Spatiotemporal variation and transport of ¹³⁷Cs in the Beibu Gulf

The temporal variation and transport of ¹³⁷Cs in the Beibu Gulf (BG) are still poorly understood. Here we measured ¹³⁷Cs concentrations in the BG water column and surface sediments during 2022. We found that ¹³⁷Cs in the BG water column was controlled by the movement and mixing of local water masses. In surface sediments, there was no correlation between the ¹³⁷Cs concentration and particle size, but there was a positive correlation with the total organic carbon. Historical data revealed that ¹³⁷Cs in the BG surface water has increased since 2015 and peaked from 2018 to 2019, which may be a consequence of the arrival of Fukushima-derived ¹³⁷Cs into the BG via the northern slope current in the South China Sea. However, more observational data are required in this regard. This result highlights the necessity of conducting long-term monitoring of ¹³⁷Cs in the Northwest Pacific marginal seas.