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doi:10.2204/iodp.proc.314315316.223.2014 Data report: concentration and sulfur isotope composition of iron monosulfide and pyrite from sediment collected during IODP Expedition 3161Natascha Riedinger2 and Benjamin Brunner3, 4AbstractIn this report, we give the concentration and sulfur isotope composition of iron monosulfide and pyrite in samples recovered during Integrated Ocean Drilling Program Expedition 316 to the Nankai Trough, Japan. We studied the impact of complex tectonic activity in this depositional system on the precipitation and occurrence of iron sulfide minerals and their isotopic composition by analyzing samples from sediment cores taken at the megasplay fault and frontal thrust. The dominating iron sulfide species is pyrite with concentrations up to 2 wt% (median = 3325 ppm, average = 4480 ppm, and minimum = 20 ppm; n = 197). The sulfur isotope composition of pyrite covers a range from –49 to +47‰ (median = –17‰ and average = –18‰; n = 117). Strongly negative sulfur isotope compositions, which are typical for open-system conditions where sulfate enriched in 34S can be exchanged with seawater, tend to be associated with relatively low pyrite concentrations. Strong enrichments in 34S in pyrite tend to be associated with high pyrite content. With one exception, boundaries between sedimentary units are associated with low pyrite content and strongly negative sulfur isotope composition. This could indicate that these boundaries may have acted as conduits for sulfate exchange between pore water and seawater for most of the time since the deposition of the sediment, preventing establishment of closed-system conditions. In contrast, within undisturbed sediment packages closed-system conditions promoted a high degree of sulfate consumption and concomitant incorporation of sulfide enriched in 34S into the pyrite fraction. In some cases, sulfate drawdown was quantitative, leading to terminal degradation of organic matter by methanogenesis and to the installation of sulfate–methane transitions. Where such transitions persisted over a substantial period of time, pyrite strongly enriched in 34S could accumulate. Iron monosulfide concentrations are generally low (median = 1.5 ppm, average = 44.8 ppm, and maximum = 1952.7 ppm; n = 195). Slightly elevated concentrations appear to be tied to specific sediment units, suggesting that their inventory of reactive iron species allows for an ongoing interplay with the sulfur cycle and thus sustaining the presence of these mineral phases. |
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