Tomoko Ohta

Blake Nose stable isotopic evidence against the mid-Cenomanian glaciation hypothesis

Detailed multitaxon stable isotope (δ 18 O and δ 13 C) data from Blake Nose (western North Atlantic) argue against a mid-Cenomanian glaciation event during the mid-Cretaceous greenhouse. Results generated are precisely correlated to sea-level changes inferred from European sequence stratigraphy using the twin δ 13 C excursions mid-Cenomanian event (MCE) Ia and MCE Ib. Microfossils analyzed (surface-dwelling to deep-dwelling planktonic foraminifera, benthic foraminifera, coccoliths) show remarkably consistent intertaxon δ 18 O and δ 13 C offsets; comparative scanning electron microscope and Sr/Ca analyses allow some δ 18 O data to be eliminated because of selective diagenesis. Across MCE Ia, the proposed interval of major glacioeustatic regression, the planktonic δ 18 O values are constant for each taxon. The absence of a mean seawater δ 18 O shift contradicts predictions for the mid-Cenomanian glaciation episode. The benthic δ 18 O records show significant fluctuations during MCE I, implying short-term variability in North Atlantic intermediate-water and deep-water circulation patterns and/or sources at that time.

Analysis of cesium isotope compositions in environmental samples by thermal ionization mass spectrometry – 1. A preliminary study for source analysis of radioactive contamination in Fukushima prefecture

Cesium was recovered from plant samples obtained from Fukushima prefecture. The isotopic ratios of 134Cs/137Cs and 135Cs/137Cs were analyzed by thermal ionization mass spectrometry with a single filament method using a TaO activator. Samples containing 5 Bq of 137Cs were analyzed with typical analytical errors of approximately 0.5% for 134Cs/137Cs and approximately 0.1% for 135Cs/137Cs. Measurements of both ratios showed profiles that were characteristic of the measurements of among other environmental samples reported in the literature. The results showed the isotopic ratios of 134Cs/137Cs and 135Cs/137Cs were applicable for the source analysis of radioactive Cs in Fukushima prefecture.

Removal of radioactive cesium, strontium, and iodine from natural waters using bentonite, zeolite, and activated carbon

Cs-134, Sr-85, and I-131 were produced by neutron irradiation of CsCl, SrCl2, and K2TeO3, respectively, using the Kyoto University Reactor. These radioactive nuclides were added to river water and seawater to prepare artificially contaminated samples, and the removal of these nuclides using bentonite, zeolite, and activated carbon was then investigated. In the river water samples, Cs-134 and Sr-85 were successfully removed using bentonite and zeolite, and I-131 was removed using activated carbon. In the seawater samples, Cs-134 was removed using bentonite and zeolite, whereas Sr-85 and I-131 were hardly removed at all by these adsorbents.

Prediction of groundwater contamination with 137Cs and 131I from the Fukushima nuclear accident in the Kanto district.

We measured the concentrations of (131)I, (134)Cs, and (137)Cs released from the Fukushima nuclear accident in soil and rainwater samples collected March 30-31, 2011, in Ibaraki Prefecture, Kanto district, bordering Fukushima Prefecture to the south. Column experiments revealed that all (131)I in rainwater samples was adsorbed onto an anion-exchange resin. However, 30% of (131)I was not retained by the resin after it passed through a soil layer, suggesting that a portion of (131)I became bound to organic matter from the soil. The (137)Cs migration rate was estimated to be approximately 0.6 mm/y in the Kanto area, which indicates that contamination of groundwater by (137)Cs is not likely to occur in rainwater infiltrating into the surface soil after the Fukushima accident.

Cherenkov counting of 90Sr and 90Y in bark and leaf samples collected around Fukushima Daiichi Nuclear Power Plant

The radioactivity of 90Sr and 137Cs in environmental samples, bark and leaf, collected around the Fukushima Daiichi Nuclear Power Plant in May 2013 was determined with the aim of investigating the migration of both nuclides using their radioactivity ratio. The radioactivity of 90Sr was determined by using Cherenkov counting of 90Y after purification using Sr resin and that of 137Cs was determined by γ-spectrometry. Quench correction in Cherenkov counting was investigated by measurements of samples spiked with purified 90Y revealed that the radioactivity could be evaluated without quench correction. The radioactivity ratio of 90Sr to 137Cs in bark samples of 4.2xa0×xa010−3 and 1.2xa0×xa010−2 was compared with the results from soil samples collected in July 2011 to show that the migration of 90Sr was slower than 137Cs in bark and tree.

235U/238U Isotopic ratio in plant samples from Fukushima Prefecture

AbstractnUranium was recovered from plant samples obtained from Fukushima Prefecture, and the 235U/238U isotopic ratio was analyzed using thermal ionization mass spectrometry. The possibility of soil and biosphere contamination with U by the nuclear accident at Fukushima Daiichi nuclear power plant was evaluated. The isotopic ratio of U in the plant samples was the same as the natural abundance, which suggests that no significant contamination with U occurred.

Front tracking of the translocation of water-soluble cesium deposited on tree leaves of plum

After the trunks of Cryptomeria japonica and Quercus serrata collected from the Fukushima prefecture were soaked in water, approximately 60xa0% of the radiocesium in the tree trunks was extracted into the water, indicating that the radiocesium in tree trunks mainly exists in a water-soluble form. The front tracking of cesium into the wood from the leaves of trees was carried out after cesium in the water-soluble form was applied to the leaf surfaces. The results demonstrated that cesium translocated from the cesium-bearing leaves, to the branch in contact with the leaves, and then into the trunk in contact with the branch.

Application of Mass Spectrometry for Analysis of Cesium and Strontium in Environmental Samples Obtained in Fukushima Prefecture

For the assessment of Fukushima Daiichi Nuclear Power Plant accident, the applicability of the thermal ionization mass spectrometry (TIMS), which is a type of mass spectrometry, was studied. For the study of the recovery/analysis method of cesium and strontium, at first, the radioactive cesium and strontium were generated by the irradiation of natural uranium at KUR. After this study, the applicability of this method to the environmental samples obtained in Fukushima prefecture was verified.

Speciation of 137Cs and 129I in Soil After the Fukushima NPP Accident

We evaluated the migration of radionuclides (131I, 129I, 134Cs, 136Cs, 137Cs, and 132Te) in the surface soil after the Fukushima nuclear accident. The radionuclides in the soil collected late March in 2011 were barely leached with ultrapure water, indicating that these are insoluble. We observed the chemical behavior of 137Cs and 129I in soil: (1) 137Cs was predominantly adsorbed within a depth of 2.5 cm from the ground surface; (2) 137Cs was hardly released from soil by the water leaching experiments that lasted for 270 days; (3) approximately, more than 90 % of 137Cs was adsorbed on organic matters and the residual fractions, while 129I was mainly fixed on the Fe-Mn oxide and organically bounded fraction. Therefore, we conclude that 137Cs and 129I in soil seldom leach into the soil water and migrate downward because of the irreversible adsorption. The shallow groundwater which residence time is short.

Isotopic Ratio of 135Cs/137Cs in Fukushima Environmental Samples Collected in 2011

The isotopic ratios of radioactive cesium derived from the Fukushima accident were determined by γ-spectrometry and thermal ionization mass spectrometry (TIMS). In order to ascertain the initial ratios at the time of the accident, environmental samples collected during 2011 were used for the analysis. Soil, litter, and seaweed were incinerated, and the cesium contained therein was adsorbed into ammonium phosphomolybdate powder. The cesium in the seawater was adsorbed into AMP-PAN resin (Eichrom Technologies, LLC); its recovery ratio was almost one without the carrier being added. Incinerated samples and the AMP-PAN resin were then measured by γ-spectrometry. The cesium solution recovered from adsorbers was subjected to TIMS measurements. The isotopic ratios of 134Cs/137Cs and 135Cs/137Cs were found to be independent of the type of sample in question, as well as the sampling location; the ratios were 0.07 and 0.36 (decay correction: 11 March 2011), respectively, which differ from the results of atmospheric nuclear tests (i.e., 0 and 2.7, respectively). This difference in the ratio of 135Cs/137Cs will contribute to estimations of the origin of radioactive contamination in the future.