Abstract
SINCE the discovery1 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 (>10 km in diameter) with the Earth1–11. Alternative explanations claim that extensive, violent volcanism12–14 can account for the Ir, and that other independent causes were responsible for the mass extinctions15,16. 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 Earth17. 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 α-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 chondrites17,18. An extraterrestrial source is the most reasonable explanation for the presence of these amino acids.
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Zhao, M., Bada, J. Extraterrestrial amino acids in Cretaceous/Tertiary boundary sediments at Stevns Klint, Denmark. Nature 339, 463–465 (1989). https://doi.org/10.1038/339463a0
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DOI: https://doi.org/10.1038/339463a0
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