Mass Extinctions Part 2: The great dying

Something vaguely resembling a cow stumbles along barren fields of ash and waste, panting, desperately searching for water that’s remotely drinkable. She has long forgotten the sight of the sun, and she’s being warmed instead by the charred ground and the flames burning away at what pastures still remained.

At one point, she had been separated from the ill and weak remnants of her herd, the once forty-strong group now dwindling down to a dozen – and once she lay down for the last time, that number got even smaller.

These are not scenes from a terrifying alien invasion movie or a disaster novel, but rather the reality of the Permian-Triassic extinction, one so infamous for its impact that it was known as the “Great Dying” by biologists and paleontologists.

In this edition of the Mass Extinctions series, we’re going to explore the next two major extinction events, the Late Devonian and the Permian-Triassic mass extinctions. Once again, we’ll be looking at what caused them, what kinds of impacts they had, and what we can learn from their stories.

Devonian

The least deadly of the five great extinction events was the Late Devonian extinction, killing “only” 40% of marine genera. This period about 372 million years ago was ‘almost’ immediately followed up by the Hangenberg event, another mass extinction that came 13 million years after. These two events have often been treated as a singular extinction, especially in the past, and signalled the transition to the carboniferous period.

Before the mass extinction occurred, plants and insects had already colonised dry land, but the majority of life’s diversity was still found in the water. Tiktaalik had just arrived on the scene. New innovations in the way of roots, seeds and stem systems allowed plants to expand far from established water sources, as the mosses and ferns of the preceding era were confined to places that had reliable moisture due to a lack of vascular transport systems. 

Picture of Tiktaalik

At the time, the continent of Pangaea (yes, the one and only!) was in the process of being formed from the collision of the two prior supercontinents Gondwana and Euramerica, effectively accounting for all the landmass of the world.

What, then, followed during this momentous period to result in nearly half of all life perishing in one fell swoop?

There seems to be a general consensus regarding the Devonian extinction being difficult to identify a single cause to, given that it took place over a rather extended duration compared to most other extinction events. One possible hypothesis is that due to the evolution of large plants with vascularity and roots, such as trees, the soil was mobilised and weathered due to being penetrated to an unprecedented depth, resulting in a great deal of runoff entering rivers and eventually making it to the ocean.

What did this mean for marine life? The nutrients from runoff would certainly have benefitted the plants and animals of the underwater world, but the greatest beneficiaries of this influx of ions and nitrates were the bacteria and algae populating the water. This led in turn to algal blooms, a term that you may already be familiar with. For those who do not recognise the phrase, an algal bloom is an event where the population and density of algae in a water body spikes, resulting in sunlight being unable to enter the water past the surface where algae floats. This in turn results in marine plants no longer being able to photosynthesise, dying, and creating a catastrophe ecosystem collapse when animals can no longer feed.

What’s that? Our old friend, anoxia, has come knocking on the door! With the death of massive numbers of animals and plants comes the overpopulation of decomposers, such as fungi and bacteria, who consume oxygen from the water. Without photosynthesising plants to replenish it, oxygen levels dip and anoxia sets in, further perpetuating the vicious cycle.

The trees may be responsible for more than just that, though. Remember the Great Oxygen Catastrophe, when too many cyanobacteria were around to photosynthesise and caused the Earth’s temperature to plummet and freeze over?

Well, it might have happened again.

It is possible that the Earth, now being covered in plants across terrestrial land where there used to be nothing but rock. These trees were capable of sequestrating carbon dioxide, and by removing the greenhouse gas from the atmosphere, they were potentially related to an ice age event that could be linked to the Devonian extinction. As an interesting note, this was also the time at which much modern coal formed, marking the beginning of the carboniferous period which was literally named after coal.

Apart from climate-related events, another proposed explanation was an asteroid impact, which may appeal to those of you who enjoy thinking about dramatic catastrophes. Impact sites in various locations such as Sweden all seem to be dated to around the time of the Devonian extinction event, but none can be conclusively linked to it. Besides rocks, other cosmic explanations for the mass extinction include supernovae* that could strip the atmosphere of ozone, exposing life to deadly UV radiation from the sun.

*Supernova referring to the catastrophic death of a large star involving the rapid ejection of most of its mass along with huge emissions of radiation.

Permian

Now, let’s talk about the big one.

Devastating more than 80% of marine species and wiping out 70% of the species on land, the Permian-Triassic extinction was unparalleled in the sheer loss of diversity it perpetrated. With the disappearance of more than half of all extant biological families, it seems the name “the Great Dying” is more than fitting.

There is a general consensus about what happened, this time: volcanism.

Igneous rocks are those formed when magma, or molten rock, cool and re-solidify; large igneous provinces (LIPs), in turn, are massive formations of igneous rock associated with a sudden large upwelling of magma from the Earth’s mantle to the crust (its outermost layer). The Siberian Traps, an LIP that dominates much of Russia’s landmass, formed very close in time to the Permian extinction, and scientists typically agree that volcanism in the area was associated with the Great Dying, with each period of high volcanism or major eruptions corresponding to the pulses of the extinction event itself.

Volcanism results in the output of enormous amounts of carbon dioxide, sulfur dioxide, methane and other greenhouse gases, resulting in enormous quantities of global warming and acid rain, disrupting the temperature and pH of the water and soil.

Besides acidification of the oceans, the sulfur compounds released by volcanoes resulted in euxinia in the water – a huge drop in oxygen levels along with the dissolution of toxic sulfur compounds, a major contributor to the deaths of marine species during the main pulses of the Permian extinction.

Beyond volcanism alone, though, other contributing factors have also been proposed, which include cosmic and atmospheric events.

For one, the sudden surge in carbon dioxide in the end-Permian was hypothesised to have been caused by factors other than volcanism alone. One suggested mechanism was via a major proliferation of methanogenic organisms in the ocean, associated with a dramatic spike in the amount of carbon stored as inorganic molecules (e.g. methane, which can be oxidised to carbon dioxide) as opposed to organic molecules such as sugar. These organisms would have completely disrupted the carbon cycle, potentially resulting in the release of vast amounts of carbon dioxide from the ocean into the atmosphere, and triggering the mass extinction.

Alternative scenarios include asteroid impacts, such as that which created the Bedout crater in Australia, or a sudden stripping away of atmospheric ozone that exposed organisms on the Earth to deadly UV radiation from the sun, as demonstrated by fossilised microspores with mutations and failed division dating from the late Permian. Exposure to radiation could very well explain these genetic anomalies.

If you’ve gotten this far, you might have realised something. Doesn’t a good deal of this sound rather familiar?

Well, yes, you could suggest that this is a mirror held up to our own reality now.

While the rate of carbon dioxide production on average throughout the whole extinction event was lower than what we see happening in the last 100 years or so of the Holocene era, the fact that it occurred in short pulses with lulls between eruptions means that the rate of greenhouse gas emission during each pulse could very well be comparable to the consequences of the human industrial revolution, and only ends up averaging out over the whole period

Perhaps this is a sign to us to reconsider where we are headed. The last time the world saw climactic upheavals of this magnitude, life itself was almost wiped off the face of the planet. Do we really hope to experience the same thing as the proto-mammals of the day?

Of course, there is hope. There always is hope, after all – the Permian extinction gave rise to Lystrosaurus, the most famous survivor of the Great Dying. Though named -saurus, this was a small mammal-like creature that proliferated and thrived in the subsequent Triassic period, eventually ending up as one of the branches that led into the current vast class Mammalia. Biota do seem to recover even after the most disastrous events.

But if we want ourselves, humanity, to succeed in survival in the long-term, we cannot afford to allow a disaster of such magnitude to strike us. Other species may dominate the Earth if we vanish because of our own actions. Is this the future we hope for, though?

Well, we haven’t quite gotten that far in this series yet. Why not stick around till the fourth and last instalment of the series, where we will finally broach the realities of modern extinction?

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21. https://www.annualreviews.org/content/journals/10.1146/annurev-earth-042711-105329 → Similarity to modern rise in Co2, climate change.