, but for marine life, the crisis only started with this, he outlines. Without the sturdiness of tree roots washed away the bottoms, into the rivers, to the sea. As a result, large amounts of nutrients ended up in the ocean. It was as if a gigantic container of manure had been emptied: algae started to bloom en masse.
Oxygen is needed to break down dead algae material, drawing the oxygen from the ocean. A similar process is in freshwater lakes in Australia observed from that time, says Van de Schootbrugge: ‘You get a bit of what we know from our summers, with blue-green algae in the swimming lakes. But on a much larger scale.’
Ocean acidification, a result of the increased amount of CO2 that dissolves in water, probably came on top of this. The entire process probably took place over the course of several hundred thousand years, as insidious as it was devastating.
It is a rough picture, admits Van de Schootbrugge. ‘The most important issue, which we are also working on in Utrecht, is how you can really link volcanism to plants and animals that go out of the pipe.’
He himself is involved in research into whether the violent volcanism could lead to mercury poisoning. ‘On the Permian-Triassic border you see a lot of fern spores and tree pollen with genetic abnormalities. These seem to link to higher mercury concentrations.’
Meanwhile, Cindy Looy is considering the idea that holes were created in the ozone layer, causing plants and animals on land to be irradiated with deadly UV light. When the Siberian Steps were created, salt deposits and rock layers full of organic material would have been heated by the magma. It may have released the same gases that caused the hole in the ozone layer in more recent years, she explains.
And indeed: if you throw the relevant rock into the oven, like Norwegian researchers did, then these ozone-destroying gases are released. In addition, plant pollen from the mass extinction era has been found around the world with deformities very similar to modern-day pollen irradiated with UV light, according to a new study. research by, among others, Looy.
Tipping Points
Why, as a scientist, would you put so much effort into an extinction wave from so long ago? ‘These are tipping points in the terrestrial system’, answers Van de Schootbrugge. “We’ve had five major extinctions, of which this is the largest. For a long time it was thought that these were sharp boundaries: everything changed in one fell swoop. We have now learned that these were very complex events, with lead times and consequences that lasted a long time.’
Interesting, because even now a tipping point may be approaching, he says, with a view to climate change, environmental pollution, deforestation and soil erosion. It will not be as dramatic as at the end of the Permian. ‘But we can learn from the past how bad things can get on Earth and how processes can reinforce each other through feedback effects.’
This shows that if many species disappear, it can take a long time for nature to recover. ‘After the Permian-Triassic extinction, but also after the more modest Triassic-Jurassic extinction, you were left with disturbed ecosystems for millions of years.’
It is difficult to draw concrete lessons from then about now, says Cindy Looy. But if the disasters of the past have shown her one thing, it’s that it doesn’t take that much to unbalance the Earth system. ‘It’s quite easy to change the atmosphere. People often do not realize how thin and vulnerable it actually is. Some think: we are not big enough to influence the earth, are we? But man is a geological force.’