GeologyRocks

I've thought for a long time that people searching for a single cause of the K-T mass extinction (can I still use the 'T'?) and indeed any others are barking up the wrong tree.

To my mind it has to be a combination of events stressing the environment such that a high percentage of species can't survive.

I'd been of the opinion that K-T was probably a combination of Chixculub ('scuse the spelling) and the Deccan Trappes but maybe the impact happened too soon before the extinction event to have been a direct cause.

Which leads me onto my next questions:
What percentage of species need to die for it to be a 'mass' extinction?
How rapidly do the extinctions need to occur? (especially given the resolution of the timescale in geology).

I believe the can of worms is now well and truly open!

Banton and Harper 1997 say that for it to be a mass extinction 10% of families and 40% of speices need to be wiped out in a short time, although how long a short time is geologically?

My guess would be in a short enough time that they could all be related to the same cause. or maybe one epoch, I don't know.

Flood Basalt Volcanism anybody? OK, ok, maybe flood basalts didn't wipe out the dinosaurs, but they may have wiped out a far larger number of species way back at the Permo-Triassic boundary - check out Andy Saunders and Marc Reichow talking about it on Radio 4...

http://www.bbc.co.uk/radio4/science/thematerialworld_20070118.shtml

http://www.le.ac.uk/gl/ads/SiberianTraps/Index.html

I thought that one of the reasons for the P-T extinction was the forming of Pangaea causing a massive loss of habitats. In particular shallow seas as there was much less coastline.

I was also taught that there was a desertification of the interior of Pangaea as occurs on most large continents today (Australia?). But just wondered if this is a conceit based on the rocks encountered in Britain? After all, the Amazon rainforest extends a long way into a continents interior? Or was Pangaea situated across the desert latitude areas?

It's occured to me that maybe the notes I took at uni weren't entirely accurate.

Maybe they said dessertification and the mass extinction was caused by obesity brought on from eating excess quantities of tiramisu and chocolate fudge cake

Come on people. What are your geology lecturers teaching you.

With reference to below:

“We now have evidence that the Chicxulub impact occurred about 300,000 years before the end of the Cretaceous and thus didn’t cause the mass extinction and, in fact, didn’t cause any species to go extinct,”

This makes perfect sence - Check out the work of Raup and Stanley, as well as Keller. Mass extinctions are step-wise. It would have taken 300,00 years for anything interested to happen. Whole groups of organisms would have died out one step after another.

The Permian-Triassic extinction was felt more keenly by marine species than land species. This is because of the coming together of Pangea and the destruction of the shallow epi-continental sea environments. If you consider four seperate squares (landmasses) and work out the total length of the sides (coasts) then this is four times larger than when you bring them altogether into one large square (continent) (I hope that is right). The more specialized species that relied on niche environments would have had no chance of survival and so a gradual homogenisation would have occurred, reducing biodiversity. The size of the continent would have also meant that it would be very hard for rain bearing weather systems to make it to the interior, this can be seen now in large shield areas such as mongolia (as al8301 says) where even though the continent isn't at the traditional desert latitudes there is a lack of rain that has led to the subsequent formation of the Gobi desert. The type of forests that were prevalent at the time might also have been unable to adapt to that type of environment. One of the reasons the rain forests are so successful is that they rely on a positive feedback loop whereby the rain that falls is rapidly evapotranspired back into the atmosphere thus condensing and forming rainclouds; broadleaf trees did not exist at this point, so maybe evapotranspiration rates were different. (am guessing on this last point!!!! :? )

Go Baylor!

sounds like sence to me!

Hi all,

Tony Hallam, Emeritus Professor of Geology at Birmingham University has written an excellent book published by Oxford University Press called: Catastrophies and Lesser Calamities - the Causes of Mass Extinction, which is quite a good read.

From this book I get the feeling that while he respects Raups veiws he doesn't necessarily agree with them. It is pretty much in the same vein as al8301's train of thought, that multiple "stressors" could have caused the event.