[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Extinction (was: Religion)



From: pjanke@maroon.tc.umn.edu

 > 
 > Stan wrote:
 > >That is far from demonstrated.  You need solid (not merely
 > >suggestive) evidence for impacts in association with other
 > >major extinctions to accomplish that.  That is not now present.    
 > 
 > I agree that the data is in need of reinforcing studies, but this
 > lineup of the big 6 with known huge craters at 4 of them is a good
 > start. ...
 > 
 > BOUNDARY                     crater      time of
 > (mya=million years ago)      diameter    impact       location
 > _________________________________________________________________________
 > Eocene/Oligocene(34mya)        100km      39+-9      Popagai, Russia
 > 
 > Cretaceous/Tertiary(65mya)    >200km      65         Chicxulub, Mexico
 > 
 > Carnian/Norian(223mya)         100km     220         Manicouagan, Quebec
 > 
 > Permian/Triassic(250mya)      >300km**   250         S. Atlantic
 > 

We went over this before.

I find the evidence that this is an impact crater singularly
unconvincing.  The evidence here is *far* less compelling than
the evidence that Chicxulub is an impact crater, and *that* one
is now in doubt!

The best you can say is that this is *maybe* a *possible*
example of what could be an impact crater.


Then there is the fact that the correlation of the Manicouagan
crater with the Carnian extinctions is not established, since
the dating is not that precise, and there is no evidence of
an ET style siderophile peak or of shocked quartz at the level
of the Carnian extinctions.

All one can say here is that the impact in question occured within
4-5 million years of the extinctions.

 > 
 > The 4 known impact structures listed above are the biggest phanerozoic
 > craters. As the dates for these structures have been refined in recent
 > years, their alignment with known mass-extinction events has become
 > evident.

Well, considering that only *two* of them are confirmed craters ...

 > It might be a statistical fluke but methinks not. Also, there
 > is indirect evidence for impacts at the other two mass extinctions
 > (P/Tr and Ordo/Silu) in the form of Iridium layers and shocked quartz.

Hmm, the last I heard the siderophile peak at the P-Tr extinction
level showed an elemental profile compatible with a terrestrial
origin.

[I think the same is true of the Ordovician/Silurian peak, but
I am not as sure about that, as I have not been following that
part of the literature closely].

swf@elsegundoca.ncr.com         sarima@netcom.com

The peace of God be with you.