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Re: astroblemes

 > Stan wrote:
 > >At present there is no known way an impact could generate an
 > >antipodal hot spot.   ...
 > The impact is not thought to be the cause of an antipodal "hot
 > Only the earth's interior can be the ultimate cause of one of
 > What the impact would do is cause a pattern of fractures in the
 > as the shock waves converge at the antipode. The shattered and thus
 > weakened crust region may facilitate massive volcanism, IF the
 > mantle is favorable towards it.

I have a problem with this as flood basalts seem to be associated
with the *initiation* of hot-spot volcanism.  (At least the
hot spot trace containing the Deccan Traps *starts* at the
Deccans, it cannot be traced further back).

Considering that there is reason to believe that hot spot
may actually originate at the *core*-mantle boundary, any
association of flood basalts with impacts seems unlikely.
[Yes, I know the core-origin is still controversial, but it does
emphasize the *depth* at which hot spots originate].

 > 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.

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].

 > >[I believe that the P-T iridium spike has been shown to have the
 > >signature expected of volcanogenic rather than extra-terrestrial
 > >origin].
 > 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.
I hesitate to ascribe to extra-terrestrial causes a siderophile
spike of this sort without strong confirming evidence.  The
limitation of the spike to China seems to be the *opposite*,
namely counter-evidence against an ET origin.

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.

 >  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
 > 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
 > subducted and erased, etc.

Yep, which is why I do not take the the lack of flood basalts
directly associated with the three earlier major extinctions
to be decisive.
 > Nevertheless, The evidence for an astrogeological origin of the P/Tr
 > anomalies is mounting. In addition to the Ir/siderophile anomaly,
 > are several other telltale clues which point to an impact:
 >   -Abundant glassy microspherules(microtektites).

This can also be volcanogenic - due to Mt St. Helen's
type exposive eruptions.
 >   -Negative stable carbon isotope perturbation,

The time span I saw for this on the chart in Gould's
new book is too long for impact originated changes.

On the scale implied there changes in geochemical and
biotic feedback processes is more likely.
 >   -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.

A *gravitic* anomoly?  *That* is the supposed P-Tr crater?

This sounds more like either a remnant microplate submerged
by the coalescence of Pangea, or the remains of a large,
perhaps defunct, hot spot.  (Perhaps a hot spot associated
with the initiation of the break-up of Pangea?)

I know of at least one submerged microplate in the mid-Pacific
(not far from the Solomons), so that is something that truly
needs to be considered.

I am going to need lots more than a gravitic anomoly
to recognise a crater there.
 > >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
 > isotope perturbation was reported between the Bellerophon and Werfen
 > formations in the South Alps. In 1987, an Ir anomaly was found at
 > P/Tr boundary type section in the Salt Range, Pakistan. In 1988, an
 > Ir/metal anomaly was described from extinction boundary sediments
 > a drill hole section from the Carnic Alps, Austria.

Hmm, that contradicts my references.  I will have to double check
the dates on my references (the finds you mention may be newer).
Unfortunately, I took them back home, so I cannot check now.
 > It seems that many are expecting to find the same worldwide massive
 > confirmation of an Ir spike that the K/T research has documented.
 > will never happen, for the reasons of intervening time stated

The problem is that even if it is worldwide, if it consistantly
has the Os-Ir ratio of volcanogenic siderophiles, it si more likely
to represent a siderophile burst out of the Siberian Traps than
a meteoric impact.

If Ir depletion is the explanation for the terrestrial ratio, the
Os-Ir ratio should vary greatly between samples in different
environments, with at least those from oxidizing environments
showing distinctly non-terrestrial signatures.
 > 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
 > is that living organisms were comparatively stable for tens of
 > of years before _and_ after the P/Tr or K/T boundaries. The faunal
 > that occurs at/around these boundaries *is* punctuation of

Not in the curves I have seen. The coral diversity actually
starts to drop at about the middle of the Permian.  (But I
attach little real significance to that, as decreased shelf
extent could easily explain that).

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).

 > >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
 > caused by a meteor impact, why doesn't he tell us? ;-)

He did.  Read the cited article.

Abrupt extinction (on the order of years to hundreds of years)
would show up as either a sharp cut-off at the boundary,
or even more likely a sharp *peak* in fossils abundance
right at the boundary (a mass death layer).  This latter
would be caused by the large number of individuals dying
in a very short time, thus increasing the number *available*
for fossilization.  Such a mass death would also most certainly
saturate the scavenger capacity, further increasing the availability
of fossils.  This might be largely offset by increased erosion,
which would show up as a non-conformity, not as a 3 m. barren
zone. Thus, a step-wise reduction in abundance is *not* to
be expected from an impact.  But this is, in fact, what is seen.

 > Seriously, even those that believe that the large impacts happened
 > 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,
 > 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.
 > Regarding the hypothetical thinning of dino fossils leading up to
 > K/T boundary: Even if this were true, it's not much to base any
 > on. NOTE: in the wake of the K/T impact(or _any_ large impact) there
 > have been a period of rapid worldwide erosion.

Yup. and this would leave *no* sediments, and *not* 3 meters
of sediments with very few bones.  (Three meters of sediments
is a *very* large amount in a riparian environment, and involves
numerous depositional events).

In fact, if the bones were more resistant to erosion than the
soil, this type of erosion might actually *enhance* the fossil
content per cubic meter of remaining sediment.

Finally, the fern spore spike following the Iridium layer
is only about a cm high, and give way to normal forest pollen.
Clearly the erosional period you mention would be restricted to
the time in which the forest cover was depleted - the fern spore
spike.  This is not at all associated with the barren interval.
 > The table of extinctions posted by Stan mixes relativley minor
 > extinction events with a few of the truly big ones.

Yep, because the timing and geological correlates of the
earlier ones is insufficiently established to add them to
the table.

 > Here's a table of the
 > largest extinction events in the fossil record and their
 > with large impacts(condensed from A Correlated History of Earth):

Of *suggested* large impacts.  Most are either poorly substantiated
or are imperfectly dated.

 > BOUNDARY                     crater      time of
 > (mya=million years ago)      diameter    impact       location
 > Carnian/Norian(223mya)         100km     220         Manicougan,

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].
 > 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?

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

The peace of God be with you.