13C enriched limestones from the Tono district, Kitakami Mountains, northern Japan

Measurements were made of the carbon and oxygen isotope ratios of 39 limestones from the Tono district in the Kitakami Mountains, northern Japan. The limestones are assigned to either the Carboniferous or the Lower Permian. The δ13C values of the limestones range from -3.2 to 10.3‰ relative to the PDB standard. Eight samples are found to have δ13C values higher than 5‰, which are quite unusual for the limestones of marine origin. The δ18O values range from 17.4 to 30.3‰ relative to the SMOW standard and they are not different from those of Paleozoic marine limestones. The limestones have been subjected to thermal metamorphism by the Cretaceous granite (Tono mass). But the carbon isotopic anomaly of the limestones cannot be ascribed to the effects of the metamorphism. Although diagenetic reactions producing methane at the expense of organic matter may form 13C enriched carbonates in pelitic rocks, it seems unlikely that the process played an important role in altering whole-rock δ13C values of the limestones. Accordingly, it is concluded that the enrichment of 13C in the limestons formed at the time of deposition. This indicates the existence of an abnormal environment of deposition during the Carboniferous and the Permian period in the South Kitakami terrane.