It was a spring without voices. On the mornings that had once throbbed with the dawn chorus of robins, catbirds, doves, jays, wrens, and scores of other bird voices there was now no sound; only silence lay over the fields and woods and marsh. (From Rachel Carson: Silent Spring)
The history of life on earth has been a history of interaction between living things and their environment. So begins the second most famous book on life: Silent Spring, by Rachel Carson. It is a book that changed how we look at the world around us, in a way bettered only by Charles Darwin’s classic, The Origin of Species. Carson’s message is the opposite of that of Darwin. The Origin of Species turns a bleak view of a winner-takes-all world into a perspective of continuous, rapid evolution and improvement of the species. In Silent Spring, unimpeded progress leads to insecticides so powerful that they kill far more than is revealed by the competing companies that create them; the struggle for company survival sacrifices the environment, and in result, risks our survival. Later, it was realized that Darwinian evolution does not provide a guaranteed path to progress. What is best for the individual may well be detrimental to the species, and thus the survival of the fittest can eventually lead to extinction of the species. The two books aren’t so far apart. In evolution’s struggle, even the winners can be losers.
In the end, Carsons’ powerful warning came just in time. Against strong opposition of the industry in question, the worst excesses were stopped. Our world is far healthier now than it was 50 years ago. Still, bad errors happen. The Bhopal pesticide disaster injured over half a million people and killed over 3000. More recently, there is the antibiotic that inadvertently killed all vultures in India. Restricted there, it is now exported to Africa instead. Some people never learn. The demise of honey bees is also fairly conclusively linked to two neonicotinoids, imidacloprid and thiamethoxam. As Carson writes: It is our alarming misfortune that so primitive a science has armed itself with the most modern and terrible weapons, and that in turning them against the insects it has also turned them against the earth. But no evidence can be strong enough for companies that make their fortune from such chemicals. In the post-modern world, companies can be larger than their governments, and if they don’t like environmental restrictions, they can buy a new government. And so the silent spring has turned to autumn.
But this has all happened before. From the demise of the dinosaurs to the disappearance of Homo Floresiensis, extinction has been a way of life. Sometimes extinctions peak: in a short amount of time much of life goes. As Carson surmised, this shows something dramatic has happened to the environment. Worst of them all was the mass extinction at the end of the Permian and start of the Triassic, 251 million year ago. This came close to sterilizing the Earth. And all because of an errant insecticide – and a Siberian volcano.
The Permian world
The continents had come together to form a supercontinent, called Pangea. It was partially dissected by an equatorial ocean, the Tethys, and surrounded by a world-wide ocean, Panthalassa. The Tethys is now gone, but Panthalassa still exists: over the ages it became our Pacific. The time was 200 million year after the Cambrian explosion. Life had multiplied; it has conquered the land, and learned to fly. A large ice age 280 million year ago was a set-back but as temperatures recovered and the earth grew warmer again, tropical abundance was everywhere. Huge forests had developed; they left coal deposits throughout the modern world. Reefs abundant with life rimmed the tropical coasts.
In the west, a sliver of sea had encroached in-land covering parts of what is now the southwestern US and Mexico. Along its shores another tropical reef developed. The famous cliffs of El Capitan and the mountains above the Carlsbad Caverns, New Mexico, are remnants of this reef. The parking lot of the Carlsbad Caverns is (reportedly) a good place to look for the fossils. Once you have found them, do go into the caves. They provide an escape from the summer heat, but are also among the most impressive in the US, and they are displayed subdued, without garishly coloured lights. It may not look like an underground fun fair but it is the real thing. Surprisingly so, after the never-ending billboards along the roads to Carlsbad. (And if you want to see the bats, June-September, the pre-sunrise return is just as good and far less crowded. You might have the bats to yourselves.)
Over 500 different types of fossils have been identified. The reef was not build by modern corals: these had not evolved yet. Instead, the frame of the reef was created by sponges, which form a light, tufa-like rock with lots of holes for other creatures to occupy: colourful fishes, rays, sharks, but also ammonites (shells with tentacles), sea urchins, and a single-celled organism growing to 10 cm across. It was a reef, but not like our reefs. The forest was equally different. The trees were huge, but were in fact giant ferns. Conifers were slowly replacing the ferns, as seeds superseded spores. There were no birds but plenty of cockroaches. Beetles began their conquest of the world. There were still trilobites in the sea. Large animals were abundant. The dominant ones were mammals: cynodont, a pack-hunting predator, lystrosaurus, a small vegetarian, and huge dimetrodon, a lizard with a big sail on top. Surprisingly in this pre-Jurassic age, the ancestors of the dinosaurs were around but in the minority against the mammalian line. It was life, but not as we know it.
The time of plenty was not to last. During the Permian, the climate had grown more hostile, as global temperatures had continued to rise. Warming is common during a supercontinent phase. During the assembling of the supercontinent, oceanic subduction zones are actively pulling the plates together. But once that has happened and the continents have collided, the intervening oceans have gone and the original subduction zones have become mountain ranges. The cessation of subduction changes the global carbon cycle. Subduction is the main way by which the Earth removes excess CO2 from the atmosphere: it dissolves in the water, becomes carbonates, sinks to the ocean floor and gets carried into the mantle, over a period of 100 million year. At the same time volcanoes slowly put recycled CO2 back into the atmosphere. Everything is nicely in balance. But once subduction ceases, when the supercontinent is in place, CO2 levels begin to rise, and temperatures in consequence go up. Thus, supercontinents tend to be hostile hothouses, without ice or snow even in the polar regions. As the Earth began to heat, the interior of the huge continent dried out and became a desert. The sea level dropped and the shallow seas dried out. Life was stressed, suffering in the desertification. Reefs were reeling with the demise of the shallow seas.
And suddenly, 251 million year ago, it was all gone. The fossil record dries up almost from one day to the next. There were two separate events, 200,000 year apart. The first one was the famous Permian extinction. The second was the early Triassic extinction. The two events hit different types of families, so that animals or plants that survived the first event fairly well, still succumbed to the second. A third wave of extinction occured a million year later. After the double, or triple, whammy, 90% of sea families were gone, and 70% of all land species disappeared. This is number of families: the number of individual creatures must have gone down far more, perhaps by 99%. If this had happened to us, the human population of the world would have been reduced to just the French (or less euro-centric, just the Thai). Even insects, which sailed through ever other extinction event, suffered extinctions.
Plants were equally hit: the forests were gone, and instead, the fossil records shows enormous amounts of fungi, presumably feasting on the dead wood. River plains changed from meandering to braided channels: this means they were flowing faster, and this is what happens if the rivers are not impeded by plant growth. Braided channels were normal before life conquered the land. With plant life gone, herbivores were in trouble. Without herbivores, predators had a hard time. Beetles survived – not much else did. This was truly the time the world came closest to sterilization. It was the ultimate silent spring.
Recovery was extremely slow. It took as long as 30 million year for the eco system to regain its previous abundance, but with an almost entirely new cast of actors. Among the many victims were the trilobites. They had survived everything the Earth had thrown at them before, but as cold-water creatures had been flagging already in the heat of the late Permian. They did not make it. But the impact of the extinction went far beyond the trilobites. The future of the world was changed.
The crater of Permian doom?
For the biggest extinction in the fossil record, worse even than the dinosaur-killing K-T event, the cause must be something dramatic. Naturally, people have looked for a major impact. There have been claims that there is evidence for this, from peculiar isotopic ratios of sulfur and carbon. But there is no debris layer, and no crater. Recently, a possible crater was located underneath the ice in Antarctica: radar showed the presence of a very large partial ring, and gravity measurements showed a mass concentration under the ground. If this is an impact crater, it is twice the size of Chicxulub, the K-T crater.
But the evidence is not sufficient. The proposed crater ring is incomplete. The crater formed very close to Australia (at least where Australia was at the time, within Gondwana) but the relevant area in South Australia has no indications for a major impact. The mass concentration (mascot) can also come from mantle activity. And finally, there is no date for the crater, so even if it is due to an impact, it can’t be linked to the Permian-Triassic boundary. At the moment, the nayes have it: the case for the Permian extinction to be linked to an impact is unconvincing. But the impact theory is not dead yet: more evidence may still be found.
The Siberian traps
But there is a much bigger smoking gun, hiding in the world’s other frozen wilderness. Siberia holds the record for the largest volcanic eruption on land known to us. And the timing of this eruption coincides exactly with the Permian extinction. The Siberian Traps make Iceland seem a children’s playground, and Yellowstone a volcanic midget.
The Siberian Traps cover much of Siberia. The exact extent is not known, as large parts are buried beneath more recent sediments. The volume is also not accurately known. The lava flows are up to 4 km deep, obviously not single flows but deposited over many separate eruptions. The area is estimated as 2 million km2, but possibly 2-3 times more, and the volume between 3 and 5 million km3. This makes it the largest flood basalt eruption for which we have decent data. If all the lava flows had entered the sea, it would have raised sea level across the globe by 20 meter. (In fact a flood basalt eruption under the sea would raise sea level by much more than this, because the inflation prior to the eruption is much more voluminous than the eruption itself).
The eruption started 257 million year ago, southeast of the Ural mountains where the first rifts opened. For a while things quieted down, but the eruption restarted around 251 million year ago, further northeast. The heavens, or rather, the ground, opened, with massive flows each 50 meter thick and traveling 100’s of kilometers. Each individual flow may have contained 1000 km3 of lava, and they occured as often as once a century. This second beginning was 300,000 year before the extinction event, and it continued until 800,000 year after. 2/3rd of the lava was erupted in the pre-extinction phase, but is not known whether this was continuous or episodic, and whether the eruption had further intensified around the time of the extinction. In any case, the close alignment between the eruption and life’s wipe-out leaves little doubt about the relation. Whether the eruption occured in pulses, and one of these pulses provided the killer blow, is up for discussion.
From space, the eruption would have been a spectacular sight, although not quite the size as shown on this BBC visualization.
What caused the Siberian eruption? It may have been a mantle plume, or edge convection caused by the thick insulation of Pangea: the eruption happened next to the Siberian craton, in a place where the lithosphere suddenly became much less deep: those may be ideal conditions for edge convection. The arguments are finely balanced. The tell-tale indicator of a mantle plume would be a dramatic inflation prior to the eruption, a huge bulge within the continent perhaps 2-3 kilometer high. But there is little evidence for such inflation, and this is the main weakness of the mantle plume model. In either case, the lava composition is from the mantle, but it is strongly fractionated, showing that the magma resided in or under the crust for a long time. Some continental crust melted into the magma.
But how did the eruption cause the mass extinction? One piece of evidence comes from other mass extinctions: only flood basalt eruptions on land seem to correlate with extinction events. Those which occured under the sea did not. This suggest the immediate cause is in the air. Was it the sulfur? Fluorine? A volcanic winter? The pieces of the puzzle are beginning to come together, and it appears it was none of these. The Permian killer was a combination of lethal global warming and anoxic oceans.
The tropical Tethys ocean had warmed substantially during the late Permian. And towards the end, slow warming became rapid warming. The rapid rise lead directly to the extinction event. The heat lasted for five million year, before finally a return to cooler climates. The temperature range was extreme. The ocean warmed by more than 10 C, to a peak temperature in excess of 35 C, possibly as high as 40 C. Very little complex life can live in water above 35 C, and no photo-synthesis can occur. The land may have reached temperatures of 50-60 C. The Great Dying was death by heat stroke.
The rapid temperature rise was clearly caused by greenhouse gases. We know this because of a change in carbon during the rise, with an increase of the fraction of ‘light’ carbon (the isotope C-12) compared to the heavier C-13. This can only be caused by the addition of a new carbon source. It is natural to blame the volcanic eruptions for this, especially since there was also an increase of ‘light’ sulfur, which is typical for the mantle. At the current time, volcanoes emit up to 600 million tons of CO2 per year. That may seem a lot, but human activity emits 40 times as much. (Note that most of the volcanic COc does not come from eruptions but from quiescent degassing of otherwise non-erupting volcanoes.) The Siberian Traps erupted 10-100 times as much lava per year as our volcanoes do; a volume of 5 million km3 would emit the same amount of CO2 as current human activity would do over 2500 year – and remember that our emissions over the past 100 year have already added one degree C to global temperatures. However, the volcanic injection of CO2 was spread out over a much longer period of time. It is not entirely clear why the Earth wasn’t able to remove the excess CO2 over that time. Somehow, it didn’t, and the increase caused lethal heating.
The global heating seems larger than can be explained solely from the Siberian Traps. Something amplified the global warming. Two suggestions have been made, and quite possibly both are right. The first is that the Siberian Traps erupted through a major coal layer and ignited it. The second is that methane hydrates at the bottom of the oceans suddenly became unstable due to the rapidly rising water temperatures, and a surge of methane entered the atmosphere. The latter seems plausible: methane is a powerful greenhouse gas.
The second killer: oxygen deprivation
As mentioned, the extinction occured in two phases. The first phase coincided with the dramatic heating. At the same time, the Tethys ocean briefly became anoxic. The extinction event was sudden, and brief: it lasted less than 60,000 year, and possibly less than 6000 year. The shortest time over which it could have happened is probably a few hundred year, limited by the speed at which ocean water mixes.
The second extinction pulse came 200,000 year later. It saw another temperature rise, but worse, the oxygen depletion now spread across Panthalassa, the world ocean surrounding Pangea. The entire ocean became a dead zone. Only the top layer of the water still had oxygen, mixed in from the atmosphere, but this region was also worst affected by the relentless heat. The oxygen loss in the ocean completed the disaster. Creatures that can live with little or no oxygen tend to favour cold water. But those creatures had already been eliminated. The oxygen deprivation caused havoc among the remaining survivors.
Why the oceans became oxygen depleted is a matter of discussion. The ocean circulation clearly had stopped working, as happens in isolated bodies of water on our Earth, such as the Caspian Sea and Loch Ness, where deeper layers become a toxic soup of methane and hydrogen-sulfide. It doesn’t affect the open oceans, although there are growing anoxic events caused by run-away algae growth, fuelled by run-off fertilizer. The ocean circulation is driven by cold surface water in the arctic regions which is denser than the warmer water below and sinks to the bottom of the sea. It forms a conveyor belt circulation, finally resurfacing a hemisphere away from the place of sinking. Models show that this sinking of cold surface water may have stopped in the increasing heat of the Permian extinction. So the ultimate cause of both parts of the disaster was the run-away greenhouse.
What was the world like during the extinction event? Very little life was left and the ecosystems were decapitated. Effectively all large animals on land and in the sea were gone. No animals, no trees, no fish – only a few survived at high latitudes. Some stunted bushes and ferns were found on land, and molluscs reigned in the sea. Stromatolites, an organised form of cyanobacteria, made a dramatic re-appearance. 2 billion year before, they had dominated the seas, but after grazers evolved they had withdrawn to regions where little else could live (such as the modern Shark Bay in Western Australia). Now they came back. Other microbialites also did well. (Microbialite is the general term for a bacterial mat that traps sediments: layered ones are called stromatolites, others are called thrombolites.) It was a world of cyano-bacteria: imagine the smell of blue-green algae across the world. Green algae, in contrast, were absent: they show a gap in their fossil record. Small snails fed on the bacteria and bacterial mats. This was the opportunity of a life time for the single cellular, and also the time when meek snails finally ruled the world.
The tropics were wet, but hot beyond belief and probably covered in little more than fungi. Rotting and decomposition would have been very fast, and created soils very poor in carbon. The silent spring seemed never-ending.
Recovery from life’s silence was indeed exceedingly slow. Conifer forests re-established themselves after 5 million year when temperatures finally moderated. Animals diversified again, spreading from the poles. But their evolution was punctuated by multiple smaller extinction events, possibly because the ecology remained unstable and became too dominated by single species, namely the ones that recovered fastest. Full recovery of the previous diversity took close to 30 million year. The large majority of families were gone, forever.
Of the surviving larger herbivores, Lystrosaurus, a pig-sized mammal-relative, became the most numerous, accounting for 90 per cent of vertebrate fossils. But it failed to diversify quickly enough and was overtaken by the archosaurs. 20 million year later, the archosaurs gave rise to the first dinosaurs. It took another 10 million year before the vertebrates on land had fully recovered. By that time the proto-dinosaurs had taken over the world. The next extinction event, 210 million year ago, cemented their dominance. The mammals withdrew into the shadows. Their chance would not come until another catastrophe, this time celestial and unpredictable, almost 200 million year later.
The shadows of the Siberian Traps were long-lasting. The worst volcanic eruption on record changed the course of evolution – it re-shaped our world. And all because of an seemingly harmless gas, turned into a killer and an insecticide: CO2.
All life depends on its environment. Interfering with the environment interferes with life, often in unpredictable ways. Bacteria can survive anything, but animals and other complex life forms can’t. Complexity comes with fragility. And the most complex of life is us.
Most volcanic eruptions, even large ones, have limited impact: a few bad years, a local wipe-out; even Toba, the largest eruption in the past million year, did not permanently damage the world. In our days, no life form has gone extinct because of an eruption. However, eruptions also depend on their environment. Their effects can be worsened if the environment is already marginal. Iceland’s Eldgja eruption, in all its immensity, appears to have had limited impact, because when it happened the climate was mild and could withstand a shock. The Laki eruption, although smaller, was more devastating because the climate had become more hostile. A stressed environment lacks shock-resistance. The Siberian Traps were massive, but they also erupted at a bad time, into an already stressed world.
But there is more to it. Sometimes, a small change can lead to series of amplifying events. It is the drop of water that makes the dam collapse. The Earth has its critical points: push the environment beyond such a limit, and suddenly the world falls apart. In our days, ocean temperatures do not go much beyond 30 C: evaporation keeps it below this. But in the Permian, the Siberian Traps caused this safety valve to be broken. The water temperature leapt up by another 10 C, and mayhem followed. Climate can only self-repair up to a point: a badly timed push can tip it over the edge. The Siberian Traps did this, and they caused a lethal greenhouse.
It is now 50 years since Silent Spring. And again we are producing a seemingly inert chemical in quantities the Earth cannot absorb. The Siberian Traps emitted far less CO2 per year than we do now; we are re-running an amplified experiment. The Siberian Traps once set the world aflame. We are still playing with its fire. Behind the crackles are the echoes of a silent spring.
Albert Zijlstra, January 2017