Dissolved organic carbon (DOC) in seawater constitutes one of Earth’s largest organic carbon reservoirs (~662 Pg C) and plays a central role in long-term carbon sequestration and climate regulation. Recent studies suggest that DOC from marginal seas of the western North Pacific may serve as a carrier of trace nutrients, thereby influencing primary production in the open ocean. However, the source and transport pathways of this allochthonous DOC remain poorly constrained. In this study, we investigated the dynamics of DOC in the western North Pacific Ocean using radiocarbon (¹⁴C) as a tracer to constrain DOC sources and mixing relationships. Stable carbon isotope (¹³C) is additionally analyzed to provide complementary information on DOC origin. Seawater samples were collected during two cruises of Hakuho Maru in 2022 and 2023. DOC was extracted using a UV oxidation-based processing flow, and its radiocarbon signatures (Δ¹⁴C) were measured with a Single Stage Accelerator Mass Spectrometer. The Δ¹⁴C results revealed three DOC endmembers: refractory DOC as a background component, labile to semi-labile DOC derived from recent primary production in the surface, and an allochthonous DOC endmember characterized by a distinct Δ¹⁴C signature relative to ambient water masses. This allochthonous DOC signal is primarily confined to the density layer of σ_θ=26.6~27.5, consistent with transport along intermediate water pathways. By integrating multiple hydrographic and biogeochemical indicators together with radium isotopes, this allochthonous DOC is inferred to originate from the Bering Sea and its continental shelf, and is transported to the open ocean on decadal timescales. Our DOC partitioning further reveals the spatial distribution of DOC degradation and the short timescales of surface DOC cycling. This study presents the first extensive Δ¹⁴C constraints on DOC in the western North Pacific, providing new insights into carbon cycling processes and the connectivity between marginal seas and the open ocean.
 

Dissolved organic carbon,Radiocarbon,western North Pacific