The vertical transport of sinking particles from the surface to the interior of the ocean is an important factor that controls the transport of materials on relatively short time scales, ranging from a few days to several tens of days. The authors have been studying the flux of sinking particles (mass of particles that settle by gravity per unit time and area) in the Japan Sea over 20 years, but there are few observations in the Tsushima Warm Current (TWC) regions. In this study, we summarize the seasonal variations of particle fluxes based on the results of mooring observations conducted near the flow axis in the TWC for about one year and discuss the supply and transport process of terrestrial suspended particles to the interior of the basin.

A sediment trap was installed at 890 m at Sta. FATO (38 °43’ N, 137° 48’ E). The trap was set to collect sinking particles for 13 periods of 26 days between June 22, 2022 and May 26, 2023. The collection bottles were filled with 5% formalin prepared with a salinity of 38‰ to prevent the decomposition of the collected sample.
After removing zooplantons from the collection jars for each period, the sinking particles were collected on a membrane filter with a pore size of 0.6 µm, and the dry weight was measured to obtain the total mass flux. The dried, crushed, and homogenized particles were first measured by a germanium semiconductor detector by γ-ray measurement, and the activity concentrations of lead-210 (²¹⁰Pb) and lead-214 (²¹⁴Pb) were measured to estimate excess lead-210 (²¹⁰Pb_ex). A part of the dried sample was heated at 450°C for more than 24 hours, and the loss of mass was determined as the concentration of organic matter. In addition, some of the dried samples were heated and decomposed in a sealed container after acid was added, silicon (Si), aluminum (Al), calcium (Ca), manganese (Mn), and lanthanum (La) and ytterbium (Yb) were measured. The aluminosilicate concentration was calculated from the Al concentration in the particles, the biogenic silicate (opal) concentration was calculated from the Al and Si concentrations, and the biogenic carbonate concentration was calculated from the Ca and Al concentrations.

The total mass flux measured in this study ranged from 198 to 564 mg m^-2 d^-1, and two maximums were observed in the winter (January to February) and spring (March to April). About 60% of the sinking particles in winter are composed of aluminosilicates. The flux of aluminosilicates that increased significantly from around December was a factor in the high total mass flux in winter. Biogenic silicates and organic matter particles dominate in spring, and the high opal flux and the corresponding increase in organic matter particle flux are presumed to be the factors for the increase in the total mass flux during this period. The range of variation and seasonal variation patterns of these mass fluxes were consistent with observations made at approximately the same depth in the western and northern Yamato Basin from 1999 to 2001 (Otosaka et al., Mar. Chem. 91, 143, 2004).
The particle flux at ²¹⁰Pb_ex tended to be higher in winter and was consistent with the variation pattern of ²¹⁰Pb deposition from the atmosphere in the region. The ²¹⁰Pb_ex flux in the particles showed a positive correlation with the Al flux over many periods, indicating that ²¹⁰Pb, along with mineral components, was transported vertically from the ocean surface to the interior of the ocean basin. However, only in winter, the Al (aluminosilicates) concentration tended to be excessive for ²¹⁰Pb.
In order to discuss the source of aluminosilicates, the La/Yb ratio used as an index to roughly classify the sources of terrestrial particles in the Japan Sea was obtained. The La/Yb ratio (8.8~10.9) among the particles obtained at 890 m at the observation point FATO was significantly lower than that of the particles obtained at the same depth in the western and northern regions of the Yamato Basin (12.8±0.9), and was close to the values in the eastern part of the Japan Basin (10.1±1.4). From these results, it was found that the aluminosilicates moving to Sta. FATO strongly reflect the characteristics of particles from the Japanese main island.
In addition, the sinking particles at the Sta. FATO had a higher Mn/Al ratio than other areas in the Japan Sea, and this trend was especially pronounced during the low particle flux period from summer to autumn. Considering that the high Mn/Al ratio is generally a characteristic of particles with long residence times in the water column, it was suggested that during the low particle flux phase, particles with high retention (relatively slow sinking velocity) were transported, while particles in the surface water were transported relatively quickly into the ocean during the winter and spring. The Mn/Al ratio in sinking particles observed at the Sta. FATO showed clear seasonal variations compared to previous observations in the Japan Sea. This is thought to be due to the sensitive response of the particle flux at Sta. FATO, a station closer to the edge of the ocean basin, to material supply from the shelf area and changes in the physical structure in the surface layers.
 

Japan Sea,Tsushima Warm Current,biogeochemical cycle,Marine sinking particles