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P/Tr impact?



Greetings,

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!"  After all, I figured, there is no more likely reason to
suspect a little chunk of asteroidal debris would ever scoop up more
extrasolar junk than a massive body like the Earth. (And PLEASE don't say
"But the asteroid is in space..." or you will be thumped! :-).
        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.

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!).

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> David Marjanovic
> > >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.

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...

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.) 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

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).

Whew!!  Must go now.

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796