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Re: extinction (reposted)



Response to Phil Bigelow's post of Sat, 17 Jan 2004 13:44:19.

Dear Phil,

I apologize once again for being so busy last week that I had little time for our listserv chats. I'm trying to meet an important deadline. This weekend, things have eased up somewhat, and I'm delighted to respond to your "challenge."

First, please allow me to correct your: "since you seem to believe that multiple iridium layers *always* means multiple iridium events." Here is my response to David Marjanovic on Thu, 08 Jan 2004 11:09:01:

"Your point on transport of iridium is one such controversial topic. I recall how at the first Snowbird I extinctions conference in 1981, the impactors claimed that iridium was refractory. Later, they had iridium migrating to where it best served their purposes. So, individual multiple "spikes" in place, or enrichments via migration of the iridium? Based on my communications with other scientists down through the years, I prefer to think of individual spikes in place. But, others feel differently."

The record demonstrates that I stated my opinion 10 days ago.

You stated: "it makes no difference to me what the opinions of the various authors were." In fact, one of the authors of the "Sequence stratigraphic setting of the Cretaceous-Tertiary boundary in central Alabama Bragg, Alabama" paper--that I have referenced several times as having three iridium spikes--is the world-renowned Peter Vail, the father of sequence stratigraphy.

Incidentally, two of my former graduate students worked with members of the Vail team while they were at Exxon.

Cordially,
Dewey McLean

----------------------------------------------

"Dewey M. McLean" <dmclean@vt.edu> writes:

Phil, for your:

> "Your list [of sites with multiple Ir layers] is not sufficiently
informative. What
 > are
 > the depositional environments at each of these sites? More
 > importantly, what is the
 > > *micro*-stratigraphy of these zones?,"

I simply do not have the time
 > to abstract this information for you.

No need to.  I have read these papers.  My question to you was
rhetorical, and it was meant as a specific challenge to you.  So far in
our dialogs, you have avoided discussing the possibility that the
stratigraphy at these multiple layer sites could have been altered by
slumping, overturning, or by other forms of redeposition (thereby
creating multiple Ir layers out of one event).  Since you say you have no
time to answer my question, then we'll just let the issue die.
As you know, I have my own informed opinion about these specific sites.
I believe that a single iridium "event" was probably redeposited into
multiple iridium layers.

 > Certainly, you will want to study the data base, methodology, and
 > conclusions drawn from each study. You can do that only by actually
 > reading each individual paper.

As I noted, I have read these papers.  For the sake of our present
discussion, it makes no difference to me what the opinions of the various
authors were.   They make their respective cases clear.  I was asking for
your opinion, since you seem to believe that multiple iridium layers
*always* means multiple iridium events.  If I have mischaracterized your
views, then it would be helpful for you to tell us which of the K-T sites
that you listed may contain redeposited iridium.