Reconstructions of paleo-environments are frequently based on stable isotope measurements. General assumptions are that a foraminifer has a stable life habitat throughout ontogeny and that the d13C of the TCO2 is constant in the mixed layer. However, because most planktic foraminifera, if not all, change their life horizon during ontogeny and because the d13C of the TCO2 is not necessarily constant in the mixed layer, the isotopic composition of the foraminiferal shells is not only dependent on water-mass properties (e.g. oligotrophic vs. eutrophic) and the geographic and climatic setting (e.g. upwelling, monsoon etc.) but to an important extent on the life history of the foraminifer. Consequently, the disequilibrium precipitation reported in the literature must be explained in terms of ontogenetic migration and so called vital effects in addition to phytoplankton growth dynamics.Modelling the effect of ontogenetic migration patterns and metabolic effects show a discrepancy with measured isotope data that needs further attention. Without the understanding of the mechanism controlling the carbon isotope signal, paleo-environmental reconstruction on the basis of carbon isotopes is not legitimate.