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doi:10.2204/iodp.proc.314315316.220.2012 Data report: distribution of iodine concentration and 129I in interstitial fluid in the Nankai Trough accretionary prism collected during IODP Expeditions 315 and 3161Hitoshi Tomaru2 and Udo Fehn3AbstractIodine concentration and 129I/I ratios were determined in interstitial fluid collected from the Nankai Trough accretionary prism during Integrated Ocean Drilling Program (IODP) Expeditions 315 and 316. Iodine concentrations increase rapidly in the uppermost 100 m below the seafloor, before concentrations reach stable values between 200 and 400 µM, reflecting the advection of iodine released during the degradation of organic matter in deep sediments. The 129I/I ratios in the interstitial fluid start just below preanthropogenic seawater values and decrease rapidly with depth to ratios ~400 × 10–15. These ratios result in ages for the potential iodine source formation between 30 and 40 Ma, which are clearly older than the host sediments and demonstrate that advection of fluids is responsible for the observed distribution of iodine at these sites. Significant increases in 129I/I ratios uphole are found just above the lithologic boundary between the overlying forearc basin and older accreted sediments. This observation suggests that only limited migration of ascending deep fluids occurs beyond the lithologic boundary, where sediment porosity drops abruptly from 60% to 50%, and that fluids move preferentially along the lithologic boundary. |
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