Distribution, stereochemistry, and stable isotope composition of amino acids in K/T boundary sediments

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, alpha -aminoisobutyric acid and isovaline, has been reported from Stevns Klint, Denmark sediments. Two hypotheses have been proposed for the occurrence of alpha -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, alpha -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 C-13 relative to terrestrial materials. The delta C-13 values of individual amino acids in the Starkville South samples are, however, depleted in C-13 in all samples, with values ranging from - 13(0)/(00) for glycine to -31 (0)/(00) for L-isoleucine. In contrast, the delta C-13 value for glycine in Murchison is + 22(0)/(00). Also, bulk organic delta C-13 and delta N-15 values for the K/T boundary sediments are substantially depleted in C-13 and N-15 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. (C) 2001 Elsevier Science B.V. All rights reserved.