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Re: P/Tr impact?



> Some thoughts on the new Becker et al. paper in Science:
>
> An extrasolar impactor? When I first got word about the extrasolar (in
fact,
> carbon star-like) istopic ratios of the noble gasses found in the
> fullerenes, I thought: "This isn't evidence for an impact; this is
evidence
> for a supernova!"

Good idea... testable: Plutonium-244 or its decay products should be present
in the boundary layer. (The hypothesis that a supernova was responsible for
K-T was falsified this way.)

> However, the values the Becker team found are consistent with those they
> find in carbonaceous condrites (which are intepreted as primeval chunks of
> the early solar system, unlike many other asteroidal bits which have gone
> through some subsequent differentiation).  Thus, carbonaceous condrites
> might concievably have retained some ancient (>4.56 Ga) material  that all
> solar system matter passed through, while active regions like the Earth
> might have long since buried or altered them.
> (Incidentally, if it was a carbonaceous condrite impact, there would not
be
> the strong iridium- or other metallic signature left from a more metallic
> body).
> In any case: no reason to suspect that the body that brought these
> fullerenes to Earth was anything other than an ordinary chunk of our own
> system.

A chondrite sounds very plausible to me... just are there [still?] such big
ones?

> But are these fullerenes really there, or really ancient?  Richard Kerr's
> accompanying news article
> (http://www.sciencemag.org/cgi/content/full/291/5508/1469 for those who
have
> access to Science online) indicates that some fullerene workers doubt
these
> structures are as preservable or durable as needed for this to be a record
> of either a P/Tr impact or (even more) material from the early solar
system.
> Furthermore, apparently few other workers have been able to find
fullerenes
> in other material (various other impacts and meteorites) previously
reported
> by Becker or her co-workers as having fullerenes.
> Furthermore, unlike what was previously reported on this list, other more
> mundane sources can produce fullerenes: forest fires, for example, or mass
> spectrometers (i.e., the tool used to find and separate them can
apparently
> generate them as well... URK!).

The unusual thing here is not the presence of fullerenes, but the presence
of ones _with extraterrestrial ratios of helium and argon isotopes in them._
This means that the argon at least was present at the time of formation of
the fullerenes (it can't get in or out by itself), and the helium was either
present, too, or something really violent happened to the fullerenes later
(it is possible to _shoot_ helium into fullerenes). Obviously, a
spectrometer on Earth can hardly generate fullerenes with extraterrestrial
noble gas atoms enclosed in them!

> > > >Becker believes that difference is
> > > >because the two space bodies that slammed into Earth had
> > > >different compositions.
> >
> > Or, maybe more probable, the impactor was so big that the explosion was
so
> > enormous that most of the material was blasted back into space ("the
> > self-cleaning impact").
> >
> This sounds very much like special pleading!!  Given that we know (from
> recovered samples) that the solar system contains all sorts of debris of
> radically different compositions, this (rather than the
> rather-oh-so-convieniently-eliminator-of-its-own-evidence self cleaning
> impact) is the more likely scenario, assuming (of course) that there
really
> was a P/Tr impactor.

Just to make sure, the self-cleaning impact is not my idea...

> Incidentally, some other thoughts on the matter:
> As with the K/T crater search, there is the very real possibility that a
> P/Tr impactor would have struck oceanic crust (given that more of the
> Earth's is floored by thin oceanic crust rather than thick continental
> crust).  This would suck mightily, as there is NO remaining ocean basin
from
> this age, due to subduction.
> However, an oceanic impact might be less likely to produce shocked quartz
> (also not yet recovered from the P/Tr), given the mafic composition of
> oceans (as opposed to granitic continental material).  Okay, I admit, this
> is also special pleading of a sort...

B-)

> Finally, some thoughts on regressions:
> There seem to be some misconceptions on this list about the effects of
> regressions.  It is true that they would cause radical habitat shifts in
any
> particular region (as the sea moves away, streams have to lengthen to
reach
> the sea, etc.)

BTW, freshwater sharks disappeared in North America sometime around the K-T,
probably because of the regression, and came back later.

> but they would also have much more global effects;
> * Change in global albedo.  As those who have worked or studied climate
> modelling know, changes in albedo (reflectivity) is a MAJOR factor in the
> climate system.  Removing lots of nice reflective surfaces and replacing
> them with more absorbant ground/trees/etc. will change  the energy budget
of
> the lower atmosphere and ground surface, which changes the amount of and
> distribution of the energy that drives weather.
> * Increased continentality of climate: Bodies of water have an
ameliorating
> effect on climate.  Remove lots of bodies of water (even shallow water),
and
> the summers get hotter and the winters colder.
> * Change in oceanic productivity: removal of a huge surface area in which
> phytoplankton  were once growing would reduce the total amount of
> productivity of the seas.
> * Change in oceanic shallow circulation

Yes. But most of this would come quite slowly, too slow to produce a mass
extinction of that size, I'd say.

> Finally, while it is true that the net change in sea level in the
Oligocene
> is comparable to the Maastrichtian Regression, the latter had the
> potentially for much greater effect.  The reason for this stems from the
> fact that the distribution of height above (or depth below) sea level is
not
> a straight line on the Earth.  The drop during the Maastrichtian managed
to
> result in a greater change in surface area covered in sea water vs. land
> (because it was starting from a point in which much of the contiental
masses
> of the Earth had epeiric seas), while the change in the Oligocene mostly
> effected steeper margins (and thus much less change in surface area).

True.