Origin of surface water in the Southern Ocean: Implications of soluble radionuclide distributions

Abstract

Concentration data of soluble radionuclides in the southern South Indian Ocean and Southern Ocean transition zone are rare or insufficient for the study of its current system. We examined the lateral surface variations in soluble natural (²²⁶Ra and ²²⁸Ra) and anthropogenic (¹³⁴Cs and ¹³⁷Cs) radionuclide activity concentrations in the surface waters in this area from November 2021 to March 2022. The surface distributions of ²²⁶Ra and ¹³⁷Cs concentrations were classified into Subantarctic Mode Water-, Antarctic Intermediate Water-, and Upper Circumpolar Deep Water (UCDW)-dominated areas along latitudinal band (40°S-65°S, 110°E-120°E). Notably, the highest ²²⁶Ra concentrations occurred along the longitudinal band (60°S-65°S, 40°E-120°E). Significantly lower ¹³⁷Cs concentrations in the Southern Ocean than those in surface waters in other global oceans were observed along with depletion of ²²⁸Ra. Additionally, ²²⁶Ra and ¹³⁷Cs concentrations appeared to show small variations between eastern and western areas (2.5-3.0 mBq/L and 0.06-0.03 mBq/L, respectively). Lateral profiles in the Southern Ocean are governed by a large contribution from deep/old waters (e.g., UCDW), with a small effect from southward transport of Subantarctic Mode Water.