基于微粒变化对崇测冰帽浅层冰芯的定年结果

冰芯年代学的建立, 对于稳定同位素定年方法失效的钻点尤为困难. 依据粒径在0.66～1.33μm之间的不可溶尘埃微粒的浓度垂向分布, 并结合阳离子Ca2 的浓度剖面变化, 实现了对崇测冰帽冰芯浅层的断代. 该冰芯钻自海拔6 532 m的冰穹顶部, 解析的18.7 m冰芯长度占到钻点冰层深度约2/5, 辅助的定年参数包括钻点表层5 a的实测净积累率和大气核试验的地层标志. 综合各种技巧定年, 崇测冰帽该冰芯覆盖的记录年代为1902-1992年, 最底部累积误差在±2 a(约为2%).     

Disastrous sediment discharge due to typhoon-induced heavy rainfall over fossil periglacial catchments in western Tokachi,Hokkaido, northern Japan

In August 2016, Typhoon 1610 (Lionrock) caused heavy rainfall in Hokkaido, which led to the discharge of a large volume of sediment and water from catchments on the eastern slope of the northern Hidaka Range. The eight catchments examined in this study are characterized by granitic lithology and late Pleistocene periglacial landforms with weakly cohesive, low-resistance periglacial debris thickly covering the weathered bedrocks. This characteristic of the landscape presumably provided a transport-limited condition where some debris flows were initiated by shallow landslides. As they moved, the debris flows grew larger through mobilization and erosion of sediment in channel beds and sidewalls. This sediment mobilization and erosion continued for an extensive distance along the course of the river. Morphological changes induced by channel aggradation and bank erosion were considerable and distinctive from upstream to downstream. Granitic periglacial sediments are amply present on the mountain slopes, river channels, and river banks in the area, likely due to the rarity of intensive rainfall events. These distinctive features of fossil periglacial catchments are important for disaster prevention and catchment-scale sediment management in sub-boreal areas, particularly in the context of climate change, which may generate more frequent and intensive rainfall events.     

Two contrasting magmatic types coexist after the cessation of back-arc spreading

Amongst island arcs, Izu–Bonin is remarkable as it has widespread, voluminous and long-lived volcanism behind the volcanic front. In the central part of the arc this volcanism is represented by a series of seamount chains which extend nearly 300 km into the back-arc from the volcanic front. These back-arc seamount chains were active between 17 and 3 Ma, which is the period between the cessation of spreading in the Shikoku Basin and the initiation of currently active rifting just behind the Quaternary volcanic front. In this paper we present new age, chemical and isotopic data from the hitherto unexplored seamounts which formed furthest from the active volcanic front. Some of the samples come from volcanoes at the western limit of the back-arc seamount chains. Others are collected from seamounts of various sizes which lie on the Shikoku Basin crust (East Shikoku Basin seamounts). The westernmost magmatism we have sampled is manifested as a series of volcanic edifices that trace the extinct spreading centre of the Shikoku Basin known as the Kinan Seamount Chain (KSC).Chemically, enrichment in fluid-mobile elements and depletion in HFSE relative to MORB indicates that the back-arc seamount chains and the East Shikoku Basin seamounts have a significant contribution of slab-derived material. In this context these volcanoes can be regarded as a manifestation of arc magmatism and distinct from the MORB-like lavas of the Shikoku back-arc basin. ⁴⁰Ar/³⁹Ar ages range from 15.7 to 9.6 Ma for the East Shikoku Basin seamounts, indicating this arc magmatism started immediately after the Shikoku Basin stopped spreading.Although the KSC volcanoes are found to be contemporaneous with the seamount chains and East Shikoku Basin seamounts, their chemical characteristics are very different. Unlike the calc-alkaline seamount chains, the KSC lavas range from medium-K to shoshonitic alkaline basalt. Their trace element characteristics indicate the absence of a subduction influence and their radiogenic isotope systematics reflect a mantle source combining a Philippine Sea MORB composition and an enriched mantle component (EM-1). One of the most remarkable features of the KSC is that their geochemistry has a distinct temporal variation. Element ratios such as Nb/Zr and concentrations of incompatible elements such as K₂O increase with decreasing age and reach a maximum at ca. 7 Ma when the KSC ceased activity.Based on the chemical and temporal information from all the data across the back-arc region, we have identified two contrasting yet contemporaneous magmatic provinces. These share a tectonic platform, but have separate magmatic roots; one stemming from subduction flux and the other from post-spreading asthenospheric melting.     

Climate model benchmarking with glacial and mid-Holocene climates

Past climates provide a test of models’ ability to predict climate change. We present a comprehensive evaluation of state-of-the-art models against Last Glacial Maximum and mid-Holocene climates, using reconstructions of land and ocean climates and simulations from the Palaeoclimate Modelling and Coupled Modelling Intercomparison Projects. Newer models do not perform better than earlier versions despite higher resolution and complexity. Differences in climate sensitivity only weakly account for differences in model performance. In the glacial, models consistently underestimate land cooling (especially in winter) and overestimate ocean surface cooling (especially in the tropics). In the mid-Holocene, models generally underestimate the precipitation increase in the northern monsoon regions, and overestimate summer warming in central Eurasia. Models generally capture large-scale gradients of climate change but have more limited ability to reproduce spatial patterns. Despite these common biases, some models perform better than others.     

Alteration processes recorded by back-arc mantle peridotites from oceanic core complexes,Shikoku Basin,Philippine Sea

We determined the mineralogical and petrological characteristics of ultramafic rocks dredged from two oceanic core complexes: the Mado Megamullion and 23°30′N non-transform offset massif, which are located within the Shikoku back-arc basin in the Philippine Sea. The ultramafic rocks are strongly serpentinized, but can be classified as harzburgite/lherzolite or dunite, based on relict primary minerals and their pseudomorphs. Strongly elongated pyroxene porphyroclasts with undulatory extinction indicate high-temperature (≥700 °C) strain localization on a detachment fault within the upper mantle at depths below the brittle–viscous transition. During exhumation, the peridotites underwent impregnation by magmatic or hydrothermal fluids, lizardite/chrysotile serpentinization at ≤300 °C, antigorite crystallization, and silica metasomatism that formed talc. These features indicate that the detachment fault zones formed a fluid pathway and facilitated a range of fluid–peridotite interactions.     

Estimating snow distribution over a large area and its application for water resources

The spatial and temporal distribution of the snow water equivalent (SWE), snow density and snow depth were estimated by a method combining remote sensing technology and degree‐day techniques over a study area of 370 000 km². The advantages of this simulation model are its simplicity and the availability of degree‐day parameters, which can be successively evaluated by referring to snow area maps created from satellite images. This simulation worked very well for estimating SWE and helped to separate the areas of thin snow cover from heavier snowfall. However, shallow snow in warm regions led to some misjudgments in the snow area maps because of the time lag between when the satellite image was acquired and the simulation itself. Vulnerable areas, where a large variation in the amount of snow affects people's life, could be identified from the differences between heavy and light snow years. This vulnerability stems from a predicted lack of irrigation water for rice production caused by future climate change. The model developed in this study has the potential to contribute to water management activities and decision‐making processes when considering necessary adaptations to future climate change. Copyright © 2007 John Wiley & Sons, Ltd.     

Characterization of the amorphous state in metamict silicates and niobates by EXAFS and XANES analyses

X-ray absorption spectroscopy using synchrotron radiation has been applied to the investigation of the coordination geometries around Y, Zr and Nb atoms in metamict zircon, gadolinite, fergusonite, euxenite and samarskite. EXAFS and XANES spectra of their crystalline counterparts and synthetic compounds including ZrO₂, Y₂O₃, YNbTiO₆, YNbO₄, LiNbO₃, and NiNb₂O₆ were also measured for comparison. Metamict zircon shows a significant decrease in its Zr-O bond distances accompanying an increase in distortion of the Zr-O coordination polyhedra as compared with crystalline zircon. On the contrary, the average Nb-O bond distances and the symmetry of the coordination polyhedra around the Nb atoms in metamict euxenite and samarskite resemble those in the crystalline euxenite. Compared with crystalline fergusonite, a decrease in the distortion of the Nb-O octahedra is observed in metamict fergusonite. The structures of the second nearest neighbors (the metal-metal interactions) are largely disrupted in the metamict specimens except for metamict zircon and samarskite with high trivalent iron concentration. Nb in metamict samarskite is in octahedral coordination by oxygen and is similar to that in euxenite.     

Seismic Signals Induced by Snow Avalanche Flow

In this paper some observations and their analysis are presented dealing with the link between seismic signals and avalanche flow. The measurements during the passage of avalanches showed a specific spindle pattern which was quite different from the one of natural earthquakes. The general trend strongly depended on the topographic features of the avalanche path, and as a result we could estimate avalanche velocity. The running power spectrum showed a meaningful shift of the dominant peak from the front to the rear part of the flow.