Amino Acid Dating

The racemization of amino acids preserved in biominerals belongs to the chemical family of dating methods, with an age range that spans the past 10⁵ to ~10⁷ years. Chemical methods differ from radioactive dating techniques in that their reaction rate depends on one or more environmental parameters, whereas radioactive decay remains constant regardless of most environmental conditions. Amino acids, derived from indigenous protein residues protected by the skeletal hardparts of organisms, survive in most environments for thousands to millions of years. The extent of racemization of these amino acids is dependent primarily on the time elapsed since death of the organism and the integrated thermal history experienced by the biominerals since death, and to a lesser extent on vital effects unique to each taxon. Amino acid geochronology (often referred to as simply amino acid racemization (AAR)) relies on the chiral nature of most amino acids. Chiral molecules are not superimposable on their mirror image. All but the simplest protein amino acid can exist in either a 'left-' or 'right-' handed configuration. When an organism dies and its biomineral hardparts are archived, nearly all of the amino acids stored within the biomineral are of the l-configuration. Over time, the indigenous amino acids racemize to their d-configuration, providing a clock. At cold sites (-10°C), racemization requires ≫10⁶ years to reach equilibrium; at hot sites (~25°C), equilibrium is reached in just over 10⁵ years. Selecting appropriate samples and following strict preparation methods increases the temporal accuracy of AAR.