Seasonal variations in settling fluxes of major components in the oligotrophic Shikoku Basin, the western North Pacific: coincidence of high biogenic flux with Asian dust supply in spring

Time-series sediment traps were deployed at ca. 500 m below the sea surface (upper) and ca. 500 m above the sea bottom (lower) at station SHIBT (29°30′N, 135°15′E, ca. 4600 m water depth) near the center of oligotrophic subtropical gyre of the Shikoku Basin in the western North Pacific from February to December 1999 and from June to September 2000 to collect settling particle samples with high time resolution (2.5 days in spring 1999). Major components of settling particles, i.e., CaCO3, Fe–Mn oxyhydroxide (ERFe-Mn), organic matter (OM), opal and lithogenic aluminosilicate (clay), were measured by stepwise leaching. Strontium isotope ratio and grain size of clay fraction were also determined. Average total mass fluxes were 18.9 and 57.4 mg m−2 day−1 in the upper and lower traps from February to early December 1999, respectively, and 3.8 and 58.0 mg m−2 day−1 in the upper and lower traps from June to September 2000, respectively. Total biogenic flux (CaCO3+OM+opal) in the upper trap accounted for 93% of total mass flux in 1999. CaCO3 was the dominant component in the upper and lower traps. Seasonal variations in settling fluxes of major components were observed for almost 1 year. High biogenic fluxes with four peaks were observed in spring, i.e., four times of spring blooms occurred in the euphotic zone from February to May 1999. Estimated export flux of particulate organic carbon from the euphotic zone was 6.2 mg C m−2 day−1 on average in 1999, and 9.0 mg C m−2 day−1 on average during the high biological production period in spring. 87Sr/86Sr ratio of clay fraction fell within the range of 0.7163–0.7188 in spring, which was consistent with that of Asian loess and dust. This indicates that the clay fraction was mainly composed of Asian dust via eolian transport in spring 1999. The high biogenic flux accompanying Asian dust coincided with Asian dust supply with time lag in early spring 1999. The coincidence of high biogenic flux with Asian dust supply suggests the possibility that high biological production in the oligotrophic subtropical Shikoku Basin might be affected by the Asian dust supply in spring.