Fluxes of 234Th, 210Po and 210Pb determined by sediment trap experiments in pelagic oceans

Sediment trap experiments were carried out in two oceans, the eastern Pacific Ocean and the Antarctic Ocean, which have very different biological productivities. The natural radionuclides, 234Th, 210Po and 210Pb were used as tracers of reactive metals. Larger particulate fluxes of these radionuclides were found in the seas where total mass fluxes were larger, although the concentrations of these radionuclides in the settling particles were somewhat smaller. The concentrations of 234Th in the settling particles varied widely and irregularly with depth, whereas the concentrations of 210Po and 210Pb in the settling particles steadily increased with increasing water depth. The ratios of 210Po/210Pb in the settling particulates were larger than unity which the ratio of 234Th/excess 210Po as larger than 234Th/210Po in the deep water. These results suggest that, when the particles sink through the water column, these radionuclides are being absorbed by settling particles in the order 234Th > 210Po > 210Pb. The observed particulate fluxes of 210Pb are about one eighth of those calculated from the disequilibria between 226Ra and 210Pb at the stations in the subtropical eastern Pacific, although the observed fluxes are the same as the calculated ones in the northern North Pacific and the Antarctic Ocean. Thus, there must be a horizontal flow carrying these reactive metals from the oligotorophic ocean to the biologically productive ocean where the metals are removed by settling particles even in deep water. (author)