Iron reduction is one of the most ancient forms of microbial respiration. This and the observation that iron reducers can grow under high temperature and pressure conditions suggests that they may play an important role in the deep biosphere. We will use stable Fe isotopes to disentangle microbial and abiotic processes involved in deep Fe cycling at IODP Site C0023 in the Nankai Trough. This will help to reach the goal of Expedition 370: ”T-Limit of the Deep Biosphere off Muroto” – the assessment of how microbial communities change with increasing sediment depth and temperature, by which factors changes are controlled, and where microbial life ceases. Dissolved iron was found at Site C0023 only within the methanic zone from 400 to 600 mbsf. The total drilling depth was 1180 mbsf. Is the Fe2+ release coupled to microbial activity? If yes, is it confined to the 200 m thick interval due to presence of reactive Fe minerals or because the microbes cannot cope with the temperatures prevailing in deeper sediments? Microbial iron reduction is known to cause pronounced enrichments of 54Fe in pore water, which should also be reflected by authigenic Fe minerals. The residual Fe pool, in contrast, becomes progressively enriched in 56Fe. Kinetic reactions of iron with sulfide enrich 56Fe in pore water, which allows a discrimination between microbial reduction and abiotic iron - sulfur interactions based on δ56Fe. As a result of different origins of incorporated Fe and different reactivities towards microbial reduction and sulfidation, Fe minerals in sediments possess different δ56Fe signatures and may show geochemical indications for microbial life. By analyzing δ56Fe of pore water and sequentially leached reactive and refractive Fe phases from Site C0023 sediments we will gain insight into the processes driving Fe2+ liberation at depth and hopefully assess links between the microbial activity and mineralogy (the presence of electron acceptors) as well as temperature.