Back in August I posted this on Facebook:
“I have been given what looks like the perfect science story. It combines geology, paleontology, astronomy, ecology and a little chemistry to produce a radical but plausible explanation for a major scientific mystery. It connects closely to the cover story of our next edition, has implications for two major political debates and does not seem to have got much coverage elsewhere.
The researcher isn’t answering his phone. Natch.”
Well eventually the researcher did answer his phone and the article is now in the November edition. It’s not the best topic I have ever covered, but I reckon it is in the top 10.
In the geological record we have evidence of five mass extinctions, along with quite a few smaller periods where the number of species disappearing jumped rapidly. We are now going through the 6th such crisis. While this one is caused by humans and the last at least partly by asteroid strike there is much debate about what caused the previous ones.
The greatest mass extinction of all is the one that occurred at the Permian-Triassic boundary. So complete is the disappearance of once widespread species from the fossil record that it is often known as The Great Dying. Perhaps the last gasp goal of the climate change movement, if all else fails, is to lose a smaller proportion of the life on Earth during the 21st Century than occurred 252 million years earlier – to make ourselves only the second most destructive force in the history of the planet.
The question this research was seeking to answer is: What was the most destructive force? Was it another, even larger asteroid? Of perhaps a supervolcano or the explosion of a nearby supernova?
UWA’s Dr Eric Tohver doubts the supervolcano theory. While there was a huge burst of volcanism around this time that greatly changed the Earth’s atmosphere it did this slowly, over a few million of years. The gas released appears to have been only about half that emitted 55 million years ago at the end of the Palaeocene era. Quite a few species vanished, and some new ones (including possibly the first primates) appeared in the recent event, but it does not rank as a mass extinction – why would something half the size have done so much more damage?
Tohver’s theory is that what happened was much more sudden, allowing no time for species to adapt. He thinks that there was an asteroid strike. The problem is that we have no record of a hole in the ground of comparable size, or greater, than the Chicxulub crater that saw off the dinosaurs. This could be because geological forces have removed any trace, but Tohver thinks he has a candidate – the Araguainha crater in Brazil.
Araguainha had been written off as a candidate for both because attempts to date it placed it several million years after The Great Dying, and because it is only 40km across, less than a quarter the diameter of Chicxulub. Dating of such events is hard, and Tohver and two colleagues (who incidentally had the cover story on the October Aus Science edition) used more accurate measures and placed the dating in a 5 million year period that includes the Great Dying.
The problem of course is how could an asteroid that left a a hole in the ground barely a 20th the size of the dinosaur-killer take out so many more species. Here Tohver parts company from his collaborators. They doubt that it was responsible, but he thinks it was. Part of the explanation, according to Tohver, is that this happened during the period of Pangea, when all the continents were together. The interiors of continents are much more prone to wild temperature swings than the coastal regions, and when most of the Earth’s land surface was interior the effect of climatic change become magnified.
But Tohver’s other reason is even more interesting and controversial. Araguainha lies in the middle of a huge oil shale field laid down almost 300 million years ago. The asteroid strike would have set off a string of massive earthquakes which he thinks released billions of tonnes of methane within the space of a few months. Temperatures would have soared, and while the total rise would have been smaller than at the end of the Palaeocene it would have happened much, much faster. Indeed while methane is far more potent a greenhouse gas than carbon dioxide, it also disappears from the atmosphere faster, so within a few decades the climate would have been back to normal.
A few decades however, is rather long to wait when the ecosystem to which you have adapted has just gone kaput. In the meantime 96% of the marine species we know of, and 70% of those on land, disappeared.
When you can’t even convince your collaborators that your theory is right you’re a long way from the scientific mainstream. Still, Tohver has been racking up evidence. For one thing the shale deposits in the area are enriched in heavier carbon isotopes compared to what would be expected. This is a sign that at some point in its nearly 300 million year history the shale has been disrupted so that much of the methane, particularly that with carbon 12 at its core, escaped. For all we know this could have happened slowly, and at some other time from the Great Dying, but it is hardly a stretch to conclude that the arrival of a bloody great meteorite in the area would be the trigger. The 1500-1600 Gt of missing methane is about the right order to trigger global temperature change of the sort we have evidence for at the end of the Permian.
So, that’s the science. What about the politics? Well the first thing this does is suggest just how vulnerable the species of the planet are to several degrees of warming over a very short period of time. That might just be relevant to anyone wanting to roll the dice on the effects of climate change. It draws a longer bow to link the issue to fracking and other unconventional methods of extracting methane. Nevertheless, it is hard, at least for me, to shake the impression that bad things happen when you trigger earthquakes in oil shale.