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Re: extinction



"Dewey M. McLean" <dmclean@vt.edu> writes:
> Response to Graydon Saunders' Wed, 14 Jan 2004 21:19:48 post.
> 
> Graydon wrote:
> 
> "How do you go from the million or so years of intermittent
> Deccan eruptions to the neat global iridium anomaly?"

 
> My posts of Wed, 07 Jan 2004 13:19:20 and Wed, 07 Jan 2004 19:10:04 
> 
> note that several K-T sections from around the world display 
> multiple 
> K-T transition iridium spikes:
> 
> 1. Nanxiong Basin, South China (6 spikes)
> 
> 2. Braggs, Alabama (3 spikes)
> 
> 3. Lattengebirge, Bavarian Alps (3 spikes)
> 
> 4. Brazos River, Texas (2 spikes)



Your list is not sufficiently informative.  What are the depositional
environments at each of these sites?  More importantly, what is the
*micro*-stratigraphy of these zones?

Until you eliminate the possibility of redeposition at each of these
sites (i.e., the creation of multple Ir layers out of one pre-existing Ir
layer), then your list tells us nothing.


> 
> Since those posts, I've recalled two more sites with multiple 
> iridium spikes:
> 
> 5. El Kef, Tunisia (2 spikes)
> 
> 6. Beloc, Haiti (2 spikes)


See my comment, above.


> The metallic iridium particles could have originated via an impact 
> event. But, who knows?


After 25 years of research, we *should* know by now.  :-(
The two possible sources for the iridium (volcanism vs. impact) should be
distinguishable by analysing the Platinum Group Metal ratios in the K-T
iridium layer.  Platinum Group Metals in meteorites are chemically and
isotopically nonfractionated.  Platinum Group metals in volcanic rocks
are highly fractionated.  Isotopic ratios of the Platinum Group Metals
should also help to distinguish between the two possible sources of the
K-T iridium.  These two studies could have been conducted (or, at least,
attempted) 25 years ago.  Have either of these two studies been conducted
yet?
 

> The K-T boundary clay is seemingly a "lag" 
> deposit whose deposition may have spanned 40,000 to 50,000 years, or 
> 
> possibly up to 100,000 years.


Are you claiming that the K-T boundary clay is a world-wide lag deposit?

This is clearly not true.  The environments where the boundary clay
exists are quite diverse.  What about bathyal deposits (DSD core sites)? 
What about swampy deposits (Brownie Butte, Montana)?   Certainly, some
places show multiple erosional/depositional cycles (and I happen to
believe that redeposition of iridium is more common than is now believed,
even in the bathyal zones), but the phenomenon is locality-dependent.

To claim that the K-T boundary clay is everywhere a lag deposit is
analogous to claiming that there was a world-wide catastrophic flood.
(and we don't want to go there).


> Volcanic debris can float for long 
> distances via marine currents.


So can the dust from impact ejecta.
As I've noted in previous posts, I have seen a lot of arm-waving and
armchair theoryizing over the last 25 years, but I have seen far fewer
studies that have attempted to actually distinguish volcanic iridium from
meteoritic iridium in the K-T layer.  I don't know if the reason is
because of a lack of research funds or if there is some psychological
need on the part of researchers to *not* do the necessary research in
order to keep the issue unresolved.  ;-)

My last comment was written only half tongue-in-cheek.

<pb>
--








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