Re: extinction

["Dewey M. McLean" <dmclean@vt.edu>]

Response to Jaime Headden's post of Wed, 07 Jan 2004 18:15:28 (below 
the dashed line).

You raise splendid points, Jaime. For now, I'll provide more 
information on possible chemical diffusion of iridium from the 
Rocchia et al. paper titled "The Cretaceous-Tertiary boundary at 
Gubbio revisited: vertical extent of the Ir anomaly" (Earth and 
Planetary Science Letters, 1990, v. 99, pp. 206-219). Following are 
some quotations from the paper:

"Diffusion is an attractive explanation for the nearly symmetrical 
wings of the Ir distribution. We cannot at present handle this 
mechanism since we do not know to which mineral phases iridium and 
other platinum group elements (PGEs) are attached. Recently, Margolis 
and Doehne have discovered almost pure platinum nuggets in the basal 
KTB clay layer. If iridium is also locked in nuggets, as is platinum, 
chemical diffusion could not produce a sizeable spread of the iridium 
distribution."

"Using the sedimentation rate of Groot et al., we find that the Ir 
anomaly above background level extends over some 200,000 yr in the 
Maastrichtian, and possibly over as much as 400,000 yr in the 
Paleocene: the event which produced the iridium anomaly seems to have 
spanned a total duration of at least some 500,000 to 600,000 yr."

"The iridium distribution is therefore compatible with a long-lived 
event, with a signature extending well outside the cm-thick clay 
boundary layer. These observations may not be in contradiction with 
the volcanic scenario. If the Deccan traps volcanism occurred near 
the KTB, as has been recently confirmed and marked the onset of the 
Reunion hotspot, and hot-spot related volcanism is able to produce 
Ir, as indicated by the observations of Zoller et al. and Toutain and 
Meyer, the observed anomaly could be related to the volcanic phases 
within chron 29R, possibly extending into chron 29N."

"In any case, the duration of the iridium-producing K-T event, if 
real, implies that a single impact scenario cannot be retained 
anymore."

Cordially,
Dewey McLean

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Dewey McLean (dmclean@vt.edu) wrote:

<In fact, multiple iridium spikes do occur at localities other than
China.>

  So we are ignoring the evidence of a efflusive rather than explosive
event at the Deccan Traps, which should NOT be able to _pump_ Ir anywhere
in the atmosphere. The distinct separation of events, by apparently
several tens to hundreds of thousands of years or millions of years
between the Late Maastrichtian and Early Danian implies episodic events of
explosive atmospheric saturation of Ir, not generalistic downfall. The
statement, "Thus, it appears that iridium was not introduced into the
atmosphere during a unique event occurring at the K-T boundary, but was
present in the atmosphere for a much longer period of time[,]" would then
appear to have little reasoning unless the earth was enshrouded for
hundreds of thousands of years, in which case existing plant life should
have been wiped out; such is not the case. The idea that easily and
repeatedly recorded cataclysmic saturations of Ir, maybe from repeated
impacts, appears to account for the shear volume of Ir in and around the
boundary, but not limited to a single event. The Ir associated with
Chixulub may not have been significant enough to acount for its singular
explosive energy, but it may account for some of a global Ir trace when
linked to the same date.

  The idea of a single explanation for the K/T boundary dieoff appears to
be waning, as is the idea there was a singular episodic dieoff, which is
good, I guess.

  What is the response on the issue of Deccan low Ir quantity and absence
of ability to saturate the atmosphere? Perhaps rather than the Deccan
Traps, the "so-called" Shiva Crater may be more to one's liking.

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