Transport and Redistribution of Radiocesium in Fukushima Fallout through Rivers.

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident released the most significant quantity of radiocesium into the environment since Chernobyl, and detailed measurements over the initial 5 years provide new insights into fluvial redistribution of radiocesium. We found that the high initial activity concentration of 137Cs-bearing suspended sediment in rivers was followed by a steep exponential decline (λ1) which extended to approximately 1 year after the accident, while the rate of initial decline in radiocesium activity concentration in water was an order of magnitude higher than rates measured after Chernobyl. Fluvial transport of 137Cs to the ocean from the Abukuma river totaled 12 TBq between June 2011 and August 2015 and almost all this radiocesium (96.5%) was transported in the particulate form. The primary sources of 137Cs were paddy fields, farmland, and urban areas [plaque-forming unit (PFU)], discharging 85% of the exported 137Cs from 38% of the watershed area. After 1 year, activity concentrations were lower and exhibited a more gradual secondary decline (λ2) which was associated with reduced radiocesium losses from PFU areas, while forest areas continue to represent more stable contaminant stores.

Dataset on the 6-year radiocesium transport in rivers near Fukushima Daiichi nuclear power plant.

Radiocesium released from the Fukushima Daiichi nuclear power plant (FDNPP) and deposited in the terrestrial environment has been transported to the sea through rivers. To study the long-term effect of riverine transport on the remediation process near the FDNPP, a monitoring project was initiated by the University of Tsukuba. It was commissioned by the Ministry of Education, Culture, Sports, Science, and Technology, and the Nuclear Regulatory Commission in June 2011, and was taken over by the Fukushima Prefectural Centre for Environmental Creation from April 2015. The activity concentration and monthly flux of radiocesium in a suspended form were measured in the project. This provides valuable measurement data to evaluate the impact of the accidentally released radiocesium on residents and the marine environment. It can also be used as verification data in the development and testing of numerical models to predict future impacts.

Factors controlling dissolved 137Cs concentrations in east Japanese Rivers.

To investigate the main factors that control the dissolved radiocesium concentration in river water in the area affected by the Fukushima Daiichi nuclear power plant accident, the correlations between the dissolved 137Cs concentrations at 66 sites normalized to the average 137Cs inventories for the watersheds with the land use, soil components, topography, and water quality factors were assessed. We found that the topographic wetness index is significantly and positively correlated with the normalized dissolved 137Cs concentration. Similar positive correlations have been found for European rivers because wetland areas with boggy organic soils that weakly retain 137Cs are mainly found on plains. However, for small Japanese river watersheds, the building area ratio in the watershed strongly affected the dissolved 137Cs concentration. One reason for this would be because the high concentrations of solutes, such as K+ and dissolved organic carbon, discharged in urban areas would inhibit 137Cs absorption to soil particles. A multiple regression equation was constructed to predict the normalized dissolved 137Cs concentration with the topography, land use, soil component, and water quality data as explanatory variables. The best model had the building land use as the primary predictor. When comparing two multiple regression models in which the explanatory variables were limited to (1) the land use and soil composition and (2) the water quality, the water quality model underestimated the high normalized dissolve 137Cs concentration in urban areas. This poor reproducibility indicates that the dissolved 137Cs concentration value in urban areas cannot be solely explained by the solid-liquid distribution of 137Cs owing to the influence of the water quality, but some specific 137Cs sources in urban areas would control the dissolved 137Cs concentration.

Asynchrony between Antarctic temperature and CO2 associated with obliquity over the past 720,000 years.

The δD temperature proxy in Antarctic ice cores varies in parallel with CO2 through glacial cycles. However, these variables display a puzzling asynchrony. Well-dated records of Southern Ocean temperature will provide crucial information because the Southern Ocean is likely key in regulating CO2 variations. Here, we perform multiple isotopic analyses on an Antarctic ice core and estimate temperature variations at this site and in the oceanic moisture source over the past 720,000 years, which extend the longest records by 300,000 years. Antarctic temperature is affected by large variations in local insolation that are induced by obliquity. At the obliquity periodicity, the Antarctic and ocean temperatures lag annual mean insolation. Further, the magnitude of the phase lag is minimal during low eccentricity periods, suggesting that secular changes in the global carbon cycle and the ocean circulation modulate the phase relationship among temperatures, CO2 and insolation in the obliquity frequency band.

State dependence of climatic instability over the past 720,000 years from Antarctic ice cores and climate modeling.

Climatic variabilities on millennial and longer time scales with a bipolar seesaw pattern have been documented in paleoclimatic records, but their frequencies, relationships with mean climatic state, and mechanisms remain unclear. Understanding the processes and sensitivities that underlie these changes will underpin better understanding of the climate system and projections of its future change. We investigate the long-term characteristics of climatic variability using a new ice-core record from Dome Fuji, East Antarctica, combined with an existing long record from the Dome C ice core. Antarctic warming events over the past 720,000 years are most frequent when the Antarctic temperature is slightly below average on orbital time scales, equivalent to an intermediate climate during glacial periods, whereas interglacial and fully glaciated climates are unfavourable for a millennial-scale bipolar seesaw. Numerical experiments using a fully coupled atmosphere-ocean general circulation model with freshwater hosing in the northern North Atlantic showed that climate becomes most unstable in intermediate glacial conditions associated with large changes in sea ice and the Atlantic Meridional Overturning Circulation. Model sensitivity experiments suggest that the prerequisite for the most frequent climate instability with bipolar seesaw pattern during the late Pleistocene era is associated with reduced atmospheric CO2 concentration via global cooling and sea ice formation in the North Atlantic, in addition to extended Northern Hemisphere ice sheets.