Effects of basic pH on amino acid racemization and leaching in freshwater mollusk shell

Quantifying the variables that affect the rate of racemization is imperative for improving the reliability of amino acid geochronology. In this paper the influence of basic pH on amino acid racemization and leaching of aspartic acid (Asp), glutamic acid (Glu), serine (Ser), and alanine (Ala) in freshwater mollusk shell is addressed. Shell fragments of modern Margaritifera falcata were immersed in buffered solutions of pH 7, 8, 9, and 10, and heated at 110 °C for eight time intervals ranging from 1 to 160 d. In addition, a suite of samples was subjected to extensive bleaching to isolate the intra-crystalline fraction of amino acids following heating in the buffer solutions. A total of 376 shell samples and 132 buffer solutions were analyzed for Asp, Glu, Ser, and Ala D/L values and concentrations, for both the total hydrolysable amino acids (THAA) and the free amino acid (FAA) populations. The results show that D/L values increase with increasing pH. Shells heated in pH 7 solution yielded the lowest D/L values, followed by pH 8 and 9 with overlapping, intermediate D/L values, and shells heated in pH 10 solution showed the highest D/L values. The effect of pH differed for each amino acid. For example, for Asp, D/L values were 29% higher in shells heated in pH 10 solution compared with pH 7, and for Glu, D/L values were 134% higher. The production of FAA increased with pH, following the same pattern as the D/L values. The intra-crystalline fraction showed similar results to the unbleached samples, suggesting that the effect of solution pH permeated the carbonate crystals. Trends in the percentage of leached FAA and THAA amino acids proved useful in tracking which amino acids were leached fastest (FAA Ser) and the most abundantly (FAA Ser and FAA Ala). Because the pH of pore fluids within shell CaCO3 naturally stabilizes at pH 8, and because our data show little difference in the rate of racemization between pH 8 and 9, we suggest that pH has a significant influence on racemization in shells only where the pH of the pore fluid is >9.