Protein and extraterrestrial amino acids in Cretaceous-Tertiary boundary sediments.

摘要

The report that the Cretaceous-Tertiary (KT) boundary sediments are greatly enriched in iridium (Ir) has generated an intense controversy concerning the origin of this signal. Although convincing evidence suggests that the Ir anomaly was generated by the collision of a large bolide with the Earth, other explanations claim that extensive, violent volcanism can account for the Ir spike. Amino acids in K-T boundary sediments are studied for the following reasons: (1) 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, for example, {dollar}alpha{dollar}-aminoisobutyric acid and racemic isovaline; (2) amino acid stereochemistry provides an excellent preservation index of organic matter in sediments.; A high-performance liquid chromatography (HPLC) method based on the reaction of O-phthaldialdehyde (OPA) and a chiral mercaptan, N-acetyl-L-cysteine (NAC), with amino acid has been developed to simultaneously determine the overall amino acid composition and enantiomeric ratios of most protein and non-protein amino acids. The OPA-NAC technique has been optimized to enhance the detection of {dollar}alpha{dollar}-alkyl amino acids, such as AIB and isovaline. HPLC detection of AIB and isovaline is further confirmed by combined gas chromatography/mass spectrometry.; The measured total amino acid concentration is 0.6-895 nmol/g in the Stevns Klint KT boundary sediments, 39-202 nmol/g in the DSDP site 577 KT boundary sediments. However, the D/L ratio of amino acids in the SK sediments is very low (for example, average D/L alanine is 0.16), while amino acids in DSDP 577 sediments are almost racemic (average D/L alanine in 577 is 0.79). Amino acid stereochemistry indicates SK sediments contain mostly recent amino acids, while DSDP 577 sediment amino acids are relatively well preserved.; Non-protein amino acids, AIB and isovaline, have been detected in carbonates immediately below and above the SK KT boundary clay, but they are not detected in the SK KT clay and DSDP site 577 KT boundary sediments. The origin of these amino acids in the KT boundary sediments is most likely to be extraterrestrial. Their high concentration in the SK sediments and their distribution on the Earth are not consistent with the single-impact hypothesis. A new impact model is adopted to explain the extraterrestrial amino acids results. It is suggested that amino acids are brought to the Earth by comet dust, and only in shallow marine or lacustrine environments can we detect extraterrestrial amino acids.