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dino extinctions and meteorites



Alex Hertzog raised the question of sulphur, meteorites and dino
extinctions.  PBS had a TV program that suggested the scenario
Alex mentions as o ne of several possibilities.

There can be little doubt that the date of the Chicxolub crater
is within the date of the K-T boundary to within 10,000 years.
It is equally certainly ONE of the impact site, probably the
largest.   My own reconstruction, given in my book (just publ-
ished two weeks ago), is th at there were about 100 impacts,
of which we can now distinguish the sites of about 12 or 13,
with Chicxolub the largest.

The Yucatan site consists of sulpur-rich rocks, but I believe
the dinos were probably all dead before the time of the impact.
The reasons are co mplex, and if you want to know why I think
this read my book.  In any case, the cause of extinction was
linked to the events that caused impacts that were slightly
later in time.  The impactors were comets, not meteotite
fragments.  The chemical signature in the K-T boundary rocks
makes that  quite clear.

ISOROPIC DATING METHODS

Isotopic dating methods are available for a great range of dates
and a great range of resolution.  C-14 is only usable back to 
60,000 years and then only under extreme conditions of experi-
mentation, using accelerator mass spectrometry (not accererated,
as someone wrote it).  I worked with the Univ or Toronto
AMS lab (and was chairman for th ree years).  Under ideal cond-
itions C-14 dating has an error range of  about 0.3 %.

Chlorine-36 dating is good for dates in the range of 1 to 5 
million years.  For longer time periods various combinations
of argon-dating or uranium series dating are available. 
Argon39-argon40 has the best resolution for a few hundred mill-
ion years, provided you use AMS.  For a few billion years 
uranium series dating methods work.  I have used lead-lead
for dating the Allende meteorite.

These are not the only methods.  Strontium-strontium is one
mentioned by Elisab eth Brouwers.  It has poor resolution,
largely because Sr can exchange freely with calcium.

A good reference for dating of older rocks is Dalrymple (1991).
The AGe of the earth.  Stanford U. P.

OTHER EXTINCTIONS       
Simon Smith raises the q uestion of impacts.  There is good
evidence for at l east 11  mass e xtinctions du ring the
phanerozoic, the biggest of which was the terminal Permian
at 284 Ga (billion years before the present).  That one 
killed off about 96% of all species in th e fossil record.
There is good evidence that at least 5 of the 11 extintions
involved an i mpact from outer space, including both the 
K-T and  terminal Permian, as well as the one in the Devon-
ian.

The idea that par  asites  could cause a mass  ex ti nction
is nonsense. 

wOBBLES OF THE EARTH

The wobble is not measured, to repl y to James Shields query.
It's inferred.  The wob le is not a su b-harmonic of any 
longer term cycle.  My I do seem to be leaving a lot of spaces
in the m i dd le o f words
DAvid





--
>From: David Brez Carlisle
bk090@Freenet Carleton.CA