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 (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. •We examined surface 134Cs, 137Cs, 226Ra, and 228Ra profiles in the Southern Ocean.•Surface 137Cs concentration was hardly influenced by nuclear test explosions.•226Ra concentration was higher than that in surface waters of other oceans globally.•Deep/old waters greatly contributed to surface water of the Southern Ocean.