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Geochemical Journal
Geochemical Journal An open access journal for geochemistry
Published for geochemistry community from Geochemical Society of Japan.

Comparative behavior of 210Pb and Ra isotopes in the waters of Tokyo Bay and Osaka Bay mixing zones

Vimonrut Kasemsupaya, Mayumi Yashima, Hiroyuki Tsubota, Yoshiyuki Nozaki
Geochemical Journal, Vol. 23, No. 3, P. 129-138, 1989


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.

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