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As part of the multiphase-multiyear Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE), the main goal of Integrated Ocean Drilling Program (IODP) Expedition 316, using the D/V Chikyu, was to investigate and understand the complex nature of tectonic activity in the accretionary prism system off Japan (Tobin and Kinoshita, 2006; Kimura et al., 2008; Strasser et al., 2009). One target of Expedition 316 was to study the impact of tectonic processes at the megasplay fault and frontal thrust area on the geochemical composition of deeply buried sediment (Screaton et al., 2009). Deep subsurface fluid flow and changes in depositional settings strongly influence the sulfur inventory in this sedimentary system (e.g., Riedinger et al., 2010).

Pyrite is often the most abundant iron sulfide in marine sediment (e.g., Goldhaber and Kaplan, 1975; Cornwell and Morse, 1987). In conjunction with its precursors (i.e., metastable iron sulfides), pyrite and its isotope composition can be used to investigate biogeochemical sulfur cycling in marine sediment (e.g., Goldhaber and Kaplan, 1974; Goldhaber et al., 1977; Howarth, 1979; Howarth and Jørgensen, 1984).

The abundance of pyrite in surface sediment is mainly controlled by the amount of buried total organic carbon (TOC) and availability of dissolved sulfate that drive microbial sulfate reduction and by the content of reactive iron, a scavenger for the produced sulfide (e.g., Jørgensen, 1977, 1982; Goldhaber and Kaplan, 1974; Berner, 1984). In surface sediment and deeply buried sediment, the interplay between sulfide production and inventory of reactive iron, which not only acts as a sulfide scavenger but can also drive oxidative sulfur cycling, exerts a major control on the formation and alteration of intermediate metastable iron sulfides such as mackinawite and greigite. These mineral phases can be transformed into pyrite by the addition of elemental sulfur (e.g., Goldhaber and Kaplan, 1975; Rickard and Luther, 2007, and references therein).

Microbial sulfate reduction produces sulfide strongly depleted in 34S, causing enrichment of remaining sulfate in 34S (Thode et al., 1961; Jones and Starkey, 1957; Harrison and Thode, 1958; Kaplan and Rittenberg, 1964; Sim et al., 2011). Oxidative sulfur cycling (e.g., with reactive iron) can transform 34S-depleted sulfide into sulfur intermediates, such as elemental sulfur or thiosulfate. The subsequent disproportionation of these sulfur intermediates into 34S-enriched sulfate and 34S-depleted sulfide (e.g., Böttcher et al., 2005) can further increase the offset between isotopically heavy sulfate and light sulfide (Canfield and Thamdrup, 1994). Overall, these processes result in the formation of dissolved sulfide, monosulfides, and pyrite that are depleted in 34S relative to their sulfate source (e.g., Jørgensen, 1979; Canfield and Teske, 1996; Bottrell et al., 2009).

Under conditions in which sulfate enriched in 34S can be rapidly exchanged with seawater sulfate, such as close to the sediment/water interface or in the vicinity of seawater fluid conduits (highly permeable sediment such as sand or gravel layers or discontinuities such as faults), isotopically light sulfides can accumulate. Isotope compositions of sulfides approaching –50‰ relative to +20‰ for seawater sulfate are not uncommon under such circumstances. In sediment where the sulfate concentration decreases because of a diminished resupply with sulfate (i.e., through diffusion), the sulfate pool becomes isotopically heavier with depth (e.g., Goldhaber and Kaplan, 1980), resulting in the precipitation of iron sulfide minerals that are less depleted in 34S. At the sulfate–methane transition, where sulfate is almost entirely consumed, iron sulfides are precipitated with the heaviest isotope composition compared to the upper sediment (Borowski et al., 2013). Under such conditions, the sulfur isotope composition of sulfides can become heavier than the isotope composition of the original sulfur source, which is often seawater sulfate.

Here we investigate the occurrence of iron sulfides in deeply buried sediment at the megasplay fault and the frontal thrust area and the relationship of the complex tectonic environment on the iron sulfide isotope signal. For this purpose, we analyzed samples from cores at the megasplay fault (Sites C0004 and C0008) and the toe of the accretionary prism (Sites C0006 and C0007) for iron sulfide phases and their isotopic signature.