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Iron monosulfide and pyrite concentrations vary strongly at the investigated sites. At Site C0004, iron monosulfide concentrations are close to the detection limit with only a slight increase at ~350 meters below seafloor (mbsf) to a maximum of ~6 ppm (Fig. F1). In contrast, pyrite concentration reaches 1.3 wt% (13,000 ppm), with four main accumulation zones at ~30, 100, 140, and 250 mbsf. Pyrite sulfur isotope compositions (δ34S-CRS) range from –49‰ to +47‰; the highest enrichment in 34S occurs in the zone dominated by mass transport (lithostratigraphic Subunit IIA) (Fig. F1). Low δ34S-CRS and pyrite concentrations are observed at the sediment/water interface and appear to coincide with transitions between different sedimentary units.

In the upper two-thirds of Unit II at Site C0006, iron monosulfide concentrations reach values of 550 ppm, whereas values are low for the remaining sediment column (Fig. F2). Except for a peak at 523 mbsf, pyrite concentrations are highest in the upper ~45 mbsf. Three features of the δ34S-CRS profile are remarkable. First, the isotope composition of pyrite shifts dramatically in the vicinity of the transition between lithostratigraphic Units I and II from –40‰ to over +20‰, coinciding with a drop in pyrite concentration. Second, δ34S-CRS displays a trend to lighter values with depth in Unit II. Third, a strong depletion in 34S was observed at the Pliocene/Pleistocene boundary followed by a maximum enrichment in 34S to 30‰ concurrent with a peak in pyrite concentration reaching almost 2 wt%.

In Hole C0007D, iron monosulfide concentrations are only elevated in the upper deposits, with values below 220 ppm followed by a downward decline (Fig. F3). Unlike iron monosulfide concentrations, pyrite concentrations are highest in the deeper sediment of Unit III, with a maximum value of ~7700 ppm. This pyrite concentration peak is accompanied by a sulfur isotope excursion from strongly negative δ34S (less than –48‰) to almost positive values (–1.6‰), succeeded by a return to strongly negative δ34S (–39‰) at the boundary with Unit IV. Overall, the sulfur isotope profile of the deeper sediment in Hole C0007D shows a resemblance to the δ34S-CRS profile at Site C0006.

Pyrite concentrations at Site C0008 are lowest between ~40 and 160 mbsf. Higher amounts of pyrite occur above and below this zone, with concentrations up to 1.5 wt% (Figs. F4, F5). Iron monosulfide concentrations show an inverse trend, with highest values (<650 ppm) in the zone of lowest pyrite concentration. The δ34S-CRS composition in Hole C0008C is nearly homogeneous throughout the sediment column, with a mean sulfur isotope composition of –3‰ (±10‰). In only two zones is pyrite slightly more depleted in 34S (~5 and 95 mbsf) (Fig. F5).

Overall, the pattern of high pyrite concentrations coinciding with very low iron monosulfide concentrations and low pyrite concentrations coinciding with elevated monosulfide concentrations suggests that two modes of operation of sedimentary sulfur–iron cycling is present in the sediment from Nankai Trough. In the high pyrite formation mode, local sulfide formation may have temporally exceeded the availability of reactive iron, despite the high amount of detrital input (Screaton et al., 2009), therefore suppressing intense oxidative–reductive sulfur cycling. In contrast, intervals with low pyrite concentrations concurrent with iron monosulfides point to an iron-dominated sequence where the excess of reactive iron rapidly consumes newly formed free sulfide either by the formation of monosulfides or by oxidation, thus limiting the alteration from iron monosulfide to pyrite.