ABSTRACT

Measurements of ²²⁸Ra, ²²⁶Ra and ²¹⁰Pb in the waters of Tokyo Bay and Osaka Bay mixing zones are presented. ²²⁸Ra concentrations and the activity ratios of ²²⁸Ra/²²⁶Ra are considerably higher in Osaka Bay than in Tokyo Bay. This probably reflects the different riverine/estuarine input of ²²⁸Ra from the two geologically different drainage zones of Tokyo Bay and Osaka Bay areas (volcanic versus granitic), because of the way in which ²²⁸Ra, a daughter of ²³²Th, is leached and transported by rivers. The Ra isotope distributions follow the trend of conservative mixing with open ocean water in which Ra isotope concentrations are low. Using a simple box-model, we estimate a value of 1.4±0.2 months for the mean residence time of Osaka Bay water and a value of 0.25 dpm cm^-2y^-1 for the ²²⁸Ra flux from Tokyo Bay sediments. In contrast to ²²⁸Ra, ²¹⁰Pb concentrations in Tokyo Bay and Osaka Bay waters are the same and nearly constant at 34 dpm/10³l, a value considerably lower than 200 dpm/10³l in the Kuroshio surface water. The non-conservative behavior of ²¹⁰Pb during mixing demonstrates that ²¹⁰Pb is rapidly removed near the front of Kuroshio Current far outside the bays, and hence there is no significant lateral input of ²¹⁰Pb from offshore to the two bays. The mean residence time of ²¹⁰Pb in seawater is also calculated to be 2 weeks for Tokyo Bay and 3 weeks for Osaka Bay.