129I and 137Cs as dual tracers: From Japan Sea to the Fram Strait
Mr Unai Abascal-Ruiz1, Dr José María López-Gutiérrez1,2, Dr Wen Yu4, Dr María Villa-Alfageme1,5
1Centro Nacional de Aceleradores (Universidad de Sevilla, CSIC, Junta de Andalucía) , , Spain, 2Dpto. Física Aplicada I, ETSIA, Universidad de Sevilla , , Spain, 3Dpto. Física Aplicada I, Escuela Politécnica Superior, Universidad de Sevilla, , Spain, 4Laboratory of Marine Isotopic Technology & Environmental Risk Assessment. Third Institute of Oceanography, Xiamen, , China, 5Dpto. Física Aplicada II, ETSI Edificación, Universidad de Sevilla , , Spain
During summer 2017 Chinese icebreaker RV Xuelong sailed from the Sea of Japan to Fukushima Dai-ichi nuclear power plant (FDNPP), Bering Strait and through the Artic Ocean to the Fram Strait and finally to the Nordic Seas. 129I (T1/2 = 15.7 × 106 years) and 137Cs (T1/2 = 30.17 years) were collected in 61 surface stations and 23 profiles in both North Pacific and Arctic Ocean. 129I was measured at the CNA in the University of Sevilla by AMS, and 137Cs results were presented in 1.
It is well known that most of the 129I discharged by the nuclear fuel reprocessing plants (NFRP) of Sellafield and La Hague travels through the North Sea, along the Norwegian coast into the Arctic. However, not many data are available on how this 129I is transported into the Pacific Ocean. In the Pacific, significant amounts of radioactive material, including 129I and 137Cs, have been released to the environment from Fukushima Dai-ichi nuclear power plant (FDNPP) after the tsunami occurred on March 11th, 2011.
Here we use 129I and 137Cs as dual tracers, together with the 129I/137Cs ratio. This study focuses on the Bering Strait, where the releases from FDNPP (Pacific) and European NFRP (Atlantic) converged. FDNPP releases were tracked through the Bering Strait into the Arctic Sea, while NFRP signal tracked the inflow of the Arctic in the Pacific Ocean.
In the Pacific, the concentrations of 129I in the vicinities of Fukushima were updated (9.68·106 – 4.83·107 at·kg-1) in 2017. 129I and 137Cs concentrations over the global fall out reference values were detected in the transect from the Sea of Japan to the Bering Strait. According to the end-member analysis, these values correspond mainly to FDNPP releases, but there is a small contribution of 129I from NFRP in the Bering Sea, associated to a small inflow of the Arctic into the Pacific Ocean. This is confirmed by the 129I/137Cs ratios of the stations inside the strait (2.07-41.07) and in Chukchi Sea and Beaufort Gyre (27.41-567.07).
As expected, 129I logically decreased from the Arctic to the North Pacific Oceans following the NFRP signal. The minimum 129I concentration in both oceans (5.96·105 at·kg-1) was found in the Bering Strait, at 1500 m. It corresponds to the lowest value associated to global fall-out and indicates the low influence of the NFRP in this Arctic region. A distinctive maximum of 137Cs (1 Bq·m-3) was found in the Chukchi and Beaufort Seas indicating the presence of FDNPP, emissions in the Artic Ocean. The 129I/137Cs ratio (2.07-41.07) in Bering Strait values lays among the FDNPP (3.88-26.44) and European NFRP (75.75-1330.1) signal. According to the water circulation time scale in the Pacific this 137Cs signal corresponded to the releases from Fukushima accident in 2011. The tracking of 137Cs and 129I/137Cs signals through the Arctic in the following years could be used to study the dynamics of the Pacific inflow into the Arctic.
1 Huang, D., Lin, J., Du, J. and Yu, T., Environ. Pollut., 256, 113386, 2020.
Unai Abascal graduated in Physics and Electronic Engineering from the UPV/EHU. Current PHD student at the University of Seville in the group of AMS at the Centro Nacional de Aceleradores (CNA, University of Seville, Spain). His research focuses on the analysis of 129I by AMS in environmental samples. Now, he is involved in a project for the study of the Arctic Ocean and its connections.