DISTRIBUTION, STEREOCHEMISTRY, AND STABLE ISOTOPE COMPOSITION OF AMINO ACIDS IN K/T BOUNDARY SEDIMENTS

Abstract

The Cretaceous/Tertiary (K/T) boundary, with an age of 65 million years before present, is associated with the mass extinctions of dinosaurs and other terrestrial and marine organisms. Evidence for the K/T boundary is reported in both marine and non-marine stratigraphic units. The presence of two exotic amino acids, α-aminoisobutyric acid and isovaline, has been reported from Stevns Klint, Denmark sediments. Two hypotheses have been proposed for the occurrence of α-Aiba and I-val in the Stevns Klint sediments: (1) they are of extraterrestrial origin; and (2) they are the by-products of the thermal alteration of coal. To test these hypotheses, samples were obtained from three K/T boundary sites, two (Starkville South and Raton Pass) located in the non-marine Raton Basin of Colorado and New Mexico and one from the marine sequence at Stevns Klint, Denmark. The samples were analyzed for amino acid distributions and stereochemistry, bulk stable C and N isotope compositions and stable C isotope compositions of individual amino acids at the Starkville South site. Protein and non-protein amino acid components are present in the K/T boundary sediments at nmol/g levels and are primarily of the l-configuration. The non-protein amino acids, α-Aiba and I-val, were detected in several of the sediment samples based on retention times and verified through coinjections of authentic standards. Protein and non-protein amino acid components of the Murchison meteorite are enriched in 13C relative to terrestrial materials. The δ13C values of individual amino acids in the Starkville South samples are, however, depleted in 13C in all samples, with values ranging from −13‰ for glycine to −31 ‰ for l-isoleucine. In contrast, the δ13C value for glycine in Murchison is +22‰. Also, bulk organic δ13C and δ15N values for the K/T boundary sediments are substantially depleted in 13C and 15N relative to bulk values for the Murchison meteorite. It is therefore suggested that the amino acids in these K/T boundary sediments are primarily of terrestrial, biogenic origin.