POC fluxes from euphotic zone estimated from ²³⁴Th deficiency in winter in the northwestern North Pacific Ocean

Dissolved and particulate ²³⁴Th, particulate organic carbon(PON), nitrogen, and chlorophyll-a concentrations in the upper 200 m of water columns from seven stations in the northwestern North Pacific Ocean were determined in the winter of 1997.The dissolved, particulate, and total ²³⁴Th activities (dissolved plus particulate)show a pronounced deficiency with respect to its parent ²³⁸U in the euphoric zone whereas the total ²³⁴Th shows a near-equilibrium at the depth greaterthan 100 m. The ²³⁴Th data are used to derive the mean residence time, export fluxes of ²³⁴Th, and fluxes of particulate organic carbon and nitrogen out of the euphotic zone. In the Subarctic Current area, the residence time of dissolved 234 Thwith respect to its removal onto particles is 40~50 d while the residence time of dissolved ²³⁴Th in the subtropical area is about 20 d due to the Kuroshio's influence.The particulate organic carbon (POC)and particulate organic nitroge(PON) fluxes in water range over 3.8~8.2 and 0.50~0.98 mmol/(m²·d),repectively,and are higher in the west region than in the east, and higher in the south than in the north. The co-influence of Kuroshio-Oyashio Currents withsupply of nutrient substances from the coastal area and the light condition are two factors determining the horizontaldistribution of POC fluxes in winter. The depth distribution of the chlorophyll-a as well as the consistence of the ratio ofPOC to PON with the Redfield ratio suggest that phytoplankton is the main contributor to the export of POC in this areain winter. The POC fluxes in the northwestern North Pacific Ocean in winter are comparable to some areas in the worldoceans in spring and summer seasons.