Ocean Drilling Program, United States

Several carbonaceous layers or fragments were recovered from sediments of Sites 1150 and 1151 on the deep-sea terrace of the Japan Trench during Leg 186. The X-ray diffraction analysis (XRD) data indicate that these are predominantly dolomitic. In this study, carbon and oxygen isotopes of these carbonates recovered at Sites 1150 and 1151 are presented. The oxygen isotope ratios of the dolomites analyzed range from +0.4 to +4.1 vs. Peedee formation belemnite (PDB) and those of calcites from +0.6 to +2.8 PDB. The isotopic composition of carbon varies from -7.0 to +12.3 PDB in dolomite and from -13.4 to -24.1 PDB in calcite. The wide range of carbon isotopic compositions indicates that the carbonate samples were formed by the decomposition of organic matter through reactions such as oxidation, sulfate reduction, and methane formation during diagenesis.