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doi:10.2204/iodp.proc.314315316.220.2012

Results

Iodine concentrations dissolved in interstitial fluid generally increase rapidly in the uppermost 100 m CSF and thereafter reach relatively constant values at ~200 µM at all sites (Fig. F1). The most pronounced increase is observed at Site C0002, where iodine concentrations reach values >400 µM but decrease to values comparable to the other sites at greater depths. Consistent with earlier results from the Nankai Trough area, interstitial fluid iodine concentrations here are exceedingly higher than that of seawater (0.4 µM), which has been interpreted as evidence for the input of deep fluids receiving iodine from degradation of marine organic matter (You et al., 1993; Fehn et al., 2003; Muramatsu et al., 2007; Tomaru et al., 2007a). Linear iodine profiles starting at ~200 m CSF at Site C0001, above 477 m CSF at Site C0002, ~150 m CSF at Site C0004, and ~100 m CSF at Site C0008 indicate that iodine from organic matter decomposed in deeper sediments dominates in these intervals and mixes with iodine from seawater in the shallower sections. There is probably only minor iodine input from host sediments because iodine concentrations in sediment and interstitial fluid do not show apparent covariation within this depth window in the Nankai Trough (Muramatsu et al., 2007).

The interstitial fluid ¹²⁹I/I ratios at Site C0001 gradually decrease downhole from 938 × 10^–15 at 41 m CSF to ~400 × 10^–15 at ~200 m CSF, providing minimum iodine ages from 10.6 to ~30 Ma (Equation 1), where the lithologic boundary between the old accretionary prism and overlying younger forearc basin sediment is located (Fig. F2). This gradient represents mixing of iodine between deep fluids (¹²⁹I/I = 400 × 10^–15) and young iodine mainly from seawater (¹²⁹I/I = 1500 × 10^–15) because iodine concentrations in this interval are modulated by seawater input (Fig. F1). On the other hand, iodine ages at adjoining Site C0002 are relatively constant at ~30 Ma throughout the sediment column (Fig. F2), deep old iodine is dominant even in the interval just below the seafloor with relatively low iodine concentrations.

The results suggest that iodine in interstitial fluid is significantly older than the host sediment because young iodine inputs that potentially increase ¹²⁹I/I ratios (decrease iodine age) are minor according to the depth profile of iodine concentrations. Significant inputs of young iodine probably from seawater are observed only in the shallow section at Site C0001. A similar co-existence of iodine with organic matter was observed in fluid from Ocean Drilling Program Leg 131 in the toe of the Nankai Trough accretionary prism, ~190 km southwest of these sites, pointing to fluid advection along the décollement zone from organic matter–rich sediments further in the accretionary prism (You et al., 1993). This observation agrees well with the occurrence of fluids much older than the host sediments at Sites C0001 and C0002.

Concerning ¹²⁹I/I variation, the old iodine flux from old accreted sediments reaches the overlying young basin sediments at Site C0002; however, old iodine dominates only in the old accretionary prism and younger iodine plays a more important role in the younger basin sediments at Site C0001. This difference probably reflects changes of fluid modes between locations, which correlates well with changes in sediment porosity, a fundamental geophysical factor controlling fluid transport (Screaton et al., 2002; Ashi et al., 2008; Kinoshita et al., 2008). A discontinuous porosity change is found at the lithologic boundary only at Site C0001, where porosity changes from ~50% above the boundary to ~60% below (Fig. F2). Because the upward fluid transport can be interrupted by the porosity drop of ~10%, fluids are more likely at this location to escape along the boundary rather than cross it. As shown in the porosity distribution, more compacted and deeper sediments at Site C0002 probably form a more uniform lithologic boundary for fluid migration than at Site C0001.

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