87Sr/86Sr ratios in inoceramids (Bivalvia) and carbonate matrix as indicators of differential diagenesis during burial. Early Maastrichtian Bay of Biscay sections (Spain and France). Potential use for chemostratigraphy?

To determine, for chronostratigraphic purposes, whether the Maastrichtian seawater 87Sr/86Sr ratio has been preserved in calcite shells, oxygen, carbon, and strontium isotope analyses have been carried out on carbonate matrix and inoceramid shell samples from deep-sea carbonate sediments at the Sopelana and Bidart sections in the Basque-Cantabrian Basin. These analyses were compared with clay-mineral distributions from the same locations. In contrast to Sopelana I samples, Bidart shells are characterised by the lowest 87Sr/86Sr ratios, the highest δ18O values, and the highest strontium contents. These geochemical trends correlate with clay distributions in these sections as mineralogical analyses, for instance, indicate that clay minerals show a higher degree of maturation at Sopelana I. Furthermore, in comparison to carbonate matrix results, and regardless of the section, inoceramid shell remains usually yield the lowest 87Sr/86Sr isotope values, the highest δ18O values, and highest strontium contents. These results suggest a palaeogeographic control on isotope exchange during burial diagenesis and that the carbonate matrix and inoceramid shells have reacted differently to the same diagenetic environment. Although inoceramid shells from Bidart show evidences of fairly good preservation and fall close to the Maastrichtian seawater 87Sr/86Sr curve, the original strontium isotope compositions have not been retained.