Re: extinction

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

Response to David Marjanovic's post of 7 Jan 2004 (below the dashed line).

Hi David, thanks for your reply. To simplify things for the list, 
I'll address only your first point in this message, and will do the 
others later.

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

(1) Braggs, Alabama: Following are some quotations from the Donovan 
et al. paper "Sequence stratigraphy setting of the 
Cretaceous-Tertiary boundary in central Alabama" (SEPM Special 
Publication no. 42, 1988):

"Of the three iridium anomalies at Braggs, the lowest occurs in the 
late Maastrichtian, the middle near the K-T boundary, and the upper 
within faunal zone NP1 (Tertiary)....The presence of iridium at these 
flooding surfaces suggests that iridium was present in the open ocean 
from the latest Maastrichtian through earliest Danian....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."

(2) Lattengebirge, Bavarian Alps: The Graup and Spettel paper 
"Mineralogy and phase-chemistry of an Ir-enriched pre-K/T layer from 
the Lattengebirge, Bavarian Alps, and significance for the KTB 
problem" (Earth and Planetary Science Letters, 95, 1989) notes that 
an almost complete K-T section at Lattengebirge, Bavarian Alps, has 
three iridium-bearing events over an extended period from latest 
Maastrichtian into early Danian. The oldest spike predates the K-T 
boundary by 14,000-9,000 years. Geochemically those spikes display 
the same signature as the K-T boundary layer, and should have the 
common source.

(3) Brazos River, Texas: The Ganapathy et al. paper "Iridium anomaly 
at the Cretaceous-Tertiary boundary in Texas" (Earth and Planetary 
Science Letters, 54, 1981) indicates two iridium spikes, one at the 
K-T boundary, and one below.

Cheers,
Dewey McLean

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

>  New information on this topic can be found in a paper by Zhao et al.
>  titled "A possible causal relationship between extinction of
>  dinosaurs and K/T iridium enrichment in the Nanxiong Basin, South
>  China: evidence from dinosaur eggshells" (Palaeogeography,
>  Palaeoclimatology, Palaeoecology, 2002, v. 178, pp. 1-17).

Thank you very much, this journal happens to be accessible. :-)

>  The authors indicate [...] Because multiple iridium spikes were
>  distributed over a temporally long Cretaceous-Tertiary duration, they
>  indicate also that the source of the iridium was likely the Deccan
>  Traps volcanism, and not an asteroid impact.

There are two big problems with this, IMHO.
Firstly, those multiple iridium spikes aren't found elsewhere, so I
suspect... hm... no, repeated leaching and redeposition by water should be
quite difficult with Ir... hm. :-]
Secondly, the Ir cannot _possibly_ have come from the Deccan traps, for 3
reasons:
- The Deccan basalts contain 0.1 ppt (zero point one parts per trillion)
iridium -- this is one of the lowest values ever measured. Ref:
James Lawrence Powell: Night Comes to the Cretaceous. Dinosaur extinction
and the transformation of modern geology, W. H. Freeman 1998
- The main phase of Deccan volcanism ended some 100,000 years before the K-T
boundary. Ref:
G. Ravizza & B. Peucker-Ehrenbrink: Chemostratigraphic evidence of Deccan
Volcanism from the Marine Osmium Isotope Record, Science 302, 1392 -- 1395
(21 November 2003). (This paper happens to confirm your idea that the Deccan
volcanism produced a greenhouse.)
- The volcanism at Deccan was effusive -- flood basalts -- and not
explosive, and therefore cannot have blasted heavy stuff like Ir high up
into the atmosphere (so that it could have fallen down all over the globe).