Inter- and intra-crystalline protein diagenesis in Glycymeris shells: Implications for amino acid geochronology

This study focuses on the mollusc Glycymeris, which is commonly used to date raised marine deposits through amino acid racemisation (AAR). We examined the effect of bleaching and heating on the amino acid composition and racemisation/epimerisation of Glycymeris shells, as well as fossil material sourced from Spain. We show that amino acid composition differs between the umbo and rim zone of Glycymeris. This difference may be attributable to sampling strategies that target distinct microstructural zones. Also, the inter- and intra-crystalline proteins in the umbo appeared to be similar, as reflected by similar amino acid composition, which varied in a similar way with time when exposed to heating. We confirm that the intra-crystalline fraction in Glycymeris does not behave like a closed system, as the leaching of amino acids and D/L values showed similar variations in unbleached and bleached samples. We propose that the sediment matrix on which the molluscs were deposited influence amino acid racemization. In contrast, the matrix does not appear to have affected protein leaching/degradation. We measured Arrhenius parameters for Asx, Glx, and Ile in Glycymeris, finding them to be similar to those of other molluscs.