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

Abstract

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.