Speaker
Description
In the marine environment, Cs-137 in the water column has been used as an effective tracer for tracking origin of water mass and circulation. To evaluate the spatiotemporal (biannual) variations of Cs-137, we investigated the vertical distributions of 137Cs and hydrography along the latitudinal (13.5° N) transect of the North Equatorial Current (NEC) region (of the Tropical Pacific) in 2024-2025, on board the Research Vessel ISABU of the Korea Institute of Ocean Science and Technology (KIOST), Cs-137 activity from 2024 to 2025 ranged from 0.43±0.08-2.95±0.49 mBq/kg in the upper water column (<500 m) [mean 1.51±0.25 mBq/kg (n=126)]. The vertical distribution of Cs-137 was higher in the high-salinity subsurface (200-300 m) than in the surface water. The observed subsurface Cs-137 maximum likely reflects isopycnal transport of recently introduced mid-latitude (30-40°N) North Pacific surface waters with elevated Cs-137 levels from the Subtropical Gyre into the North Pacific Tropical Water (NPTW). Such distribution through intrusion and mixing processes may have contributed to the development of the subsurface maximum (200-300 m depths), which is consistent with previously reported Pacific-wide Cs-137 distribution patterns. While the peak activity of Cs-137 was observed at depths of 100-200 m in 2024, it was at 200-300 m in April and November 2025. In 2024, the North Pacific experienced a transition from El Niño to La Niña. Therefore, it is thought that the overall deepening of NPTW together with the thermocline is due to the accumulation of Tropical Surface Water (TSW) driven by the strengthening of the northeast trade winds. These findings suggest that Cs-137 in the water column can be a useful tracer of the behavior, circulation, and redistribution of water mass fluctuations due to climate variability and other anthropogenic radioactive contaminants in the North Pacific.