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. 2023 Dec 27;13(1):22963.
doi: 10.1038/s41598-023-49083-4.

Temporal changes of 137Cs concentrations in the Far Eastern Seas: partitioning of 137Cs between overlying waters and sediments

Katsumi Hirose  1 , Pavel P Povinec  2
Affiliations

Temporal changes of 137Cs concentrations in the Far Eastern Seas: partitioning of 137Cs between overlying waters and sediments

Katsumi Hirose et al. Sci Rep. .

Abstract

Deep-ocean sediments, similarly to seawater, are important reservoirs of 137Cs, an anthropogenic radionuclide with a relatively long half-live found in the Earth system. To better understand the geochemical behaviour of 137Cs in the ocean, we examined the temporal changes of 137Cs activity concentrations in the overlying waters and in sediments from the Far Eastern Seas (Sea of Japan, SOJ, and Okhotsk Sea, OS) during the period of 1998-2021. The 137Cs activity levels showed exponential changes during the observed period. The decay-corrected change rates of 137Cs in deep waters of SOJ exhibited a slow increase, while 137Cs levels in seawater and sediment in OS decreased gradually. This reflects a topographical difference, as SOJ is a semi-closed sea, whereas OS receives continuously inflow of subarctic waters. It was confirmed that 137Cs released after the Fukushima Dai-ichi Nuclear Power Plant accident was rapidly transported into the deep waters of the SOJ. To elucidate the transfer processes of 137Cs from seawater to sediment, we discussed the temporal changes of the partition coefficients (Kd) of 137Cs between the overlying water and the surface sediment. In shallow areas (< 1500 m water depth), Kd values were almost constant within the sampling periods, although the temporal changes in the Kd values occurred in deeper waters (> 2500 m depth). The Kd values increased with increasing depth, which may reflect a pressure effect as a possible mechanism. These findings suggest that chemical processes may be important factors controlling the transport of 137Cs between seawater and sediment, although more complicated phenomena occurred in deep waters and sediments of the SOJ (> 3000 m depth).

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Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1
Figure 1
Schematic map and sampling points of the Sea of Japan and Okhotsk Sea.
Figure 2
Figure 2
Temporal changes of 137Cs activity concentrations in overlying waters of the SOJ and OS.
Figure 3
Figure 3
(A) Vertical profiles of 137Cs activity concentrations in overlying waters of the SOJ and OS in January 1998. Pre-Fukushima era (1998–2010): red closed circles, full data use: pale blue closed circles. (B) Apparent change rates of 137Cs in overlying waters of the SOJ and OS. Pre-Fukushima era (1998–2010): red closed circles, full data use: pale blue closed circles.
Figure 4
Figure 4
Temporal changes of 137Cs activity concentrations in sediments of the SOJ and OS.
Figure 5
Figure 5
(A) Vertical profiles of 137Cs activity concentrations in sediments of the SOJ and OS. Pre-Fukushima era (1998–2010): red closed circles, full data use: pale blue closed circles. (B) Apparent change rates of 137Cs in sediments of the SOJ and OS. Pre-Fukushima era (1998–2010): red closed circles, full data use: pale blue closed circles.
Figure 6
Figure 6
Relationship between apparent change rates of 137Cs in overlying waters and sediments. Red closed circles: Bottom Water (> 2800 m depth) in the SOJ, blue closed circles: shallow water and JSPW (100–2000 m depth).
Figure 7
Figure 7
Temporal changes of partition coefficients (Kd) of 137Cs between overlying waters and sediments of the SOJ and OS.
Figure 8
Figure 8
Vertical variability of change rates of partition coefficients of 137Cs between overlying waters and sediments of the SOJ and OS. Pre-Fukushima era (1998–2010): red closed circles, full data use: pale blue closed circles.
Figure 9
Figure 9
Depth dependency of partition coefficients of 137Cs between overlying waters and sediments of the SOJ and OS. Red open circle: western North Pacific (data from Nagaya and Nakamura).
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