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Re: Milankovich Cycles



>And here I was worried that PaleoNet was getting a little dull lately!

>Being a skeptic about these notions of extraterrestrial forcing, I'd >like
>to point out (along with Roy Plotnick) that the 26 m.y. extinction
>periodicity Tom alludes to has a lot of critics ranging from >statisticians
>to systematists.

Ah. Nice to see your still around Norm! 
No allusion at all!  I was merely pointing out another cycle in my list of
cycles and cited Rampino, who attempted to couple periodic asteroid showers
with the solar system's motion through the galaxy. I don't think anyone
 accepts the impact aspect of the model including _myself.

>  And the dates on many (most?) impact craters >are much
>more imprecise than the biostrat. ages for the biotic data.

While true,  there are renewed attempts to obtain better age data and the
Chicxulub structure comes to mind in this regard.   And not nescessarily from
(some) of the biostrat. people, but isotopically.

 >Moreover, the
>simple demonstration of an impact having occurred at any particular >time is
>not sufficient to demonstrate a causal link between the impact event >and
>elevated levels of extinction intensity. 

Agreed. While the timing of an impact may or may not play a significant role
in elevating extinction intensity the SIZE of an impact and the type if
target rock could.  

> Even the most cursory examination
>of the data reveals many cases in which well-dated impacts are not >closely
>associated with heightened levels of extinction (e.g., the Late >Eocene
>impacts; see Prothero and Berggren, 1992)

I'm not even trying to imply that every impact had a mass extinction
associated with it. Having said that,  I feel that the possibility exists for
such a scenario to occur and the evidence (hard rock)  for the terminal
Cretaceous event is the strongest  I know of in support of that scenario. 


> as well of cases in >which bona
>fide mass extinction events are not associated with unambiguous >impact
>markers (e.g., the Late Devonian event: BTW the current consensus >is that
>Ir anomalies are NOT unambiguous evidence of impact occurrence; >see
Colodner et al., 1992, Nature; Sawlowicz, 1993, P3; Wang et al., 1993,
Geology).

I would not yet write off the Frasnian-Famennian or latest Devonian
extinction  yet.  If you like I can comment in more detail off net.

>  The assertion that extraterrestrial factors (a polite euphemism
>for impacts?) cause global extinctions cannot be tested unless the >precise
>mechanisms responsible for those extinctions are specified AND >the fossil
>record examined to determine whether it conforms to the >predictions of
>those mechanisms.  If you try this for most fossil faunas you run >into big
>problems in trying to get the predictions of your mechanisms to >provide a
>detailed and unique fit to the observations

Each extinction event is unique unto itself. I would contend that the
consequences of say, the K-T impact would not nescessarily apply to an
earlier impact of equal magnitude.  Let me reiterate that I do not  contend
that every extinction is associated with an impact(s) but that impacts can be
associated with some extinctions. The problem as you say is in testing the
models. The cometary bombardment of Jupiter earlier this year at least gives
us a "ballpark" idea of the effects of an impact in lieu of actually waiting
for a big one to hit the earth ;-)

________________________________________________________
References of Relevant Interest:

Geotimes; May 1995. p11 
              Model links ancient impact to volcanic eruptions. 
    
This was a blurb reprinted from the November 1994 issue of Sandia Science
News. It involves a paper by Mark Boslaugh et al , "Axial Focusing of Energy
from a Hypervelocity Impact on Earth".

Sandia super computer simulations now show that mjor episodes of flood
volcanism can be induced on the ANTIPODE (i.e. Deccan) side of the earth. 
________________________________________________________

Meisel, Thomas et al,  1995, Combined osmium and strontium isotopic study of
the Cretaceous-Tertiary boundary at Sumbar, Turkmenistan: A test for an
impact vs. a volcanic hypothesis. Geology, v.23, no.4, p.313-316

Also in the same month's (April) issue:

Longoria, L.F., and Gamper, M.A., 1995, Planktonic foraminiferal faunas
across the Cretaceous-Tertiary succession of Mexico: Implications for the
Cretaceous-Tertiary problem. Geology, v.23, no.4, p.329-332

________________________________________________________

Schuaraytz, B. C., Sharpton, V. L., and Marin, L. E., 1994, Petrology of
impact-melt rocks at the Chicxulub multiribg basin, Yucatan, Mexico. Geology,
v.22, no.10, p.868-872

Barrera,E,. 1994, Global environmental changes preceeding the
Cretaceous-Tertiary boundary: Early-late Maastrichtian transition. Geology,
v.22, no.10, p. 877-880

See also the "Forum" section p.953 of the same volume as above for a comment
and reply by Smit,Alvares et al vs Keller, MacCleod et al and a second C&R by
Koeberl vs. Lyons, Officer et al.
_________________________________________________________

Perry, E., Marin, L., McClain, J., Guadalupe, V., 1995. Ring of Cenotes
(sinkholes), northwest Yucatan, Mexico: Its hydrogeologic characteristics and
possible association with the Chicxulub impact crater. Geology v.23, no.1, p.
17-20
_______________________________________________________
McRoberts, C. A., Newton, C. R., 1995. Selective extinction among
end-Triassic of European bivalves. Geology, v.23, no.2, p. 102-104
________________________________________________________
Kauffman, E. G., and Erwin, D. E., Surviving mass extinctions. Geotimes.
March 1995

Toon, O.T., and Zahnle, K.,  All impacts great and small. Geotimes, March
1995.

In fact the entire March issue of Geotimes is devoted to the subject of mass
extinction and a must read.
________________________________________________________


 (e.g., ammonites gone >from the
>record prior to the K/T impact debris, fish sailing through the same
>"catastrophe" seemingly unaffected). 

I guess that depends on whose paper you cite regarding the ammonite
extinction (one I don't have in front of me).  For another time!

Two apologies; 1) For the length of this post and 2) for the atrocious
spelling. The latter is due to a rather crappy keyboard and exacerbates the
fact that I "hunt and pick" type!  

                                  Regards,
                                  Thomas R. Lipka
                                  Paleontological/Geologoical Studies