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

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 (Ra-226 and Ra-228) and anthropogenic (Cs-134 and Cs-137) radionuclide activity concentrations in the surface waters in this area from November 2021 to March 2022. The surface distributions of Ra-226 and Cs-137 concentrations were classified into Subantarctic Mode Water-, Antarctic Intermediate Water-, and Upper Circumpolar Deep Water (UCDW)-dominated areas along latitudinal band (40 degrees S-65 degrees S, 110 degrees E-120 degrees E). Notably, the highest Ra-226 concentrations occurred along the longitudinal band (60 degrees S-65 degrees S, 40 degrees E-120 degrees E). Significantly lower Cs-137 concentrations in the Southern Ocean than those in surface waters in other global oceans were observed along with depletion of Ra-228. Additionally, Ra-226 and Cs-137 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.