[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: P/Tr impact?



At 10:49 AM -0500 2/23/01, Thomas R. Holtz, Jr. wrote:
Some thoughts on the new Becker et al. paper in Science:

        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.

Both comets and asteroids are possible; we know very little about asteroid composition. Another possibility raised earlier is a nearby supernova. What the isotopic ratios seem to indicate is that the buckyballs formed near a star in a region of high pressure. That material could have been processed through the solar system or arrived directly. The only thing known about their age is that they have been on Earth since the Permo-Triassic boundary.



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.

These are interesting questions.

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!).

These sources would have had terrestrial isotopic ratios for helium and argon. If Becker's results are correct, the buckyballs from the P-Tr do not.



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...

Iridium, shocked quartz, and spherules -- all found at the KT -- are all missing or extremely rare at the Permo-Triassic boundary. (There are a very few possible shocked quartz grains.) Peter Ward says there is no distinctive formation such as the K-T 'boundary clay' at the Permo-Triassic; the rocks don't look similar. These are all qualitative findings, but they all seem to indicate that whatever happened at the Permo-Triassic boundary was both qualitatively and quantitatively different than at the KT. A comet impact in the deep sea might meet those criteria, but so could other events.


One thing I have not seen addressed is the condition of the Permian fauna at the boundary. We have had long debates over whether or not the dinosaurs were in decline at the KT. What was going on with the Permian fauna? Why were extinction rates higher for marine fauna than terrestrial vertebrates?

This is an interesting paper, but I don't think it has "the" answer.

-- Jeff Hecht