K-T Ir

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Date
Mickey Rowe gave the wrong reference on that iridium paper. I tracked it
down. The correct reference is:

Anbar, A. D., Wasserburg, G. J., Papanastassiou, D. A. & Anderson, P. S.,
1996. Iridium in natural waters. Science, 273, 1524-1528. (The 13 Sept 1996
issue).

I'd missed this study which is quite interesting in terms of providing
background for the interpretation of Ir anomalies. Ir anomalies are not
currently thought to be incontrovertable evidence for or against an impact.
The big news in this paper is the estimation of Ir residence times in sea
water, which turn out to be between 2,000 and 20,000 years. The total span
of time represented by the Gubbio Ir anomaly (and apparently accepted by
these authors) is 10,000 to 100,000 years. Please note than in many
previous discussions K-T Ir anomalies have been portrayed has representing
hours to days to (at most) months of depositional time. The Anbar et al.
(1996) paper concludes that given the long residence time of Ir in
seawater, a massive injection of Ir from a terrestrial (volcanic) or
extraterrestrial source could account for the Ir anomaly thicknesses
observed at some K-T sections.

I wouldn't say that the thickness of the Ir anomaly was a key criticism of
the "one-shot asteroid" hypothesis. It played a role in the arguments
advanced by...

Rocchia, R., Boclet, D., Bronté, P., Jéhanno, C., Chen, Y., Courtillot, V.,
Mary, C. & Wezel, F., 1990. The Cretaceous-Tertiary boundary at Gubbio
revisited: vertical extent of the Ir anomaly. Earth and Planetary Science
Letters, 99, 206-219.

..., but it was not the only argument contained in that paper and (as far
as I can see) the Anbar et al. (1996) study does not rule out the Robin et
al. (1990) interpretation.

As you might expect there has been much published on the stratigraphy and
geochemistry of Ir in the last dozen years or so. If you are interested in
the topic you might want to also check out

Hansen, H. J., Rasmussen, K. L., Gwozdz, R. & Kunzendorf, H., 1987.
Iridium-bearing carbon black at the Cretaceous-Tertiary boundary. Bulletin
of the Geological Society of Denmark, 36, 305-314.

Lerbekmo, J. F., Sweet, A. R. & St. Louis, R. M., 1987. The relationship
between the iridium anomaly and palynological floral events at three
Cretaceous-Tertiary boundary localities in western Canada. Geological
Society of America Bulletin, 99, 325-330.

Tredoux, M., De Wit, M. J., Hart, R. J., Linsay, N. M., Verhagen, B. &
Sellschop, J. P. F., 1988. Chemostratigraphy across the Cretaceous-Tertiary
boundary and a critical assessment of the iridium anomaly. Journal of
Geology, 97, 585-605.

Crockett, J. H., Officer, C. B., C., W. F. & Johnson, G. D., 1988.
Distribution of noble metals across the Cretaceous/Tertiary boundary at
Gubbio, Italy: Iridium variation as a constraint on the duration and nature
of Cretaceous/Tertiary boundary events. Geology, 16, 77-80.

Robin, E., Boclet, D., Bonté, P., Froget, L., Jéhanno, C. & Rocchia, R.,
1991. The stratigraphic distribution of Ni-rich spinels in
Cretaceous-Tertiary boundary rocks at El Kef (Tunisia), Caravaca (Spain)
and Hole 761C (Leg 122). Earth and Planetary Science Letters, 107, 715-721.
(this reference also contains a discussion of the Ir anomalies in these
sections).

Colodner, D. C., Boyle, E. A., Edmond, J. M. & Thomson, J., 1992.
Post-depositional mobility of platinum, iridium and rhenium in marine
sediments. Nature, 358, 402-404.

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

Wang, K., Attrep, M., Jr. & Orth, C. J., 1993. Global iridium anomaly, mass
extinction, and redox change at the Devonian-Carboniferous boundary.
Geology, 21, 1071-1074.

Elliot, D. H., Askin, R. A., Kyte, F. T. & Zinsmeister, W. J., 1994.
Iridium and dinocysts at the Cretaceous-Tertiary boundary on Seymour
Island, Antarctica: implications for the K-T event. Geology, 22, 347-355.


Norm MacLeod



>Date: Fri, 25 Oct 1996 17:10:50 -0700 (PDT)
>Reply-To: mrowe@indiana.edu
>Originator: paleonet@ucmp1.berkeley.edu
>Sender: paleonet@ucmp1.berkeley.edu
>Precedence: bulk
>From: "Mickey P. Rowe" <mrowe@indiana.edu>
>To: Multiple recipients of list <paleonet@ucmp1.berkeley.edu>
>Subject: Re: This McLean Stuff
>X-Comment: PaleoNet Mailing List
>
>
>Tom DeVries is not alone; I too have wondered about the deafening
>silence that Dewey's messages have received here.  I know that someone
>at _Science_ pays attention to this forum because it's been mentioned
>there before.  I wonder if anybody there would like to provide an
>editorial response here.  I also note that in the September 27th
>issue, _Science_ didn't fully review Officer and Page's book, but at
>least showed the moral courage to describe it and mention that they
>are portrayed badly by the non-asteroid sides of the debate.  Doesn't
>anybody have anything else to say on the subject?
>
>Pathological science aside, I'm sure that those of us who still aren't
>convinced one way or another would appreciate more discussion of the
>data.  The September 20th issue of _Science_ has an article about the
>retention time of Ir in rivers and oceans (I think it's entitled
>"Iridium in Natural Waters").  The article doesn't purport to show
>that the K-T Ir spike is extra-terrestrial in origin, but it does
>infer that the boundary layer's thickness does not require a long term
>terrestrial source.  The layer's thickness is one of the key arguments
>that the Ir came from volcanos rather than a one-shot asteroid, right?
>Is there any possibility that we can get going a discussion of the
>science underlying the debate?
>
>--
>Mickey Rowe     (mrowe@indiana.edu)
>



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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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