Seasonal export fluxes of particulate organic carbon from ~(234)Th/ ~(238)U disequilibrium measurements in the Ulleung Basin1 (Tsushima Basin) of the East Sea1 (Sea of Japan)

摘要

Export fluxes of particulate organic carbon (POC) were estimated from the ~(234)Th/ ~(238)U disequilibrium in the Ulleung Basin1 (UB) of the East/Japan Sea1 (EJS) over four seasons. The fluxes were calculated by multiplying the average POC/~(234)Th ratio of sinking particles larger than 0.7 μm at 100- and 200-m water depths to ~(234)Th fluxes by the integrated ~(234)Th/ ~(238)U disequilibrium from the surface to 100-m water depth. In spring, the ~(234)Th profiles changed dramatically with sampling time, and hence a non-steady-state ~(234)Th model was used to estimate the ~(234)Th fluxes. The ~(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 ~(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 ~(234)Th fluxes were moderate. However, these values might have been significantly underestimated, as the ~(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~(-2) day~(-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 UB1, 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.