We present a comprehensive data set of organic and inorganic geochemistry from a lower Cretaceous pelagic bedded chert succession of the Shimanto accretionary belt in the Yokonami Peninsula (Kochi, Japan). Based on stable isotopic composition of total organic carbon (δ¹³C_org), in conjunction with radiolarian biostratigraphic data, we propose that a 1.3-m-thick interval within the examined section is correlative with Tethyan Selli Level (Apennines, Italy), a sedimentary expression of oceanic anoxic event (OAE) 1a. Specifically, the δ¹³C_org record illustrates a discernible negative shift and subsequent positive excursions upsection, a pattern that resembles the typical δ¹³C_org pattern across OAE 1a reported from various sites such as the Mediterranean Tethys and Pacific seamount flanks. Our δ¹³C_org record from the deep Pacific basin supports the idea that the δ¹³C variation across OAE 1a was induced by a significant perturbation of global carbon cycle. The slight increase in total organic carbon contents of sediment deposited during OAE 1a suggests slight or no expansion of oxygen-deficient water mass in the overlying water column. Rare earth elements and lead isotopic compositions indicate relatively higher contributions of volcanic or hydrothermally altered components before and after OAE 1a. The volcanic or hydrothermal source may be associated with emplacement of the Ontong Java Plateau during the early Aptian, or tectonically induced hydrothermal alteration associated with the formation of the accretionary complex.