Migration area of the Tsushima Warm Current Branches within the Sea of Japan: Implications from transport of 228Ra

Mashita

et al. 2023

Sci Rep

We studied the spatiotemporal variations in 134Cs, 137Cs, and 228Ra concentrations at the sea surface off southeastern Hokkaido, Japan (off-Doto region) from 2018 to 2022 using low-background γ-spectrometry. The 134Cs concentrations in the off-Doto region, decay-corrected to the date of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, exhibited wide lateral variation each year (e.g., 0.7–1.1 mBq/L in 2020). By studying the 228Ra concentrations and salinity, this variation was explained based on the current mixing patterns. Furthermore, the 134Cs concentrations in the waters highly affected by the Oyashio Current (OYC) gradually increased from 2018 to 2020, and subsequently decreased in 2022. This implies that the water mass maximally contaminated with 134Cs was transported back to the side of the Japanese islands 10 years after the FDNPP accident along with counter-clockwise currents (e.g., the OYC) in the northern North Pacific Ocean. The 134Cs concentrations in the OYC-affected waters in the off-Doto region in 2020 were ~ 1/6 times those in the 134Cs-enriched core of waters off the western American Coast in 2015, which can be ascribed to dilution via spatial dispersion during subarctic current circulation. Overall, we elucidated the ocean-scale subarctic current systems in the northwestern North Pacific Ocean, including water circulation timespans.

Section: Resultsmentioning

confidence: 99%

“…Additionally, the lateral distributions of the concentrations of 228 Ra—a natural and soluble radionuclide with a half-life of 5.75 years and a conventional tracer used for studying water currents 11 , 12 —have been employed to study the transport patterns of radiocesium in the seas around Japan 13 .…”

Section: Introductionmentioning

confidence: 99%

Subarctic-scale transport of 134Cs to ocean surface off northeastern Japan in 2020

Mashita

et al. 2023

Sci Rep

“…2 b). Owing to the short half-life of 228 Ra, high concentrations of this isotope can be attributed to the mixing of seawaters that have been in contact with the shallow continental shelf and coastal sediments, as observed in the East China Sea 32 , 33 . In this study, the high 228 Ra concentrations were predominantly ascribed to the continual supply of 228 Ra from the coasts of Southeast Asia.…”

Section: Resultsmentioning

confidence: 99%

Unique current connecting Southern and Indian Oceans identified from radium distributions

Hanaki

et al. 2022

Sci Rep

We examined the spatial variations in 226Ra and 228Ra (activities) concentrations from the surface to a depth of 830 m in the Indian and Southern Oceans from December 2019 to January 2020. 226Ra concentrations at the surface increased sharply from 30° S to 60° S along a ~ 55° E transect (1.4–2.9 mBq/L), exhibiting small vertical variations, while 228Ra decreased southward and became depleted in the Southern Ocean. These distributions indicated the ocean-scale northward lateral transport of 226Ra-rich and 228Ra-depleted currents originating from the Antarctic Circumpolar Current (ACC). 226Ra concentrations indicated that the fractions of the ACC at depths of 0–800 m decreased from 0.95 to 0.14 between 60° S and 30° S. The ACC fractions in the subantarctic western Indian Ocean were higher than those previously reported in the eastern Indian region, indicating preferential transport of the ACC. The fractions obtained were approximately equivalent to those in the western Indian Ocean in the 1970s. This could be attributed to the minimal southward shift of the polar front due to global warming over the last 50 years.

“…The 228 Ra concentrations at the surface sharply decreased toward the Southern Ocean (~0.1 mBq/L) via the subantarctic region. Owing to the short half-life of 228 Ra, high concentrations of this isotope can be attributed to the mixing of seawaters that have been in contact with the shallow continental shelf and coastal sediments, as observed in the East China Sea 30,31 . In this study, the high 228 Ra concentrations observed were predominantly ascribed to the continual supply of 228 Ra from the large and shallow continental shelves and coastal sediments.…”

Section: Spatial 228 Ra Pro Lesmentioning

confidence: 99%

Spatial Distributions of 226Ra and 228Ra in the Indian and Southern Oceans in 2020 and Their Implications for Unique Currents

Hanaki

et al. 2021

Preprint

We examined the spatial variations in 226Ra and 228Ra concentrations from the surface to a depth of 830 m in the Indian and Southern Oceans during December 2019–January 2020. Notably, 226Ra concentrations at the surface increased sharply from 30°S to 60°S along an ~55°E transect (1.4 to 2.9 mBq/L), exhibiting small vertical variations, while 228Ra became depleted, particularly in the Southern Ocean. These distributions indicated the ocean-scale northward lateral movements of 226Ra-rich and 228Ra-depleted currents originating from the Antarctic Circumpolar Current (ACC). Using 226Ra concentrations, the fractions of the ACC at depths of 0–800 m were estimated to decrease from 0.95 to 0.14 from 60°S to 30°S through 0.56 at 43°S. The fractions in the subantarctic area the western Indian Ocean were higher than those previously reported from the eastern, indicating the preferential transport of the ACC. The fractions obtained were approximately equivalent to those in the western Indian section in the 1970s. This could be attributed to the minimal effects of the southward shift of the polar front due to global warming over the last 40 y, implying no notable changes in soluble material transport systems from the Southern Ocean to southern Indian Ocean.