ABSTRACT

A sediment trap experiment and stable carbon and nitrogen isotope analysis of sinking organic particles have been performed in the northeast Japan Sea from August 1998 to October 1999 in order to confirm the relationship between seasonal variability of phytoplankton production and the isotope composition of sinking particles. The sediment trap deployed at 1090 m has successfully recovered samples recording organic particles sinking for a year round. Increases of POC and total mass fluxes suggested that blooming occur twice a year in late autumn and early summer in this region. It was shown that the seasonal variation of δ¹³C and δ¹⁵N of sinking particles could be explained by the general pattern controlling isotope ratios of phytoplankton so that appearance of isotope fractionation on photosynthesis and nitrate uptake might play a most important role to change δ¹³C and δ¹⁵N of phytoplankton in tightly linked with nutrient supply and production rate in the mixed layer. The δ¹³C and δ¹⁵N of the surface sediment were significantly heavier than those of sinking particles and showed a consistent trend of vertical isotopic change with the results reported in previous works. Compiled such vertical trend we addressed that carbon and nitrogen isotopic discrimination between sinking particles and the surface sediment might be qualified by the bottom depth due to isotope discrimination in sinking process but further modification can occur by regional factors such as import of allochthonous substances. Finally we stress about the implication of such factor to apply isotopic proxies to paleoceanographic research.