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 δ 13C 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 δ 13C value for glycine in Murchison is +22‰. Also, bulk organic δ 13C and δ 15N 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.