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doi:10.2204/iodp.proc.301.203.2008

Samples and analytical procedures

Sediment samples were obtained using the advanced piston corer and maintained under a nitrogen atmosphere during the squeezing process. After squeezing, sediment samples were freeze-dried and then crushed into a fine powder and homogenized with an agate mortar and pestle.

Reagent-grade deionized water (18 MΩ) was obtained using a Milli-Q purification system. High-purity acids (HF, HNO₃, HCl, and HBr) used in this study were Tamapure-AA-100 grade (Tama Chemicals Co., Ltd., Tokyo) and were used as received or diluted with reagent water.

Approximately 100 mg samples of finely powdered sediment were dissolved using a mixture of HF and HNO₃. Dissolution was performed on a hotplate (~100°C; overnight) in closed 7 mL polytetrafluoroethylene (PTFE) Savilex beakers with 1.5 mL of 14 M HNO₃ and 3 mL of 29 M HF. The sample solution was evaporated to dryness on a hotplate. The residue was then treated overnight with 1 mL of 6 M HCl at ~90°C. After evaporation to dryness, the residue was dissolved in ~1 mL of 0.6 M HBr.

Pb was separated from the sample solution by conventional HBr/HCl techniques using PTFE microcolumns (Manhes et al., 1978), Muromac AG 1-X8 anion-exchange resin (100–200 mesh; ~0.06 mL resin bed; Muromachi Technos Co., Ltd., Tokyo), and two column passes for sample purification.

Fractionation correction for Pb isotope measurement was done using the ²⁰⁷Pb–²⁰⁴Pb double spike technique. The ²⁰⁷Pb–²⁰⁴Pb double spike (²⁰⁷Pb and ²⁰⁴Pb = ~5 ppm; ²⁰⁷Pb/²⁰⁴Pb = 0.992001) of the University of the Ryukyus was made by mixing of single ²⁰⁷Pb (²⁰⁷Pb = 92.81%) and ²⁰⁴Pb (²⁰⁴Pb = 99.73%) spike solutions. Single spikes were prepared by dissolution of metal-form Pb purchased from Oak Ridge National Laboratory (USA), with 0.3 M HNO₃. Calibration of double spike isotopic composition was performed relative to the National Institute of Standards (NIST) standard reference material (SRM)-982 Pb, assuming ²⁰⁸Pb/²⁰⁶Pb is 1.00016 for this standard (Todt et al., 1996; Thirlwall, 2000). Calculation for mass bias correction followed the method in the appendix of Galer (1999), which assumes a linear mass fractionation law.

Following Woodhead et al. (1995), the chemically separated Pb sample was dissolved in dilute HNO₃ and split between the natural and mixed runs immediately before loading onto a previously outgassed flat Re filament. The double spike solution (~1 μL was added directly to the "mix" filament, and 3 μL of emitter solution, similar to Gerstenberger and Haase (1997), was added to both natural and mixed filaments, with mixing encouraged by repeatedly sucking the mixture back into the loading pipette.

Pb isotope composition was determined using a Finnigan MAT262 mass spectrometer at the University of the Ryukyus. Data acquisition was performed on ²⁰⁸Pb ion beam intensities between 1.5 × 10^–11 and 2.5 × 10^–11 A. During the experiment period, NIST SRM-981 Pb yielded average values of 16.940 ± 0.011 (2 σ; N = 4) for ²⁰⁶Pb/²⁰⁴Pb, 15.497 ± 0.004 ²⁰⁷Pb/²⁰⁴Pb, and 36.718 ± 0.004 ²⁰⁸Pb/²⁰⁴Pb. These values compare well with the preferred values obtained by double spike (e.g., Thirlwall, 2000) and triple spike (Galer and Abouchami, 1998) techniques. Total procedural Pb blanks were generally <30 pg. Because blank contribution is significantly low (<<0.003%) relative to Pb amount (several measured isotopic ratios are negligible.

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