Redistribution and export of contaminated sediment within eastern Fukushima Prefecture due to typhoon flooding

Tropical cyclones expose river basins to heavy rainfall and flooding, and cause substantial soil erosion and sediment transport. There is heightened interest in the effects of typhoon floods on river basins in northeast Japan, as the migration of radiocaesium‐bearing soils contaminated by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident will affect future radiation levels. The five main catchments surrounding FDNPP are the Odaka, Ukedo, Maeda, Kuma and Tomioka basins, but little quantitative modelling has been undertaken to identify the sediment redistribution patterns and controlling processes across these basins. Here we address this issue and report catchment‐scale modelling of the five basins using the GETFLOWS simulation code. The three‐dimensional (3D) models of the basins incorporated details of the geology, soil type, land cover, and used data from meteorological records as inputs. The simulation results were checked against field monitoring data for water flow rates, suspended sediment concentrations and accumulated sediment erosion and deposition. The results show that the majority of annual sediment migration in the basins occurs over storm periods, thus making typhoons the main vectors for redistribution. The Ukedo and Tomioka basins are the most important basins in the region in terms of overall sediment transport, followed by the other three basins each with similar discharge amounts. Erosion is strongly correlated with the underlying geology and the surface topography in the study area. A low permeability Pliocene Dainenji formation in the coastal area causes high surface water flow rates and soil erosion. Conversely, erosion is lower in an area with high permeability granite basement rocks between the Hatagawa and Futaba faults in the centre of the study area. Land cover is also a factor controlling differences in erosion and transport rates between forested areas in the west of the study area and predominantly agricultural areas towards the east. The largest sediment depositions occur in the Ogaki and Takigawa Dams, at the confluence of the Takase and Ukedo Rivers, and at the Ukedo River mouth. Having clarified the sediment redistribution patterns and controlling processes, these results can assist the ongoing task of monitoring radioactive caesium redistribution within Fukushima Prefecture, and contribute to the design and implementation of measures to protect health and the environment. Copyright © 2016 John Wiley & Sons, Ltd.     

Air-sea gas transfer in a shallow,flowing and coastal environment estimated by dissolved inorganic carbon and dissolved oxygen analyses

We estimated gas exchange rates in Kabira Reef at Ishigaki Island, southwest Japan, using a mass balance calculation with dual “biological” tracers: dissolved inorganic carbon (DIC) and dissolved oxygen (DO). The nighttime results allowed us to obtain reasonable gas transfer velocity k _w values, all of which exceeded those obtained in wind-dominant studies. The difference is likely due to the contribution of turbulence generated by the interaction between the current and bottom topography. The k _w obtained during high tides is consistent with that reported by Raymond and Cole (2001), whereas k _w during low tides is significantly higher, which seems to be caused by enhanced friction with the bottom of the reef and/or bubble-induced gas transfer by wave breaking at the reef crest.     

Ecology and restoration techniques for Sargassum beds in the Seto Inland Sea,Japan

Due to the reduction and degradation of coastal areas in Japan by land reclamation and anthropogenic perturbations, from the point of view of conservation of the coastal environment, the restoration of Sargassum beds is essential. Between 1978 and 1991, 6400 ha of seagrass and seaweed beds have been lost along the Japanese coast, of which Sargassum beds were 22%. New techniques for Sargassum bed restoration are summarized based on three coastal engineering techniques. (1) Construction of shallow and gentle sloping bottom substrata have been shown to be effective for the reestablishment of 'management-free seagrass and Sargassum beds' on developed coasts. (2) Seeding or transplanting using artificial substratum for extension of nursery and fishing grounds around natural Sargassum beds. (3) Periodic transplanting of Sargassum plants using artificially produced seedlings is effective to produce niches to allow faunal re-colonization in severely polluted and sparsely vegetated area. However, prior to implementation, the suitability and limitations of these three techniques requires to be ascertained for effective Sargassum bed restoration.     

Relationships between Kuroko volcanogenic massive sulfide (VMS) deposits, felsic volcanism, and island arc development in the northeast Honshu arc, Japan

The northeast (NE) Honshu arc was formed by three major volcano-tectonic events resulting from Late Cenozoic orogenic movement: continental margin volcanism (before 21?Ma), seafloor basaltic lava flows and subsequent bimodal volcanism accompanied by back-arc rifting (21 to 14?Ma), and felsic volcanism related to island arc uplift (12 to 2?Ma). Eight petrotectonic domains, parallel to the NE Honshu arc, were formed as a result of the eastward migration of volcanic activity with time. Major Kuroko volcanogenic massive sulfide (VMS) deposits are located within the eastern marginal rift zone (Kuroko rift) that formed in the final period of back-arc rifting (16 to 14?Ma). Volcanic activity in the NE Honshu arc is divided into six volcanic stages. The eruption volumes of volcanic rocks have gradually decreased from 4,600?km³ (per 1?my for a 200-km-long section along the arc) of basaltic lava flows in the back-arc spreading stage to 1,000?C2,000?km³ of bimodal hyaloclastites in the back-arc rift stage, and about 200?km³ of felsic pumice eruptions in the island arc stage. The Kuroko VMS deposits were formed at the time of abrupt decrease in the eruption volume and change in the mode of occurrence of the volcanic rocks during the final period of back-arc rifting. In the area of the Kuroko rift, felsic volcanism changed from aphyric or weakly plagioclase phyric (before 14?Ma), to quartz and plagioclase phyric with minor clinopyroxene (12 to 8?Ma), to hornblende phyric (after 8?Ma), and hornblende and biotite phyric (after 4?Ma). The Kuroko VMS deposits are closely related to the aphyric rhyolitic activity before 14?Ma. The rhyolite was generated at a relatively high temperature from a highly differentiated part of felsic magma seated at a relatively great depth and contains higher Nb, Ce, and Y contents than the post-Kuroko felsic volcanism. The Kuroko VMS deposits were formed within a specific tectonic setting, at a specific period, and associated with a particular volcanism of the arc evolution process. Therefore, detailed study of the evolutional process from rift opening to island arc tectonics is very important for the exploration of Kuroko-type VMS deposits.     

Trace-element characteristics of east–west mantle geochemical hemispheres

Recent statistical analyses on the isotopic compositions of oceanic, arc, and continental basalts have revealed that the Earth's mantle is broadly divided into eastern and western hemispheres. The present study aimed to characterize the isotopically defined east–west geochemical hemispheres using trace-element concentrations. Basalt data with Rb, Sr, Nd, Sm, Pb, Th, and U in addition to the isotopic ratios ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb were selected mostly from the GEOROC and PetDB databases. A total of 4787 samples were used to investigate the global geochemical variations. The results show that the wide trace-element variations are broadly explained by the melting of melt-metasomatized and fluid-metasomatized mantle sources. The larger amount of the fluid component derived from subducted plates in the eastern hemisphere than that in the western hemisphere is inferred from the basalts. These characteristics support the hypothesis that focused subduction towards the supercontinent created the mantle geochemical hemispheres.     

Assessing a decade of phosphorus management in the Lake Mendota, Wisconsin watershed and scenarios for enhanced phosphorus management

A phosphorus (P) budget was estimated for the watershed of Lake Mendota, Wisconsin, to assess the effects of nutrient management on P accumulation in the watershed soils. We estimated how nutrient management programs and legislation have affected the budget by comparing the budget for 2007 to a budget calculated for 1995, prior to implementation of the programs. Since 1995, inputs decreased from 1,310,000 to 853,000 kg P/yr (35% reduction) and accumulation decreased from 575,000 to 279,000 kg P/yr (51% reduction). Changes in P input and accumulation were attributed primarily to enhanced agricultural nutrient management, reduction in dairy cattle feed supplements and an urban P fertilizer ban. Four scenarios were investigated to determine potential impacts of additional nutrient management tactics on the watershed P budget and P loading to Lake Mendota. Elimination of chemical P fertilizer input has the greatest potential to reduce watershed P accumulation and establishment of riparian buffers has the greatest potential to prevent P loading to Lake Mendota.     

Calcareous duricrust, Al Qasim Province, Saudi Arabia: Occurrence and origin

In Al Qasim Province, Saudi Arabia, calcareous pedogenic duricrust overlies the Upper Permian carbonates and clays of the Khuff Formation, and ranges in thickness from 60 cm to 2 m. It consists of white sandy duricrust capped by brown, nodular porous duricrust. Petrographic investigation led to recognition of three lithofacies that show spatial distribution: 1- structureless sandy calcareous mudstone, 2- brecciated-nodular calcareous mudstone, and 3- pisolitic calcareous mudstone. The diagenetic changes in duricrust are aggrading neomorphism and formation of calcite rims around quartz grains. Microprobe chemical analysis was performed on the diagenetically altered material. The development of the duricrust is controlled by four factors: the climate changes, the type of underlying rocks, source of carbonate and biological effects. The repeated changes from semi-arid climate with alternating wet and dry seasons are the main controlling factor, in generation of duricrust development. During humid periods, descending water carries the dissolved carbonates downward as bicarbonates, whereas dry and interpluvial period, the carbonate-rich solutions are carried upwards by capillary or evaporation. The claystone and sandstone interbeds possibly affect the development of calcareous duricrust, more than the sandstones and dolostones, because the former have the ability to attach calcium carbonate to form the duricrust. The development of calcareous duricrust is mainly attributed to the Pleistocene and Early Holocene alternating humid and arid periods that prevailed over the Arabian Peninsula during this time.     

Rock magnetic analyses as a tool to investigate diversity of drift pumice clasts: An example from Japan's 2021 Fukutoku-Oka-no-Ba (FOB) eruption

The crystallization of nanolites within magma chambers has recently raised a strong interest due to their impact on increasing melt viscosity and triggering magmatic eruptions. In 2021, the Fukutoku-Oka-no-Ba (FOB) underwater eruption produced large quantities of pumices that eventually formed rafts drifting at the surface of the ocean to the East coasts of Japan. Pumices collected along the shore shortly after grounding show various colors, microscopic and Raman analyses made by Yoshida et al. (Island Arc, 31, 1, 2022) revealed the presence of magnetite nanolites in some of them. In this study, we explore the magnetic properties of a batch of pumices of different colors from the FOB eruption, aiming to refine characterization of iron oxide nanolites. We used various analytical techniques such as SEM and FEG-SEM observations, EDS-X analyses, and rock magnetic experiments, including thermomagnetic analyses, hysteresis curves, coercivity analyses and FORC measurements. Our findings reveal that the iron oxides present in the FOB samples are Ti-magnetite, with minor amounts of Mg and Al. The magnetic crystals show a wide range of sizes, from extra small iron oxide nanolites (ESION) in the pumices with the lighter colors, to more bulky grains reaching the micrometer size in some of the dark color samples, significant diffusion is inferred in that case. Consequently, the magnetic characterization of iron oxide crystals within the Fukutoku-Oka-no-Ba pumices reveals varying stages of nucleation, dissolution, growth, and diffusion processes, providing evidences for the heterogeneous state of the magma during the eruption.     

Early Carboniferous HP metamorphism in the Hida Gaien Belt,Japan: Implications for the Palaeozoic tectonic history of proto‐Japan

We report two new eclogite localities (at Kanayamadani and Shinadani) in the high‐P (HP) metamorphic rocks of the Omi area in the western most region of Niigata Prefecture, Japan, which form part of the Hida Gaien Belt, and determine metamorphic conditions and pressure–temperature (P–T) paths. The metamorphic evolution of the eclogites is characterized by a tight hairpin‐shaped P–T path from prograde epidote–blueschist facies to peak eclogite facies and then retrograde blueschist facies. The prograde metamorphic stage is characterized by various amphibole (winchite, barroisite, glaucophane) inclusions in garnet, whereas the peak eclogite facies assemblage is characterized by omphacite, garnet, phengite and rutile. Peak P–T conditions of the eclogites were estimated to be ~600°C and up to 2.0 GPa by conventional cation‐exchange thermobarometry, Ti‐in‐zircon thermometry and quartz inclusion Raman barometry respectively. However, the Raman spectra of carbonaceous material thermometry of metapelites associated with the eclogites gave lower peak temperatures, possibly due to metamorphism at different conditions before being brought together during exhumation. The blueschist facies overprint following the peak of metamorphism is recognized by the abundance of glaucophane in the matrix. Zircon grains in blueschist facies metasedimentary samples from two localities adjacent to the eclogites have distinct oscillatory‐zoned cores and overgrowth rims. Laser ablation inductively coupled plasma mass spectrometry U–Pb ages of the detrital cores yield a wide range between 3,200 and 400 Ma, with a peak at 600–400 Ma. In the early Palaeozoic, proto‐Japan was located along the continental margin of the South China craton, providing the source of the older population of detrital zircon grains (3,200–600 Ma) deposited in the trench‐fill sediments. In addition, subduction‐related magmatism c. 500–400 Ma is recorded in the crust below proto‐Japan, which might have been the source for the younger detrital zircon grains. The peak metamorphic age was constrained by SHRIMP dating of the overgrowth rims, yielding Tournaisian ages of 347 ± 4 Ma, suggesting subduction in the early Carboniferous. Our results provide clear constraints on the initiation of subduction, accretion and the development of an arc‐trench system along the active continental margin of the South China craton and help to unravel the Palaeozoic tectonic history of proto‐Japan.     

On mantle drag force for the formation of a next supercontinent as estimated from a numerical simulation model of global mantle convection

Three‐dimensional spherical mantle convection was simulated to predict future continental motion and investigate the driving force of continental motion. Results show that both the time required (≥300 Ma from the present) and the process for the next supercontinent formation are sensitive to the choice of critical rheological parameters for mantle dynamics, such as a viscosity contrast between the upper and lower mantles and a yield strength of the lithosphere. From all the numerical models studied herein, mantle drag force by horizontal mantle flow beneath the continents may mostly act as a resistance force for the continental motion in the process of forming a new supercontinent. The maximum absolute magnitude of the tensional and compressional stress acting at the base of the moving continents is in the order of 100 MPa, which is comparable to a typical value of the slab pull force.