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From: cramer@kontron.UUCP
Newsgroups: sci.bio,sci.astro
Subject: Re: Mass extinctions
Message-ID: <1489@kontron.UUCP>
Date: Tue, 31-Mar-87 19:04:39 EST
Article-I.D.: kontron.1489
Posted: Tue Mar 31 19:04:39 1987
Date-Received: Fri, 3-Apr-87 06:41:38 EST
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Xref: utgpu sci.bio:201 sci.astro:779

> The volcanic hypothesis is not dead yet; a recent Nature (376 #6109;
> 12 March 1987) prints a long review article by Officer, Hallam, Drake,
> and Devine entitled "Late Cretaceous and paroxysmal Cretaceous/Tertiary
> extinctions" espousing it.  Officer et al. do take care to say
> "We wish to emphasize that this article predominantly advocates
> a particular point of view and may be criticized in giving less
> attention to alternative models."
> 
> They do not discuss osmium at all (even though the abstract mentions
> "iridium and other associated elements.")  I take this to be
> a defect.
> 
> There are at least three ideas attending the work of the Alvarezes
> and others, and that following it:
> 
> 1) A large meteor/comet struck the earth at the K/T boundary 65
> million years ago, as suggested by the iridium-rich layer
> and perhaps other things.
> 
> 2) This event was responsible for the mass extinctions around that
> time ("killed the dinosaurs").
> 
> 3) Similar events occurred periodically before that.
> 
> One doesn't have to buy these as a package, even though they're
> frequently offered that way.  Moreover, they are all still in
> controversy.  From what I've been able to gather, the meteor
> idea is reasonably well supported but by no means certain.
> 
> The second is not especially well supported in any direct way;
> many paleontologists believe that the extinctions were occurring
> for quite a while both before and after the "instant" of the
> putative meteor.
> 
> The third seems to be on slightly shaky ground after a strong start.
> The reality of the periodicity is questionable, and all of the
> proposed mechanisms (e.g. the Nemesis planet or dark star,
> the oscillations of the Solar system through the galactic
> central plane) have been attacked on physical grounds.

It's time for cross-fertilization.

I was reading the Encyclopedia Brittanica article, "Meteorites" the other
night, and noticed a number of interesting details that might well explain
meteoritic origin of extinctions with rough periodicity and also explain
how the event Alvarez *et. al.* are interested in could be in the middle
of the extinctions, without damaging the essential hypothesis.

1. No great surprise, but the estimates for one of the Canadian astroblemes
is 17,500,000 megatons.  (No, not 17.5 megatons -- 17.5 gigatons).

2. Cosmic ray effects can be used to date how long a meteor has been detached
from a larger body.  Stony meteors show a peak at 5 x 10^6 years, with a
tail out to 40 x 10^6 years.

3. Meteors in Earth-crossing orbits originating in the asteroid belt are
likely to be removed from such orbits pretty quickly (in geologic terms),
and thus what we are seeing are probably the result of a significant
collision 5 x 10^6 years ago, with a few from older collisions.  The
results of really old collisions (in the asteroid belt) are likely to 
have been swept out of Earth-crossing orbits so long ago that we are 
unlikely to see meteors as a result of these collisions.

The conclusions I want to draw are that:

1. Alvarez *et. al.* may have identified one very large meteorite event
that was part of a stream of meteors resulting from a collision of large
bodies in the asteroid belt >65 million years ago.  A great many meteors,
some quite large, could explain why the event Alvarez *et. al.* have
postulated is in the middle of the extinctions -- not at the beginning.

2. Periodicity of extinctions may be irregular because not only does
SOMETHING have to be regularly interfering with the orbits of the 
asteroid belt, but just the right combinations of asteroids have to be
in orbits that can collide and produce short-term (a few hundred thousand
years) increases in meteor collisions with Earth.

> I'm most emphatically a non-expert in these issues and can't
> really judge them.  The new ideas are most exciting, but I
> don't think they have yet carried the day.
> 
> 	Dennis Ritchie

Just another amateur also.

Is that THE Dennis Ritchie?

Clayton E. Cramer