Even if extensive lab and fieldwork has been done to determine geo- or biogeochemical reactions that precipitate uranium ore, dominant pathways of uranium accumulation in anoxic marine sediments remain to be demonstrated. As well iodine transformation below sediment-water interface lacks of definitive explanation. In this work, we utilize bioreactors for undisturbed sediment to validate possible mechanisms and extract kinetics information that applies to in situ conditions in order to feed diagenetic model. Here, we are presenting our first results in uranium and iodine from our study sites in the Bay of Biscay (France) and in the Thau lagoon (France). We observe that uranium is trapped in anoxic marine sediments (cores from Bay of Biscay) during our bioreactor experiment under anoxic conditions. The concentration of uranium decreases drastically from the injection concentration, to almost zero, in 20 hours. For iodine, it is not as clear as for uranium, because the concentration is still the same from the beginning of circulation of the injection solution, until the end of the experiment (96 hours). In order to decide wether iodine is trapped in marine sediments or transformed in one of its compounds, we also study its speciation at the reactor output.