Distribution of (210)Po and export of organic carbon from the euphotic zone in the southwestern East Sea (Sea of Japan)

Abstract

The distribution of the natural radionuclide (210)Po in the water column along a horizontal transect of the continental shelf, slope and deep basin regions of the East Sea (Sea of Japan), a marginal sea of the Northwest Pacific Ocean, was investigated, and its behavior is described here. The settling fluxes of particulate (210)Po in the deep basin along with (210)Pb, (234)Th and biogenic matter were also determined. (210)Po inventories in the water column were observed to decrease from winter to summer in all stations, probably due to increased influx of (210)Po-poor Kuroshio Water of the Northwest Pacific Ocean during summer. Vertical profiles of dissolved and particulate (210)Po along with the settling fluxes of particulate (210)Po in the deep basin station have enabled us to evaluate temporal variations and residence times of (210)Po. In the slope and basin, activities of dissolved (210)Po generally decreased from the surface to the bottom water, with maximum activity just below the subsurface chlorophyll a maximum at 50-75 m depth in spring and summer. These subsurface peaks of dissolved (210)Po activity were attributed to the release of (210)Po from the decomposition of (210)Po-laden biogenic particulate organic matter. In the deep basin, despite the decrease in total mass flux, the sinking flux of particulate (210)Po was higher in the deeper trap (2000 m) than in the shallower one (1000 m), probably due to scavenging of dissolved (210)Po from the water column during particle descent and/or break-down of (210)Po-depleted particulate matter between 1,000 m and 2,000 m depths. In general, the ratios of the particulate phase to the dissolved phase of (210)Po (K(d)) increased with depth in the slope and basin stations. (210)Po removal from the water column appears to depend on the primary productivity in the upper waters. There is an inverse relationship between Kd and suspended particulate matter (SPM) concentration in the water column. From the (210)Po activity/chlorophyll a concentration ratios, it appears that sinking particles arriving at 1000 m depth were similar to those in the surface waters.