Tectonic reconstruction of batholith formation based on the spatiotemporal distribution of Cretaceous–Paleogene granitic rocks in southwestern Japan

The spatiotemporal distribution of Cretaceous–Paleogene granitic rocks in southwestern Japan is investigated to understand the origin of the granitic batholith belt and to reconstruct the tectonic setting of emplacement. New U–Pb zircon ages for 92 samples collected from a region measuring 50 km (E–W) by 200 km (N–S) reveals a stepwise northward younging of granitic rocks aged between 95 and 30 Ma with an age‐data gap between 60 and 48 Ma. Based on the spatiotemporal distribution of granite ages, we examine two plausible models to explain the pattern of magmatic activity: (i) subduction of a segmented spreading ridge and subsequent slab melting (ridge‐subduction model), and (ii) subduction with a temporally variable subduction angle and corresponding spatial distribution of normal arc magmatism (subduction angle model). We optimize the model parameters to fit the observed magmatism in time and space, and compare the best‐fit models. As to ridge subduction model, the best‐fit solution indicates that the spreading ridge started to subduct at approximately 100 Ma, and involved a 45‐km‐wide section of the ridge segment, a subduction obliquity of 30°, and a slow migration velocity (～1.6 cm/y) of the ridge. These values are within the ranges of velocities observed for present‐day ridge subduction at the Chile trench. On the other hand, the best‐fit solution of subduction angle model indicates that the subduction angle decreases stepwise from 37° at 95 Ma, 32° at 87 Ma, 22° at 72 Ma, to 20° at 65 Ma, shifting magmatic region towards the continental side. These results and comparison, together with constraints on the geometry of the tectonic setting provided by previous studies, suggest that the ridge subduction model better explains the limited duration of magmatism, although both models broadly fit the data and cannot be ruled out.     

Spreading of river water in Suruga Bay

We have investigated the spreading of river water in Suruga Bay by performing numerical experiments and conducting field surveys with drifting buoys. There are clear seasonal variations in the large river discharges into the bay: increased discharge in the rainy summer season and decreased discharge in the dry winter season. The numerical model reproduces the main feature that has been observed in the actual sea: the river water extends gradually from the northwestern to the southeastern regions in the bay, especially in summer. The river water spreading is greatly influenced by the bottom topography of the bay: the Fuji River water spreads over a deep continental slope as a surface-advected plume and extends well offshore, since a large bulge (anticyclonic eddy at the river mouth) extends well offshore and effectively transports the river water offshore. On the other hand, the Oi River water tends to flow parallel to isobaths (along a coastline) on a shallow continental shelf as a bottomadvected plume. Moreover, the influences of seasonal variations in the stratification and a bay-scale, wind-driven circulation are also investigated. Trajectories of the drifting buoys, which were released around the Fuji River mouth, certainly suggest that the bulge exists there.     

The insoluble carbonaceous material of CM chondrites: A possible source of discrete organic compounds under hydrothermal conditions

Abstract— We report on the molecular analyses of the water‐ and solvent‐soluble organic compounds released from the insoluble organic material (IOM) of the Murray meteorite upon treatment with weight‐equivalent amounts of water and under conditions of elevated temperature and pressure. A varied suite of compounds was identified by gas chromatography‐mass spectrometry (GC‐MS). C₃‐C₁₇ alkyl dicarboxylic acids and N‐ and O‐containing hydroaromatic and aromatic compounds were found in the water extracts. The solvent extracts contained N‐, O‐, and S‐containing aromatic compounds, a large number of their isomers and homologs, and a series of polycyclic aromatic hydrocarbons (PAHs) of up to five rings, together with noncondensed aromatic species such as substituted benzenes, biphenyl, and terphenyls as well as their substituted homologs, and hydrated PAHs. Isotopic analyses showed that residue IOMs after hydrothermal treatment had lower deuterium and ¹⁵N content than the untreated material (ΔD = ?833‰ and Δ¹⁵N = ?24.1) but did not differ from it in ¹³C composition. The effect of the hydrothermolytic release was recorded in significant differences between the NMR spectra of untreated and residue IOM. A possible relation to common precursors for the dicarboxylic acids found in the IOM and bulk extracts is discussed.     

Estimation of vertical profiles of leaf drying times after daytime rainfall within a Bornean tropical rainforest

South‐east Asian Bornean tropical rainforests have large and complex canopy structures. To clarify how forest structure affects wet‐canopy evaporation, it is necessary to consider wet‐canopy evaporation processes within the forest canopy, such as vertical profiles of canopy drying time. In a previous study a method was proposed that utilizes sap flow measurements to estimate canopy drying time after rainfall (CDTobs) during daytime. In this present study, the method was applied to estimations of the vertical variations in CDTobs in 11 individual trees of various heights, ranging from 2·7 m to 53 m. The study derived vertical profiles and showed that the lengths of CDTobs in lower canopy trees were 2–4 h longer than those in the upper canopy trees. The new method for CDTobs profiles presented in this study, which is available for validation of multi‐layer biosphere‐atmospheric models, is a useful illustration for clarifying wet‐canopy evaporation processes in tropical rainforests. Copyright © 2008 John Wiley & Sons, Ltd.     

High-temperature annealing of amoeboid olivine aggregates: Heating experiments on olivine–anorthite mixtures

Amoeboid olivine aggregates (AOAs) are composed of forsteritic olivine, Fe,Ni-metal, and Ca,Al-rich nodules consisting mainly of Al-diopside, spinel, and anorthite. Although the textures, shapes, and mineralogy of AOAs are consistent with their being aggregates of nebular condensates, some textures are in conflict with equilibrium condensation calculations, indicating that AOAs were not produced by a simple one-stage condensation. To examine the origin of the constituent minerals within AOAs and their textural relationships, we performed heating experiments using mineral mixtures analogous to those in AOAs. Isothermal and cooling experiments on forsterite + anorthite mixtures reveal that a high-Ca pyroxene phase forms via the incipient melting of the two minerals. Comparative studies of heating experiments performed using the mineralogy of AOAs suggest that Al-diopside in AOAs can be produced from a small degree of melting of forsterite and anorthite. The formation of Al-diopside in this way is consistent with the annealing textures observed in AOAs, and it may account for the discrepancy between the observed mineralogy of AOAs and the results of equilibrium condensation calculations, the occurrence of two types of diopside (Al,Ti-rich diopside and Al-diopside), and the variable Al₂O₃ content of Al-diopside.     

Interannual variation of evapotranspiration in an eastern Siberian larch forest

In a deciduous larch forest in eastern Siberia, the mean and standard deviation of the total evapotranspiration (E) during May to September (day of year (DOY) = 121–274) for 2003–2006 were 181.5 and 26.4 mm, respectively. The interannual variation (IAV) in the total E was caused by the IAV in E for the canopy‐foliated period (DOY = 164–253), not by the IAV in the dates of leaf expansion and leaf fall. For the years with higher total E, E in the canopy‐foliated period was consistently higher, which corresponded to the higher soil water content in these years. Copyright © 2012 John Wiley & Sons, Ltd.     

The Tsenkher structure in the Gobi-Altai, Mongolia: Geomorphological hints of an impact origin

The Tsenkher structure, in the Gobi-Altai region of Mongolia, was studied using a wide array of remote sensing data and field observations. The structure has a shallow, 3.6 km wide, central depression bordered by a near-circular ridge (putative crater rim) with breaches to the northwest. The central depression is obliterated partially by fluvial infill through these breaches. Outside the ridge, the western side is a rugged terrain, but the eastern half is characterized by a concentric outer ridge that occurs at about one radius distance from the inner ridge. Although intrusion, salt tectonics and maar crater hypotheses cannot be completely ruled out, its morphology strongly implies an impact origin for the Tsenkher structure. If of impact origin, it has a well-preserved morphology and its position in the basin fills indicates that the formation may have occurred relatively recently, sometime during the late Tertiary or Quaternary. The outer ridge morphology is similar to rampart craters on Mars, whose formation has been attributed to fluidization of a water-rich target layer and ejecta materials, or to atmospheric entrainment and deposition of ejected materials. However, other hypotheses including erosional remnant of ejecta blanket, erosional scarp of structural rim uplift, multi-ring or deeply eroded crater rim of a peak-ring crater are also possible at this stage. A complex fluvial and probable lacustrine history is envisaged within the Tsenkher structure. The structure is also associated with archaeology, including Paleolithic and Bronze Age remains.     

Effects of slope gradient on runoff from bare-fallow purple soil in China under natural rainfall conditions

Purple soil is highly susceptible for overland flow and surface erosion, therefore understanding surface runoff and soil erosion processes in the purple soil region are important to mitigate flooding and erosion hazards. Slope angle is an important parameter that affects the magnitude of runoff and thus surface erosion in hilly landscapes or bare land area. However, the effect of slope on runoff generation remains unclear in many different soils including Chinese purple soil. The aim of this study was to investigate the relationship between different slope gradients and surface runoff for bare-fallow purple soil, using 5 m × 1.5 m experimental plots under natural rainfall conditions. Four experimental plots(10°, 16°, 20° and 26°) were established in theYanting Agro-ecological Experimental Station of Chinese Academy of Science in central Sichuan Basin. The plot was equipped with water storage tank to monitor water level change. Field monitoring from July 1 to October 31, 2012 observed 42 rainfall events which produced surface runoff from the experimental plots. These water level changes were converted to runoff. The representative eight rainfall events were selected for further analysis, the relationship between slope and runoff coefficient were determined using ANOVA, F-test, and z-score analysis. The results indicated a strong correlation between rainfall and runoff in cumulative amount basis. The mean value of the measured runoff coefficient for four experimental plots was around 0.1. However, no statistically significant relationship was found between slope and runoff coefficient. We reviewed the relationship between slope and runoff in many previous studiesand calculated z-score to compare with our experimental results. The results of z-score analysis indicated that both positive and negative effects of slope on runoff coefficient were obtained, however a moderate gradient(16°-20° in this study) could be a threshold of runoff generation for many different soils including the Chinese purple soil.     

Monitoring of DSP toxins in small-sized plankton fraction of seawater collected in Mutsu Bay,Japan, by ELISA method: relation with toxin contamination of scallop

Monitorings were conducted on DSP toxins in mid-gut gland of scallop (mouse assay), cell numbers of toxic dinoflagellate species of Dinophysis, and diarrhetic shellfish poisoning (DSP) toxins in small-sized (0.7-5 microm) plankton fraction of seawater collected from surface (0 m) and 20 m depth at a station in Mutsu Bay, Aomori Prefecture, Japan, in 2000. A specific enzyme-linked immunosorbent assay (ELISA) was employed for the analysis of DSP toxins in small-sized plankton fraction using a mouse monoclonal anti-okadaic acid antibody which recognizes okadaic acid, dinophysistoxin-1, and dinophysistoxin-3. DSP toxins were detected twice in the mid-gut gland of scallops at 1.1-2.3 MU (mouse units) g(-1) on 26 June and at 0.6-1.2 MU g(-1) on 3 July, respectively. Relatively high cell densities of D. fortii were observed on 26 June and 11 September, and may only contribute to the bivalve toxicity during late June to early July. D. acuminata did not appear to be responsible for the toxicity of scallops in Mutsu Bay in 2000. ELISA monitoring of small-sized plankton fraction in seawater could detect DSP toxins two weeks before the detection of the toxin in scallops, and could do so two weeks after the loss of the bivalve toxicity by mouse assay. On 17 July, toxic D. fortii was detected at only small number, <10 cells l(-1), but DSP toxins were detected by the ELISA assay, suggesting a presence of other toxic small-sized plankton in seawater. For the purpose of reducing negative impacts of DSP occurrences, monitorings have been carried out hitherto on DSP toxins of bivalve tissues by mouse assay and on cell densities of "toxic" species of Dinophysis. Here we propose a usefulness of ELISA monitoring of plankton toxicity, especially in small-sized fraction, which are possible foods of mixotrophic Dinophysis, as a practical tool for detecting and predicting DSPs in coastal areas of fisheries grounds of bivalve aquaculture.     

Chitinozoans and scolecodonts from the Silurian and Devonian of Japan

Silurian and Devonian chitinozoans and scolecodonts are recorded from strata of the Hida‐Gaien Terrane, central Honshu. Silurian chitinozoans include Eisenackitina, Bursachitina, and the species Angochitina elongata. The latter provides a precise biostratigraphical tie between the Japanese succession and the Type Ludlow Series of the Welsh Borderland, UK, and indicates a Ludlow age (Gorstian or early Ludfordian) for the upper member of the Yoshiki Formation in the Ichinotani Valley. Chitinozoans from other Yoshiki Formation localities contain other specimens of Lagenochitinidae. Scolecodonts are more common than chitinozoans in the palynological residues, but are mostly represented by fragments or minor apparatus elements with a low biostratigraphical value. However, material from the Yoshiki and Fukuji Formations includes several species of Mochtyella as well as representatives of Oenonites, Kettnerites, Lunoprionella, Vistulella? and possibly other placognath taxa. Scolecodonts are also present in the Silurian Middle Member of the Gionyama Formation, Kyushu, including a well‐preserved jaw of Pistoprion, and fragments of putative Kettnerites and Oenonites. These finds of scolecodonts suggest close similarity in assemblages between Paleozoic polychaete faunas of Japanese terranes and those of Baltica‐Laurentia.