Agents of extinction
Why have some monster volcanoes triggered global catastrophes while others have not? KATE RAVILIOUS reports.
WELCOME TO the late Permian. It’s 253 million years ago and life on Earth is thriving.
THE OCEANS ARE A SHOWPIECE of biodiversity, sporting trilobites, molluscs, coral and fish. On land, animals and plants have moved away from the swamps to stake out their lives on dry land. Blunt-horned reptiles the size of cows, known as pareiasaurs, crash through vast conifer and fern forests, chased by sleek, fast predators known as gorgonopsians – mammal-like reptiles that are relatives of the lineage that gave rise to mammals.
Fast-forward one million years and Earth is a barren wasteland. Entire ecosystems have disappeared. Rocks all over the world bear witness to the catastrophe with an abrupt change in the appearance of their layers – the so-called Permian-triassic transition. Below the transition there is limestone, coal and an abundance of Permian species. Immediately above it’s mostly lifeless shale – only 5% of species have made it through. In the oceans, the coral reefs and most species have vanished. Tiny clams and snails are the main survivors. On land, the forests have gone, and with them the insects and animals. It’s referred to as the “Great Dying” and it is the biggest mass extinction our planet has ever experienced.
What caused it? Geologists have identified the smoking gun: the Siberian Traps in Northern Siberia. Traps, the Swedish word for stairs, refers to the stepped appearance of lava flows that oozed from a vast rift in the Earth’s crust for nearly one million years. Flowing in fits and starts, like a chocolate fountain, it covered a region the size of Western Europe in lava one kilometre thick.
Earth was hell. Ash filled the skies and sulphurous acid rain poured down. Chlorine and bromine gases tore a hole in the ozone layer, letting UV light through to shred the DNA of land animals and plants. Volcanic CO2 and methane emissions sent global temperatures soaring.
For land species, it was probably the assault from UV light that tipped them over the edge. But in the oceans, it was the heat. The seas turned into a hot bath with temperatures reaching 40 degrees. As water heats, it loses its ability to dissolve gases like oxygen. The heat also drove bacterial blooms that consumed oxygen. The loss of oxygen tuned the oceans into a dead zone; sea life suffocated, even the hardy trilobites. Overall, life was set back 300 million years. Undisturbed by animals, the ocean floors became covered in bacterial slime, just as they had been during the Precambrian era.
Our planet produces one of these monstrous “flood basalt” eruptions roughly every 30 million years. They occur, scientists believe, when portions of the ocean floor sink into Earth’s mantle, leaving magma nowhere to go but up. Compared with the brief, explosive volcanic eruptions we are familiar with today, flood basalts are in a class of their own.
But not all of them are as deadly as the Siberian Traps. A similar eruption burst forth about 60 million years ago in the landmass now covered by the North Atlantic. It created the spectacular rock columns known as the Giant’s Causeway in Northern Ireland, as well as the soaring cliffs of Baffin Island on Canada’s northeast flank and western Greenland. Again, a vast area of the Earth was deluged in magma and the climate warmed. Yet on this occasion, things were different. Instead of the mass extinction of reptiles and forests, crocodiles moved poleward to bask on Alaskan beaches beneath swaying palms.
What makes one mega-eruption catastrophic to life on Earth, while another brings only minor setbacks? It’s a question that has puzzled Paul Wignall, a geologist at the University of Leeds in the UK. A renowned mass extinction detective, Wignall travels the world in search of the clues left by these global catastrophes, gathering evidence to understand what caused them. Now he thinks he has spotted a curious link between the arrangement of Earth’s continents and the potential of a volcanic eruption to cause mass extinction. Kate Ravilious recently spoke with Wignall, and he explains the evolution of his idea and what past extinctions may teach us about a warming Earth.
IT’S REFERRED TO AS THE ‘ GREAT DYING’ AND IT IS THE BIGGEST MASS EXTINCTION OUR PLANET HAS EVER EXPERIENCED.
KATE RAVILIOUS: You’ve been studying mass extinctions for more than 30 years. What attracted you to this line of research?
PAUL WIGNALL: I find mass extinctions fascinating. They’re the worst disasters in the history of this planet and tell us how our world responds to severe crises. When I first became interested in mass extinctions in the early 1980s, the idea that the dinosaurs were killed by a meteorite – now widely accepted – was new, and one of the most exciting theories in science.
KR: In your recent book, The Worst of Times, you suggest that mass extinctions are more likely when the continents are arranged in one big landmass – a supercontinent. How did you arrive at this theory?
PW: Ever since I started work in this field, I’ve noticed that the correlation between volcanism and extinction events comes and goes, and I’ve wondered why. Around seven years ago, I was working on a 260-million-year-old mass extinction event and its link with an eruption in southwest China, known as the Emeishan Traps. By dating the fossils found just underneath the lava, we were able to show that the timing of the eruption matched the timing of the extinctions seen elsewhere around the world.
Having convinced ourselves that this eruption triggered the mass extinction, it struck me that there was a string of at least six volcano-driven mass extinctions in the distant past, including the Great Dying, and that they all coincided with the existence of a single supercontinent known as Pangea that began to break up about 175 million years ago. I began to wonder if the existence of the supercontinent made the impact of the volcanoes worse than they might otherwise have been.
KR: Can you walk us through the logic of why this relationship likely exists?
PW: Fragmented continents are better able to remove CO2 from the atmosphere than a single supercontinent. It’s all to do with rocks reacting with CO2 and trapping it chemically as bicarbonate ions which are washed into streams and eventually shallow seas. Sea creatures take up the bicarbonate into their shells, and when they die, carbon is buried for the long-term as limestone (calcium carbonate). The conveyer belt that moves CO2 from the atmosphere into rock minerals is rainwater which dissolves the gas and, being slightly acidic, breaks down the rock so that carbon-trapping chemical reactions take place.
The conveyer belt works well with small continents because nowhere is too far from the sea, so they have wetter climates and therefore can move more CO2 into limestone. Small continents also have more shallow seas due to their trailing continental shelves, so they provide more habitat for limestone-building creatures. Stick all the continents together into a giant land mass and you end up with vast interior deserts, and fewer shallow seas. It is easier to end up with runaway global warming in a supercontinental world.
KR: So of all the factors that go with megavolcanism, such as corrosion of the ozone layer and acid rain, you’re suggesting it’s actually the rising CO2 levels that tip life over into a mass extinction?
PW: Yes, I think this is the crucial factor. Global warming has many harmful consequences: the heat itself can be a real problem, especially in lower latitudes, and knock-on effects like oxygen loss in the oceans are also devastating to life. No doubt there are other factors involved too, and ozone destruction, even for short intervals, can be harmful particularly for terrestrial plant life.
KR: In hindsight, the relationship between supercontinents, volcanism and mass extinctions seems obvious. Why hadn’t anyone spotted it before?
PW: Up until now, most research has fallen into two camps: either volcanism did it, or something else – like a meteorite impact – did it. In fact, I think the relationship between volcanoes and extinctions is more nuanced. There was always a problem with the “volcanoes-did-it” hypothesis because there are plenty of giant eruptions not linked with extinctions. A supercontinent is the missing link in the chain.
KR: What theories had other scientists put forth about why some volcanoes triggered mass extinctions and others did not?
PW: Around a decade ago, Henrik Svensen of the University of Oslo came up with the idea that some volcanoes push magma through “juicier” [more carbon-rich] areas of the Earth’s crust than others. As the magma goes through the crust it bakes the rocks around it, and if that rock happens to be coal or shale it releases additional CO2. By contrast, when magma passes through ancient, dry crust, it doesn’t produce this extra pulse of greenhouse gases.
It is a good and valid hypothesis. But there are several cases that fail to fit the theory, such as the 60-million-year-old North Atlantic eruption, which passed through coal deposits. But it isn’t associated with a significant mass extinction event.
THERE WAS A STRING OF AT LEAST SIX VOLCANODRIVEN MASS EXTINCTIONS AND THEY ALL COINCIDED WITH THE EXISTENCE OF A SINGLE SUPERCONTINENT.
KR: What’s the best evidence to support your idea?
PW: I think it’s the 100% correspondence between volcanism and extinction during the time of Pangea. That’s a six-out-of-six hit rate. Since Pangea broke apart there have been another six major phases of eruption, but only one coincides with a mass extinction – and that’s at the end of the Cretaceous, when a giant meteorite hit.
KR: So how do you explain that extinction event, which occurred when the continents were spread out?
PW: The Cretaceous-paleogene mass extinction, which wiped out the dinosaurs 66 million years ago, occurred at a time when Earth’s continents were widely dispersed. The extinction coincides with the eruption of the Deccan Traps, flooding the west coast of India with one million cubic kilometres of lava. But it also coincides with a giant meteorite impact at Chicxulub in Mexico. Disentangling the role of the Deccan eruption and the meteorite impact is really tricky, but it is likely that the combination of the two events was what made it so fatal to life on Earth. If the meteorite impact hadn’t occurred at the same time I think the dinosaurs might have made it through.
KR: What have critics said about this idea of a link between extinctions and supercontinents?
PW: Volcanic extremists believe that all large-scale volcanism causes extinctions and they downplay the role of Pangea because it only existed for 80 million years. It is true that all the largest volcanic eruptions caused climate change. But those that occurred at the time of Pangea were much more intense and it is only these that coincide with widespread extinctions. Some scientists also criticise the theory because as we go deeper back in time, there are some mass extinctions that aren’t as clearly related to volcanism – for instance the Ordovician-silurian extinction 443 million years ago. But the situation [there] was different, because there were no plants on Earth, so the carbon cycle didn’t work in the same way.
KR: What key evidence are you lacking to support your hypothesis?
PW: There’s lots we don’t know about the actual details of mass extinction. Some are caused by oxygen starvation in the ocean and warming, but others are more enigmatic. We particularly need to know more about the environmental changes on land during extinction events.
KR: What are the challenges in trying to understand this? PW: Ideally we’d like to have more examples
IF THE METEORITE IMPACT HADN’T OCCURRED AT THE SAME TIME I THINK THE DINOSAURS MIGHT HAVE MADE IT THROUGH.
of massive volcanism, but once we go back beyond a few hundred million years, the lava fields tend to have been lost to erosion and so [those events] have to be inferred from other things. The dating techniques available for these ancient eras are also much less precise, making it harder to line up the dates of volcanic activity with the dates of known extinction events.
KR: The most notable example of a mass extinction during the period of Pangea was the end-permian extinction 252 million years ago, which wiped out most life on Earth and is the worst extinction in Earth’s history. Why was it so singularly catastrophic?
PW: It’s difficult to answer that – a trite answer would just say that there was a lot more gas released from volcanoes during that crisis.
But it may have triggered runaway global warming. It’s the only mass extinction to have seen the complete loss of terrestrial forests. Forests are a great way of removing CO2 from the atmosphere, and without them the crisis in effect got past the point of no return.
KR: How certain is it that the mass extinctions you’ve got in your sights were in fact triggered by volcanism? Other researchers have suggested that asteroid impacts, cosmic ray blasts or reversals of Earth’s magnetic field could all be culprits.
PW: Yes, much effort is being expended on looking for evidence of other extinction styles but none have yet received much support. One of the aspects of mass extinctions we understand better nowadays is that they occurred over several thousand years. This makes it difficult to invoke a “single strike” cause like an impact or cosmic ray blast from a supernova.
KR: Why is it valuable to understand why some historic volcanoes triggered mass extinctions while others did not?
PW: It gives us much more insight into how life on Earth has evolved. The gigantic volcanic eruptions that occurred when Earth only had one massive supercontinent – Pangea – caused a series of crises that fundamentally changed the course of life and removed vast numbers of animals and plants that would otherwise have thrived. By contrast, the dinosaurs evolved during a climate-buffering phase as Pangea was splitting apart. Perhaps that explains how they took centre stage for such a very long time. Massive volcanic eruptions from the past are also the only natural process to provide an analogue for our current impact on the atmosphere.
KR: What can we learn from these climate-induced extinctions of the distant past about how modern-day warming will affect life?
PW: We are pumping CO2 into the atmosphere at a far faster rate than any previous volcanic eruption, and species are going to have to shift poleward to cope with warming – something that happened after the North Atlantic volcanism 60 million years ago. However, shifting polewards is more difficult for today’s wild plants and animals because natural habitats are so fragmented by urban build-up and agriculture.
In the oceans, the clear story from past mass extinctions is that global warming goes hand-in-hand with oxygen starvation. That’s because global warming results in less oxygen dissolved in the water. The increased temperatures favour the rapid decay of plankton and other plant life, which uses up the oxygen. Modern oceans are showing the first signs of these changes, and modelling predictions suggest that we may only be a few hundred years away from seeing large expanses of lifeless seas.
A warm climate-induced mass extinction is probably inevitable, but our dispersed arrangement of continents will help to moderate the climate change. If we lived on a supercontinent world, it would most likely be much worse. We’ve now passed the peak of dispersed continents – that was around 100 million years ago – and we are heading towards the next supercontinent. Africa and India have both bumped into Asia and started to form the beginnings of the next supercontinent. In around 100 million years from now we can expect all the other continents to have joined up with Asia too, and we’ll be in the next supercontinent cycle.
MODELLING PREDICTIONS SUGGEST THAT WE MAY ONLY BE A FEW HUNDRED YEARS AWAY FROM SEEING LARGE EXPANSES OF LIFELESS SEAS.