We’ve reached the end of the Permian Period, and the end of the Paleozoic Era, which we’ve been plodding through since the first of February. This ending is marked by the most devastating extinction event in the entire history of the earth. It’s the end of the world as we knew it, and lots of species don’t feel fine.
Most estimates say that at least 90%, maybe 95%, of all marine species and 70% of all land animals did not survive this mass extinction. Trilobites, fusulinids, blastoids, rugose corals, and eurypterids were gone for good. Every other marine group was decimated – bryozoa, crinoids, ammonites, brachiopods were all hard hit. On land, most of the large amphibians disappeared, and even the large trees that made forests were gone. Even insects were impacted. And most of the early therapsids, the mammal-like animals including the dimetrodons, also disappeared, although some of their descendents survived.
So what caused all that? After the “smoking gun” of a catastrophic impact was identified and finally accepted as the event that put an end to the dinosaurs, at the end of the Cretaceous Period, it became popular to look for impact events to relate to other extinctions. There are some at about the end of the Permian, notably the Bedout feature offshore northwestern Australia. Its age is essentially identical to the end-Permian extinction, but it’s not entirely accepted as an impact structure. Also, there’s geophysical evidence in Wilkes Land, Antarctica, that can be interpreted as a really big crater. It got a lot of attention in the popular press a few years ago as the potential cause of the Permian extinction. But it’s under the ice – we have no rocks from it – and its age is pinned down to only 100 million to 500 million years ago – hardly a close match for the Permian extinction. Also, there’s no conclusive geochemical evidence, such as the famous iridium layer associated with the end-Cretaceous impact, to support a huge Permian impact. I really think that for now, impact hypotheses for this event have no weight.
Much more interesting is the close correlation in time with the vast Siberian basalt flows that we talked about yesterday. Such volcanism could put acid aerosols into the atmosphere – consider Laki, the volcano that erupted in Iceland in 1783, whose aerosols put poisonous fogs as far away as France. Volcanoes also introduce ash into the atmosphere, which might create a brief nuclear winter, a quick cool-down, but they also inject carbon dioxide into the atmosphere, which would soon lead to greenhouse conditions and the warming that we observe. Warmer conditions could have led to melting of methane hydrates in the sea floor, and releasing methane would add to the greenhouse effect.
Earlier this year, the popular press was inundated with headlines like “Bug farts nearly killed us” – a reference to an idea that methane-generating organisms, archaea, microorganisms that were once considered to be bacteria, were the culprits. They are now classified at the same level as the kingdoms of plants and animals. The idea is that these critters evolved a new way of growth, just about at the end of the Permian, that led to exponential reproduction and an associated generation of huge amounts of methane, producing the greenhouse conditions and acidification and anoxic waters that we know were present. There are some aspects of carbon isotope ratios that this idea explains better than the concept of volcanism as the cause of the extinction, but the jury is still out on this, I think.
I think that the Siberian basalt flows probably have the strongest support as the cause for the Permian extinction, but even that isn’t really a smoking gun. At least not yet.
As with so much in geology – and this is what keeps it interesting – it’s not completely certain that this was even one “event.” You’ve heard me refer to mountain-building “events” that lasted tens of millions of years. Geologists tend to play pretty loosely with our time scales, even though we recognize the value of accurate age dating. Often, we just don’t have it. So consequently there’s some debate about the nature of the Permian extinction. There is really no doubt that the final blow, at about 251 million years ago, was very much a “spike” in terms of geologic time. But such a spike could represent most of a million years, or as few as 200,000 years, and some scientists suggest that most of the Great Dying took place within a span as short as 20,000 years – and that really would be essentially instantaneously, geologically speaking. The other view would have the final blow coming as the culmination of a 15-million-year period of climate change that was punctuated by other more short-lived episodes. There isn’t really any doubt that there was another specific event near the end of the Middle Permian, at about 260 million years ago – we mentioned that the other day in connection with the last trilobites – and there seems to be a decent correlation with it and a glacial pulse that lowered sea level. At this point we could almost call that a “conventional” cause for extinction. But was it an isolated event or was it, and the big one at the end, all part of some more general set of changes? We don’t really know, but obviously everything from glacial advances to aerosols from volcanoes to ocean circulation patterns caused by Pangaea’s size and shape and position – all those things could be part of the cause for the greatest extinction the world has ever seen.
As you can imagine there is a vast amount of work that has been done on this topic, so please look at this as just the barest summary of possibilities.
* * *
On a far smaller scale than the Permian extinction, on August 31, 1886, a strong earthquake struck a very unexpected place – Charleston, South Carolina. The magnitude is estimated at 7.3 – huge by any estimation. It was felt as far away as Boston, where the shaking rang church bells. Almost every building in Charleston was severely damaged or destroyed, and at least 60 people were killed. The area had a long history of low-level seismic activity, just as it does today. But why is there any seismicity? It’s far from plate margins and volcanic zones. It’s really challenging to explain these earthquakes that occur far from areas of active fault motion, but I think the current explanation for the Charleston quake is that there was significant faulting in this area when the Atlantic Ocean opened, and that the 1886 quake was a rejuvenation of that faulting related to the ongoing pull-apart of the Atlantic. Right now in our calendar, we’re at the end of the Permian, and South Carolina is still firmly attached to Africa. It didn’t begin to break apart until the Jurassic, which we’ll get to in October.
Tomorrow, the Triassic Period and the Mesozoic Era begin.
—Richard I. Gibson
Graphic from Wikipedia under GFDL
Palaeos mass extinctions