Shell amino acid racemization dating

Amino Acid Racemization Dating of Bone and Shell... 

Although the first demonstration that amino acid racemization took place in fossils used mollusc shells (23), the application of this reaction in dating these materials has been extensively investigated only recently (19,20). Work on Mercenaria (19), Chione (20), and other species (24) has tested the application of racemization dating to fossil mollusc shells from geological contexts and Indian shell middens. These and other studies have shown that there are problems with amino acid racemization dating of carbonaceous fossils which are not encountered with bone. Reversible first-order racemization kinetics which are observed in bone... 

Amino acid racemization dating of mollusc shell has not been investigated as extensively as that of fossil bone. Recently several studies have used d/l enantiomeric ratios to resolve stratigraphic relationships (24) and to date geological deposits by the calibration procedure (19,20). Racemization dating has also been applied to midden shell from archaeological sites with limited success (20). 

Kimber R.W.L., Griffin C.V. (1987) Further evidence of the complexity of the racemization process in fossil shells with implications for amino acid racemization dating. Geochim. Cosmochim.Acta 51, 839—46. 

Amino Acid Dating Techniques depend on the "rates of hydrolysis reactions of proteins and racemization, epimerization, and decomposition reactions of amino acids [they have] been applied to the age-dating of fossil bone, teeth, and shell. Activation energies range from near 20 kcal per mole for hydrolysis reactions to around 30 kcal per mole for racemization... 

This work demonstrated that AAR could give reasonable dates from smaller samples of bone than were necessary for radiocarbon, and had a time depth of at least 70 000 years, and possibly more if one of the more slowly racemizing amino acids such as alanine was used. The key paper came in 1974 (Bada et al., 1974), which published dates of between 6000 and 48 000 BP for various samples of human bone from the Californian coast (Table 8.1). The SDM (San Diego Museum) samples from site W-2 were from a shell midden near La Jolla excavated in 1926. Subsequently, it appears that 19 individual burials were recovered in a rescue operation from this site, known as La Jolla Shores SDM-16755 is thought to refer to more than one individual (La Jolla Shores I and II), with a third (La Jolla Shores III) identified as SDM-16740 (Taylor et al., 1985 Table 8.1). Site W-34 was located between Del Mar and Solano Beach, from a shell midden which had been largely destroyed by coastal... 

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