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doi:10.2204/iodp.proc.338.203.2015

Methods and materials

The Nankai Trough (Fig. F1) is a sediment-filled trench located south of Japan where the Philippine Sea plate is subducting beneath the Eurasia plate at ~4.1–6.5 cm/y (Miyazaki and Heki, 2001; Seno et al., 1993). Sediments in the Shikoku Basin on the Philippine Sea plate are accreted to the Eurasia plate at the deformation front, and the accretionary prism is formed on the Eurasia plate. Around the Nankai Trough, large earthquakes accompanied by tsunamis have occurred repeatedly according to historical records (Ando, 1975; Baba et al., 2006; Hori et al., 2004). In Stages 1 and 2 of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) project, eight riserless drilling sites and one riser drilling site were drilled during Integrated Ocean Drilling Program (IODP) Expeditions 314, 315, 316, 319, 322, 332, and 333 (Fig. F1A) (Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009; Saffer et al., 2009; Underwood et al., 2010; Expedition 332 Scientists, 2011; Expedition 333 Scientists, 2012). In this study, during NanTroSEIZE Stage 3, a slope of the accretionary prism was drilled during IODP Expedition 338 at two sites. IODP Site C0021 is located on a mass transport deposit, and IODP Site C0022 is located on the hanging wall of a megasplay fault (Fig. F1B). At Site C0021, drilling was between 80 and 195 meters below seafloor (mbsf), and mass transport deposits were observed at 100–115 and 135–180 mbsf. A thick sand layer was observed at 180–195 mbsf (Moore et al., 2014). Site C0022 was drilled to 440 mbsf and penetrated the megasplay fault at 100 mbsf, where sediment in the core was deformed about 15 m above and below this level (Moore et al., 2014).

Extraction of interstitial waters from sediment followed the procedure described in the “Methods” chapter (Strasser et al., 2014). The protocol adheres fundamentally to routine IODP protocols (Expedition 315 Scientists, 2009; Expedition 319 Scientists, 2010), modified during the Ocean Drilling Program (Gieskes, 1991; Murray et al., 2000). Sediment samples were placed into nitrogen-filled glove bags, and their outer surface was scraped off using cleaned spatulas to avoid contamination by water from the circulating drilling mud. Samples were then squeezed by a Manheim-type titanium squeezer (Manheim, 1966) to a maximum pressure of 30 MPa. The extracted interstitial water was filtered with a 0.45 µm disposable filter and stored in high-density polyethylene sample bottles that had been previously washed with trace metal grade 12 M HCl and Milli-Q water. These were then acidified with 6 M HCl with a ratio of 0.4 vol%.

B concentrations in interstitial waters were measured on board the ship using procedures described in the “Methods” chapter (Strasser et al., 2014). Aliquots of 500 µL were measured using an inductively coupled plasma–atomic emission spectrometer (ULTIMA 2; Jobin Yvon Horiba). Samples were diluted by 20 times, and the matrix was matched to that of seawater by artificial seawater for accurate analysis. A yttrium solution was added as an internal standard to correct for instrumental drift. The precision was ±2.5%.

To measure B isotope ratios in interstitial waters, B was isolated based on the procedure of Wang et al. (2010) using a sealed upside-down beaker. The method uses a microsublimation technique to desalinate sample water, which is independent of matrix effect (Wang et al., 2010). A 30 µL aliquot sample was placed at the center of an upward-facing beaker cap, a conical beaker was placed upside-down on the cap creating a tight seal, and the resulting assemblage was heated at 100°C overnight, or about 15–17 h. B and moisture condensed within the apical end of the beaker were dissolved in 0.15 M HNO₃ + 0.015 M HF to yield a solution with a B concentration of 50 parts per billion. This solution was introduced into a multicollector inductively coupled plasma–mass spectrometer (Neptune Plus; Thermo Fisher Scientific) to measure B isotope ratios. The isotope ratio was expressed as δ¹¹B, the deviation from the international standard material NBS SRM951. Precision was within ±0.7‰. The running error during each sample measurement was listed in Table T1; 0.05–0.07‰ (2σ).

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