Distribution of local 137Cs anomalies on the seafloor near the Fukushima Dai-ichi Nuclear Power Plant

An estimated 3.5 ± 0.7 × 1015 Bq of 137Cs is thought to have been discharged into the ocean following the melt down at Fukushima Dai-ichi Nuclear Power Plant (F1NPP). While efforts have been made to monitor seafloor radiation levels, the sampling techniques used cannot capture the continuous distribution of radionuclides. In this work, we apply in situ measurement techniques using a towed gamma ray spectrometer to map the continuous distribution of 137Cs on the seafloor within 20 km of the F1NPP. The results reveal the existence of local 137Cs anomalies, with levels of 137Cs an order of magnitude higher than the surrounding seafloors. The sizes of the anomalies mapped in this work range from a few meters to a few hundreds of meters in length, and it is demonstrated that the distribution of these anomalies is strongly influenced by meter scale features of the terrain

Five-minute resolved spatial distribution of radiocesium in sea sediment derived from the Fukushima Dai-ichi Nuclear Power Plant

AbstractThe spatial distributions of radiocesium concentration in sea sediment to a core depth of 14 cm were investigated in the offshore region from the Fukushima Prefecture to the northern part of the Ibaraki Prefecture in February and July 2012, at a spatial resolution of 5 min of latitude and longitude. The concentrations in the area south of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) were generally higher than those in the area north of it. In the southern area, a band of especially high concentration with a width about 20 km was present in the region shallower than 100 m, and a narrow minimal concentration band was found along the 200-m isobaths. In more than half of all cases, the vertical core profiles of radiocesium concentration generally showed an exponential decreasing trend with depth. However, in the area north of the FDNPP, where the radiocesium concentrations tended to be very low, radiocesium concentrations that had similar or larger magnitude compared with those of the most-surface layer were often found in deeper layers. Relatively good correlations were found between radiocesium concentrations and grain sizes of the most-surface sediment. The vertical profile of radiocesium concentration also had a relationship with grain size. In other case, the radiocesium concentration in the sediment seems to have had a dependence on the radiocesium concentration in bottom seawater, suggesting that the quantity of radiocesium supplied and the grain size were major factors determining the spatial distribution pattern of the radiocesium concentration after the FDNPP accident

Mean and seasonal circulation of the south Indian ocean estimated by combining satellite altimetry and surface drifter observations

The mean velocity field of south Indian Ocean has been derived by combining high resolution maps of sea level anomalies and the surface drifter data from the Global Drifter program from 1993 to 2012 with a resolution of 1/3 × 1/3 degrees in latitude and longitude. The estimated mean velocity field exhibits strong western boundary currents, zonal currents and eastern boundary currents. The Agulhas Current shows a velocity of above 1.5 m s-1 at around 35°S. The distribution of energy associated with the fluctuating motion and the mean flow illustrates that mesoscale variability are particularly relevant in the Mozambique Channel, south of Java and around 40°S. Advection of mesoscale features along the western boundary is evident in the distribution of eddy fluxes. The long-term average monthly surface velocity field exhibits large variations in surface currents. The most change is observed in the South Equatorial Current which shows spatial and temporal variation

Population Subdivision of Japanese Flounder Paralichthys olivaceus in the Pacific Coast of Tohoku Japan Detected by Means of Mitochondrial Phylogenetic Information

This study deals with mitochondrial phylogenetic information of Japanese flounder in the Pacific coast of Tohoku Japan to estimate the genetic population subdivision that was undetectable by conventional population statistics. We determined complete sequences of mitochondrial NADH dehydrogenase subunit-2 (ND2) and subunit-5 (ND5) genes for 151 individuals from northern (Aomori and Iwate prefectures, 40–41°N) and southern (Miyagi and Fukushima prefectures, 37–38°N) waters. Samples from both waters showed high genetic diversity, including 126 haplotypes. These haplotypes were located at mixed and nested positions on an inferred phylogenetic tree, and traditional F-statistics indicated no significant population divergence (φST = −0.00335, p > 0.05), corroborating our previous study. Three variable sites, however, showed significant base composition heterogeneity between samples from the northern and southern waters (Fisher’s exact-test, p < 0.01). Nucleotide substitutions at the three sites converged on an apical clade, which consisted of the five southern individuals, whereas its sister clade consisted only of the three northern individuals. This phylogenetic information corroborates previous ecological studies indicating the presence of separate stocks in the northern and southern waters

Experimental Estimate of Geostrophic Current from Trajectory Data of Sea-Surface Drifting Buoys

Sea-surface drifting buoy directly observes Lagrangian velocity variation. In this study, estimate process for long-term velocity component from buoy trajectory data was examined for geostrophic current through a virtual tracer experiment. In low-pass filtering of the path of particles, the long-term velocity component was detected stably and had high agreement with true value by using temporal window function, better than that by using spatial window which is based on the movement distance of tracer. In temporal smoothing process, the spatial smoothing scale is automatically determined by temporal smoothing scale and sampled velocity. The estimated velocity also depends on the adopted window function and its window width