I have been debating the basic premise of anthropomorphic influence on the habitat of Earth over time. Lots of areas into which this discussion has drifted are good debate fodder, but one element has me stumped, with respect to basic answers, rather than implications, and inferences.
There have been some half dozen or more “extinction events” during the history of life on earth. I say some half dozen because it turns out that even the exact number is not something on which the various disciplines of study can entirely agree. Well, yeah, reading your data over half a billion years has its limits. I get that. But there is one little fact I find elusive, and yet I see references to other assumptions which seem to assume some level of knowledge about it.
How long did the events last? Did the dinosaurs die out in a period best measured in hours, days, weeks, months, years, decades, centuries, millennia, or was it a matter of millions of years? The Permian extinction is certainly vast in terms of loss of diversity, but was it a cataclysm, or a process? (My most exacting sources differ on the date of the period by more than four million years, and make no mention of its duration.) I actually don’t want to get into the causality issue, but it may be inevitable, to support particular estimates.
What I want is a fairly well supported estimates on the length of time that whatever forces killed off so many genera took to do so. Cites are critical, since this particular debate forum really distrusts “the Internet” as a source, unless you can provide institutional ownership information for the website. (Suspicious bastards, but not necessarily unreasonable, given the actual content of the Internet.)
Tris
You’re going to get different answers for each of the mass extinctions, since the causes have usually been different for each one. Further, these events often seem to be due to a confluence of several different negative forces happening to take place at nearly the same time. The answers will also be different for different groups of organisms in each extinction event.
For the end Cretaceous (K-T) event, it’s pretty well established that an asteroid impact was at least partly to blame. Some extinctions linked to the impact seem to have taken place suddenly, most likely over a period of months to years. Others may have stretched out longer, probably over centuries to a few thousand years.
However, there were also massive outflows of lava taking place in India (the Deccan Traps) at approximately the same time which also could have had very severe effects. Also, sea levels were falling, which would have had adverse effects on marine life. This one-two-three punch almost certainly made the extinctions even worse than they would have been otherwise. And such extinctions could have taken place over tens to hundreds of thousands of years.
The causes of the Permian extinction are much more poorly understood, and there remains a great deal of controversy. However, it seems clear that multiple causes were at work at the same time, including the consolidation of the supercontinent Pangea (which decreased habitat for marine organisms), massive volcanic eruptions, changes in ocean chemistry including very low oxygen levels, very intense global warming (perhaps triggered by the volcanic eruptions), and possibly an asteroid or meteorite impact on top of it all.
The Permian event was pretty certainly far more prolonged than the K-T event, taking hundreds of thousands to a few million years. In fact, there may have been two major pulses of extinctions 5 million years apart. It seems that the Earth was just having a very very bad mega-year back then.
As for cites, a lot of this is based on technical literature that is not available on the internet, at least not for free. Wiki has some decent summaries of the evidence and controversies on both extinctions, but some of it is not well-sourced.