The variability of critical shear stress, friction angle, and grain protrusion in water-worked sediments

The erodibility of a grain on a rough bed is controlled by, among other factors, its relative projection above the mean bed, its exposure relative to upstream grains, and its friction angle. Here we report direct measurements of friction angles, grain projection and exposure, and small-scale topographic structure on a variety of water-worked mixed-grain sediment surfaces. Using a simple analytical model of the force balance on individual grains, we calculate the distribution of critical shear stress for idealized spherical grains on the measured bed topography. The friction angle, projection, and exposure of single grain sizes vary widely from point to point within a given bed surface; the variability within a single surface often exceeds the difference between the mean values of disparate surfaces. As a result, the critical shear stress for a given grain size on a sediment surface is characterized by a probability distribution, rather than a single value. On a given bed, the crtitical shear stress distributions of different grain sizes have similar lower bounds, but above their lower tails they diverge rapidly, with smaller grains having substantially higher median critical shear stresses. Large numbers of fines, trapp.ed within pockets on the bed or shielded by upstream grains, are effectively lost to the flow. Our calculations suggest that critical shear stress, as conventionally measured, is defined by the most erodible grains, entrained during transient shear stress excursions associated with the turbulent flow; this implies a physical basis for the indeterminacy of initial motion. These observations suggest that transport rate/shear stress relationships may be controlled, in part, by the increasing numbers of grains that become available for entrainment as mean shear stress increases. They also suggest that bed textures and grain size distributions may be controlled, within the constraints of an imposed shear stress and sediment supply regime, by the influence of each size fraction on the erodibility of other grain sizes present on the bed.     

Flume experiments on the alignment of transverse, oblique, and longitudinal dunes in directionally varying flows

For more than a century geologists have wondered why some bedforms are orientated roughly transverse to flow, whereas others are parallel or oblique to flow. This problem of bedform alignment was studied experimentally using subaqueous dunes on a 3–6-m-diameter sand-covered turntable on the floor of a 4-m-wide flume. In each experiment, two flow directions (relative to the bed) were produced by alternating the turntable between two orientations. The turntable was held in each orientation for a short time relative to the reconstitution time of the bedforms; the resulting bedforms were in equilibrium with the time-averaged conditions of the bimodal flows. Dune alignment was studied for five divergence angles (the angle between the two flow directions): 45°, 67–5°, 90°, 112–5° and 135°. The flow depth during all experiments was approximately 30 cm; mean velocity was approximately 50 cm s^-1 and mean grain diameter was 0–6 mm. Each experiment continued for 30–75 min, during which time the flume flow was steady and the turntable position changed every 2 min. At the end of each experiment, water was slowly drained from the flume and dune alignment was measured. Transverse dunes (defined relative to the resultant transport direction) were created when the divergence angle was 45° and 67–5°, and longitudinal dunes were created when the divergence angle was 135°. At intermediate divergence angles, dunes with both orientations were produced, but transverse dunes were dominant at 90°, and longitudinal dunes were dominant at 112–5°. One experiment was conducted with a divergence angle of 135° and with unequal amounts of transport in the two flow directions. This was achieved by changing the orientation of the turntable at unequal time intervals, thereby causing the amount of transport to be unequal in the two directions. The dunes formed during this experiment were oblique to the resultant transport direction. These experimental dunes follow the same rule of alignment as wind ripples studied in previous turntable experiments. In both sets of experiments, the bedforms developed with the orientation having the maximum gross bedform-normal transport (the orientation at which the sum of the bedform-normal components of the two transport vectors reaches its maximum value). In other words, the bedforms develop with an orientation that is as transverse as possible to the two flows. In those cases where the two flows diverge by more than 90° and transport equal amounts of sand, bedforms that are as transverse as possible to the two separate flows will be parallel to the resultant of the two flow vectors. Although such bedforms have been defined by previous work as longitudinal bedforms, they are intrinsically the same kind of bedform as transverse bedforms.     

Boron isotope variations in the atmosphere

We report here the first measurements of boron isotope ratios in the maritime atmosphere together with those of precipitation. The δ¹¹B values of atmospheric condensates in the western North Pacific and Japanese coast and snow in Tokyo range from −12.8 to +5.1‰ and from −0.4 to +0.4‰, respectively, which are significantly lower than those of rainwater (+18.9 to +34.7‰) collected mostly over the North Pacific. Since the ¹¹B/¹⁰B ratios of the atmosphere are lower than those of volcanic emissions (δ¹¹B=+2.3 to +21.4‰), we must seek sources for atmospheric boron other than volcanism. We postulate that the sea may be an important supplier for atmospheric boron under some dynamic conditions and that boron isotope fractionation during evaporation from seawater and removal from the atmosphere may account for the large variations of ¹¹B/¹⁰B ratios observed in the atmosphere and precipitation.     

High‐pressure polymorphs of magnesian orthopyroxene from a shock vein in the Yamato‐000047 lherzolitic shergottite

Abstract– We found a simple thin shock vein, less than or equal to about 60 μm in width and 1.8 mm in length, in the poikilitic area in the Yamato (Y‐) 000047 lherzolitic shergottite. The shock vein occurs only in magnesian Ca‐poor clinopyroxene, which may have transformed from orthopyroxene during the pressure increase at the shock event. The shock vein consists of (Mg_0.8,Fe_0.2)SiO₃ pyroxene polymorphs, such as columnar akimotoite, two kinds of pyroxene glasses, dendritic akimotoite, and framboidal pyroxene glass, in the order from the periphery to the center. The compositions and textures suggest that columnar akimotoite in the periphery of the shock vein crystallized from solid‐state phase transition of clinopyoroxene during the cooling of the vein, and the remains in the shock vein solidified from shock‐produced melt. The glass includes two kinds of massive glass in the vein and framboidal glass in the vein center. The framboidal glass is the most magnesian and may have been vitrified from perovskite crystallized from high‐pressure melt produced at high temperature ≥3000 °C and high‐pressure 23–40 GPa. Dendritic akimotoites in the vein center metastably crystallized from residual shock melt. The formation sequences of the constituent phases in the shock vein happen in the following order: columnar akimotoites, rim glass, center glass, framboidal glass, and dendritic akimotoites. The increase of the Raman intensity of 660–670 cm^?1 in the order of rim glass, center glass, and framboidal glass suggests that the formation of the pyroxene chain proceeds faster in the vein center than in the vein rim due to its slower cooling. The finding of the shock vein consisting merely of high‐pressure polymorphs of pyroxene, akimotoite, and framboidal glass (vitrified perovskite) is the first reported among all Martian meteorites.     

How to use the groundwater resources at the area polluted by organoarsenic compounds for Old Japanese Army Toxic Gas Weapon in Japan

从医学地质学角度看,在日本地质污染的地方如何使用地下水资源,我们面临许多问题.本文讨论了地质污染学(geo-pollution science)作为医学地质学中一门学科的概念以及在受二苯亚砷酸污染的地方成功使用农业地下水的案例.     

Timing of collision of the Kohistan–Ladakh Arc with India and Asia: Debate

The Kohistan–Ladakh Arc in the Himalaya–Karakoram region represents a complete section of an oceanic arc where the rocks from mantle to upper crustal levels are exposed. Generally this arc was regarded as of Jurassic–Cretaceous age and was welded to Asia and India by Northern and Southern Sutures respectively. Formation of this arc, timings of its collisions with Asia and India, and position of collision boundaries have always been controversial. Most authors consider that the arc collided with Asia first during 102–75 Ma and then with India during 55–50 Ma, whereas others suggest that the arc collided with India first at or before 61 Ma, and then the India–arc block collided with Asia ca 50 Ma. Recently published models of the later group leave several geological difficulties such as an extremely rapid drifting rate of the Indian Plate (30 ± 5 cm/year) northwards between 61–50 Ma, absence of a large ophiolite sequence and accretionary wedge along the Northern Suture, obduction of ophiolites and blueschists along the Southern Suture, and the occurrence of a marine depositional environment older than 52 Ma in the Indian Plate rocks south of the Southern Suture. We present a review based on geochemical, stratigraphic, structural, and paleomagnetic data to show that collision of the arc with Asia happened first and with India later.     

Unique Geochemistry of Sedimentary Iron Deposit Formed by Biologically Induced Mineralization

Abstract: Major, trace, and rare earth element contents were determined for sedimentary iron ores from the Gunma iron deposit. The deposit was precipitated from a spring water on the hillside of the active Kusatsu-Shirane Volcano. The ores are mainly composed of goethite and jarosite with various proportions of silicified andesitic detritus. Microbial fossils are often preserved well in goethite-rich ores. Goethite was likely precipitated by both inorganic and biogenic (biologically induced mineralization; BIM) processes, whereas jarosite was precipitated inorganically from the spring water. Si, Ti, Al, Mn, Mg, Ca, Na, Co, Rb, Y, Zr, Nb, Hf, Ta, U, and middle-heavy REEs (Sm-Lu) in the ores are dominantly derived from a detrital component (silicified andesitic rocks). On the other hand, Zn and V are likely to have been inorganically coprecipitated with goethite. Preferential uptake of P, Sc, Cu, Mo, Ba, Th, and light REEs occurs in the BIM ores. Unique enrichment of these elements is a promising indicator of biomineralization for ancient sedimentary iron deposits.     

Noble gases in pillow basalt glasses from the northern Mariana Trough back-arc basin

Noble gas concentrations and isotopic compositions have been measured in eight samples of pillow basalt glasses collected from seven different localities along 250 km of the Mariana Trough spreading and rifting axis. The samples have uniform and mid-ocean ridge basalt (MORB)-like ³He/⁴He values of 9–12 × 10^–6 (6.4–8.6 times atmospheric) despite large variations in ⁴He. Concentrations of the noble gases Ne, Ar, Kr, and Xe show much smaller variations between samples, but larger variations in isotopic compositions of Ne, Ar, and Xe. Excess radiogenic ²¹Ne is observed in some samples. ⁴⁰Ar/³⁶Ar varies widely (atmospheric to 1880). Kr is atmospheric in composition for all samples. Some samples show a clear excess ¹²⁹Xe, which is a well-known MORB signature. Isotopic compositions of the heavier noble gases (Ar, Kr, and Xe) in some samples, however, show more atmospheric components. These data reflect the interaction of a MORB-like magma with an atmospheric component such as seawater or of a depleted mantle source with a water-rich component that was probably derived from the subducting slab.     

Nitrogen‐isotopic compositions of IIIE iron meteorites

Abstract— The (compositionally) closely related iron meteorite groups IIIE and IIIAB were originally separated based on differences in kamacite bandwidth, the presence of carbides only in the IIIE group, and marginally resolvable differences on the Ga‐Ni and Ge‐Ni diagrams. A total of six IIIE iron meteorites have been analyzed for C and N using secondary ion mass spectrometry, and three of these have also been analyzed for N, Ne, and Ar by stepped combustion. We show that these groups cannot be resolved on the basis of N abundances or isotopic compositions but that they are marginally different in C‐isotopic composition and nitride occurrence. Cosmic‐ray exposure age distributions of the IIIE and IIIAB iron meteorites seem to be significantly different. There is a significant N‐isotopic range among the IIIE iron meteorites. A negative correlation between δ15N and N concentration suggests that the increase in s?15N resulted from diffusional loss of N.     

Origin of polymict breccias on asteroids deduced from their pyroxene fragments

Abstract— Cumulate eucrite, noncumulate eucrite, and diogenite meteorites are considered to have come from the crust of one (or similar) parent asteroid. Howardites are regarded as regolith breccias of eucrites and diogenites, and polymict eucrites are regarded as polymict breccias of eucrites. These polymict breccias show many textural and chemical features. In order to gain a better understanding of the origin of polymict breccias and the origin of their components, we investigated four polymict breccias, Yamato (Y)-791439, Y-791192, Y-82009, and Y-82049 with a scanning electron microscope (SEM) equipped with a chemical mapping system, and by electron probe microanalysis (EPMA). We analyzed all pyroxene grains with chemical maps, classified them by chemical composition, and observed their chemistry and mineralogy in detail. The characteristics of pyroxenes suggest that the polymict breccias were generated by gathering locally ordinary eucrites and cumulate eucrites. The chemical-evolutionary features of the pyroxenes (such as homogenization, chemical zoning, and exsolution lamellae) suggest that there were at least two long annealing events and one short (or low-temperature) annealing event, separated by mixing events. Local heterogeneity on the asteroidal crust is also suggested.