Seasonal δ13C and δ15N variations of sediment-trap particles in the Japan Basin and Yamato Basin (the East/Japan Sea)

Study of sediment-trap deployments in the East/Japan Sea showed that distinct seasonal variations in particulate organic carbon (POC) fluxes of intermediate-water sediment-traps clearly corresponded to chlorophyll a concentration changes estimated from SeaWiFS data and that the regional difference in POC fluxes may be attributed to the different surface-water hydrographic conditions characterized by distribution of diatom biomass in terms of biogenic opal content. Thus, the prominent high POC flux periods (e.g., March) were strongly related to the enhanced surface-water phytoplankton bloom. Distinct seasonal δ¹³C and δ¹⁵N variation of settling particles of the intermediate-water sediment-traps strongly supports notion of seasonal primary production in surface waters recorded by POC fluxes. δ¹³C values tended to increase during productive seasons, indicating the direct link between [CO₂]_aq of surface water and δ¹³C in plankton organic matter. The low δ¹⁵N values were attributed to isotopic fractionation of diatom and most phytoplankton species during uptake, and subsequently the gradual increase of δ¹⁵N values seemed to be related to the degree of nutrient utilization in surface waters. Deep-water sediment-traps exhibited similar variation pattern to the intermediate-water sediment-traps. However, their total and POC fluxes were higher than those of intermediate-water sediment-traps during some months (e.g., April and May), thus, indicating that the lateral delivery of particles should be considered in the deep-water sediment traps. Nonetheless, settling particles collected by bottom-water sediment-traps is of the same nature through δ¹³C and δ¹⁵N values of settling particles as that of intermediate-water sediment-traps, being representative of present-day sediment accumulation across the basin, despite the lateral transportation in the bottom water.