Abstract Accelerator mass spectrometry offers for the first time the opportunity to date hand picked planktonic (surface dwelling) and benthic (bottom dwelling) foraminifera from deep sea cores. This opens up the possibility to reconstruct temporal changes in the rate of deep ocean ventilation. There is, however, a serious problem in interpreting such results. Bioturbation of the “soil” zone in the upper sediment has two subtle influences which lead to biases in the results. One has to do with changes with time in the abundances of the forams in newly deposited sediment. The other has to do with dissolution effects on the forams during their residence within the bioturbated layer. Mixing models are presented in this paper which illustrate the effects of these interactions on the 14C age difference between benthic and planktonic species.