Isotopic and chemical intra-shell variations in low-Mg calcite of rudist bivalves (Mollusca-Hippuritacea): disequilibrium fractionations and late Cretaceous seasonality

Isotopic (delta(13)C, delta(18)O) and elemental (Mg, Sr, Mn, Fe) compositio ns were analysed in sclerochronological profiles of several shells of late Cretaceous rudist bivalves from Greece, Turkey, Somalia and the Arabian Pen insula. The preservation of original compositions of low-Mg calcite of oute r shell layers is indicated by constant and high Sr, generally low Fe and M n, and the preservation of fibrous-prismatic ultrastructures. Cyclic variat ions in delta(18)O and Mg are interpreted to reflect. seasonal temperature/ salinity cycles and, thus, annual growth increments. In shells of Torreites , amplitudes of correlated delta(13)C and delta(18)O cycles cannot be relat ed to reasonable palaeotemperatures or salinity. This isotopic pattern refl ects vital fractionations of an extent which is unknown from modern bivalve s. In contrast, almost identical ranges and amplitudes of delta(18)O cycles are observed in 13 shells of five species from Santonian-Campanian localit ies in central Greece and northern Turkey, suggesting that seasonal variati ons in environmental conditions were recorded without significant vital fra ctionations. The effect of seasonal salinity changes on delta(18)O of the s hells is evaluated, and mean palaeotemperatures are constrained within the range of 30-32.5 degrees C. The annual range of temperature was estimated t o be 7 degrees C, assuming a constant salinity. This agrees with other isot opic proxies of Late Cretaceous palaeotemperatures, and with global circula tion models which predict higher low-latitude sea-surface temperatures than the present ones.