Three hundred fifty-five sediment samples from Site 997 were examined for geochemistry and mineralogy. The sampling interval was ~150 cm because the sediments were collected from every available section of cores from Holes 997A and 997B. Sediment samples of approximately 5 cm3 were twice washed with deionized water and then recovered in wet condition by centrifuging. They were dried at room temperature and then powdered in an agate mortar. A portion of each powdered sample was sent to the University of Tokyo for mineralogical and major element analysis (Lu et al., Chap. 14, this volume). A 0.1000 g subsample was used for the trace element analyses.
To prevent precipitation of insoluble CaF2 in the mixed acid digestion, the powdered samples were soaked in 1.0 M acetic acid solution for two nights at room temperature to recover the soluble carbonate fraction (mainly of calcite and dolomite). This solution was separated by centrifugation, and the residual solids were dried in an oven below 90șC.
The residual solid was washed with deionized water and then soaked in 3 mL of HCl + HNO3 (1:1 by volume) acid overnight. They were then digested completely by adding 4 mL of HClO4 + HF (1:2 by volume) solution in a tightly sealed 7-mL Teflon PFA screw-cap beaker, heated for 12 hr on a hot plate below 180șC, and then evaporated to nearly dryness.
The sample solutions for both the acid-soluble and residual fractions were prepared in the same way. The residues were dissolved with 5-mL (1 + 1 by volume) HNO3 by microwave heating for short time, and 5 mL of 4 ppm indium solution (1000 ppm Cica atomic absorption standard) was added as an internal standard. The final solution was diluted to 1:1000 by mass. For each determination batch, the solution of the Geological Survey of Japan (GSJ) reference sample JB-1 (Quaternary basalt; Imai et al., 1995) was used as a standard.
Three major elements (Mg, Ca, and Fe) for the acid-soluble fraction were determined using a SEIKO SPS-1200 inductively-coupled plasma atomic emission spectrometry (ICP-AES) at the Geological Survey of Japan, Tsukuba, Japan. Analytical run precision was within ± 3% for these elements; details of the method are described in Imai (1987).
Twenty-two trace elements (P, Sc, V, Cr, Mn, Co, Ni, Cu, Zn, As, Rb, Sr, Y, Zr, Nb, Mo, Cd, Ba, Hf, Pb, Th, and U) and 14 rare earth elements for both the acid-soluble and residual fractions were determined by Yokogawa Analytical Systems PMS-200 inductively-coupled plasma mass spectrometry (ICP-MS) at the GSJ. All the samples were analyzed twice if a duplicate solution was available; average values are reported. Analytical run precision is often <2% of the reported values of the reference samples JB-1, JB-3 (basalt), JA-1 (andesite), and JG-1 (granite). Reproducibilities of the trace and REE elements are less than 6%. More detailed descriptions of the procedure are given in Imai (1990).