Protein diagenesis in archaeological gastropod shells and the suitability of this material for amino acid racemisation dating: Phorcus lineatus (da Costa, 1778)

The inter-and intra-crystalline fractions of the topshell Phorcus lineatus recovered from modern specimens and shells from archaeological sites in Northern Spain covering Neolithic, Mesolithic, and Upper Magdalenian periods were examined for amino acid composition and racemisation over time. The main loss of proteins from the inter-crystalline fraction occurred within the first 6000 years after the death of the organism. In contrast, the intra-crystalline fraction isolated by bleaching-with a different protein composition to that of the inter-crystalline fraction-appeared to behave like a closed system for at least 12.6 ka, as reflected by the lack of a significant decrease in amino acid content. However, changes in the relative composition of the amino acids present in these shells occurred during this period. The concentration of aspartic acid remained almost constant with age within the intra-crystalline fraction and its contribution to the total amino acid content also remained the same. Good correspondence was obtained between Asx D/L values in unbleached and bleached samples and age, thereby allowing the dating of archaeological sites and the determination of chronometric age.