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Preparation of samples for elemental and isotopic analysis

HCl, HF, and methanol were obtained from Wako Pure Chemical Industries. Procedures for preparing samples are shown in Figure F1. Sediment samples were dried at 45°C. Basalt samples were trimmed by at least 5.0 mm to prevent surface contamination. The trimmed, inner basalt samples were ground into powder. Carbonates and sediment breccias were prepared for analysis in the same way as the basalts. To remove carbonates, 10 g of powdered sample was washed with 6 M HCl at least 20 times, until the yellow iron contamination in the supernatant solution was clear. A group of these washed samples was then dried at around 70°C. Recovered acid-insoluble matter was washed with deionized water (“milli-Q” = 18.2 MΩ) until the pH of the supernatant solution was neutral. Finally, the neutralized insoluble matter was washed with methanol and dried at around 70°C. Sediment breccias and carbonates were collected between basaltic layers at 163.04 and 163.70 meters below seafloor (mbsf) in Hole U1382A and from 117.15, 145.93, and 154.04 mbsf in Hole U1383C. These samples were prepared the same way as the basalt samples. To extract kerogen, splits of the 6 M HCl-washed basalt samples were further washed with 1 M HCl/9 M HF. HCl/HF demineralization treatment is a conventional technique to extract kerogen (Durand and Nicaise, 1980). The acid-insoluble residue was rinsed with milli-Q water and methanol and then dried at around 70°C.

All samples were analyzed twice by elemental analyzer (Flash EA 1112; Thermo Fisher Scientific K.K.)/isotope ratio mass spectrometer (Delta Plus Advantage; Thermo Fisher Scientific K.K.) (EA/IRMS) at Kochi Core Center, Japan. We analyzed about 10 mg of sediment samples and 50 mg of basalt samples to measure carbon content and δ13C of total carbon (TC). Additionally, we analyzed about 50–100 mg of the organic (6 M HCl treated) and kerogen (1 M HCl/9 M HF treated) basalt samples to measure carbon contents and δ13C. All samples were analyzed in Sn capsules. Elemental and isotopic compositions were calibrated relative to sulfanilamide and L-alanine, respectively. We also analyzed only Sn cups for blank analysis. Carbon was not detected in only Sn cups. The analytical error for determination of carbon contents was within 0.005 wt%. The precision of δ13C was better than 0.1‰.