Transport paths of radiocesium and radium isotopes in the intermediate layer of the southwestern Sea of Okhotsk

Abstract

The spatial distributions of ¹³⁴Cs, ¹³⁷Cs, ²²⁶Ra, and ²²⁸Ra in/around the southwestern Sea of Okhotsk were examined in July 2019 and July 2021. Wide variations in the concentrations of these radionuclides were detected at the surface, including 0.2-0.7 mBq/L of ¹³⁴Cs (decay-corrected to the date of the Fukushima Dai-ichi Nuclear Power Plant accident), which indicated a large mixing ratio between the Soya Warm Current and East Sakhalin Current/Okhotsk Sea Surface Water. The Intermediate Cold Water at depths of approximately 30-300 m was subjected to the effects of ²²⁶Ra-rich and ²²⁸Ra-poor intermediate (or deeper) seawater. Moreover, the ¹³⁴Cs concentrations were maximum in 2021 (approximately 0.6 mBq/L), which most probably resulted from the increase in ¹³⁴Cs concentrations in the southward dense shelf water along the eastern Sakhalin Island along with the effect in the Okhotsk Sea Intermediate Water originating from the western subarctic water (e.g., the East Kamchatka Current) in the Pacific Ocean.