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Re: extinction



It is easy to make two or more sedimentary layers out of one layer by the
process of redeposition.  I have seen places in central Washington State
where stream sedimentation has eroded the 1980 Mout St. Helens air-fall
ash and pumice and then redeposited it down-slope.  This creates multiple
ash layers along the stream banks.  Two or more ash layers, clearly
separated by years of intervening sedimentation, yet each layer came from
one air-fall event from May 18, 1980.

One can replicate such a scenario in a contraption called a "sediment
box".  A "sediment box" is essentially a 10-20-gallon glass aquarium tank
that is filled with 2 layers of very fine sand of 2 different colors. 
The aquarium is placed on a tiltable board and is rocked back and forth. 
Or a steady current is induced on one side of the tank and allowed to
drain out the other end.  I have demonstrated this phenomenon to
students.  It never ceases to be a show-stopper.

Now try the same experiment, but use 2 colored sediments of different
sizes and densities.  The above effect is magnified.  By creating various
currents, you can make one layer into a zig-zag pattern that maintains
its continuity up-section.  In outcrop, if this pattern is of a large
enough scale, and if a geologist has a limited amount of outcrop to
study, the zig-zag morphology wouldn't be obvious and it would appear to
be two or more distinct layers.

Another experimental device called a "wave tank" could also be used.

K-T boundary sedimentation processes would make a neat little Senior
Thesis project, and the empirical data collected from sediment box and
wave tank experiments has the potential to make a real contribution. 
Since the iridium is usually found in a clay layer, the student should
use two clays of different colors.

Before the multiple impact hypothesis is widely accepted, the
single-impact/redepostion hypothesis should first be falsified.  This
would involve studying the *micro*stratigraphy of K-T outcrops where the
layer(s) are well-defined, looking for evidence of current-imposed
micro-structures and textures.  Obviously, this is more along the lines
of Masters- or PhD-level work.

<pb>
--


On Thu, 8 Jan 2004 11:09:01 -0500 "Dewey M. McLean" <dmclean@vt.edu>
writes:
> Response to David Marjanovic's post of Thu, 08 Jan 2004 12:08:43 
> (below the dashed line).
> 
> Perceptive questions, David, and I'll do my best to respond to 
> them.
> 
> The longer I research the K-T extinctions, the more I despair that 
> much of anything will be "proven" concerning cause of the 
> extinctions 
> for long into the future. The reason is that nearly every aspect of 
> 
> the K-T geobiological record is controversial. Many new 
> multidisciplinary data, and interpretations that most scientists 
> agree upon, are requisite to meaningful progress in discovering the 
> 
> actual cause(s) of the extinctions.
> 
> Your point on transport of iridium is one such controversial topic. 
> I 
> recall how at the first Snowbird I extinctions conference in 1981, 
> the impactors claimed that iridium was refractory. Later, they had 
> iridium migrating to where it best served their purposes. So, 
> individual multiple "spikes" in place, or enrichments via migration 
> 
> of the iridium? Based on my communications with other scientists 
> down 
> through the years, I prefer to think of individual spikes in place. 
> 
> But, others feel differently.
> 
> Just because scientific papers are several years old does not 
> invalidate them unless they have been shown to be incorrect. Nor, 
> does it excuse scientists for ignoring the data and interpretations 
> 
> in them.
> 
> Cordially,
> Dewey McLean
> 
> ----------------------------------------
> 
> >>  In fact, multiple iridium spikes do occur at localities other 
> than China.
> 
> >  Hm. These are still rather few -- though the wide geographic 
> distribution
> >  could mean something --, so that, without knowing the local 
> sedimentologies,
> >  I wonder if some bioturbation and groundwater percolation could 
> have
> >  multiplied the peaks or at least smeared them out. It's also 
> interesting
> >  that all 3 papers are quite old. Is there a reason why this topic 
> has
> >  seemingly disappeared from the literature?
> 
> 
> 


"Let's get this train outa here.  Those damn bees might be back any
minute." - General Slater, from the movie "The Swarm"


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