Hokuto Iwatani

Size distribution studies of 137Cs in river water in the Abukuma Riverine system following the Fukushima Dai-ichi Nuclear Power Plant accident

The occurrence of (137)Cs in size fractionated samples in river water from the Abukuma River system, (the Kuchibuto and Abukuma Rivers, five sampling events for three sites) was studied from June 2011--approximately some three months after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident until December 2012. The total concentration of (137)Cs (mBq/L) in river water was generally high at the upper stream site in the Yamakiya District within the evacuation/off-limits zone. The (137)Cs concentration was about 1Bq/L for the first sampling campaign (June 2011) at all sites, but then decreased substantially to about one-tenth of that by the time of a second sampling campaign (November or December 2011). The (137)Cs in the <0.45 μm fraction was present exclusively as a dissolved species rather than as a species adsorbed on suspended solids or complexed with organic materials. The contribution of the dissolved fraction ranged from 1.2 to 48.9% (averaged 20%) of the total concentration of (137)Cs throughout the observation period. The maximum contribution of (137)Cs was found in the silt size fraction (3-63 μm), which can be explained by the relatively large Kd values and the suspended solids (SS) concentration of this size fraction. Although the concentration (Bq/g) of (137)Cs in each size fraction did not show any significant trends and/or variations for any of the sampling campaign, Kd values for each site increased with time. Furthermore, it was found that the Kd values decreased with distance from the headstream in the off-limits zone. Thus, the data acquired in this study give an overview of the radiological situation for Fukushima including temporal and spatial variation of radiocaesium in a natural riverine system, within a few years after the accident.

Local distribution of radioactivity in tree leaves contaminated by fallout of the radionuclides emitted from the Fukushima Daiichi Nuclear Power Plant

We analyzed fresh and dead leaves collected in forests in Fukushima after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, using autoradiography. Both fresh and dead leaves of Cryptomeria japonica were contaminated by radionuclides (134Cs and 137Cs). Contamination of the fresh leaves was possibly attributed to interception of radionuclides by tree canopies, whereas the dead leaves indicated the direct deposition of radionuclides by fallout and/or washout of radionuclides intercepted by tree canopies. Translocation of radiocesium from a contaminated branch to new leaves growing after the FDNPP accident was not clearly observed, although transfer of radiocesium from leaf parts to male flowers occurred. Fallen leaves of Quercus serrata, which started growing after the FDNPP accident, did not show radioactivity, indicating that significant amounts of translocation from other parts to new leaves did not occur. Fallen leaves of Q. serrata collected from a litter showed hot spots originating from direct fallout. Needles of Pinus densiflora were also contaminated by fallout. Leaching with pure water removed soluble fractions of radiocesium and hot particles from the surface of the contaminated leaves, but significant amounts of radioactivity remained. This means that foliar absorption occurred in both fresh and dead leaves. Further leaching experiments using surfactant and acetone could not remove the remaining radiocesium from the leaves. The leaching experiments indicate that radiocesium in the contaminated leaves is strongly fixed in leaf tissues and is not readily released unless leaf tissues are decomposed.

Investigation of Spatial Distribution of Radiocesium in a Paddy Field as a Potential Sink

Surface soils, under various land uses, were contaminated by radionuclides that were released by the Fukushima Daiichi Nuclear Power Plant accident. Because paddy fields are one of the main land uses in Japan, we investigated the spatial distribution of radiocesium and the influence of irrigation water in a paddy field during cultivation. Soil core samples collected at a paddy field in Fukushima showed that plowing had disturbed the original depth distribution of radiocesium. The horizontal distribution of radiocesium did not show any evidence for significant influence of radiocesium from irrigation water, and its accumulation within the paddy field, since the original amount of radiocesium was much larger than was added into the paddy field by irrigation water. However, it is possible that rainfall significantly increases the loading of radiocesium.

Response of subtropical submarine-cave ecosystem to Holocene cave development and Asian monsoon variability

Abstract. A submarine cave is a unique environment that is dark, food limited, semi-isolated from the outside, and sheltered from wave action. However, our knowledge of the long-term change in submarine-cave ecosystems remains limited. We document here the community-scale responses toward long-term change in a submarine cave, Daidokutsu in Okinawa in southern Japan. Using both metazoans (ostracods and bivalves) and protozoans (larger benthic foraminiferans) in two sediment cores obtained from the cave, we reconstruct the faunal and diversity changes of the past 7 Kyr. All taxonomic groups showed long-term, gradual linear change of faunal composition from predominantly open-water taxa to predominantly cave taxa, and ostracods showed short-term variability of species diversity. The long-term faunal trend probably reflects gradual isolation of the cave ecosystem due to coral reef development (i.e., development of the cave ceiling) during periods of the Holocene transgression and subsequent sea-level highstand. The short-term diversity changes show substantial similarity to centennial- to millennial-scale Holocene Asian monsoon variability. Ostracod species diversity peaks tend to correspond with periods of strong East Asian winter monsoons. The results indicate that limestone submarine-cave ecosystems, an important cryptic habitat, developed gradually during the Holocene and may be sensitive to rapid climate changes.

An enigmatic Holocene podocopid ostracod from a submarine cave, Okinawa, Japan: ‘living fossil’ or adaptive morphotype?

Submarine cave faunas remain poorly understood, and only a few stygobite podocopid ostracods are known. We describe an enigmatic submarine cave ostracod, Tabukicypris decoris gen. et sp. nov., from Holocene sediment cores taken from the Daidokutsu submarine cave off Ie Island, Okinawa, Japan. This species is endemic to the submarine cave environment. At least part of its enigmatic morphology, especially the distinct pore clusters in the anterior and posterior parts, is probably an adaptation to the submarine cave environment. There is no clear morphological evidence to indicate that Tabukicypris decoris is a ‘living fossil’. The morphological characters of this stygobite indicate three possible origins of this species: from a deep-sea, interstitial or tropical open shallow-marine environment. http://zoobank.org/urn:lsid:zoobank.org:pub:25E37E49-C0A0-4A49-A8D8-E4AE9D85DB74

Occurrence of a rare puncioid ostracod, Promanawa konishii (Nohara, 1976), in Recent sediments of the East China Sea

A left valve of Promanawa konishii (Nohara, 1976) was found in the modern surface sediment of the East China Sea, off the western Iheya-Izena Islands, SW Japan (Fig. 1). The specimen was collected from St 447 (26.98°N, 127.72°E, 512 m water depth) during the GH09 cruise by the Geological Survey of Japan, Agency of Industrial Science and Technology (AIST). Promanawa konishii has a laterally compressed semi-elliptical carapace with a wide frill around the entire ventral margin and a long straight hinge, uniquely characteristic of the superfamily Puncioidea which includes the extant genera Manawa, Promanawa and Puncia (Hornibrook, 1949; McKenzie & Neil, 1983), considered by some to represent a surviving lineage of the order Palaeocopida, all others of which have been extinct since the end of the Palaeozoic (Hornibrook, 1949; Swanson, 1991). Living specimens of Manawa were described from a water depth of 17 m off Goat Island, New Zealand, by Swanson (1989, 1991). According to this study, the nauplius larvae of Manawa have a dome-shaped single carapace that is later divided into two valves jointed by hingement during the growth stages; however, ostracods usually have two such valves …

Intermediate-water dynamics and ocean ventilation effects on the Indonesian Throughflow during the past 15,000 years: Ostracod evidence

The Indonesian Throughflow (ITF) is thought to influence thermohaline circulation dynamics and is important for understanding global climate and the marine ecosystem. The physical and chemical properties of North Pacific Intermediate Water (NPIW) and the underlying deep water incorporated into the ITF appear to be the result of climate-related preconditioning in the North and South Pacific. Thus, these high-latitude source waters play an important role in the Indo-Pacific oceanography. Here, we present the results of down-core faunal analyses of fossil ostracods (Crustacea) that we argue reflect NPIW variability in the central part of the Makassar Strait in the ITF over the past 15 k.y. The results show that the warmwater and low-oxygen–water fauna, and species diversity, rapidly increased at ca. 12 ka, reaching maxima during the Younger Dryas (YD). We interpret the faunal change and the diversity maximum at ca. 12 ka as a response to the stagnation of intermediate water due to the decline in ITF intensity during the YD. After ca. 7 ka, the ostracod faunal composition clearly changed from a relatively shallower, warmer, and low-oxygen fauna to a relatively deeper, colder, and high-oxygen fauna. Our interpretation is that the ostracod fauna was responding to the deglacial–early Holocene sea-level rise and the ventilation variations due to the mixing of the NPIW and the underlying deep water. The intermediate-water environment and the ecosystem in the ITF could have been driven by the intensification of the influence of the underlying deep water, caused by changes in the southern high-latitude source due to the latitudinal displacements of the southwesterly winds.