Long term variations of 137Cs and 90Sr distribution in the Sea of Japan

Anthropogenic radionuclides were released into the world's oceans as a result of large-scale weapons testing in the late 1950s and early 1960s. These anthropogenic radionuclides have long lifetimes and are still present in the oceans today. The behavior of long-lived radionuclides is useful for seawater circulation in the global ocean.

In this study, we focus on the spatiotemporal variations in the ¹³⁷Cs and ⁹⁰Sr activity concentrations in the surface seawater and vertical distributions. Most of data were included into the database, Historical Artificial radioactivity database in Marine environment, Global integrated version 2021 (Inomata and Aoyama, 2023). The ratio of ¹³⁷Cs to ⁹⁰Sr from large-scale nuclear tests was reported by Harley et al. (1965) to be 1.45. These ratios were in agreement with the average value of the ratio found in open ocean seawater (1.43±0.70) (Bowen et al., 1974). However, the ratios analysed in the present study did not always consistent with the reported values. This may be due to the different behaviour of ⁹⁰Sr and ¹³⁷Cs, e.g. ¹³⁷Cs is mainly present in the dissolved form, whereas ⁹⁰Sr is incorporated into particles in seawater. Furthermore, the ⁹⁰Sr/¹³⁷Cs ratio may be closely related to seawater circulation. However, as data on ⁹⁰Sr are very scarce compared to ¹³⁷Cs, detailed analysis of long-term variations in the ¹³⁷Cs/⁹⁰Sr ratio is limited to the North Pacific Ocean and its marginal sea.

The results showed that the ¹³⁷Cs/⁹⁰Sr ratio varied in the northern North Pacific and adjacent waters (northern North Pacific Ocean (NNPO), western North Pacific Ocean (WNPO), Sea of Japan (SOJ), Sea of Okhotsk (OKH) and East China Sea (ECS)). The apparent half-life (Tap) of ⁹⁰Sr in ECS were longer than those in other area. Considering that a longer Tap indicates a larger inflow into the area and a shorter Tap indicates a lower inflow, it can be assumed that the OKH has sources of ¹³⁷Cs and ⁹⁰Sr. At some SOJ stations, the ratio of ⁹⁰Sr/¹³⁷Cs was large at depths greater than 2000 m. Furthermore, inventories of ¹³⁷Cs and ⁹⁰Sr have increased since 1990 at the SOJ stations. The increase in inventories may be attributed to the dumping of radioactive material.