Testing the effect of bleaching on the bivalve Glycymeris: A case study of amino acid geochronology on key Mediterranean raised beach deposits

• First test of the closed-system behaviour on the bivalve Glycymeris for AAR dating.
• Open-system behaviour detected in bleached fossil Glycymeris but not in experimental samples.
• Key MIS 5 deposits in Sardinia, Mallorca and Tunisia show similar D/Ls for shells.

This work focuses on the bivalve Glycymeris, commonly used in classic amino acid racemisation (AAR) studies in the Mediterranean, as part of an extensive assessment of the suitability of intra-crystalline protein diagenesis (IcPD) dating. We test the closed system behaviour (a prerequisite for IcPD studies) of Glycymeris through bleaching and heating studies, as well as fossil material sourced from Last Interglacial deposits in Sardinia (Calamosca), Mallorca (Palma Bay) and Tunisia (Hergla). Although bleaching appears to be effective in isolating an intra-crystalline fraction of proteins, this fraction may not behave as a closed system in all molluscan taxa, as we verify here for some of the fossil Glycymeris. We show that open-system behaviour can be identified by analysing the co-variance between FAA and THAA fractions and the relative THAA composition and we also provide further evidence that, for taphonomic reasons, dating the molluscan fauna does not necessarily yield age information for the sedimentary units when these contain derived shells.