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Re: more astroblemes

>Well, there are flood basalts associated with the last *two*
>of the five truly major extinction events.  And the previous
>three are so old that the evidence for flood basalts at those
>times may be hard to find.  [Dating is still quite uncertain
>for most Paleozoic sequences - especially when dealing with
>time spans on the order of 2-5 million years, that is the
>error bars are larger than the relevent time spans].

Granted, the Deccan and Siberian traps must be considered in any serious
attempt to understand the faunal turnovers at those times.

Two unrelated comments on the above:
1) What you say about the difficulty finding evidence for very
   old flood basalts holds doubly true for impact structures.

2) The errors on dating Paleozoic sequences can be used to argue
   against such statements as "many/most life forms were already on 
   their way to extinction by the time of an alledged comet impact".
   There may be something to diversity trends over many millions of
   years, but it seems like alot of anti-asteroid theorists have
   rapidly developed an extraordinary knowledge of the 2-3 million 
   years prior to either the K/T of P/Tr boundaries. 

>>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.
>This is the result I meant.
>Is this few centimeters in marine or terrigenous sediment?
>In terrestrial environments sedimentation is often strongly
>pulsed, especially in riparian beds.  Under these conditions
>a few centimeters might well represent in excess of a thousand
>years.  That seem adequate time to me for volcanogenic siderophile
>enrichment - especially if the eruptions were frequent and high
>in siderophiles.

Sorry, its marine.

>...which is why I do not take the the lack of flood basalts
>directly associated with the three earlier major extinctions
>to be decisive.

For the same reason, I do not take the lack of known impact structures
to be decisive.

>But even the other diversity curves start dropping about
>10-15 MY befor the P-Tr boundary.  And some groups did not
>go extinct until some 4-5 MY *after* the boundary.  (The
>Early Triassic faunas are generally transitional in nature
>between Permian and typical Mesozoic faunas).

This is all consistent with a large asteroid/comet impact. 

>> 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.
>To some degree yes, but the duration of the effects could scarcely
>exceed a few thousand years.  This is *far* too short a time to
>show up as a 3 m. barren interval.  I mean, after all, the whole
>point is the *abruptness* of the effects.

This is where we disagree on the effects of a massive impact. There are
immediate, direct effects which will be over in less than a year. There
are other, more interdependant effects that are not merely the result of
the asteroid hitting, but also involve the earth's response to them.
Examples of these lingering effects would include climate shifts, new
faunas changing the balance, possible associated volcanism. The world
would take perhaps a few million years to return to equilibrium.

Many anti-asteroiders create a paradigm that says that if a big comet hits, 
the effects must terminate within a few thousand years. I cite Bell's 
theorem as a scientific basis as to why this is not necessarily true. 
Massive impacts, in the long run, may be viewed as catalysts of evolutionary
change. They have both an initial effect that is immediate and severe, and 
leave a legacy of Darwinian struggle for perhaps millions of years after.
To quote Douglas Erwin: "The organisms you see in a tidal pool today are
a consequence of the Permo-Triassic extinction".

>> Carnian/Norian(223mya)      100km     220         Manicougan, Quebec
>My sources cite no corroborating evidence for this one.
>[The table I modified does not even *mention* it, or any
>increase in siderophiles or tektites at this level].

You might want to try "Late Triassic extinctions and the Origin of
Dinosaurs", by Michael J. Benton, Science, v260, 7 May 1993.
BTW, this structure is still somewhat visible as a circular lake
with a central peak.

>> Ir anomalies have been documented at *all* of the above horizons.
>Not according to my references.  Have these discoveries occured
>in the last 3-4 years?

A quick pass shows all my sources in the 1990-1994 interval, with
exceptions being the early work by Alvarez, Raup and Sepkoski, etc.