Particulate organic carbon fluxes estimated from ²³⁴Th deficiency in winters and springs in the northwestern North Pacific

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Activities of particulate and dissolved ²³⁴Th and concentrations of particulate organic carbon (POC) were measured in the northwestern North Pacific in winters and springs. The export fluxes of POC from surface waters were estimated by using ²³⁴Th as a tracer. The POC fluxes in winter showed a wide variation (20–190 mg-C m^-2d^-1) and were higher in the western region than in the eastern region. The influence of the Oyashio Current and the supply of substances from the continent to the study area also appeared to be higher in the western region. Therefore, the horizontal distribution of POC fluxes in winter was influenced by continental materials. In the spring bloom, the fluxes of POC were much higher (up to 520 mg-C m^-2d^-1), as were the e-ratios (up to 70%). In the northwestern North Pacific, the efficiencies of export of POC from the euphotic zone were higher than in other areas of the world ocean. The increase in POC fluxes in spring was considered to depend not only on the increased abundance of phytoplankton, but also on the transition of phytoplankton species, as evidenced by the relationship between ²³⁴Th adsorption rates and the concentrations of POC.