Unique transport paths of ¹³⁷Cs from the Indian to Southern Oceans

To assess ocean-scale transport systems, we examined the latitudinal cross-sectional distribution of ¹³⁷Cs activity concentrations in the Indian and Southern Oceans between December 2019 and January 2020 using low-background γ-spectrometry. At 0°-20°S, ¹³⁷Cs concentrations exhibited a gradual decrease below the mixing layer (1-0.1 mBq/L). However, the concentrations steeply decreased toward the Southern Ocean along a transect of 30°-60°S (from 0.8 to 0.02 mBq/L) with minor vertical variation at each site. For the ¹³⁷Cs inventories (0-800 m depth) from 15 to 600 Bq/m², a maximum value was recorded at 30°S, indicating the downwelling of ¹³⁷Cs as a reservoir for the Subantarctic Mode Water. The significantly low concentrations (0.02 mBq/L) at 60°S suggest minimal transport of ¹³⁷Cs to the Southern Ocean. These findings assist in understanding ¹³⁷Cs circulation patterns and provide valuable insights into the transport pathways of soluble contaminants.