PATTERNS OF RACEMIZATION AND EPIMERIZATION OF AMINO-ACIDS IN LAND SNAIL SHELLS OVER THE COURSE OF THE HOLOCENE

The patterns of racemization of six amino acids (alanine, proline, aspartic acid, methionine, glutamic acid, and phenylalanine) and of epimerization of isoleucine over the course of the Holocene (the last 10,000 years) were studied in a series of 38 radiocarbon-dated samples of the land snail Trochoidea seetzeni from the Negev Desert in southern Israel. The D/L ratios of each of the amino acids show a strong correlation with age (r = 0.84-0.94) and thus good age predictive ability. The patterns in faster-racemizing amino acids (proline, methionine, aspartic acid, and phenylalanine) do not conform to first-order kinetics. Transformation to parabolic kinetics (time transformed to square root to time) linearizes these faster-racemizing amino acids, except for aspartic acid (which shows a very high initial rate of racemization). After transformation, each amino acid shows an equally good correlation with age (r = 0.91-0.93). The D/L ratios of the various amino acids covary very strongly, even after the covariation due to sample age is removed from the data set. Thus, analysis of more than one amino acid provides largely redundant information on sample age. Nevertheless, because of differences in racemization rates, some amino acids provide better time resolution in different time ranges-aspartic acid is especially useful for very young samples, and glutamic acid and isoleucine for older samples. Neither the depth of burial of the samples nor the burial mode (fluvial deposition vs. burial by rodents in their burrows) appears to affect the rate of racemization or epimerization.