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Irridium spikes (was Re: Astro turf)



LN Jeff (martz@holly.ColoState.EDU) wrote:     

> As I understand it, the clay layer at the K-T boundary only caught
> Walter Alvarez's eye because of its placement.  I am forced to
> wonder, if a impossibly huge analysis of sedimentary rocks across
> geologicic history was conducted, exactly how many anomalous
> irridium spikes would turn up.

You're not the first person to wonder.  Many moons ago (April 26,
1995) Phil Bigelow mused on the same thing here on this list.  I'm not
sure how much of what he wrote can now be considered out of date, but
if you want to look, point your web browser at:

http://www.cmnh.org/fun/dinosaur-archive/1995Apr/0283.html

An excerpt:

] In all three of these sites, a clay layer was either containing the
] spike, OR, the clay layer was adjacent to the spike. This implies
] (at least to me) a typical sampling bias that unfortunately is
] unavoidable in such "in search of" expeditions. The clay looked
] unusual, so the researchers looked at it more closely. The only
] other alternative would be to do what foram and nannofossil experts
] call "continuous sampling"...and these guys will probably tell you
] that you don't want to do that. It is hard, slow, and expensive,
] because you are looking at an entire core of rock, rather than a
] unusual clay layer. As far as I know, no one has attempted
] continuous sampling for iridium in these continental sediments.

--
Mickey Rowe     (mrowe@indiana.edu)