Variations in racemization/epimerization ratios and amino acid content of Glycymeris shells in raised marine deposits in the Mediterranean

In this study we compare the organic geochemistry of fossil and modern Glycymeris shells. Amino acids were preserved within the shells. The amino acid content of the shells was similar at all the sites studied. Amino acid racemization and epimerization of Glycymeris shells are suitable techniques for dating Pleistocene raised marine deposits. As reported in other studies, we found that isoleucine epimerization analysis has a greater capacity to discriminate between sites of different age than glutamic acid and aspartic acid. However, particular constraints regarding the use of amino acid dating concern intrashell variability, so to avoid divergent results, it is necessary to sample the same part of the shell, namely the complex cross lamellar region near the umbo. The dating of high-energy coastal marine deposits calls for extensive field work in order to ensure the collection of a large number of samples in order to obtain robust results and reject spurious values. Shell accumulations on the shore-line are conditioned by several factors. The high coefficients of variation for epimerization values and their distribution pattern can be attributed mainly to time-linked taphonomical processes (time-averaging) that gave rise to the shell-bearing bed. However, reworking from former highstand sea level deposits, which usually occupy higher topographic levels, is not a common occurrence. Therefore, as a result of time-averaging and post-depositional processes, it is difficult to identify substages in stacked shell beds in raised beach deposits belonging to the same marine oxygen isotope stage by means of amino acid racemization/epimerization.