The 129I geochronology of marine pore water is useful for the understanding of the origin of methane in gas hydrates because of the close association between I and marine organic materials responsible for methane generation. We report 129I/I ratios in pore waters from three deep cores in the eastern Nankai Trough gas hydrate field, two located on the outer ridge and one in the forearc basin. As in previous studies of gas hydrate fields, I ages of pore water are consistently older than those of the host sediments. For the first time, however, the results demonstrate that the potential I source formations vary considerably across the forearc setting: While I at the basin site reaches ages close to 50 Ma, all I ages at the two ridge sites are <32 Ma. The latter two sites also demonstrate the influence of I from formations younger than 10 Ma. The results suggest that I and, by association, methane on the outer ridge are derived mainly from Miocene to Pliocene forearc sediments through the active faults, while the main source for the forearc basin is the old accretionary wedge related to an earlier subduction configuration of Eocene age, which acts as a backstop in the current subduction system.