Amino Acid Racemization

Amino acids exist in two mirror-image forms, left-handed (L) and right-handed (D). Living organisms build proteins almost entirely from L forms, so a fresh sample has a D/L ratio near zero. After death, the molecules slowly interconvert, raising the D/L ratio toward an equilibrium value over time. Because this racemization proceeds at a measurable chemical rate, the measured D/L ratio in a fossil shell or bone records how long ago the organism died. The clock is reset by the organism's own biochemistry at death, and it 'ticks' chemically thereafter, which is why it is a kinetic rather than a radioactive method.

Racemization is a chemical reaction, and like most chemical reactions it speeds up with temperature, following the Arrhenius law — its rate can more than double for a ten-degree rise. This means a sample buried in a warm climate reaches a given D/L ratio much faster than an identical sample in a cold one. Because the relevant quantity is the effective temperature integrated over the whole burial history, AAR ages are only as good as the thermal history that can be reconstructed. Johnson and Miller emphasize that constraining temperature, ideally by calibrating against an independently dated sample from the same setting, is essential for quantitative ages.

It can do both, but its strength differs by use. With an independently dated calibration sample of the same taxon, skeletal material, and thermal environment, the effective rate constant can be fixed and AAR yields quantitative absolute ages. Without such calibration, AAR is most reliable for relative dating and correlation: consistent D/L ratios order deposits and link separated outcrops in an aminostratigraphy, and outliers flag reworked material. Many archaeological applications use AAR precisely in this relative, screening, and correlation role, reserving costlier methods like radiocarbon for samples the racemization data identify as worth dating absolutely.