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Re: Re: "No Bolides!"

>There is virtually no more doubt in the geological community of those who 
>have examined the data that there was a huge bolide impact so close to 
>the stratigraphically-determined end of the Cretaceous that its timing 
>cannot be distinguished from that end point.  I believe that the claim 
>that core data from Chicxulub simply shows volcanism is no longer being 
>promoted by those (e.g. Charles Officer) who once did so.  The claim by 
>marine micropaleontologists (e.g. Gerta Keller) that the presumed impact 
>did not quite correspond to plankton extinctions also seems to have been 

Not to be nitpicky, but I have not seen retractions by either of these
workers, nor have I seen any published or presentation refutations of
Officer's data (at least).

Indeed, there are two things to consider with the Officer's interpretation
of the structure.  On the one hand there is the chemical composition of the
melt layers themselves, which Officer considers to be incompatible with a
bulk melting of the bedrock.  (This can be confirmed or rejected by
additional mineralogical tests).

Secondly, and much more intriguing to this soft rock worker, is the claim
that the disturbed layers are *overlain* by carbonates containing late
Campanian and Maastrichtian microfossils.  If true, this would require the
impact/volcano/whatever to have occurred 12 million years or more before the
K/T boundary.

It is the latter comment that got me thinking: neither side seems to have
published their microfossil data, as far as I have seen.  So, here is a
professional challenge to those who have Chixculub core samples:

PUBLISH YOUR DAMN MICROFOSSILS!!  This would be a relatively simple matter:
a few plates from each horizon immediately above the disturbed layer, and
census data table of the same would suffice.  This would certainly help
settle the matter.

[Please note: even if Chixculub itself turns out to have occurred in the
mid-Campanian, there is still abundant independant evidence demonstrating an
impact took place at the K/T boundary.  On the other hand, if Chixculub was
a mid-Campanian impact, it would suggest a) giant bolide collisions are more
common than previously supposed and b) giant bolide collisions are
insufficient to cause mass extinctions, since the late Campanian
(immediately following Chixulub, if Officer's dates are correct) is the peak
of known dinosaurian diversity in at least North America and Asia).

>take a back seat to what the fossil record actually shows.  If you don't
>like the data from the fossil record, then you should have a simple 
>explanation for why the data are so misleading.  The data indeed SUGGEST 
>that extinctions occurred at essentially the same time as the impact.  In 
>my reading, I have found that proponents of the 
>"bolide-impact-causing-extinction" hypothesis have only a little 
>difficulty in explaining what became extinct and what survived.  The 
>hypothesis is still being refined in the classic iterative process that 
>is science.

Just to put this in: I do think the K/T impact (which was probably
Chixculub, if the age data confirm such a date) was resonsible for the
terminal Cretaceous extinctions.  However, the "rock dropping establishment"
(rde) has refused to acknowledge the paleontological data of other
extinction events prior to the K/T boundary.

If these other extinctions (including taxa as diverse as helmet crested
lambeosaurines and centrosaurine dinosaurs, rudist clams, and a few
zooplankton lineages) had occurred much earlier in the Late Cretaceous, I
don't think anyone would argue that the timing of that date was a
preservational artifact. However, since these mid-Maastrichtian extinction
events are (relatively speaking) close to the K/T boundary, there are some
who refuse to acknowledge the data that support this.

The Campanian and Maastricthian were a time of great change, including the
draining of the epicontinetnal seaways, transition from gymnosperm to
angiosperm dominance among the tree habit, and the beginnings of the Deccan
Traps flood basalts.  Some or all of these might have been related to the
pre-boundary extinctions.

This does not detract from the importance of the K/T boundary and the work of
the many people on those centimeters surronding that event.

Just my $0.02

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661