The Permian Extinction is one of the most profound changes to the ecosystem or life has seen at all ever. Life changed. Not merely with respect to the various species that were alive, but how the ecosystem was organized. The trends that had been pretty consistant in the Paleozoic were completely dumped. it was as though someone had come in and hit the HUGE reset button on the earth's lifeforms and their habitat.
In the process, it seems that somewhere between 60 - 90% of all living species - dependng on who you believe - went extinct during the Permian Triassic Extinction. The previous extinctions were nothing in comparison. People in the not too distant past used to point to the Cambrian Explosion (and subsequent extinction?) has the most profound impact. That extinction is lessening in magnitude with each passing year of research. There are now some paleo types suggesting there wasn't even an extinction event at all. However, that's another topic. The Permian was profound. It freed the waterways from the amphibians. It set in motion the events that led to the Age of Dinosaurs. It even set about events that led to the cynodonts becoming true mammals and thus led to you and me. There will be another post, another time for discussing the End Permian Ecosystem. This post will discuss the causes of what happened. It's not as simple as the most studied mass extinction either (the KT Event).
What were the theories about what caused the mass extinction? Well, there were a few. The longest reigning one was that the oceans receded to very low levels. The second theory that went along with the first theory was that because all the continents had come together into Pangaea, life that had been confined to certain parts of the world were not able to spread all over causing greatly increased competition and associated die-offs (the NorAm/SoAm exchange of animals due to the Panama Straits closing was the source of this theory). The next big theory was what people expected to see after investigating the KT Event: a bollide impact. The next theory was that the volcanos did it: the
Siberian Traps to be specific. There have been other extinction theories that have eben related to environmental degradtion.
Douglas Erwin once suggested that it was the Midnight Express: a little bit of everything did it because it it would take a lot to do such damage.
It seems that the cause was pretty straight forward after digging through the data. A concensus has grown and a theory has been largely adopted by those studying the event. Unfortuntaely, it deals away with the ide that a single source would be the cause of mass extinctions. You could not point to the sky and say if we cleaned out all the asteroids and comets that we wouldn't face another mass extinction. There is no One True Cause. All the evidence points away from a bollide impact. Rather the whole thing seems to have been kicked off by vulcanism. More properly, those evil Siberian Traps. However, it is a pretty complex chain of events that lead from there to there and a lot of explaining to do.
First off, there were actually
two events at the end of the Permian. There was one that was ten million years before the second. The kill ratio was some 20% of life took a hit. That's pretty hefty. It's not as hefty as what following. Life seemed to be recovering pretty well, if the fossil evidence can be believed. Then came a long, long dying that seemed to almost end life on Earth.
What caused the long dying seems to a complex operation as to what happened. The root cause was the Siberian Traps. They belched lava and carbon dioxide over nearly a million years. The amount of lava they belched has no parallel. Not even the
Deccan Traps come close. The important part was the CO2 though. It apparently was enough to warm the planet 5 degrees C above what it was already: which was already warmer than now by a considerable bit (a few degrees C).
From there, the oceans warmed. This had two effects. The oceans of the Permian seem to have been very stratified. That is to say that the oceans did not intermix from the bottom depths to the ocean surface. There were very distinct layers. Additionally, as the world warmed, the ice caps melted. This decreased the salinity of the top waters and made the stratification worse. This caused the bottom of the ocean to be anoxic (ie very little or no oxygen dissolved in the various layers). The warming made the anoxic conditions
worse. Warm water holds less oxygen than does cold. Life in the water would be...less than comfortable. With the melting of the very extensive ice caps, the oceans would rise and the anoxic layers would be brought up to depths that would originally have been habitat for the Permian marine organisms (This is Hallam's regression-transgression-anoxia theory).
Then the warmed ocean unleashes a nasty surprise for the surface and atmosphere: we get methane hydrate burps. There is a large amount of methane sequestered in the ocean in the
form of ices. When the water warms too much, the methane is released from the ice. This makes it way into the atmospehre. Methane is a much
better greenhouse gas than carbon dioxide. This ended up causing a further 5 degree C atmospheric rise. This happened very quickly and helped to pummel the End-Permian lifeforms even more. If the
simulations (PDF) of
Jeff Kiehl and
Christine Shields of
NCAR hold up to the fossil record, this means that the methane in the atmosphere end up
whacking the ozone and unleashes a lot of normally blocked ultraviolet light [
here too] above and beyond the norm which helps kill off the terrrestrial ecosystems as well.
As if to add insult to injury, the hypoxic-anoxic oceans
encouraged growth of anaerobic bacteria that produce lots of hydrogen sulfide. Hydrogen sulfide is nasty stuff and highly toxic. It would help kill even more in the oceans than even oxygen deprivation would. It would, futhermore, bubble out of the oceans and kill right and left anything that took a whif on land. However, to make things even worse, Kiehl et al state that it increases that length of time that the methane remains in the atmosphere. This increases the amount and heating and away we go.
In the end, what killed life at the end of the Permian? It was a negative feedback that in effect, even in such a hellaciously hot climate (72 C at the equator!), snowballed. If ever there was a time that we, the planet Earth, was dangerously close to tripping over the edge into a run away greenhouse, that would have been it. Not the Eocene. The mechanisms that regulate the CO2 content in the atmosphere broke down, got depressed, and nearly shot itself in the head; in effect nearly ending all life.
To me, based on all the evidence that's coming in about the PT Extinction, it seems that what happened is pretty settled. Details and refinements need to be done. We're a long ways from being able to close the book, say it's been written, and that we completely understand why life nearly died then. We are far closer and the alternative scenarios are, with the pace that the supporting science for the above theory is growing, falling further and further behind.
The new question, that probably ought to be asked sometime in the next decade is not why life died at the PT Extinction, but rather:
Why did it survive at all?
After all, if the carbon cycle choked and died, how did it get restarted? It looks like in the case of Venus, it didn't and that world died. Why did it not here? Perhaps a hint is in Erwin's book,
Extinction, where he states that a colleague of his noticed that the fossils that people have been attributing to the '
fungal spike' looked a lot more like algae fossils to him. Perhaps the humblest of photosynethesizers saved us all. That's a bit of research for people in the future to determine though.
Next time out on the PT Event will be discussing the land ecology. Why was this event so important to us?
