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Re: craters and the k/t



Sherry (Flint Weller) wrote (9/8/96; 10:42p):

>Norm King wrote:

>>I think the presumed tsunami deposits are actually more in dispute than 

>>the subsurface feature that is hypothesized to be the impact crater. 

>Can't tsunamis also be caused by the movments of the ocean plates? 
>There was quite a bit of volcanic activity around the K-T...

The Gulf Coast K/T tsunami deposits (if that is indeed what they are) are 
on a scale unparalleled in the rock record.

>>It just happens to be so close to the K-T boundary that its position
>>is indistinguishable from that level.

>As I understand it, Chixalub is a bit too old for the K-T. Didn't they
>find undisturbed sediment from the later Maastrican (sp?) *inside* the
>thing? If it was volcanic or extraterrestrial, anything from that age
>should have been *blown out*.

I don't believe they found undisturbed Maastrichtian sediments within the 
crater.  One question is how to show sediments are really _undisturbed_.  
Others are how far "inside" is "inside", and what do you mean by 
"inside"?  Sediments deposited over the top could now have settled into 
the crater by the eventual compaction of debris below them, so they now 
lay opposite older Maastrichtian sediments in the crater wall.  Or, other 
sediments may have slumped into the crater in large pieces that appear 
undisturbed.  Yet another question is whether a K/T boundary as 
identified on the basis of marine microfossils (as at Chicxulub) would 
match a K/T boundary as identified on the basis of terrestrial fossils, 
if it is the dinosaur's demise on land that we're ultimately intersted 
in.  Still another question is, how did they date the Maastrichtian 
sediments in the crater?  Could the biostratigraphic indices be reworked? 

>>The data suggest a classic impact crater, with many (all?) of the
>>subtleties that should be observable for a buried impact crater.
>>People who know how to assess these things claim that the presumed
>>crater is large enough for the impact to have had an effect on the
>>environment that could have adversely affected life on a global
>>scale.

>Of course, this is were we get to the parts of the impact theories I
>have the most trouble with. Where is the evidence that an impact
>would cause such long term damage??

There will be no totally convincing evidence until we have the misfortune 
of witnessing a similar event.  That doesn't mean that we should make no 
attempt to predict what the effects should have been.

>Large impacts in the past have never caused mass extinctions-or any 
>extinctions.  It's all guesswork backed up with more guesswork.

I think you are assuming your conclusion.  You can define guesswork any 
way you want, but the hypothesis here is the simplest for all of the data 
presented so far. 

>>Recently, a chunk of meteorite was brought up in a core from the
>>eastern Pacific from a location that would have been almost directly
>>down range from the impact, given the trajectory which had been
>>calculated for the impacting object based on independent data.

>But we find meteorites all the time all over the Earth. How can they
>really tell?

This is only the second meteorite ever recoverd in any DSDP or ODP core, 
anywhere.  It came from the cm or so thick K/T layer in that downrange 
core.  The question might be, "Is that just a coincidence?"  Might be, 
but I don't like these one in a million coincidences.

>> I don't know about the rest of you, but finally the number of
>>observations that fit together so nicely in the impact scenario
>>become so great that I find it impossible to believe that they are
>>all coincidences.

>Well, I think if you take into consideration volcanic activity and the
>fossil record, the impact theory is pretty weak. 

I need something more specific to respond.

>As far as the fossil record goes, we find a history of decreasing
>diversity and numbers approching the K-T. 

At least some of this (marine fossils) has been determined to be what is 
called "Signor-Lipps effect," which means that not every taxon will be 
found right up to the boundary in question; in fact, most won't.  In 
other words, those taxa did not die out at their highest observed 
occurrence, prior to the main event.  We just haven't found the taxa in 
those strata, possibly due to preservational problems.  This is a 
difficult problem to assess, but a statistical technique has been applied 
to the K/T boundary showing at least some Signor-Lipps effect in the 
data.

>We also never have found an increase of fossils after the K-T that would 
>suggest a sudden massive death event. Also, why would the effects of the 
>impacts effect some animals and not others?

I think this is simply not a correct assessment of the situation.  To my 
knowledge, the pattern of recovery of the marine fauna/flora above the 
K/T boundary has been shown to be consistent with what is expected.  I 
believe also that palynomorph data are consistent.

>[snip]...to prove cause and effect, the timing has to be *just
>right*. That is really hard to pin down. 

That's why I am always careful to say that the Chicxulub crater and 
associated effects are so close to the boundary that they cannot be 
distinguished from it on the basis of any unequivocal data.  I realize 
that claims of a-synchroneity are made from time to time, but it doesn't 
take long for counterclaims to those claims to appear.
 
>Oh well, there has to be a skeptic in every crowd, huh? It's nice to
>talk about this stuff without fistfights occurring for a change.

This is the only way, when you realize that some new information might be 
found tomorrow that disproves everything, and we all need to be receptive 
to it.  Of course, you really have to find something important to do 
that.  Just one unexamined observation won't be enough to do it.  What I 
mean by "unexamined observation" is that some data may not be as 
pertinent as originally thought, for reasons that someone else is bound 
to come along and tell you about.  The situation will evolve, rather than 
changing overnight.

>My biggest gripe about the article is that it is dangerous to assume 
>theories that are still under considerable research. 

Hypotheses can be used to design additional research.  In fact, that is 
_normal_ in science.  You go out and "test" the hypothesis, by seeing 
whether predicted observations can be made.  The only danger would be 
that you are so close-minded that you see what you want to see, and 
ignore counter evidence.  In this case, they really weren't assuming 
their hypothesis to be proved--they were wanting to see if additional 
observations either support it or refute it.  The writer of the article 
just couldn't quite make that clear.

>Also, I'd like to see a few non-impact articles in these magazines. 
>But hey, it's not as dramatic. You need those exploding dinosaurs on
>the cover to sell issues. :)

You're right about the drama.  But it may be significant that non-impact 
articles are few and far between.  Non-impactors haven't made a synthesis 
that accounts for everything that was going on at the end of the 
Cretaceous.  Some have even said that nothing unusual was going on--that 
there was no mass extinction.  That may be, but I think what we see at 
first glance, which is a mass extinction, is likely to have some 
connection to reality.  Too many observations have to be explained away 
otherwise.  As I've said here before, I'm waiting for the "nothing 
happened" synthesis, too!

*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*
Norman R. King                                       tel:  (812) 464-1794
Department of Geosciences                            fax:  (812) 464-1960
University of Southern Indiana
8600 University Blvd.
Evansville, IN 47712                      e-mail:  nking.ucs@smtp.usi.edu