Behavior of ⁷Be in Funka Bay, Japan, with reference to those of insoluble nuclides, ²³⁴Th, ²¹⁰Po and ²¹⁰Pb

Abstract

⁷Be in seawater and sediment trap samples in Funka Bay, Japan was determined together with ²¹⁰Pb, ²¹⁰Po and Th isotopes. The sampling was carried out in summer in 1977 and 1979, when the Bay constituted a closed system for mixing with water outside the Bay. The concentration of ⁷Be of atmospheric origin in seawater did not monotonously decrease with depth, but showed a weak maximum in the intermediate layer. The layer also contained larger amounts of ²³⁴Th, ²¹⁰Po and ²¹⁰Pb in summer, 1979. The results obtained from sediment trap samples suggest that significant portions of these nuclides and some other heavy metals are transported downward by particulate matter. The residence time of Be in Funka Bay (mean depth 60m) was calculated to be 90–160 days from the activity ratio of ⁷Be/²³⁴Th of the trapped material and the removal rate of ²³⁴Th from seawater obtained from the disequilibrium with ²³⁸U. These facts also support the TSUNOGAI and MINAGAWA's settling model for the removal of isoluble metals from seawater rather than the irreversible removal from seawater and the rapid transport of these metals in particles.