If there is ever a competition for best-named geological phenomenon, the Great Dying is surely a contender.
Over a relatively short period of time some 70% of vertebrates living on land and around 90% of ocean species were killed off. The end-Permian mass extinction, as it is more formally known, was quite simply the biggest disaster ever to hit life on Earth.
Until around a decade ago, the trigger for this deadliest of catastrophes 252 million years ago was often presented as the greatest murder mystery of all time, with scientists offering up some half a dozen “suspects”.
More recently, advances in dating techniques and new geological evidence have provided a very prominent smoking gun. Most earth scientists now agree the greatest of the Earth’s “Big Five” mass extinctions was triggered by 1 million years or so of intense volcanic activity.
Mystery solved then? Not entirely.
Yes, somewhere in the range of 5 million cubic km of lava spewed out across what is now northwestern Siberia – enough to cover the Earth’s surface to a depth of about 10 metres – and it did so shortly before the start of the mass extinction. This triggered the release of huge volumes of greenhouse gases that drove global warming and critically disrupted the Earth’s life support systems.
The end-Permian mass extinction was quite simply the biggest disaster ever to hit life on Earth
However, the precise details of how this caused so many life forms to die out remain the subject of scientific discussion.
These debates are no mere academic dispute. The catastrophic event played a major role in shaping the flora and fauna we see today. Moreover, there are clear parallels between the environmental changes that occurred back then and those being seen today. Some say improving our understanding of a time when life almost died could boost the long-term survival prospects of our species.
In 1980, Luis and Walter Alvarez, a father and son team at the University of California, Berkeley offered new and compelling evidence that the most famous of the mass extinctions – the one that did for the dinosaurs 66 million years ago – was the result of a massive asteroid impact. This triggered a wave of interest in the causes of the other mass extinctions, including the much larger end-Permian one.
Back then, western “extinction hunters” had less evidence to go on because many sites with sections of rock of the right age were in locations that were difficult to access, such as China and Russia. This did not stop them offering up a variety of theories.
The idea at the time was that the end-Permian mass extinction was spread over millions of years
Some pointed to the knock-on effects of the formation of the super-continent Pangaea, such as the reduced extent of shallow marine environments that are home to most marine species. Others emphasised evidence of severe oxygen depletion in late Permian rock samples and falling sea levels – both of which could explain why marine species were so hard hit.
Still others proposed there had been a massive release of methane from the sea floor – while others maintained that the intense volcanism that left behind so much volcanic rock in Siberia must have been a factor. Each successive lava flow spread over the preceding one, producing what today is a series of step-like hills. This feature earns the lavas their common name – the Siberian Traps – from the Swedish word “trappa” meaning steps.
“The idea at the time was that the end-Permian mass extinction was spread over millions of years,” says Paul Wignall, a geologist at the University of Leeds, who published a book about mass extinctions called The Worst of Times in September 2015.
After collecting samples in the Dolomites, Italy, Wignall and Anthony Hallam, of the University of Birmingham, concluded in a paper published in 1992 that the extinction in fact lasted more like tens of thousands of years.
We just don’t have the geological evidence for an impact
This shorter time frame encouraged some to look for a short, sharp catastrophe to explain the extinction – which seemed to suggest an asteroid impact as the cause.
In favour of the idea, some researchers pointed to rare grains of shocked quartz in Australia – sand grains that were suggested to have been beneath the site of the proposed impact, and that had been physically stressed by the experience. Other researchers, working in Antarctica, found helium and argon with isotopic ratios similar to those in a type of carbon-rich meteorite formed in the early solar system.
Yet no impact crater was found and these claims were soon being challenged.
“We just don’t have the geological evidence for an impact or any other major event capable of causing an extinction on this scale except the Siberian Traps,” says Jonathan Payne, a geologist who specialises in mass extinctions at Stanford University in California, US.
As dating techniques improved and more samples were gathered, a consensus formed around volcanism as the primary culprit – particularly when other scientists concluded the Siberian Traps were formed over about 1 million years, not 10-50 million years, as previously believed.
The death toll from the Siberian Traps was extreme. The effects were greatest in the oceans, especially on the sea floor.
Some 40% of late-Permian insect families were wiped out
Many groups were wiped out entirely, including one of the earliest known arthropod groups – the trilobites – as well as primitive rugose and tabulate corals, and the nut-shaped blastoid echinoderms – relatives of today’s sea urchins and starfish. Others, such as brachiopod shellfish, bryozoan “moss animals”, squid-like ammonoids, and flower-like crinoids, lost most of their species.
The swimmers fared better, though: acanthodian and placoderm fishes went extinct but many other fish and the eel-like conodonts survived relatively unscathed.
Land-based organisms were also devastated. Numerous major groups were wiped out including the gorgonopsians, the dominant predators of the time with sabre tooth cat-like fangs, and the large, bulky herbivororous pareiasaurs.
According to Dmitry Shcherbakov of the Paleontological Institute in Moscow, some 40% of late-Permian insect families were wiped out. Many equatorial groups, such as cockroaches and cicadas, moved north as temperatures rose.
Among plants there is evidence that forest species virtually disappeared.
Many groups of the dominant seed-producing gymnosperm plants went into decline. No coal was produced for around 10 million years, indicating the extinction of peat-forming plants. Another sign of uniquely deadly nature of the devastation among plants is the “fungal spike” – a huge increase in fossilised fungi spores.
“This is interpreted as the result of an explosion of fungi living on dead or dying trees,” says Barry Lomax of the University of Nottingham, UK. “It’s something that’s not been seen at any other geological boundary.”
Enormous volumes of both greenhouse and other harmful gases were belched out with the lava
Analysis of precisely what died out and when it did so offers some of the best clues to explain how the Siberian Traps had such a massive impact.
When Wignall and colleagues at the China University of Geosciences in Wuhan looked in detail at samples that revealed the fates of 537 marine species in China, they found 92% were wiped out. More usefully, however, they discovered the extinctions came in two phases separated by around 180,000 years.
The first of these was especially deadly for shallow water inhabitants like corals, seafloor-dwelling microscopic animals called fusulinids and plankton called radiolarians. Extinctions during the second pulse were focused in the ocean depths.
New species evolved fairly rapidly after the first pulse, but the recovery was much slower following the second – suggesting the longer term causes that undermined the basis of so many ecosystems occurred later in the crisis. Other evidence from plant remains recovered in both Greenland and Antarctica support the idea of it having been a double whammy mass extinction.
So what was it about the volcanism in Siberia that wreaked so much devastation to life in its various Permian guises?
Most importantly, enormous volumes of both greenhouse and other harmful gases were belched out with the lava. These included large quantities of CO2 and sulphur dioxide that helped raise temperatures.
Most geologists now blame the land-based devastation on the large-scale release of noxious CFC-like gases
Heating up the oceans would have reduced their capacity to hold oxygen, and currents that normally bring oxygen to the depths would have slowed or stopped. This lack of oxygen is widely thought to have been a primary cause of marine extinction – something revealed by rock samples at the border between the two geological periods at sites across the world.
“Almost everywhere you look the sediments switch from oxygen-rich, life-rich to oxygen-poor, life-poor,” says Mike Benton, a palaeontologist at the University of Bristol, and author of the 2003 book When Life Nearly Died. “It’s just an absolute line as thin as a knife blade you can just point to.”
Some have emphasised that as the increased CO2 levels in the atmosphere were taken up by the oceans, the water would have become more acidic, making it harder for many marine organisms to make their shells. Higher CO2 levels would also force ocean animals to work harder to take in oxygen and expel CO2.
“Many people talk about a deadly trio of warming, acidification and deoxygenation,” says Payne, who argues the acidification could have lasted tens of thousands of years. “All of these things affect the ability of marine animals to function because of the way they impact metabolism and oxygen use. My take is they were probably equally important.”
Wignall, however, downplays the role of ocean acidification. “There is no doubt you will acidify the surface of the ocean, but most organisms are quite capable of still making their shells because they make them within their bodies not in contact with ambient sea water,” he says.
Heating up the oceans would have reduced their capacity to hold oxygen – a primary cause of marine extinction
Warming would have had significant impacts on land too – but could probably not have accounted for extinctions on the scale of those seen among animals and plants at the time. Most geologists now blame the land-based devastation on the large-scale release of noxious CFC-like gases like chloromethane.
These gases are believed to have been generated when layers of coal and salt were heated up as magma forced its way up to the Siberian surface. This would have caused huge ozone layer destruction, triggering significant increase in exposure to the sun’s harmful ultraviolet B radiation.
Evidence to support this theory came in 2004, when Henk Visscher at the University of Utrecht in the Netherlands reported evidence of a substantial increase in levels of fossilised mutated spores from lycopsids, or club mosses, at the time of end-Permian mass extinction.
Others have proposed that environmental stressors such as increased aridity – not increased radiation exposure – could have caused the mutations.
Lomax, however, backs Visscher’s theory. “There have been other periods of prolonged aridity and we don’t see evidence of those being associated with plant spore mutations, so it seems more logical for it to be linked to increase in exposure to UVB radiation.”
In effect we are creating or forcing an end-Permian mass extinction
The gases coming from the volcanoes would have generated carbolic, sulphuric and other acids that would have fallen as acid rain, adding another environmental hazard. This illustrates how environmental causes could kill off species both directly and indirectly.
“The loss of plants from UV radiation and acid rain would have removed the basis of the food chain on land, leading firstly to death from starvation for herbivores, which in turn would have removed food sources for carnivores,” says Benton.
Many believe such ecosystem interconnections are among the things 21st-Century humans should bear in mind as our activities drive carbon dioxide concentrations in the atmosphere to the highest levels since measurements began. The end-Permian mass extinction does offer contemporary lessons, but they don’t necessarily provide easy ammunition to one side or another.
“In effect we are creating or forcing an end-Permian mass extinction,” says Wignall. “However it takes a long time to heat up oceans and the models suggest the oceans will be in trouble in 200-300 years in terms of dissolved oxygen content and we might see issues related to ocean circulation in a couple of thousand years. Who knows what we’ll be doing by then.”
Payne points out that the end-Permian mass extinction can be seen as beneficial to life in the long run – the total number of species on Earth ultimately rose to a new high after the event – but that the time scales involved mean that’s no reason for complacency.
“The biggest extinction event we have in the history of life has a lot in common with the environmental changes occurring today and that we anticipate in the next 100 to 1,000 years,” he says. “In fact in the long run, it had a stimulating effect on ecosystem diversity, but the recovery took millions of years to kick in, so loss of diversity is not something that should be thought of as useful or relevant to human society.”
If this is still one crime scene, it’s an increasingly complex one
In his 1993 book The Great Paleozoic Crisis, US palaeobiologist Doug Erwin likened the task of evaluating the potential causes of the end-Permian mass extinction to the position facing Hercule Poirot in Murder on the Orient Express. Agatha Christie’s detective eventually concludes that all the passengers on the train were involved in the murder.
Wignall, in his recent book, describes the Poirot “everyone did it” conclusion as “lazy”. He instead turns to Sarah Lund, the star of the Danish crime drama The Killing, for analogy. In the first series, a growing list of possible suspects for a murder are presented as episodes go by, before she finds the one single killer. Wignall’s killer is volcanism, along with its knock-on effects of warming, ocean de-oxygenation and ozone destruction.
Yet as scientists get access to more and more data of greater and greater accuracy, attempting to separate out the differences in the precise combinations of causal factors behind extinctions in different ecosystems, groups and species may not provide straightforward answers. If this is still one crime scene, it’s an increasingly complex one with large numbers of bodies killed with a variety of different but inter-dependent weapons.
“Environmental causes can have multiplicative rather than just additive effects, making it hard to isolate the effect of one versus the other,” says Payne.
This is not a failure of science, more an indictment of our tendency to demand easy answers of it. Perhaps it is time for the murder mystery as an analogy for the end-Permian mass extinction to become extinct.