Most living organisms use amino acids exclusively of the L-configuration.
Upon death and removal of biologic constraints, the L-amino acids begin to racemize to their D-configuration.
If the reaction rate can be calibrated using an independently dated nearby site with similar thermal history, absolute dates can be inferred through calibration.
Because the rate of amino acid racemization is controlled by the temperature experienced by the fossil since burial (rather than when it was living), an evaluation of site temperature is required for geochronological interpretation.
To minimize the effect of seasonal and diurnal temperature fluctuations, samples should remain buried at least 1 m and preferably 2 to 3 m throughout their post-depositional history.
Two general approaches are used to convert D/L ratios to an absolute time scale: The first is a calibrated approach in which the D/L ratios are used to interpolate between, or extrapolate beyond, the known ages of independently dated samples within a restricted geographic/ oceanographic area, where temperature histories are similar.
In the second approach, the effects of time and temperature on the extent of racemization are determined in modern shells subjected to high-temperature laboratory experiments, and in Holocene samples whose ages are known from 14C analysis and whose temperature history can be inferred based on instrumental data. R., eds., Quaternary geochronology: methods and applications, American Geophysical Union, Washington D.