Stable isotope evaluation of the origins of amino acids in fossils

Abstract

The racemization reactions of amino acids in Quaternary fossil materials have been exploited for absolute dating^1,2, stratigraphic correlation^3,4 and palaeothermometry⁵. Recent work has involved the isolation of amino acid constituents of proteinaceous material from fossil bones for ¹⁴C dating⁶ and for establishing ancient dietary preferences by determining the stable nitrogen and carbon isotopic compositions of fossil collagens^7,8. Such investigations are often hindered by a lack of control over the indigeneity of these compounds, as the ubiquitous nature of amino acids generally precludes the determination of the absolute authenticity of amino acids in fossils by conventional means. However, the amino acids in organisms have been found to have distinct stable isotopic compositions^9,10. Here we report experimental results which indicate that after the death of an organism, low-temperature diagenetic reactions such as racemization should not cause a significant shift in the stable carbon or nitrogen isotopic compositions of the resultant D- and L-amino acid enantiomer products. We therefore propose that a comparison of these enantiomers from individual amino acids isolated from fossils may provide, for the first time, a method for establishing the absolute indigeneity of these compounds.