Age variation of pore water iodine in the eastern Nankai Trough, Japan: Evidence for different methane sources in a large gas hydrate field

Abstract

The 129 I 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 129 I/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