The Kellwasser events in the Upper Devonian Frasnian to Famennian transition in the Toc Tat Formation,northern Vietnam

Upper Devonian carbonates of the Toc Tat Formation in the Si Phai Pass area of Dong Van District, northern Vietnam were deposited in carbonate platform, slope, and basin environments. These carbonates yield abundant conodonts indicative of the Palmatolepis nasuta, Pa. linguiformis and Pa. triangularis zones, the Frasnian–Famennian stage boundary being identified by the first occurrence of Pa. triangularis. Two positive carbon isotope excursions are recognized, the lower excursion peaking in the interval of the lower to middle Pa. nasuta Zone, whilst the upper excursion peaks just above the local Frasnian–Famennian boundary. Based on the biostratigraphy, these excursions equate to the Lower and Upper Kellwasser events. Locally, tentaculitoid taxa (Nowakia, Styliolina, Homoctenus, and Metastyliolina?) are abundant in the interval of the Pa. nasuta Zone, but show a drastic decline in abundance before the Lower Kellwasser Event, and only two taxa survived into the Famennian.     

Pressure and temperature dependence of the viscosity of a NaAlSi<Subscript>2</Subscript>O<Subscript>6</Subscript> melt

The viscosity of a silicate melt of composition NaAlSi₂O₆ was measured at pressures from 1.6 to 5.5 GPa and at temperatures from 1,350 to 1,880°C. We employed in situ falling sphere viscometry using X-ray radiography. We found that the viscosity of the NaAlSi₂O₆ melt decreased with increasing pressure up to 2 GPa. The pressure dependence of viscosity is diminished above 2 GPa. By using the relationship between the logarithm of viscosity and the reciprocal temperature, the activation energies for viscous flow were calculated to be 3.7 ± 0.4 × 10² and 3.7 ± 0.5 × 10² kJ/mol at 2.2 and 2.9 GPa, respectively.     

In situ observation of a garnet/perovskite transition in CaGeO<Subscript>3</Subscript>

We used an in situ measurement method to investigate the phase transition of CaGeO₃ polymorphs under high pressures and temperatures. A multi-anvil high-pressure apparatus combined with intense synchrotron X-ray radiation was used. The transition boundary between a garnet and a perovskite phase at T = 900–1,650 K and P = 3–8 GPa was determined as occurring at P (GPa) = 9.0−0.0023 × T (K). The transition pressure determined in our study is in general agreement with that observed in previous high-pressure experiments. The slope, dP/dT, of the transition determined in our study is consistent with that calculated from calorimetry data.     

Mobility-dependent response of aquatic animal species richness to a wetland network in an agricultural landscape

Management of wetland connectivity is important for biodiversity conservation. In the modern agricultural landscape, the natural connections between floodplain wetlands have been greatly altered. Agricultural ditches and channelized streams are widely distributed in floodplains, which may contribute to the maintenance of wetland connectivity and biodiversity. To determine how these watercourse networks affect wetland biodiversity, we examined the relationship between the species richness of aquatic animals and wetland connectivity, with a special focus on species mobility. From July to August 2011, fish and aquatic insects were collected from 24 wetlands in northern Japan. To determine the degree of wetland connectivity, we assessed the relative importance of individual wetlands in maintaining the entire wetland network using two connectivity indices: hydrologic connectivity via watercourses and spatial connectivity defined as Euclidian distances between wetlands using graph theory. We found that only high mobility groups of both taxa could enhance species richness in either a hydrologic (fish) or spatial (insect) wetland network. The species richness of insects with high-flying ability was found to increase as spatial connectivity increased. Furthermore, the species richness of fish with high-swimming ability was positively influenced by hydrologic connectivity, most likely because highly mobile species were able to reach suitable habitats and migrate from source populations in a wetland network owing to their good mobility. Our findings indicate that hydrologic network is important for maintaining biodiversity as well as spatial connectivity. It is important to focus conservation efforts on key wetlands with high hydrologic and spatial connectivity in future wetland management.     

Stage-V copepodites of Calanus sinicus and Calanus jashnovi (Copepoda: Calanoida) in mesopelagic zone of Sagami Bay as identified with genetic markers, with special reference to their vertical distribution

We applied genetic makers to identify Calanus species occurring in Sagami Bay, Japan, in order to investigate their vertical distribution in the upper 1000 m. First, interspecific genetic distances of three gene loci, mitochondrial small ribosomal RNA (srRNA), nuclear internal transcribed spacers 1 (ITS1) and 2 (ITS2), were estimated from morphologically distinguishable adult females of Calanus sinicus, Calanus jashnovi and Calanus pacificus that were collected from Sagami Bay, the Kuroshio Extension and the Oyashio region, respectively. The highest levels of interspecific genetic distance were observed in srRNA, followed by ITS1 and ITS2. The intraspecific genetic distances within C. sinicus were much lower than the interspecific genetic distances, indicating that DNA sequences in these loci are consistent with the morphological differences. This information was used as a criterion for species identification based on DNA sequence variation, and allowed us to identify the fifth copepodites (CVs) or younger stages of these species. Next, the vertical distribution of Calanus species was investigated in Sagami Bay in May 2006, on the basis of a stratified sampling in the upper 1000 m. By applying the genetic markers, 23 individuals comprising all copepodite stages were allocated into either C. sinicus or C. jashnovi, and the small- and large-sized CVs were identified as C. sinicus and C. jashnovi, respectively. The total abundance of C. sinicus was highest at 0-50 m and decreased with depth. On the contrary, CV individuals of C. sinicus were abundant not only in 0-50 m but also below 200 m with minimum occurrences in 150-200 m depth. C. jashnovi was much less abundant than C. sinicus and comprised of only CIV and CV which occurred in the upper 100 m and deeper than 50 m depths, respectively. The abundance of C. sinicus in the 1000-m water column of Sagami Bay was at a level comparable to that in shelf waters, suggesting the importance of off-shelf individuals in the biological production and organic transport in the respective areas.     

日本虎杖覆盖对连作的影响——日本西阿瓦陡坡土地农业系统传统耕作评价

禾本科物种(例如银草、芦苇)在日本传统的地膜覆盖农业中普遍使用,西阿瓦地区是全球重要的农业遗产系统(GIAHS)所在地。传统上,这里的农民使用银草覆盖农业土壤。但当地农民从长期的经验中了解到,日本虎杖覆盖更适合该地区茄科作物的种植。目前尚不清楚日本虎杖覆盖物为何对茄科作物的种植有益。在本项研究中,我们假设日本虎杖覆盖物可能会有效避免连续种植马铃薯的危害。因为过去每年都会种植两次本地马铃薯,因此,我们调查了日本虎杖覆盖对连续种植马铃薯以及番茄种植后对马铃薯产量的影响。在2018年进行的田间试验中,我们首先在连续马铃薯栽培的土壤和未耕种的土壤中比较了日本虎杖覆盖、银草覆盖和无草覆盖(对照)对马铃薯产量的影响,结果表明日本虎杖覆盖减少了马铃薯产量的降低程度。其次,我们比较了番茄种植后的土壤和未耕种的土壤中的虎杖覆盖和无草覆盖对马铃薯产量的影响,结果再次表明了日本虎杖覆盖减少了马铃薯生长和产量下降的程度。这些发现表明用日本虎杖覆盖有助于避免连续种植马铃薯的风险。     

Sedimentation of terrigenous red soils in coastal area adjacent to an estuary and coral reefs,the Okkubi River estuary,Okinawa Island,Japan

Sedimentation process of fine-grained terrigenous red soils was observed on the seafloor by coral reefs near an estuary using a paired mooring of turbidity meter and sediment trap or a mooring of sediment trap and current meter. Silty clays contained in the bottom calcareous sands were mainly resuspended and trapped in both surface and bottom layer traps, at noncatastrophic normal sedimentation periods. In addition, silty clays were supplied to this mooring site by inflowing river. Turbidity and the flux rate determined by sediment traps show certain relationship accompanied with the coefficient, which is changed by precipitation, current, and other conditions. Resuspension process caused by a typhoon was recorded as both core sequence of sediment trap and time-series data of bottom current. Maximum velocity of 49.5 cm/s was recorded from bottom currents resulting from the passing of attyphoon at the mooring site. A graded sand layer is interbedded in dark-gray, silty clay and considered to be a resuspended sediment resulting from the passing of the typhoon. Flux of the resuspended sediments by the typhoon was estimated from this core sequence and compared to the flux observed at the normal sedimentation.     

The structure of monomeric and dimeric uranyl adsorption complexes on gibbsite: A combined DFT and EXAFS study

We investigated the structure of uranyl sorption complexes on gibbsite (pH 5.6-9.7) by two independent methods, density functional theory (DFT) calculations and extended X-ray absorption fine structure (EXAFS) spectroscopy at the U-L_III edge. To model the gibbsite surface with DFT, we tested two Al (hydr)oxide clusters, a dimer and a hexamer. Based on polarization, structure, and relaxation energies during geometry optimization, the hexamer cluster was found to be the more appropriate model. An additional advantage of the hexamer model is that it represents both edges and basal faces of gibbsite. The DFT calculations of (monomeric) uranyl sorption complexes show an energetic preference for the corner-sharing versus the edge-sharing configuration on gibbsite edges. The energy difference is so small, however, that possibly both surface species may coexist. In contrast to the edge sites, sorption to basal sites was energetically not favorable. EXAFS spectroscopy revealed in all investigated samples the same interatomic distances of the uranyl coordination environment (R_U-Oax ≈ 1.80 Å, R_U-Oeq ≈ 2.40 Å), and towards the gibbsite surface (R_U-O ≈ 2.87 Å, R_U-Al ≈ 3.38 Å). In addition, two U-U distances were observed, 3.92 Å at pH 9.7 and 4.30 Å at pH 5.6, both with coordination numbers of ∼1. The short U-U distance is close to that of the aqueous uranyl hydroxo dimer, UO₂(OH)₂, reported as 3.875 Å in the literature, but significantly longer than that of aqueous trimers (3.81-3.82 Å), suggesting sorption of uranyl dimers at alkaline pH. The longer U-U distance (4.30 Å) at acidic pH, however, is not in line with known aqueous uranyl polymer complexes. Based on the EXAFS findings we further refined dimeric surface complexes with DFT. We propose two structural models: in the acidic region, the observed long U-U distance can be explained with a distortion of the uranyl dimer to form both a corner-sharing and an edge-sharing linkage to neighboring Al octahedra, leading to R_U-U = 4.150 Å. In the alkaline region, a corner-sharing uranyl dimer complex is the most favorable. The U-O path at ∼2.87 Å in the EXAFS spectra arises from the oxygen atom linking two Al cations in corner-sharing arrangement. The adsorption structures obtained by DFT calculations are in good agreement with the structural parameters from EXAFS analysis: U-Al (3.394 Å), U-U (3.949 Å), and U-O (2.823 Å) for the alkaline pH model, and U-Al (3.279 Å), U-U (4.150 Å), and U-O (2.743 Å) for the acidic pH model. This work shows that by combining EXAFS and DFT, consistent structural models for uranyl sorption complexes can be obtained, which are relevant to predict the migration behavior of uranium at nuclear facilities.     

Melting relationships in the system Fe-Feo at high pressures: Implications for the composition and formation of the earth's core

A reconnaissance investigation has been carried out on melting relationships in the system Fe-FeO at pressures up to 25 GPa and temperatures up to 2200° C using an MA-8 apparatus. Limited studies were also made of the Co-CoO and Ni-NiO systems. In the system FeFeO, the rapid exsolution of FeO from liquids during quenching causes some difficulties in interpretation of textures and phase relationships. The Co-CoO and Ni-NiO systems are more tractable experimentally and provide useful analogues to the Fe-FeO system. It was found that the broad field of liquid immiscibility present at ambient pressure in the Co-CoO system had disappeared at 18 GPa, 2200° C and that the system displayed complete miscibility between molten Co and CoO, analogous to the behaviour of the Ni-NiO system at ambient pressure. The phase diagram of the system Fe-FeO at 16 GPa and from 1600–2200° C was constructed from interpretations based on the textures of quenched run products. The solubility of FeO in molten iron is considerably enhanced by high pressures. At 16 GPa, the Fe-FeO eutectic contains about 10–15 mol percent FeO and the eutectic temperature in this iron-rich region of the system occurs at 1700±25° C, some 350° C below the melting point of pure iron at the same pressure. The solubility of FeO in molten Fe increases rapidly as temperature increases from 1700 to 2200° C. A relatively small liquid immiscibility field is present above 1900° C but is believed to be eliminated above 2200° C. This inference is supported by thermodynamic calculations on the positions of key phase boundaries. A single run carried out on an Fe₅₀ FeO₅₀ composition at 25 GPa and 2200° C demonstrated extensive and probably complete miscibility between Fe and FeO liquids under these conditions. The melting point of iron is decreased considerably by solution of FeO at high pressures; moreover, the melting point gradient (dP/dT) of the Fe-FeO eutectic is much smaller than that of pure iron and is also smaller than that of mantle pyrolite under the P, T conditions studied. These characteristics make it possible for melting of metal phase and segregation of the core to proceed within the Earth under conditions where most of the mantle remains below solidus temperatures. Under these conditions, the core would inevitably contain a large proportion of dissolved FeO. It is concluded therefore, that oxygen is likely to be the principal light element in the core. The inner core may not be composed of pure iron, as often proposed. Instead, it may consist of a crystalline oxide solid solution (Ni, Fe)₂O.