Seasonality in the flux of natural radionuclides and plutonium in the deep Sargasso Sea

Abstract

A record of radionuclide fluxes at a deep-ocean station near Bermuda (32°05′N, 64°15′W) was obtained from analysis of a 3-year collection of sediment-trap samples. The trap was placed at a depth of 3200 m, 1000 m above the sea floor, and the samples were recovered at 2-month intervals. Concentrations of 238 U, 234 U, 232 Th, 230 Th, 228 Th, 231 Pa, 210 Po, and 239, 240 Pu were measured in the trapped material. Most of the radionuclide activity was found in the <37- gmg m sieved fraction. All of the radionuclide fluxes showed seasonal variations that were in phase with the variations in total sediment flux, which were shown in earlier work to be closely tied to the annual cycle of primary production in the overlying surface water. The seasonal variations are especially noteworthy for 230 Th and 231 Pa, considering that most of their production occurs in the water column below the euphotic zone. Evidently the seasonal influence is transmitted downward by the varying particle flux so that radionuclide scavenging rates at depth, as well as at the surface, are affected. It is suggested that this could be brought about by seasonal variations in the flux of marine snow or in the rate of fecal-matter production in the deep-water column. Fluxes of 230 Th and 231 Pa integrated over the annual cycle yielded a trapping efficiency of 105 ± 17% for the PARFLUX sediment trap used in this investigation.