Sinking fluxes of particulate U-Th radionuclides in the east sea (Sea of Japan)

Abstract

A record of radionuclide fluxes at a deep marginal sea of the Northwest Pacific Ocean (39 degrees 40' N 132 degrees 24' E, Japan Basin, East Sea/Sea of Japan) was obtained from analysis of a 1-year continuous collection of sediment-trap samples. The trap was placed at a depth of 2800 m, 500 m above the sea floor, and the samples were recovered at the end of one year. Concentrations of (238)U, (234)U, (232)Th, (230)Th and (228)Th were measured in the trapped material. All of the radionuclide fluxes showed seasonal variations that were in phase with the variations in total particle flux, which were shown in earlier work to be closely tied to the primary production in the surface water. The formation of authigenic (238)U appears to be less than in other open ocean regimes. The residence time of particulate thorium isotopes were of the order of a year, resulting in an average settling rate of a meter per day. Comparison of the measured (230)Th(ex) flux with that of the theoretical production in the overlying water column yielded about 40% surplus of (230)Th, indicating that lateral advection contributes (230)Th to this sampling site.