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Re: astroblemes

Stan wrote:
>At present there is no known way an impact could generate an
>antipodal hot spot.  And even if it were possible, the time
>sequence would be wrong - hot spots are *deep* structures
>(penetrating at least the asthenospere), and any hot spot
>produced by an impact would take *many* millions of years to
>reach the surface.

The impact is not thought to be the cause of an antipodal "hot spot". 
Only the earth's interior can be the ultimate cause of one of those.
What the impact would do is cause a pattern of fractures in the crust 
as the shock waves converge at the antipode. The shattered and thus
weakened crust region may facilitate massive volcanism, IF the underlying
mantle is favorable towards it.

Stan wrote:
>But, given the neraly perfect correlation of flood basalt volcanism
>and major (and minor) extinctions, it seems that flood basalts are
>more likely a major player in extinctions than impacts.

This is probably true for minor extinctions, but probably false
for major ones.

>[I believe that the P-T iridium spike has been shown to have the
>signature expected of volcanogenic rather than extra-terrestrial

The Ir in the P/Tr boundary clay from Meishan, China is over 10x the
layers above or below it. It is highly unlikely that a volcano could
produce that kind of anomaly is such a short time(the clay is only a
few cm thick). The Os-Ir ratio looks more terrestrial, but might be
explained by Ir depletion in a strong reduction environment. Lets not
forget that we are talking about 250 million years ago for the P/Tr
boundary(compared to only 65 million for the K/T). It will be alot 
more difficult to document the P/Tr as the evidence has been eroding,
decaying, etc. for so long. This is to be expected in general, the
farther back you go in time, the more difficult to find evidence for
impacts, as the craters and evidence for them will have eroded or been 
subducted and erased, etc.

Nevertheless, The evidence for an astrogeological origin of the P/Tr
anomalies is mounting. In addition to the Ir/siderophile anomaly, there 
are several other telltale clues which point to an impact:

  -Abundant glassy microspherules(microtektites). 

  -Negative stable carbon isotope perturbation, 
  -A 300km+ wide gravity anomaly near the Falkland Islands with
   surrounding fold belts all dating to around 250mya. This is
   Rampino's hypothetical site and is admittedly controversial.

Stan wrote:
>Now that I have my references handy, I can post my more definitive
>(and longer) rebuttal of the 'meteorites cause most extinctions'
One good rebuttal deserves another :-)

>The final 10 to 15 million years of the Permian were characterized
>by the formation of the Siberian Flood Basalts, and by extensive
>volcanism in China (the probably cause of the Chinese siderophile
>spike - since that spike is restricted to China).

Not true. In 1986, an Ir anomaly and an obvious shift of stable carbon 
isotope perturbation was reported between the Bellerophon and Werfen 
formations in the South Alps. In 1987, an Ir anomaly was found at the 
P/Tr boundary type section in the Salt Range, Pakistan. In 1988, an 
Ir/metal anomaly was described from extinction boundary sediments from
a drill hole section from the Carnic Alps, Austria. Ultimately, drill-
hole sections may become the best place to look for P/Tr evidence as
it is so meagerly exposed at the surface.

It seems that many are expecting to find the same worldwide massive 
confirmation of an Ir spike that the K/T research has documented. This 
will never happen, for the reasons of intervening time stated above. 
This gun is no longer "smoking" like the K/T is. You can find contiguous 
K/T deposition all over the place, but it's difficult to even find a P/Tr 
section that's not an uncomformity. 

Stan wrote:
>Certainly, the known pattern of extinction seems to be a gradual
>one across the last few million years of the Permian rather than
>an abrupt termination.

When you look at the fossil record over geologic time, the P/Tr *is*
an abrupt event.  Many anti-asteroid theorists make the error that 
everything has to die off in a very short time. The point you are missing 
is that living organisms were comparatively stable for tens of millions
of years before _and_ after the P/Tr or K/T boundaries. The faunal turnover
that occurs at/around these boundaries *is* punctuation of equilibria. 

Stan wrote:
> [table deleted - see original]
>Note the pattern here, the two factors with the *highest* correlation
>with extinctions are flood basalts and ocean anoxic events.  Most
>substantial extinctions are associated with *both* factors.
>Dr. Williams also points out that this pattern is *not* what
>would be expected from a catastrophic extinction caused by a
>meteor impact (where the fossils should continue in full
>abundance right up to the iridium spike, and even perhaps
>show a peak in abundance at the spike due to mass death).

If Dr. Williams "knows" what to expect from a catastrophic extinction
caused by a meteor impact, why doesn't he tell us? ;-)
Seriously, even those that believe that the large impacts happened at
the K/T or P/Tr don't claim to understand exactly what happens after 
that. You can assume an extreme amount of environmental stress, with
some things going extinct right away, some lingering for a while, many
new faunas emerging to fill the recently voided niches, etc.

Regarding the hypothetical thinning of dino fossils leading up to the
K/T boundary: Even if this were true, it's not much to base any conclusions
on. NOTE: in the wake of the K/T impact(or _any_ large impact) there would 
have been a period of rapid worldwide erosion. Midwesterners heard about 
things like "hundred year flood" in the last few years. This would be 
a once-in-50-million-year erosion event worldwide. Does Dr. Williams
factor that into his model?

The table of extinctions posted by Stan mixes relativley minor 
extinction events with a few of the truly big ones. Here's a table of the 
largest extinction events in the fossil record and their correlations
with large impacts(condensed from A Correlated History of Earth):

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         Manicougan, Quebec
Permain Triassic(250mya)      >300km*    250         S. Atlantic

Frasnian/Fammenian(367mya)     100km     365         Taihu Lake, China

Ordovician/Silurian(439mya)    No known crater 


* The P/Tr structure in the S. Atlantic is not a confirmed crater.

The pattern above indicates the larger the astrobleme, the greater the
magnitude of the faunal turnover/extinction event. The Ordovician/Silurian
event may have produced a crater as big or bigger than the K/T(Chicxulub)
structure. IMHO, finding the sites for the P/Tr and the Ordo/Silu craters
are two holy grails of geology.

Ir anomalies have been documented at *all* of the above horizons. The
further back you go in time, the more scant and confusing the data becomes 
as the ratios between siderophiles change in poorly understood ways.
Shocked quartz and/or microtektites have also been found at most, if not 
all of the above horizons.