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The Cretaceous--Paleogene Boundary Clays At Højerup And Woodside Creek Show Anomalous Enrichments Of Iridium Compared With The Marine Sedimentary Rocks. For The Average Iridium Content Of 465 Ppb In Ci Chondrite The Estimate Of The Carbonaceous Chondritic Proportions In The Decarbonated Iridium-Rich Boundary Layers, Based On The Integrated Iridium Fluencies, Is About 26 % At Højerup And 65 % At Woodside Creek. These Proportions Are Most Likely Too High Due To A Significant Ir Influx From The Nearby Marine Or Continental Site To These Sections. [ABSTRACT FROM AUTHOR]

Since the discovery nearly a decade ago that Cretaceous/Tertiary (K/T) boundary layers are greatly enriched in iridium, a rare element in the Earth's crust, there has been intense controversy on the relationship between this Ir anomaly and the massive extinction of organisms ranging from dinosaurs to marine plankton that characterizes the K/T boundary. Convincing evidence suggests that both the Ir spike and the extinction event were caused by the collision of a large bolide (greater than 10 km in diameter) with the Earth. Alternative explanations claim that extensive, violent volcanism can account for the Ir, and that other independent causes were responsible for the mass extinctions. We surmise that the collision of a massive extraterrestrial object with the Earth may have produced a unique organic chemical signature because certain meteorites, and probably comets, contain organic compounds which are either rare or non-existent on the Earth. In contrast, no organic compounds would be expected to be associated with volcanic processes. Here we find that K/T boundary sediments at Stevns Klint, Denmark, contain both alpha-amino-isobutyric acid [AIB,(CH3)2CNH2COOH] and racemic isovaline [ISOVAL, CH3CH2(CH3)CNH2COOH], two amino acids that are exceedingly rare on the Earth but which are major amino acids in carbonaceous chondrites. An extraterrestrial source is the most reasonable explanation for the presence of these amino acids.