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We measured dissolved radiocesium ( 134 Cs and 137 Cs) in surface seawater collected in the western subarctic area of the North Pacific Ocean, Bering Sea, and Arctic Ocean in 2019 and 2020. The radiocesium released from the accident of the Fukushima Dai-ichi nuclear power plant (FNPP1) in 2011 was still observed in these areas (∼2 Bq m -3 decay-corrected to the date of the accident). In 2019/2020, the FNPP1-derived radiocesium concentrations in the Bering Sea and the Chukchi Sea, which is a marginal sea of the Arctic Ocean connecting the Bering Sea to the Arctic Ocean, were within the range of those observed in 2017/2018. On the other hand, the FNPP1-derived radiocesium was detected in the Arctic Ocean farther north of the Chukchi Sea in 2019/2020 for the first time. This was probably derived from the long-range transport of the FNPP1-derived radiocesium from the North Pacific coastal area of Japan to the Arctic Ocean through the Bering Sea during the past decade. The transport of the FNPP1-derived radiocesium from the Bering Sea to the western subarctic area in 2019/2020 is not clear, which implies the retainment of the FNPP1-derived radiocesium within the Bering Sea., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

The spatial distributions of 134 Cs, 137 Cs, 226 Ra, and 228 Ra in/around the southwestern Sea of Okhotsk were examined in July 2019 and July 2021. Wide variations in the concentrations of these radionuclides were detected at the surface, including 0.2-0.7 mBq/L of 134 Cs (decay-corrected to the date of the Fukushima Dai-ichi Nuclear Power Plant accident), which indicated a large mixing ratio between the Soya Warm Current and East Sakhalin Current/Okhotsk Sea Surface Water. The Intermediate Cold Water at depths of approximately 30-300 m was subjected to the effects of 226 Ra-rich and 228 Ra-poor intermediate (or deeper) seawater. Moreover, the 134 Cs concentrations were maximum in 2021 (approximately 0.6 mBq/L), which most probably resulted from the increase in 134 Cs concentrations in the southward dense shelf water along the eastern Sakhalin Island along with the effect in the Okhotsk Sea Intermediate Water originating from the western subarctic water (e.g., the East Kamchatka Current) in the Pacific Ocean., (Copyright © 2022 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Dinoflagellates in the family Symbiodiniaceae are intensively investigated as algal symbionts of corals and other invertebrates, underpinning coral reef ecosystems as primary producers. Diversity, including regional diversification, of free-living communities is less studied. In this study, an environmental Symbiodiniaceae community at an isolated island, Okinotori Island, Japan, was investigated to determine whether the community is endemic or common with other locations near continents and major ocean currents. Symbiotic algae in common corals at the island were the same type as those of the corals from other Japanese waters. In the environmental samples, genera Symbiodinium (formerly clade A), Cladocopium (clade C), Durusdinium (clade D), and clades F (including Freudenthalidium), G, and I, were identified through analysis of internal transcribed spacer region 2 of nuclear ribosomal RNA gene (ITS2) sequences. Interestingly, some sequences found were genetically different from those of previously reported genera/clades. These unknown sequences were genetically included in the Symbiodiniaceae linage, but they were differentiated from the previously known nine clades. The sequences formed a cluster in the phylogenetic tree based on 28S nrDNA. These sequences were thus considered members of a novel clade in the family (clade J). In total, 120 kinds of ITS2 sequences were produced; while 10 were identical to previously reported sequences, the majority were highly divergent. These genetically unique Symbiodiniaceae types, including novel clade J, may have evolved in isolation and reflect the environmental characteristics of the Okinotori Island., (Copyright © 2021 Elsevier Inc. All rights reserved.)

The spatial variations of low-level 134 Cs concentrations (activities) in seawater off the Japanese Archipelago, particularly in the eastern East China Sea (ECS), in 2018 and 2019 were examined. The 134 Cs concentrations, decay-corrected to the date of the Fukushima Dai-ichi Nuclear Power Plant accident, in seawaters were 0.5-2.0 mBq/L. High 134 Cs concentrations (1.1-2.0 mBq/L) of the Kuroshio Current subsurface water (densities of 25-26σ θ ) in the eastern ECS could indicated the contribution of the subtropical mode water from the Pacific Ocean side, and total column inventories were 330-426 Bq/m 2 . In contrast, as indicated by the same 134 Cs concentration level at the surface of the eastern ECS and Sea of Japan, larger portions of the subsurface waters remained in the ECS and Yellow Sea side in response to the existence of the shallow Tsushima Strait., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

To assess the delivery mechanism of radiocesium emitted from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), we examined vertical profiles of (134)Cs, (137)Cs, and (228)Ra concentrations and the (228)Ra/(226)Ra ratio in the water columns off the Sanriku Coast in the northwestern Pacific Ocean, in July 2012, along with their surface lateral variations in July 2009. Radiocesium concentrations exhibited maximum peaks (3-5 mBq/L for (134)Cs) at depths of 100-200 m, accompanied by high (228)Ra concentrations (0.6-0.8 mBq/L) in comparison with shallower depths (∼0.4 mBq/L). Taking the circulation patterns of currents in the area into account, it was inferred that radioactive depositions were supplied to the (228)Ra-rich Tsugaru Warm Current Water (TWCW) in the offshore area of the Sanriku Coast following the FDNPP accident, and that after the spring of 2011, this water (∼26.5σθ) was covered by lower density surface water, which helped intrude its way to depths of 100-200 m., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Ninety-six seawater samples were collected between May 2011 and March 2012 at 6 sites along the Shimokita Peninsula and the northern Sanriku coast, 250-450 km north of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP). Cesium-134 and (137)Cs concentrations were determined by low-background γ-spectrometry. During May-June 2011, (134)Cs and (137)Cs concentrations in surface waters decreased from 1.0-2.8 to 0.7-1.5 mBq/L and from 2.1-3.9 to 1.9-3.0 mBq/L, respectively. These decreases were due to diffusion and advection in the ocean after atmospheric input of the FDNPP-derived radionuclides. However, in July-August 2011, the concentrations of both radionuclides in the water samples collected on the Pacific side of the Shimokita Peninsula and the northern Sanriku coast exhibited 30-50-fold increases (∼40 mBq/L for (134)Cs and ∼50 mBq/L for (137)Cs) over concentrations observed at these sampling sites in June 2011 in contrast to the gradual decreases in the concentrations on the Tsugaru Strait side of the Shimokita Peninsula. These results suggest that radiocesium-contaminated waters offshore in the Pacific Ocean were transported to coastal regions along the Pacific side of the Shimokita Peninsula and the northern Sanriku coast by ocean currents., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

A total of 37 seawater samples were collected at 10 sites along the coastline of the Northern Sanriku and Tsugaru Strait, 250-450 km north of the Fukushima Dai-ichi Nuclear Power Plant in April-December 2009 and May-June 2011, and analyzed for (134)Cs and (137)Cs activities using low-background γ-spectrometry. The (134)Cs and (137)Cs activities measured in these samples in May 2011 were found to be 2-3 mBq/L and 2.5-4 mBq/L, respectively. By June, these values had decreased by 25-45%/month and 5-30%/month, respectively. These results can be plausibly explained by surface infusion of these isotopes into the sea by atmospheric transport from Fukushima and their subsequent reduction by water migration to off-shore and deeper regions., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

High-precision Mg isotope measurements by multiple collector inductively coupled plasma mass spectrometry were applied for determinations of magnesium isotopic fractionation of biogenic calcium carbonates from seawater with a rapid Mg purification technique. The mean δ(26)Mg values of scleractinian corals, giant clam, benthic foraminifera, and calcite deep-sea corals were -0.87‰, -2.57‰, -2.34‰, and -2.43‰, suggesting preferential precipitation of light Mg isotopes to produce carbonate skeleton in biomineralization. Mg isotope fractionation in deep-sea coral, which has high Mg calcite skeleton, showed a clear temperature (T) dependence from 2.5 °C to 19.5 °C: 1,000 × ln(α) = -2.63 (±0.076) + 0.0138 (±0.0051) × T(R(2) = 0.82, p < 0.01). The δ(26)Mg values of large benthic foraminifera, which are also composed of a high-Mg calcite skeleton, can be plotted on the same regression line as that for deep-sea coral. Since the precipitation rates of deep-sea coral and benthic foraminifera are several orders of magnitude different, the results suggest that kinetic isotope fractionation may not be a major controlling factor for high-Mg calcite. The Mg isotope fractionation factors and the slope of temperature dependence from deep-sea corals and benthic foraminifera are similar to that for an inorganically precipitated calcite speleothem. Taking into account element partitioning and the calcification rate of biogenic CaCO(3), the similarity among inorganic minerals, deep-sea corals, and benthic foraminiferas may indicate a strong mineralogical control on Mg isotope fractionation for high-Mg calcite. On the other hand, δ(26)Mg in hermatypic corals composed of aragonite has been comparable with previous data on biogenic aragonite of coral, sclerosponges, and scaphopad, regardless of species differences of samples.