This dataset includes the carbon isotopic composition of dissolved inorganic carbon (δ13C-DIC) of sediment cores that were retrieved from Site C0023 (Hole A) during International Ocean Discovery Program (IODP) Expedition 370 in the Nankai Trough offshore Japan in the Pacific Ocean (Drilling vessel Chikyu). Site C0023 was established on 17 September 2016. Coring terminated on 3 November 2016.

The carbon isotopic composition of DIC (δ13C-DIC) was determined via isotopic ratio monitoring-gas chromatography mass spectrometry (irm-GC-C-MS; Thermo Finnigan Delta Plus XP isotope-ratio mass spectrometer (IRMS) connected to TRACE GC Ultra gas chromatograph) according to Ijiri et al. (2012) at the Kochi Institute for Core Sample Research, JAMSTEC, Japan. The δ13C-DIC values are expressed relative to VPDB (Vienna Pee Dee Belemnite). The standard deviation from repeated carbon isotope analysis of the laboratory standard (NaHCO3 solution) was <0.2‰.