The basis for the technique is the fact that almost all amino acids change from optically active to optically passive compounds (racemize) over a period of time.
Aspartic acid is the compound most often used because it has a of 15,000-20,000 years and allows dates from 5,000-100,000 years to be calculated.
This newer method converts a stable form of potassium (39K) into 39Ar while irradiated with neutrons in a nuclear reactor.
SYNONYMS OR RELATED TERMS: amino-acid dating; aminostratigraphy; amino-acid racemization, amino acid racemization CATEGORY: technique DEFINITION: A method of absolute (chronometric) dating which is hoped to fill the gap between radiocarbon dates and potassium-argon dates.
On multi-period sites, however, it calls for much more meticulous recording since the stratigraphy is revealed one layer at a time.
Stratigraphy is the study of the relative chronology of the Earth's strata.
Time since recrystallization is calculated by measuring the ratio of the amount of 40Ar accumulated to the amount of 40K remaining.
The long half-life of 40K allows the method to be used to calculate the absolute age of samples older than a few thousand years.
For example, the oldest burials are likely to be those nearest the settlement, the top of a hill, or some other favored position.
The later ones will be progressively further out as the cemetery expands.