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Re: astroblemes

>1. Why is it an error to assume that astrobleme extinctions require
>quick die-offs? The environmental effects of astroblemes would last a
>few years to perhaps a few tens of years at most. Critters that
>survived the environmental effects would seem to be out of danger

It's not an error. There will be *some* and perhaps many quick die-offs, 
while others might linger until new equilibriums are established before
going extinct. There will be brand new ecosystems with new food chains 
and new rules. Some organisms will sail right through with little or no 
disruption(I'm not sure there's any catastrophy that will kill of the 
tartigrades). It looks like <10% made it through the P/Tr and perhaps
50% made it through the K/T. The point is that there will be some
immediate extinctions, and some happening as much as a few million
years later as equilibrium is re-established.

>2. I thought the point was made that living organisms were NOT stable
>for the few millions of years before the proposed astroblemes, but were
>already dying-off when the astrobleme struck.

Do you believe we have adequate knowledge of the fossil record to 
make assertions like "most organisms were already in decline in the few 
million years prior to the P/Tr boundary" when we are talking about 250 
million years ago? The preserved record is a series of micro-snapshots 
that, IMHO don't lend themselves to this detailed a conclusion. What I 
would conclude about the P/Tr boundary is that there was a worldwide faunal 
turnover, whereby over 90% of animals above the species level fail to cross 
the boundary and proliferate into the Triassic. Something very catastrophic
happened, as this is virtually a one-time event in Earth's History. There
is mounting evidence for a very large astrogeological component in this
story, in spite of many who would rather write them out of the script.

>Don't know about Dr. Williams, but geologists in general do. The
>thinning of dinosaur bones WOULD be conclusive. Rapid erosion (as would
>be expected from an oceanic astrobleme impact) leaves its own kind of
>signature known as an unconformity. The entire stratagraphic sequence
>would be missing, not just the dinosaur bones. What the anti-astobleme
>folks argue is that the rocks are there, but the bones are not.

The dinos may have been in decline before the K/T, or they may have been
in a heyday. You will not find a consensus on this, because the amount of
analysis required has not been done and may be impossible. There are many
factors which govern why fossils form, or don't form. Any arguements that
begin with "because I can't find Y fossils, therefore X" should be 
viewed with a skeptical eye. Most of the dinos that ever lived did not

>> 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.
>This seems like a contradiction. If the data are scant, confusing, and
>poorly understood, how can *anomalies* be documented?
>Care to clarify these arguments?

Okay, lets begin with the K/T: Ir anomalies have been discovered at dozens
of sites around the world. The Os/Ir ratios are about what you would expect
for an astrogeological origin.

The P/Tr: Ir anomalies have only been documented at 3 localities that I'm
aware of. The Os/Ir ratio is not what would be expected for an astro-origin.
This trend seems to hold(i.e. ratios out of balance) as you go back further
in time and analyze the Os/Ir ratios. The simplest explanation for this
would be that the Ir is NOT extraterrestrial. I don't believe this is the
case, because there are worldwide extinction events associated with 
these boundaries, as well as physical evidence such as microtektites which
point to astrogeological events. At the very least, mechanisms such as
Ir depletion in a reduction environment should be studied to see if these
ratios might be reconciled with astrogeological hypotheses.

In summary: 
The data becomes scant(dozens of K/T Ir anomaly sites compared to 3 P/Tr). 
The ratios are confusing because we don't yet understand what sort of 
reduction, leaching, etc. might skew Os/Ir ratios over intervals of hundreds
of millions of years. 

I will concede the possibility that there is nothing here to understand, 
i.e the ratios are what they are because they were derived from terrestrial 
sources, I just don't believe that's the case.