Variation in amino acid epimerization rates and amino acid composition among shell layers in the bivalve Chione from the Gulf of California

Variation in the isoleucine epimerization rate and amino acid composition and concentration among and within the three layers comprising the shell of the marine bivalve Chione fluctifraga from the northern Gulf of California was determined in a time series of ten shells, from modern back to 900 yr BP (reservoir-age-corrected radiocarbon age). Differences in epimerization rates of up to 30% were found between various portions of the shells. In samples taken from the middle layer of the shell (two different positions) and the hinge area of the inner layer, D-alloisoleucine/L-isoleucine (A/I) values showed excellent age prediction ability, whereas in samples from the outer layer and the central part of the inner layer, A/I values showed greater variability. Isoleucine epimerization rates were found to differ between sampling positions within both the inner and outer layers of the shell. Average rate differences were also found among layers. The amino acid composition of the three layers is rather similar (with Asp, Glu, and Gly the most abundant amino acids) but variable. Significant differences in amounts of amino acids were found, with the middle layer showing the lowest amounts and the inner layer the highest amounts. Careful choice of sampling position may improve the accuracy of age estimates from amino acid racemization.