Evaluation of diagenetic effects upon 87Sr/86Sr ratio in rudist and Chondrodonta sp. shells from Upper Aptian – Lower Albian red limestones of Ereño (Bizkaia)

87Sr/86Sr isotopic ratios measured on rudist and Chondrodonta sp. shells from upper Aptian – lower Albian red limestones of Ereño (Bizkaia) reveal different degrees of diagenetic alteration when compared to coeval values. Chondrodonta sp. and requienid shells show a better degree of chemical preservation, whereas caprotinid and monopleurid shells seem to be more affected by diagenetic fluids. The same conclusion can be obtained when 87Sr/86Sr values are plotted against cationic contents in Sr, Na, Mg, Fe and M