Re: Gondwana Buster!

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Those interested in the record of imapcts on the Earth's surface might want
to take a look at

Grieve, R. A. F. 1991. Terrestrial impact: the record in the rocks.
Meteoritics 26:175-194.

Grieve has definitely done the most comprehensive work in this area and I
suspect the meteoritics paper contains more info. than the GSA Today
article Tom cited.

I'll gladly cede the P-Tr debate to Doug Erwin, whose book (Erwin, D. H.
1993. The Great Paleozoic Crisis: Life & Death in the Permian. Columbia
University Press. New York.) is, at least for my money, the most
authoritative current compilation, and simply point out that the data
reviewed by Doug are somewhat at odds with the points made Paul Janke and
reproduced below.

>
>To summarize some of the evidence for a major P/Tr impact:
>-a very severe global mass extinction event featuring abrupt,
> conspicuous faunal changes in the fossil record.(Worldwide)

The faunal changes were neither abrupt or synchronous. Aside from Doug's
review those interested in the paleontological evidence might want to take
a look at...

Stanley, S. M. and Yang, X. 1994. A double mass extinction at the end of
the Paleozoic era. Science 266:1340-1344.

>-siderophile/geochemical anomolies with probable astrogeological
> causes. (China, Pakistan, South. Alps)

I'm not sure what "probable" means here. There is an extensive literature
on Ir geochemistry now and my reading of the consensus is that such
anomalies are not universally accepted as evidence for impact. In fact, at
a recent meeting Glenn Izett seemed to downplaying the role of any iridium
data in the impact debates. The most recent review of the topic of which I
am aware is...

Sawlowicz, Z. 1993. Iridium and other platinum-group elements as
geochemical markers in sedimentary environments. Palaeogeography,
Palaeoclimatology, Palaeoecology 104:253-270.

>-shifts in stable carbon isotope perturbation. (China and South. Alps)

Shifts in d13C are rather common and can arise for many different reasons.
Erwin (1993) contains a good discussion of this.

>-a sharp peak in microspherules of probable impact origin(China)

Can we have a reference on this? Glass alters to clay over time and most of
the "spherules" that have been found at the K-T boundary are clay
sphereules. Clay sphereules can be formed by many different processes
(organic and inorganic) and it seems a bit of a stretch to simply assume
that all clay sphereules were originally glass of a highly particular
composition. Moreover, a recent paper by Robin et al. in Geochimica et
Cosmochimica Acta seems to reopen the entire question of geochenically
distinguishing between some tektite and volcanic glass compositions. The
situation must be even more ambiguous at the P-Tr boundary because of its
greater age.

>-a possible smoking cannon(crater) in the South Atlantic with unusual
> coeval metamorphism and folding in the surrounding area. This is
> *not* the Aranguainha Dome structure which at only 40km diameter
> was only a bump in the road in comparison, although the "Dome" might
> be a related impact site from the same parent body(S/L 9 style).

Possible, but as Janke states the evidence is not compelling.

>-possible induced volcanism. The time correlation and relationship
> between GB and the Siberian Traps might be analagous to Chicxulub
> and the Deccan Traps.

There has been a suggestion that the Deccan Traps might have been induced
by the Chixculub impact, but there is no hard evidence in favor of this
position. Indeed, for this model to have a chance of working there must be
tight synchrony between the onset of Deccan volcanism and the timing of the
impact. Given the errors on radiometric dating, as well as problems posed
by the presence of Maastrichtian microfossils in the marls above the
Chixculub breccia, it is doubtful that this issue can be pinned down with
the requisite certainty. Once again, the situation must be even more
ambiguous for the P-Tr case.

It seems that all this discussion about a "Gonwana buster" impact is more
wishful thinking than anything else. If the authors of the Pan Terra Wall
Chart are going to list a possible imapct as the cause of the P-Tr biotic
extinction and tectonic reorganisation I think it behooves them to present
a more credible scientific case than this.


Norm MacLeod



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Norman MacLeod
Micropalaeontological Research
N.MacLeod@nhm.ac.uk (Internet)
N.MacLeod@uk.ac.nhm (Janet)

Address: Dept. of Palaeontology, The Natural History Museum,
         Cromwell Road, London, SW7 5BD

Office Phone: 0171-938-9006
Dept. FAX:  0171-938-9277
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