The shells of the oyster Crassostrea sp. of the Somocuevas beach preserve EsMT patterns in the hinge region that can be used to establish paleoambiental conclusions, like the presence of periods of annual transit in the, so called, Zonas de Transición (ZT). The interaction of diagenetical fluids with the shell through the ZT was favoured by the existence of a microporosity that was developed in zones rich in organic matter, related to periods of low rate or cease of the shell growth, giving rise to an enrichment in diagenetic elements: Mg, Fe and Mn in the cements that filled up this microporosity. The cements that filled the cameras, originally empty, are enriched in Fe, Mn and Mg, with respect to the regular foliated calcite of the shell, which indicates that a diagenesis in reducing conditions, with mixing-water contributions, has taken place