Non-equilibrium isotopic compositions of shells of planktonic foraminifera in the Mediterranean sea

The deviation from isotopic equilibrium has been evaluated for eight foraminiferal species from the Mediterranean Basin. A detailed study of some forms presenting an outer calcitic crust in late ontogenetic stages and in sedimented specimens allows us to propose another hypothesis for the biological fractionation: it should be a function of metabolic processes which condition the secretion of the tests.