Estimation of POC and PON fluxes from euphotic zone based on 234Th deficiency in winter in the northwestern North Pacific Ocean

The fluxes of carbon, nutrients, and associated elements involved in the biogeochemical cycles out of the euphotic zone are important in the study of global CO2 change. The Th-234 method, developed and elaborated in recent years, has been proved to work best in the euphotic zone for estimation of particle export, especially when using non-steady state models. Dissolved and particulate Th-234, particulate organic carbon (POC) and nitrogen (PON), and chlorophyll-a concentrations in the upper 200 in of water columns from seven stations in the northwestern North Pacific Ocean were determined in winter in 2003. The POC and PON fluxes in winter ranged. 3.8 - 8.2 mmol m(-2) d(-1) and 0.50 - 0.98 minol m(-2) d(-1), respectively, and were higher in the west region than in the east as well as higher in the south than in the north. The co-influence of Kuroshio-Oyashio Currents with supply of nutrient substances from the coastal area and the light condition were two factors determining the horizontal distribution of POC flux in winter. The depth distribution of the chlorophyll-a as well as the consistence of the POC to PON ratio with the Redfield ratio suggested that phytoplankton was the main contributor to the export of POC in this area in winter.