Seasonal export fluxes of particulate organic carbon from ^sup 234^Th/^sup 238^U disequilibrium measurements in the Ulleung Basin^sup 1^ (Tsushima Basin) of the East Sea^sup 1^ (Sea of Japan)

Export fluxes of particulate organic carbon (POC) were estimated from the ^sup 234^Th/^sup 238^U disequilibrium in the Ulleung Basin^sup 1^ (UB) of the East/Japan Sea^sup 1^ (EJS) over four seasons. The fluxes were calculated by multiplying the average POC/^sup 234^Th ratio of sinking particles larger than 0.7 μm at 100- and 200-m water depths to ^sup 234^Th fluxes by the integrated ^sup 234^Th/^sup 238^U disequilibrium from the surface to 100-m water depth. In spring, the ^sup 234^Th profiles changed dramatically with sampling time, and hence a non-steady-state ^sup 234^Th model was used to estimate the ^sup 234^Th fluxes. The ^sup 234^Th flux estimated from the non-steady-state model was an order of magnitude higher than that estimated from the steady-state model. The ^sup 234^Th fluxes estimated using the steady-state model showed distinct seasonal variation, with high values in summer and winter and low values in autumn. In spring, the phytoplankton biomass had the highest value, and primary production was higher than in summer and autumn, but the ^sup 234^Th fluxes were moderate. However, these values might have been significantly underestimated, as the ^sup 234^Th fluxes were estimated using the steady-state model. The POC export fluxes estimated in autumn were about four times lower than those in other seasons when they were rather similar. The annually averaged POC flux was estimated to be 161 ± 76 mgC m^sup -2^ day^sup -1^, which was somewhat lower than that in highly productive coastal areas, and higher than that in oligotrophic regions. The export/primary production (ThE) ratios ranged from 7.0 to 56.1%, with higher values in spring and summer and lower values in autumn and winter. In summer, a high ThE ratio of 48.4 ± 7.0% was measured. This may be attributed to the mass diatom sinking event following nitrate depletion. In the UB^sup 1^, the annually averaged ThE ratio was estimated to be 34.4 ± 12.9%, much higher than that in oligotrophic oceans. The high ThE ratio may have contributed to the high organic carbon accumulation in the UB^sup 1^.[PUBLICATION ABSTRACT]