Turbidity currents on the deep seafloor are caused not only by slope failure during submarine earthquakes, but also by floods and storms. Therefore, understanding the origin and depositional processes of turbidites is important in paleoseismic studies based on deep-sea turbidites. We investigated the origin and depositional processes of Holocene deep-sea turbidites in the Nankai Trough by analysis of the sedimentary organic matter they contain. We analyzed the organic content of sediments in three forearc basin environments on the landward slope of the trough: in a deep-sea submarine fan fed by sediments from a submarine canyon, at the mouth of that submarine canyon, and in a deep-sea basin without an associated submarine canyon. To determine the origin of these sediments, we compared the composition of the organic matter contained in them with the organic content of nearby fluvial, deltaic, bay, and submarine slope deposits. Turbidite mud (Et) and hemipelagite (Eh) deposited in a submarine fan during the lowstand to early transgression stage are rich in terrigenous organic matter similar to that of bay sediments, suggesting that the fan consists mainly of terrigenous sediments that were initially deposited near the shore and later transported to the deep seafloor. The composition of organic matter in canyon mouth sediments deposited during the highstand stage is different from that of sediments deposited in the submarine fan during lowstand to early transgression stages, indicating a reduced extent of the depositional area on the deep seafloor during the highstand stage and suggesting that the origin of the lowstand to early transgressive sediments was different from that of the highstand sediments. Sediments deposited in three forearc environments were overlain by hemipelagite characterized by low proportions of terrigenous organic matter during highstand stages, including the present day. These hemipelagites have been transported seaward by slope failures associated with submarine earthquakes and redeposited on the deep basin floor. The composition of organic matter in Et is similar to that of Eh in each of the three forearc environments, indicating that Et and Eh had the same origin despite their different transport processes. Our study shows that the composition of organic matter in sediments provides a useful tool for determining the origin and depositional processes of deep-sea sediments including turbidites. (C) 2012 Elsevier B.V. All rights reserved.