This is not the original post that I wanted to write. I wanted to specifically cover my rant on YAGUMETs. Or as the rest of you have seen me call them 'Yet Another Grand Unified Mass Extinction Theory', but in the plural. However, since Dr Keller - ever the anti-bollide guerrilla - sent out yet another press release in support of vulcanism as the culprit for the KT Mass Extinction killer, I thought I would shift gears and try to do a comparison of the PT Extinction and KT Extinction events to try to make a point. Since Dr Keller favors vulcanism, I am going to make the assumption for this post that the Permian Extinction, which has been pretty strongly linked to the effects of massive vulcanism, is a 'proven' fact and its signature is the gold standard for vulcanism being the cause. Even though I am thoroughly convinced that the KT Extinction was caused by Chicxulub. I am going to assume we're unproven here and compare the evidence and signatures of the KT to the PT. I will return to the YAGUMETs post for my next big paleo post, but for now, let's get on with the show.
The Permian Extinction was not a pretty picture. I sketch it out here at least moderately well. It was almost certainly caused by the prolonged and quite nasty eruptions of the Siberian Traps, a truly terrifyingly large and prolonged flood basalt eruption. The evidence that supports this had a very distinctive signature: certain events happened and we think we know why. If we use this signature as a baseline for comparing other extinctions to the PT Extinction we really ought to be able to see if that other signature fits the same profile. If it does, then it seems very likely that the extinction was caused by the same mechanism. With that in mind, we'll approach and compare the KT to the PT!
One of the very distinctive and confusing items about the PT Extinction was that it seemed like it was a very long term affair. This was because careful collections and biostratigraphy had not been done until relatively recently. The reason for appearing to be a very drawn out affair was there were actually multiple pulses of mass death. The pattern of pulses looks something like a drum beat that gets louder and fades. In fact, the last pulses take place actually after most of the dying took place and end up geochronologically in the Triassic. It's very, very distinctive and almost certainly a result of the eruptions and their resultant ecological complications tripping past some thresholds. The the multi-pulse extinction attribute seems to be a very important one for vulcanism caused extinctions and makes perfect sense given how it kills. How does this compare to the KT Event's pattern of extinction? The fact of the matter is that it doesn't match at all. There seems to have been a minor mass extinction ten million years prior to the KT Boundary, but it does not seem to have reduced the number of dinosaur genera present and even had the effect of the mammals diversifying. That might be argued that the prior minor extinction might be the first pulse of the KT, but...there's no subsequent pulses into the Cenozoic! The KT Extinction was a quick and bloody affair: there appears to be only a single pulse involved.
The Permian Extinction killed a lot of everything. By some counts as much as 90% of everything died. That included plants, animals, etc. As a result, and also as a consequence of what was changed environmentally because the the Siberian Traps, life took a very long time to recover. There were sections that have no fossils at all. While the extinction was less nasty in the terrestrial environment - approximately 70% of everything died there - beds of the Karoo in South Africa, one of the best places for chronologically uninterrupted PT Event sections, there are no fossils at all for over 5 million years. Life would not recovery the diversity of the Late Permian for a very long time. Delayed biotic recovery would be another good attribute for identifying vulcanism induced extinctions. The KT Extinction had, at the most, a 300k year recovery period. For something that killed 70% of everything, this is remarkably fast and completely unlike the PT Event. The diversity of life would increase very, very fast after the KT Event. This would also lean towards the idea that the KT Extinction had a different mechanism for the root cause.
During the PT Event, it appears that the Siberian Traps belched tons of carbon dioxide. This caused a massive upswing in temperature especially with the addition of the forcing caused by methane as well. The upswing appears to have been in the ten Celsius range. While the temperature dropped down, it remained high throughout the Triassic and into the Jurassic. This is supported by the isotope measurements and mineral deposits. On the other hand, the climate that crosses the KT Boundary as geologically recorded seems to be pretty stable. It was warm and stayed warm. There doesn't seem to have been a wild change over a prolonged period as it was with the PT Extinction.
Differences in Marine Biotic Extinctions
One of the most profound differences in the KT and PT Extinctions can be found in what died first and what was most effected in the oceans. During the PT Extinction, the benthic organisms - those that live on the seabed - were effected first AND most severely. As you rose up from the depths of the sea, the extinction got less and less severe - though, truthfully, that's something of a hair's difference, the PT Extinction was just damnably horrible - until you reached the terrestrial environment where it was the least severe. The key point though here is that the benthic environment & ecology was mopped across the floor. The reason for this is that the oxygen level dropped down to a anoxic state and full of hydrogen sulfide: the chemical analyses of the PT sediments support this. The KT Extinction couldn't be more different. The organisms effected the most were those that were those that were living in the euphotic zone: photoplankton, ammonites, the great marine reptiles, etc. The benthic organisms were far less impacted. This is a VERY strong indication that something different happened at each extinction event.
Extinctions in the Terrestrial Realm
In the terrestrial environment the Permian Event seems to have been a little uncertain as to what allowed organisms to survive the Great Dying. Truthfully, not enough research has gone into trying to find some sort of common theme that might unite what survived and what did not here. From the point of view of what is currently known, there is no common theme. It appears currently that the killing didn't favor one type of organism over another. This would be what Raup called a 'Field of Bullets' scenario. It truly remains to be seen though. On the other hand, it appears that at least for KT Event this was rather different. There was a definite pattern involved. At first it was thought that it was a weight limit: no more than 50 kg survived the KT Extinction. That hasn't quite turned out to be true: the cut off wasn't mass or weight, but metabolic requirements. Rather it was a metabolic requirement. If your required more calories than a certain amount - the exact amount uncertain right now - then you got whacked for sure. The prime example of this is the Sebecosuchidae, a form of Mesosuchia (or Mesoeucrocodylia, if you're more accurate). the Sebecosuchidaes were largish terrestrial carnivores that made the cross-over from the Mesozoic to the Cenozoic. IIRC, they were over 6m in length (Darren Naish has an excellent article on the subject at Tetrapod Zoology). This means that there is a definite and solid pattern. The KT Extinction was more selective in what died than the PT Extinction. However, there was still an element of randomness to the groups that were within the surviving criteria; frex, 50% of mammals died out and they were well within the metabolic requirements for survival. Lady Luck still had her due. However, this is still very different than in the PT Extinction as far as the research so far has indicated.
Omega: In the End
Now, I have run out of time. I hope that by detailing some of the differences between the KT Mass Extinction and the PT Mass Extinction. I have tried to avoid the usual arguing points: evidence of impact, charcoal traces, geochronology of the Deccan Traps, etc. I have been trying to point out the other diagnostic characteristics that are often overlooked. The fact of the matter is that these two extinctions killed in very different ways. These two extinctions effected organisms of different types. They cannot be caused by the same mechanism. Extinction events are not just something that has a geochronologically convenient 'explanation.' That explanation has to fit the pattern of that extinction as well as the time frame.
I will do a detailed KT Mass Extinction post in the relatively near future, but that will come after the YAGUMETs post. Now let's see if anyone comes along and beats me with a big stick.
1. Which makes me wonder why it's, well, the Triassic if we are defining the extinction event as the demarker, but I am not a geologist.