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Re: extinction

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.

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).