Organic carbon and nitrogen contents and their isotopic compositions in surficial sediments from the East China Sea shelf and the southern Okinawa Trough

The southern Okinawa Trough has been noted as an important depocenter for the particulate organic matter from the East China Sea shelf. In order to determine the major sources of sedimentary organic matter in the southern Okinawa Trough, we analyzed surficial sediments from the East China Sea shelf and the southern Okinawa Trough for organic carbon and nitrogen contents and their isotopic compositions in the decarbonated fraction. Distributions of total organic carbon content, δ13Corg and nitrogen content all show a similar spatial pattern, resembling the distribution of fine-grained sediments. The coastal belt of elevated organic carbon content extends southward from the Changjiang mouth and veers offshore towards the southern Okinawa Trough just north of Taiwan, implying a pathway for channeling fine-grained sediments from the inner shelf to the depocenter. This distribution pattern is consistent with the shelf circulation. The isotope compositions (δ13Corg, δ15N) of sediments from the southern Okinawa Trough fall between those of riverine particulate organic matter and the mid-outer shelf sediments, but overlap with those of the inner shelf sediments. In addition, the previously reported δ13Corg values of sinking particles collected by sediment traps in the southern Okinawa Trough are also close to those of the inner shelf sediments, especially during high-flux conditions. Therefore, the isotopic evidence strongly supports the notion that a major fraction of the sedimentary organic matter in the southern Okinawa Trough may originate from the inner shelf.