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Re: extinction



"David Marjanovic" <david.marjanovic@gmx.at> writes:

Dewey McClean wrote:
> > The alkali basalts of Anjar, in the western periphery of Deccan
> > province, have iridium concentration as high as 178pg/g


> Interesting. 178 ppt is still far below the Ir anomalies in K-T 
> sediments,
> but clearly different from 0.1 ppt! :-) Many thanks for this 
> information.
> 
> (Gubbio: some 9 ppb, a site in eastern Montana: some 2.6 ppb. 
> Source: a
> popular book by David M. Raup which reproduces fig. 4 and 10 of L. 
> W.
> Alvarez, PNAS 80, 627 -- 642 [1983].)



The Deccan alkali basalts' 178 parts per trillion iridium concentration
is close to *background concentration* for some K-T boundary clay sites
around the globe.

Here are some typical global Ir concentrations in the K-T boundary layer.
 Note that these concentrations are given in ppb and not in parts per
trillion:


1) Lance Creek, Wyoming (Lance Formation): Iridium anomaly is "21 parts
per billion".
Reference: Bohor, B.F., D.M. Triplehorn, D.J. Nichols, and H.T. Millard,
Jr. 1987.
Dinosaurs, spherules, and the "majic" layer: A new K-T boundary clay site
in Wyoming.
Geology 15: 896-899.


2) Hojerup Church site, Stevns Klint, Denmark (Fish Clay): "...the Ir in
the boundary layer
residue rises by about a factor of 160 over the background level (the
background level is
~0.26 ppb). A 1-cm thickness of this layer would have about 72 X 10-9
gram of Ir per
square centimeter." Reference: Alvarez et al. 1980. Extraterrestrial
cause for the
Cretaceous-Tertiary extinction. Science 208(4448):1095-1108.

3) Bottaccione Gorge, Acqualagna, Petriccio, and Contessa sites, Gubbio,
Italy (Scaglia
Rossa Formation): "...iridium increases by a factor of about 30 in
coincidence with the K-T
boundary....iridium abundance is 5.5 parts per billion (ppb) up to 9.1
ppb....with a steady
[iridium] background level of ~0.3 ppb...." The average iridium
concetration in all of the
Gubio boundary clay sites was slightly greater than 6 ppb. Reference:
Alvarez et al. 1980.
Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science
208(4448):1095-1108.


4) Gosau Basin, Austria (Upper Gosau Formation): "Iridium anomaly of 14.5
p.p.b. (parts per 1 X 109)...The marly limestone underlying the K/T
boundary layers....has an iridium
content of less than 0.5 p.p.b......A marked increase of carbon, as
determined by
microanalyses, is also apparent in the K/T boundary layers." Reference:
Preisinger et al.
1986. The Cretaceous/Tertiary bound in the Gosau Basin, Austria. Nature
322:794-798.

<pb>
--








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