地球化学

Carbonate minerals are ones of the most ubiquitous authigenic minerals in the sedimentary rocks. Therefore, we studied these minerals in order to clarify the retardation effect of radioactive elements, Am and Cm due to the incorporation of rare earth elements into carbonates. The light rare earth element concentrations of carbonates in the sedimentary rocks in Tono uranium mine area, central Japan, are high compared to those of the bulk rock. Since light rare earth elements are chemical analogues to Am and Cm, it is likely that carbonates easily incorporate them. Therefore, it is expected that incorporation of Am and Cm into carbonates can retard long-term migration of these elements. Calculations of the partitioning of rare earth elements between carbonates and groundwater suggest that the ionic radius and charge influence significantly the incorporation of Am and Cm into carbonates under actual underground geological environment.

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Lead-210 dating method was applied to the sediment cores collected in Ago Bay, Mie prefecture, Japan where the seawater and bottom sediments have been contaminated due to the active pearl cultivation carried out for more than 100 years. A contamination with pearl cultivation was observed on the vertical distribution of the organic carbon (OC) and nitrogen (N), and the OC/N ratio suggesting marine plankton was a major source as the organic contaminants. A surface mixed layer (SML) was observed on the excess ²¹⁰Pb vertical distribution and the underlying part, below the SML, was divided into two layers with different sedimentation rates, suggesting the change of sedimentary condition in the late 1960s. The transition of the sedimentation rate is supposed to occur in connection with the change of pearl-production activity in Ago Bay. The surface mixed layer of ¹³⁷Cs has expanded to the deeper layer than that of the excess ²¹⁰Pb and the inventory of ¹³⁷Cs was significantly smaller compared to that of the excess ²¹⁰Pb. These facts suggest different behavior of ²¹⁰Pb and ¹³⁷Cs in the marine environment after they were transferred from the freshwater environment as soil-particle associated radionuclides.

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