Palaeoenvironmental implications and diagenesis of inoceramid shells (Bivalvia) in the mid-Maastrichtian beds of the Sopelana, Zumaya and Bidart sections (coast of the Bay of Biscay, Basque Country)

Inoceramid bivalve shells from the mid-Maastrichtian beds of the Sopelana, Zumaya and Bidart sections in the Basque Country, were tested for their post-depositional diagenetic alteration. Petrography, cathodoluminescence (CL) and scanning electron microscope (SEM) observations together with geochemical analyses, indicate that the coarsely prismatic calcite of the shells from the Sopelana and Zumaya sections have undergone diagenetic alteration, without there being substantial textural changes in the original prismatic microstructure. A generalized bright yellowish to red colour is seen in all the samples observed by CL. The δ18O values obtained from the shell prisms range from −2.94‰ to −5.18‰, with a mean value of −3.6±0.5‰ in the Sopelana section, whereas in the Zumaya section they range from −1.88‰ to −4.71‰, with a mean value of −3.2±0.3‰. These values are clearly lower than those from the Bidart section (from −0.32‰ to −1.48‰; mean value −0.7±0.3‰). Other evidence of diagenetic alteration is inferred from both whole-shell geochemistry and microprobe analysis. A complementary zonation in the distribution of `diagenetic tracersʹ can be detected from the inner shell layer (ISL) to the outer shell layer (OSL) of inoceramid shells, together with a rare earth element (REE) enrichment in the shells which have lighter δ18O values. In contrast, the inoceramid bivalve shells and their host-rock from the Bidart section were less altered. The calculated mean palaeotemperature (15°C) and the value of 13°C estimated from the non-luminescent middle shell layer seem to be consistent with palaeolatitudes about 30°N for deep marine waters of mid-Maastrichtian time, as the last step of the long-term climatic cooling of the Late Cretaceous `greenhouseʹ.