Thorium and protactinum isotope distribution in waters of the Japan Sea

Abstract

New data on the distribution of Th and Pa isotopes in the Japan Sea are presented based on analysis of large volume seawater samples. The isotopes 230Th and 231Pa show no systematic change in concentration with depth. Their box model residence times, based on mean concentration, are 10 years for Th and 59 years for Pa. In deep waters, the 230Th concentration becomes nearly constant which may result from rapid mixing of the water column and enhanced removal of the isotope at the sediment-water interface. The activity ratios of 228Th/ 228Ra in deep waters are often higher than 1.0, indicating that particulate 228Th scavenged from surface waters and transported downward by settling is a significant source for 228Th in the deep water. In shallow waters, there is always excess 230Th relative to the simple box-model prediction based on in situ production by uranium decay and removal by particulate scavenging. This excess 230Th is probably attributed to the terrestrial source supply to the surface water, of which the exact mechanism needs to be investigated further. The activity ratio of 230Th/ 231Pa is approximately 2 in the water column which agrees, perhaps fortuitously, with a single reported value for the seafloor surface sediment.