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Gregory S Paul wrote:

>Koeberl et al. (1997 - Mod Geol 25:731-734) claim to have discovered a giant
>South African impact crater at or near the Jurassic/Cretaceous boundary. Its
>size is uncertain, it may be from 70 to as much as 340 km across. In
>comparison the Yucatan crater is 180-280 km across. It is therefore possible
>that the biggest impact during the nonavian dinosaur era did not occur at the
>end of their reign, but in the middle of it. Of course, it is possible that
>other super impacts occured that we yet no nothing of because the crater was
>on the ocean floor and has long been subducted, and no one has sampled the
>entire Mesozoic sequence for iridium etc. layers so far as I know.
>If the size of the J/K impact proves to match or exceed that of the K/T
>event, then the problem with the impact hypothesis of dinosaur extinction is
>an obvious one...

There are a couple of qualifications to be made to that report (and I write
as one who interviewed Koeberl a few months back when writing a story on
the impact for _New Scientist_).

One is that the size of both the South African and Yucatan craters are
uncertain. The South African crater is severely eroded; Chicxulub is
buried. Comparable structures are not well-defined and easy to identify.
Sharpton has published estimates of Chicxulub size of 300 kilometers, based
on structures surrounding the crater, but those are not well accepted.
Koeberl told me that the diameter was "at least" 70 kilometers, based on
their preliminary work. That could be the whole thing; it could be just the
inner part. So it's quite possible that the South African crater is
significantly smaller (and the global 'environmental impact' of the crater
probably is more proportional to the square of the diameter than the
diameter itself, so a crater 3 times the diameter would have nearly 10
times the effect).

A second is the nature of the impact zone. The Yucatan target rock was
anhydrate, which ejected a nasty mixture rich in sulfates into the
atmopshere, along with plenty of carbon dioxide, which would have had
serious long-term effects. The impact was on submerged continental shelf,
so there must have been large amounts of water superheated. The South
African target rock was typical continental rock, which would have much
less severe effects when vaporized and tossed into the atmosphere.

Add those two factors together, and you may explain the differnce between
the modest extinctions at the end of the Jurassic and the devastation at
the end of the Cretaceous. It's certainly possible that other factors
played a role. Other environmental stresses could have left dinosaur
populations and diversity low. It will be very interesting to see how this
one plays out, and to see if the dates match the end-Jurassic extinctions.
-- Jeff Hecht,