Effects of the Upstream-Flow Regime and Canyon Aspect Ratio on Non-linear Interactions Between a Street-Canyon Flow and the Overlying Boundary Layer

Large-scale structures within a rough-wall boundary layer generated over a cube array have recently been linked to small-scale fluctuations close to the roughness through a dynamical mechanism similar to amplitude modulation. Demonstrating the existence of this mechanism for different roughness types is a crucial step towards the development of a generic model for wind fluctuations in the urban canopy. Here the influence of the upstream roughness geometry (two-dimensional (2D) and three-dimensional (3D)) and planform packing density (\( \lambda_{p} \)) and street-canyon aspect ratio on the non-linear interactions between large-scale momentum regions and the small scales induced by the presence of the roughness is studied within a wind tunnel using combined particle-image velocimetry and hot-wire anemometry. A multi-time delay linear stochastic estimation is used to decompose the flow into large scales that participate in modulation and the remaining small scales. Using three different upstream roughness configurations composed of either 3D cubes or 2D rectangular blocks it is shown that the upstream roughness configuration has an influence on the non-linear interactions in the rough-wall boundary layer. Analysis of the turbulence skewness decomposition shows a change in the location of the maximum of the term \( \overline{{u_{L}^{\prime} u_{S}^{\prime 2}}} \), which represents the influence of the large-scale momentum regions on the small scales, whilst the temporal correlation shows a modification of the interaction located closer to the roughness with a change from 3D to 2D roughness. Furthermore, a two-point spatio–temporal correlation demonstrates that the non-linear relationship is significantly modified in the wake-interference-flow regime compared to the skimming-flow regime. Through skewness decomposition and temporal correlations the canyon aspect ratio is shown to have no influence on the non-linear interactions, indicating that the mechanism depends only on the flow developing upstream. Finally, although the upstream roughness configuration is shown to influence the non-linear interactions, the nature of the mechanism remains the same in all configurations.     

A geophysical transect of the Adélie Margin,East Antarctica

In 1982, a geophysical survey of the Antarctic margin, including multichannel seismic, gravity, magnetic and bathymetric surveying, was carried out off Adélie Coast-Wilkes Land and in the eastern Ross Sea. Of the 5000 km of lines recorded, 3000 km were in the Adélie Coast area. Lines ATC 101–102, approximately following meridian 138°E, show the first complete transect of the Adélie Coast margin from the Southeast Indian abyssal plain at DSDP Site 269 to the continental shelf. These lines reveal a thick sedimentary series divided into three main acoustic units by two major unconformities considered to be Upper Eocene (42 Ma) and late Oligocene (25 Ma). Oceanic or continental basement can be traced under the whole area, and the ocean-continent boundary clearly lies beneath the lower continental slope. A deep, high-amplitude, low-frequency horizon, extending under the oceanic basement for over 300 km of line, is considered to be the Moho. Our interpretation supports a recent revision be Cande and Mutter proposing an early Upper Cretaceous opening between Australia and East Antarctica.     

New high-pressure and high-temperature metal/silicate partitioning of U and Pb: Implications for the cores of the Earth and Mars

In order to quantify possible fractionation of U and Pb into a metallic core, we have performed piston cylinder and multi-anvil press experiments at high pressure (up to 20 GPa) and high temperature (up to 2400 °C) and obtained the distribution coefficient D_{metal-silicate} and the exchange partition coefficient K_{metal-silicate} for these elements between metal and silicates (mineral or liquid). and depend strongly on the S content of the metallic phase, and also on the oxygen fugacity, in agreement with an effective valence state of 4 for U in silicates and 2 for Pb in silicates. and show no discernable pressure and temperature trend. U remains lithophile even at high pressure and high temperature but its lithophile nature decreases at very low oxygen fugacity. From our experimental data, it was possible to calculate the U and Pb contents of the cores of Mars and Earth under core-mantle equilibrium conditions at high pressure and high temperature. From the D_{metal-silicate} of the present study, we obtained that: 0.008 ppm < Pb_{in the core} <4.4 ppm, and 0.0003 ppb < U_{in the core} < 0.63 ppb, depending on whether the metal is S-free or S-saturated respectively, and if the mantle was molten or solid during the segregation process of the Earth’s core around ΔIW-2. For Mars, based on a core segregation process around ΔIW-1, we obtained that: 0.005 ppm < Pb_{in the core} < 3 ppm, and 0.00002 ppb < U_{in the core} < 0.05 ppb, depending on the metallic composition: S-free or S-saturated respectively.Our results suggest that the low concentration of Pb in the terrestrial mantle could not be explained by an early Pb sequestration in the Earth’s core even if S is the dominant light element of the core. If we assume a magma ocean scenario, U might produced a maximum value of 1.5% of the total heat budget of the core with a segregation occurring below ΔIW-3. The values found in the present study for U in the Martian core suggest that the magnetic field activity of Mars before ∼0.5 b.y. after its formation would be difficult to ascribe to the decay of U alone.     

Formation of todorokite from vernadite in Ni-rich hemipelagic sediments

Todorokite is considered to form from vernadite in nature and commonly concentrates nickel. However, this mineralogical transformation has never been imaged nor explained mechanistically, and its effect on the uptake of nickel has never been quantified at the molecular-level. We have characterized these reactions at the macroscopic, microscopic, nanoscopic and atomic scales in a marine manganese concretion by combining transmission electron microscopy, electron and X-ray microprobe analysis, powder and micro X-ray diffraction, and Mn and Ni K-edge EXAFS spectroscopy. The concretion was collected during the Ticoflux II expedition near the Nicoya Peninsula, Costa Rica, and is representative of Mn deposits in hemipelagic sediments. It consists of 5 to 25 μm aggregates, shaped like sea-urchins, with a core of 7Å-vernadite (1.0 wt% Ni), a rim of 10Å-vernadite (3.8 wt% Ni), and an outermost region of todorokite fibers (1.9 wt% Ni) that extend outwards. The crystallites of 7Å-vernadite are single- to bi-layered, with hexagonal layer symmetry (a = b = 2.83 Å), and an average structural formula of . The crystallites of 10Å-vernadite contain 10 to 20 layers semi-coherently stacked in the ab plane and uniformly separated in the [0 0 1] direction by ∼9 Å due to the intercalation of hydrated Mg²⁺ cations. The average structural formula of 10Å-vernadite is if the layers contain vacancy sites, or alternately , if they contain Mn³⁺. The average formula of todorokite is .A genetic model is proposed based on combining these new data with previously published results. The thermodynamically unstable 7Å-vernadite transforms via dissolution-recrystallization to semi-ordered Mg-rich 10Å-vernadite. Nickel is released from dissolved biogenic silica or reduced organic matter, and taken up mainly in the Mn layer of 10Å-vernadite. Interlayer magnesium serves as a template to the further topotactic transformation of 10Å-vernadite to todorokite. The dimension of the todorokite tunnels in the [0 0 1] direction is uniform and determined by the size of the hydrated Mg²⁺ ion (8.6 Å). The tunnel dimension in the [1 0 0] direction depends on the density of Mg²⁺ in the interlayer and the superstructure of the phyllomanganate layer. If the parent phyllomanganate contains high amounts of Mg²⁺ (i.e., high layer charge), or Mn³⁺ and Mn⁴⁺ cations ordered following the Mn³⁺-Mn⁴⁺-Mn⁴⁺ sequence as in synthetic triclinic birnessite, then the tunnel dimension is ideally 3 × 3 octahedral chain widths in both crystallographic directions. Otherwise, the tunnel dimension is incoherent in the [1 0 0] direction (i.e., T(3,n) tunnel structure), as has been observed in all natural todorokites. Natural todorokite is defective because the precursor natural phyllomanganates either have a layer charge deficit below 0.33e per octahedral site, or rarely are triclinic birnessite. The abundance of Mg in seawater and its key role in converting phyllomanganate to tectomanganate with T(3,n) tunnel structure explain why todorokite is common in marine ferromanganese oxides, and seldom present in terrestrial environments. The topotactic phase transformation described here is the only known route to todorokite crystallization. This implies that all natural todorokites may be authigenic because they are formed in situ from a phyllomanganate precursor.     

Neutral Hydrogen and Its Emission Lines in the Solar Corona

Since the Lyman-\(\alpha\) rocket observations of Gabriel (Solar Phys. 21, 392, 1971), it has been realized that the hydrogen (H) lines could be observed in the corona and that they offer an interesting diagnostic for the temperature, density, and radial velocity of the coronal plasma. Moreover, various space missions have been proposed to measure the coronal magnetic and velocity fields through polarimetry in H lines. A necessary condition for such measurements is to benefit from a sufficient signal-to-noise ratio. The aim of this article is to evaluate the emission in three representative lines of H for three different coronal structures. The computations have been performed with a full non-local thermodynamic-equilibrium (non-LTE) code and its simplified version without radiative transfer. Since all collisional and radiative quantities (including incident ionizing and exciting radiation) are taken into account, the ionization is treated exactly. Profiles are presented at two heights (1.05 and 1.9 solar radii, from Sun center) in the corona, and the integrated intensities are computed at heights up to five solar radii. We compare our results with previous computations and observations (e.g. L\(\alpha\) from Ultraviolet Coronal Spectrometer) and find a rough (model-dependent) agreement. Since the H\(\alpha\) line is a possible candidate for ground-based polarimetry, we show that in order to detect its emission in various coronal structures, it is necessary to use a very narrow (less than 2 Å wide) bandpass filter.     

Determination of the earth rotation parameters from optical astrometry observations, 1962.0–1982.0

The optical astrometric data of the years 1962–1982 have been reduced once again at the Bureau International de l’Heure (BIH) in order to redetermine the Earth Rotation Parameters (ERP). This new reduction is based on serie largely revised by the stations since their use in the operational work of theBIH, and on some series which were not available previously. A total of 113 stations is considered, totaling nearly 500,000 measurements of time or latitude. TheERP are determined at five-day intervals. A new approach is developed: the catalog and local errors are analysed and corrected as group unknowns, which values are adjusted together with the main unknowns. The results obtained in the new reduction are compared to other series obtained by astrometry and space geodesy.     

Experimental transport of Si,Al and Mg in hydrothermal solutions: an application to vein mineralization during high-pressure,low-temperature metamorphism in the French Alps

A study of hydrothermal vein mineralization in meta-argillites subjected to high-pressure, low-temperature metamorphism reveals that ferromagnesian (e.g., chlorite) and pure aluminosilicate (e.g., pyrophyllite) mineralization can be correlated with regimes of increasing and decreasing temperature, respectively. An experimental study of the transport of silica, aluminum and magnesium in hydrothermal solutions has been undertaken to simulate variations in the physical conditions during metamorphism and the accompanying mass transport in a closed system. Thermodynamic and kinetic analysis of the experimental results indicates that local equilibrium among aqueous and mineral phases controls the distribution and composition of hydrothermal vein mineralization and that vein mineralogy can be used to infer the sense of variation of pressure and temperature during metamorphism.     

The essential contribution of indigenous knowledge to understanding natural hazards and disaster risk: historical evidence from the Rwenzori (Uganda)

Natural Hazards - The integration of indigenous knowledge into understanding disasters from natural hazards is hitherto hampered by the limited conceptualization of the process that shapes...     

Development of crystallographic preferred orientations by nucleation and growth of new grains in experimentally deformed quartz single crystals

Deformation experiments have been carried out to investigate the effect of dynamic recrystallisation on crystallographic preferred orientation (CPO) development. Cylindrical samples of natural single crystals of quartz were axially deformed together with 1 vol.% of added water and 20 mg of Mn₂O₃ powder in a Griggs solid medium deformation apparatus in different crystallographic orientations with compression direction: (i) parallel to <c>, (ii) at 45° to <c> and 45° to <a> and (iii) parallel to <a>. The experiments were performed at a temperature of 800 °C, a confining pressure of 1.2 GPa, a strain rate of  10^− 6 s^− 1, to bulk finite strains of  14–36%. The deformed samples were analysed in detail using optical microscopy, electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Two different microstructural domains were distinguished in the deformed samples: (i) domains with undulatory extinction and deformation lamellae, and (ii) domains with new recrystallised grains. Within the domains of undulatory extinction, crystal-plastic deformation caused gradual rotations of the crystal lattice up to  30° away from the host orientation. New recrystallised grains show a strong CPO with c-axis maxima at  45° to the compression direction. This is the case in all experiments, irrespective of the initial crystallographic orientation. The results show that c-axes are not continuously rotated towards the new maxima. The new grains thus developed through a mechanism different from subgrain rotation recrystallisation. New grains have a subeuhedral shape and numerous microcavities, voids, fluid channels and fluid inclusions at their grain boundaries. No host control is recorded in misorientation axes across their large angle grain boundaries. New grains might have been created by nucleation from solution in the μm-scale voids and microfractures. The CPO most likely developed due to preferred growth of the freshly precipitated grains with orientations suitable for intracrystalline deformation at the imposed experimental conditions.