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Re: K/T Extinctions. Volcanic-Greenhouse or Impact?



Matt wrote:

> I recently had to write an 2000 word essay on the following topic. I
> got very interested in the K/T boundary and have decided to put my
> post here where you scientist lot can rip it shreds for me and point
> out all my over assuptions.
>

Overall, it looks fairly good to this amateur's eyes.  I see a few things that 
I'm fairly sure are wrong, though.

> Due to the incident 47% of the genera on the planet
> packed their bags and quitely became extinct, taking with them 76% of
> the species around at the time Including 96% of all marine species.
> This mass extinction also removed 90% of the marine calcareous
> nannoplankton which probably had an influence on the outcome of the
> event, and will be discussed in the conclusion of this essay.

The _Permian_ extinction killed off 96% of marine species, but the K-T 
extinction was not nearly so destructive.  David Raup ("Extinction: Bad Genes 
or Bad Luck?") says that the loss rate was about 38% for marine genera, a bit 
higher for land animals, and land plants don't seem to have been nearly as
badly affected.  .

Regarding the iridium anomaly, one of the major indicators of an impact, not 
volcanic, origin for the iridium is that the iridium isotopic ratio does not 
match Earthly iridium, even volcanic iridium.  However, it does match closely 
with the isotopic ratio for iridium found in iron and stony-iron
meteorites.  This is something that an awful lot of people seem to forget 
about, but it's a critical point because it all but rules out a terrestrial 
origin for the iridium anomaly.

> Volcano-Greenhouse Theory
>
>         Dewey M. McLean coupled the Deccan Traps volcanism to the K-T
> extinctions in 1979 originating the volcano-greenhouse extinction
> theory. The idea behind this theory is that CO2 degassing from the
> mantle associated with the Deccan Traps added vast amounts of extra
> carbon into the carbon cycle. The cycle would not have been able to
> remove this extra CO2 and it would remain in the atmosphere. CO2 is a
> green-house gas. The Deccan Traps main eruptions occured 65 million
> years ago and at the around the same time other volcanoes and sources
> had become active, such as the Cameroon volcanic line, (66 million
> years ago.)

The Deccan Traps explanation is made a bit less likely by the fact that the 
eruptions included at least four major flow episodes, and the dinosaurs didn't 
vanish until the interval between the third and fourth eruptive episodes.  Some 
Indian sites reveal a distinct pattern: the third Deccan eruptive
episode, then some sediments with dinosaur fossils, then the iridium anomaly.  
Next come more sediments, this time devoid of dinosaurs, and then the fourth 
cycle of Deccan eruptions.

>         The extra CO2 in the atmosphere would also directly result in
> the death of some species. For example, humans live in an environment
> with an amtorperic CO2 content of 0.01%, if the CO2 percentage rose
> to 0.03% parts of the human brain would cease to function properly
> death would soon follow.
>

CO2 is not that efficient a toxin.  Old-time submariners during World War 2 
proved the hard way that humans can live, at least temporarily, in an 
atmosphere with as much as one or two percent CO2.

-- Jon W.