Water masses labeled with global fallout 137Cs formed by subduction in the North Pacific

We provide a first cross section of the 137Cs concentration along 165°E longitude in the western North Pacific. The 137Cs profile is characterized by several subsurface cores with high 137Cs, including two 137Cs concentration maxima at 20°N, one at 250 m (σθ ≈ 25.5) and one at 400–500 m (σθ ≈ 26.0) depths. The shallower maximum is in the density range of North Pacific Subtropical Mode Water (NPSTMW) and the deeper one is in the density range of Lighter Central Mode Water (LCMW). The main 137Cs cores, therefore, were formed by movements of NPSTMW and LCMW in the interior ocean during the past four decades. The 137Cs has been transported from subarctic region to subtropics and tropics as a result of subduction.