In the time interval datable by radiocarbon, and at the temperatures of most archeological sites, a substantial amount of racemization of aspartic acid takes place. By determination of the amount of racemization of aspartic acid in bones from a particular location which have been dated by the radiocarbon technique, it is possible to calculate the in situ first-order rate constant for interconversion of the L-and D enantiomers of aspartic acid. Once this "calibration" has been calculated, the reaction can be used to date other bones from the deposit that are either too old to be dated by radiocarbon or that are too small for radiocarbon dating. The only assumption required with this approach is that the average temperature experienced by the "calibration" sample is representative of the average temperature experienced by older samples. This "calibration" technique is used herein to date bones from the Olduvai Gorge area in Tanzania, Africa.Only L-amino acids are commonly found in living organisms. However, recent studies have shown the occurrence of D isomers in fossil materials. The amino acids in fossil shells (1-3), sediments (4-6), and bones (7,8) are partially racemized, with the amount of racemization increasing with the age of the fossil. This racemization reaction has obvious applications in geochronology, and it appears that the reaction can be used to date deep-sea sediments (4-6) and fossil bones (7,8) found in certain environments.Most studies have concentrated on isoleucine. The epimerization of L-isoleucine produces the nonprotein amino-acid Dalloisoleucine. (For the reaction involving isoleucine, the process is more properly termed epimerization rather than racemization.) rrsoleucine and D-alloisoleucine are directly separable on an automatic amino-acid analyzer. In contrast, in order to determine the amount of racemization of other amino acids, a suitable diastereomeric derivative must first be synthesized. The isoleucine epimerization reaction in bone has a half-life (i.e., the time required for the ratio of alloisoleucine to isoleucine to reach 0.345) at 200 in excess of 100,000 years (8), and evidence suggests that the reaction can be used to date fossil bones too old to be datable by radiocarbon. The rate of the amino-acid racemization reaction is dependent upon temperature. Therefore, in order to use the reaction to calculate ages which are reasonably accurate, some estimate of the temperature history of a fossil bone must be available. Due to the problem of temperature uncertainties, the isoleucine epimerization reaction can apparently only be used to date fossil bones found in certain environments (7,8) where kasp is the first-order rate constant for interconversion of the L and D enantiomers of aspartic acid. In bone, this reaction has a half-life (i.e., the time required for the ratio of D-to iaspartic acid to reach 0.333) at 200 of about 15,000-20,000 years (Bada, J. L., Kvenvolden, K. A. and Peterson, E., in preparation). Thus, in the time interval datable by radiocarbon, a sub...

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By determining the extent of racemization of aspartic acid in a well-dated bone, it is possible to calculate the in situ first-order rate constant for the interconversion of the L and D enantiomers of aspartic acid. Collagen-based radiocarbon-dated bones are shown to be suitable samples for use in "calibrating" the racemization reaction. Once the aspartic-acid racemization reaction has been "calibrated" for a site, the reaction can be used to date other bones from the deposit. Ages deduced by this method are in good agreement with radiocarbon ages. These results provide evidence that the aspartic-acid racemization reaction is an important chronological tool for dating bones either too old or too small for radiocarbon dating. As an example of the potential application of the technique for dating fossil man, a piece of Rhodesian Man from Broken Hill, Zambia, was analyzed and tentatively assigned an age of about 110,000 years.The amino acids commonly found in living organisms consist mainly of the L-isomers. However, fossil materials have been found to contain D-amino acids, and the proportion of Damino acids to L-amino acids increases with the age of the fossil (1-9). The reaction responsible for this conversion is termed racemization. Each amino acid (with the exception of glycine) undergoes this process, some much faster than others. For example, in bone the racemization half-life (time required for D: L ratio to reach 0.333) at 200 for aspartic acid is about 15,000 years (9); at the same temperature the reaction for isoleucine has a half-life in excess of 100,000 years (8).The amino-acid racemization reaction has important applications in anthropology and geochronology. Recent studies have shown that the reaction can be used to date deepsea sediments (4-6) and fossil bones (7-9). Because of the much slower reaction rate, the racemization reaction can be used to date fossil materials too old for radiocarbon dating. Unfortunately, one limitation of this method is that the racemization reaction is temperature dependent. Thus, in order to date materials using the degree of racemization, some estimate of the temperature history of the region where the fossil was found must be available.Recently it has been shown (9) that by determining the extent of racemization of aspartic acid in a bone which has been dated by radiocarbon it is possible to calculate the in situ first-order rate constant for interconversion of the L-isomers and D-isomers of aspartic acid. Once this "calibration" has been carried out for a site, the racemization reaction can be used to date other bones from the area which are either too old or too small for radiocarbon dating. This "calibration" procedure eliminates the need for evaluating the temperature history of a bone before it can be dated using the amino-acid racemization reaction. The only assumption required, in using this approach, is that the average temperature experienced by the "calibration" sample is representative of the average temperature experienced by other samples from t...

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