A biological dating method used on human and animal bone and carbonate shells. Within living organisms amino acids are molecularly structured in what is known as the L‐isomer configuration, but at death they restructure to what is known as the D‐isomer configuration through a process known as racemization. This restructuring takes place relatively slowly, so that from a zero point at the time of death the D/L ratio gradually increases so that equilibrium is reached after anything from 150 000 years to more than 2 million years. The main problem with the technique is that the rate of racemization varies according to temperature and groundwater conditions so that regional differences according to latitude can be seen as well as, over long periods, uneven rates of change through time. This means that turning the D/L ratios into absolute ages is difficult, although as data about a particular area is built up fairly reasonable sequences can be developed.
Subjects: Chemistry — Archaeology.