Dynamical evolution of the Oort cloud: I. A Monte Carlo simulation

We have studied the long-time dynamical evolution of a population of comets surrounding the Solar System at a large distance. Orbital changes are caused by random passing stars. We first emphasize the need for a new simulation because of the lack of completeness of previous analytical and numerical studies. Then the solar neighborhood is modeled by a sphere of 1 pc in radius, which stars cross at random in direction and distance. The geometry of the encounters allows us to compute the impulse gained by the star and the Sun, in the context of an impact approximation. Then we determine the change of orbital elements for a population of comets and follow the evolution of the frequency distribution for the five Keplerian elements. Clouds are selected in such a way that we test the two main hypotheses for the origin of the Oort cloud, and also the regions of stability in an aphelion-eccentricity diagram. We show that stellar perturbations randomize the cloud and prevent one from inferring the initial cloud configuration from the current distribution. Clouds are depleted by the diffusion of comets into the planetary regions, where they become planet-influenced comets or are ejected from the Solar System. The diffusion of aphelion toward interstellar regions proves to be the major source of cometary loss. Direct ejection to hyperbolic orbits amounts to 9% of the originally population over the age of the Solar System. Finally the current and original cloud populations are estimated at 1.8 × 10¹² and 2 × 10¹³ comets and we discuss these results.     

The Multiscale TROPIcal CatchmentS critical zone observatory M-TROPICS dataset I: The Nyong River Basin,Cameroon

Simultaneously acquiring time series of climate, hydrology and hydrochemical data over decades on river systems is pivotal to understand the complex interactions involving rock, soil water, air and biota in the Critical Zone, to build integrated modelling and to propose predictive scenarios. Among the Critical Zone Observatories (CZOs) implemented in the past 25 years, only a few are located in the humid Tropics despite the importance of these regions in terms of population density, fast-changing land use, biodiversity hotspots, biomass stock on continents, size of river systems, etc. Since 1994, weathering and erosion processes and fluxes have been investigated at both local (experimental watershed) and regional scales in the Nyong River Basin (Cameroon) which belongs to the Critical Zone Observatories network named Multiscale TROPIcal CatchmentS (M-TROPICS). The data shared by M-TROPICS in Cameroon are: (1) rainfall; (2) air temperature, air relative humidity, wind speed and direction, and global radiation; (3) stream and river water level; (4) pH, electrical conductivity, water temperature and suspended particulate matter (SPM) concentration; (5) major ion, alkalinity and dissolved organic carbon (DOC) concentrations. The dataset already contributed to describe the water partitioning in these tropical humid watersheds, to better understand the factors controlling chemical weathering and physical erosion in tropical ecosystems, particularly the role of organic matter. The dataset also contributed to calculate elemental weathering fluxes and saprolite production rate and to propose denudation rates on tropical cratonic landscapes. Hydrological modelling allowed quantification of the geographical water sources contributing to streamflow. DOC data were used to determine greenhouse-gas emissions and carbon budgets from African inland waters. However, long-term solute concentrations at the outlet of a small tributary of the Nyong River exhibit non-stationary behaviour over the last 26 years. The processes governing those fluctuations are not yet fully understood and might be related to changes in the hydrological regime, land-cover and land-use. The latter highlights the need for longer time-series and continued support for CZOs particularly in the humid tropics.     

High pressure polymorphism of dicalcium silicate Ca2SiO4. A transmission electron microscopy study

Ca₂SiO₄ dicalcium silicate has been transformed at high pressure in a diamond-anvil cell (DAC) coupled with a YAG laser heater, in order to study the high-pressure modifications of this compound. Starting material was the olivine form of Ca₂SiO₄ (γ-polymorph). Several samples have been synthesized at loading pressures of 4.5, 10 and 15 GPa respectively, at room temperature. Other samples have been obtained at pressures ranging between 4.5 and 45 GPa and temperatures estimated to be about 2500 °C. The study of the quenched high pressure and/or high temperature phases has been performed using analytical transmission electron microscopy (ATEM) and X-ray diffraction (XRD). All the polymorphs of Ca₂SiO₄ usually produced with high temperatures, including α-Ca₂SiO₄, have been observed in the samples recovered from the high-pressure experiments. The α′-Ca₂SiO₄ and α-Ca₂SiO₄ polymorphs have been obtained at ambient conditions for the first time without stabilizing impurities. A new modification of α′-Ca₂SiO₄ has also been synthesized. Finally, the breakdown at high-pressure and temperature of Ca₂SiO₄ into CaSiO₃ and CaO is reported.     

High-pressure Raman spectroscopic study of Mn2GeO4, Ca2GeO4, Ca2SiO4, and CaMgGeO4 olivines

We present a Raman spectroscopic study of the structural modifications of several olivines at high pressures and ambient temperature. At high pressures, the following modifications in the Raman spectra are observed: 1)?in Mn₂GeO₄, between 6.7 and 8.6?GPa the appearance of weak bands at 560 and 860?cm^?1; between 10.6 and 23?GPa, the progressive replacement of the olivine spectrum by the spectrum of a crystalline high pressure phase; upon decompression, the inverse sequence of transformations is observed with some hysteresis in the transformation pressures; this sequence may be interpreted as the progressive transformation of the olivine to a spinelloid where Ge tetrahedra are polymerized, and then to a partially inverse spinel; 2)?in Ca₂SiO₄, the olivine transforms to larnite between 1.9 and 2.1?GPa; larnite is observed up to the maximum pressure of 24?GPa and it can partially back-transform to olivine during decompression; 3)?in Ca₂GeO₄, the olivine transforms to a new structure between 6.8 and 8?GPa; the vibrational frequencies of the new phase suggest that the phase transition involves an increase of the Ca coordination number and that Ge tetrahedra are isolated; this high pressure phase is observed up to the maximum pressure of 11?GPa; during decompression, it transforms to a disordered phase below 5?GPa; 4)?in CaMgGeO₄, no significant modification of the olivine spectrum is observed up to 15?GPa; between 16 and 26?GPa, broadening of some peaks and the appearance of a weak broad feature at 700–900?cm^?1 suggests a progressive amorphization of the structure; near 27?GPa, amorphization is complete and an amorphous phase is quenched down to ambient pressure; this unique behaviour is interpreted as the result of the incompatibilities in the high pressure behaviour of the Ca and Mg sublattices in the olivine structure.     

Complex conductivity response to microbial growth and biofilm formation on phenanthrene spiked medium

Several laboratory studies have recently demonstrated the utility of geophysical methods for the investigation of microbial-induced changes over contaminated sites. However, it remains difficult to distinguish the effects due to the new physical properties imparted by microbial processes, to bacterial growth, or to the development of bacterial biofilm. We chose to study the influence of biofilm formation on geophysical response using complex conductivity measurements (0.1-1000 Hz) in phenanthrene-contaminated media. Biotic assays were conducted with two phenanthrene (PHE) degrading bacterial strains: Burkholderia sp (NAH1), which produced biofilm and Stenophomonas maltophilia (MATE10), which did not, and an abiotic control. Results showed that bacterial densities for NAH1 and MATE10 strains continuously increased at the same rate during the experiment. However, the complex conductivity signature showed noticeable differences between the two bacteria, with a phase shift of 50 mrad at 4 Hz for NAH1, which produced biofilm. Biofilm volume was quantified by Scanning Confocal Laser Microscopy (SCLM). Significant correlations were established between phase shift decrease and biofilm volume for NAH1 assays. Results suggest that complex conductivity measurements, specifically phase shift, can be a useful indicator of biofilm formation inside the overall signal of microbial activity on contaminated sites.     

Mapping rainstorm erosion associated with an individual storm from InSAR coherence loss validated by field evidence for the Atacama Desert

Extreme high-magnitude and low-frequency storm events in arid zones provide the necessary runoff to entrain sediments from source areas and therefore dictate the linkages between hillslopes and channels. Nevertheless, the erosive impact of large storms remains difficult to predict. Most of the uncertainty lies in the lack of topographic change maps associated with single hydro-meteorological events. Consequently, event-based erosion models are poorly constrained and their extrapolation over long time periods remains uncertain. In this study, a 15-month Sentinel-1A coherence time series, optical and field data are used to map the spatial patterns of erosion after the 5-day storm occurred on March 2015, in the Atacama Desert. The coherence change detection (CCD) analysis suggests that temporal loss of coherence is related to variations in soil moisture, while permanent loss of coherence is related to modification of soil texture by erosion and sedimentation. Importantly, permanent loss of coherence is more apparent on gentle rather than steeper slopes, likely reflecting differences in regolith cover and thickness. These findings can contradict the landscape models predicting higher erosion on steeper hillslopes. The CCD technique represents a promising tool for analysing and modelling sediment connectivity in arid areas, giving a clear picture of the relation between sediment sources and sink pathways. © 2020 John Wiley & Sons, Ltd.     

Groundwater beneath the urban area of Khan Younis City,southern Gaza Strip (Palestine): assessment for multi-domestic purposes

The present study research investigation is aimed to assess the groundwater quality for the urban area in Khan Younis City, southern Gaza Strip, for multi-domestic purposes. The physicochemical analysis of the groundwater wells shows the major ions in the order of Na⁺ > Mg²⁺ > Ca²⁺ > B³⁺ > K⁺ and Cl^? > HCO₃ ^? > SO₄ ^2? > NO₃ ^2? > F^? > PO₄ ^3?. Groundwater quality is classified as very hard-brackish water type. Ninety-five percent of the wells are classified as saline water type with high NO₃ ^2? concentrations. Based on water quality index (WQI), the groundwater falls into one of three categories: fair water (10%), poor water (15%), very poor (45%), and worst (30%). The high WQI values are because of high Na⁺, Cl^?, SO₄ ^2?, and NO₃ ^2? concentrations, while synthetic pollution index (SPI) values indicate that most about 80% of the wells are seriously polluted. Langelier Saturation Index (LSI) indicates that most of data are either slightly scale forming or corrosive water or slightly corrosive but non-scale forming, and 75% of the wells are suitable for construction purposes (have SO₄ ^2? concentrations <300 mg/L). The groundwater reaches alarming situation, where almost chemically unsuitable for drinking purposes and the water to be used after proper treatment such as desalination.     

Geodynamic aspects of the basic intrusions of central and Western Nepal

A petrological study of the basic rocks of magmatic activity in the Pokhara and Beri Khola regions of Central and Western Nepal suggests that the volcanic activities were not related to collision or continental subduction.     

Total Mercury Distribution and Volatilization in Microcosms with and Without the Aquatic Macrophyte Eichhornia Crassipes

Mercury (Hg) is one of the most toxic pollutants and spreads in the environment according to its affinity to several compartments. Aquatic macrophytes, such as Eichhornia crassipes, are known as sites for accumulation of Hg and methylmercury formation. The objective of this research was to observe Hg distribution among air, water and whole plants of the macrophyte E. crassipes for 17?days. The distribution of a single ²⁰³Hg spike was evaluated by gamma spectrometry. Two experiments, with and without macrophytes, were made, and the compartments analyzed for the presence of Hg were air, 0.2-μm filtered water, suspended and settled particles, roots, leafs, petioles and adsorption on the desiccators walls. ²⁰³Hg was detected in all analyzed compartments, and the highest total Hg concentrations were found in the roots and particles of the incubations with and without macrophytes that retained in average 68 and 34?% of added Hg, respectively. On the other hand, the lowest concentrations were found in air for both incubations, with higher volatilization (up to 2.5?% of added Hg) in the absence of macrophytes. The lower Hg values in leafs and petioles suggest this plant has mechanisms of Hg retention in the roots. Results suggest this macrophyte promotes changes in the Hg cycle since it attracts most Hg present in water and particulate to its roots and settled particles underneath and also reduces Hg volatilization.     

Experimental and numerical study of the effect of variable bathymetry on the slow-drift wave response of floating bodies

An experimental campaign is reported on the slow-drift motion of a rectangular barge moored at different positions along an inclined beach, at waterdepths ranging from 54 cm to 21 cm, and submitted to irregular beam seas. The beach is achieved by inclining the 24 m long false bottom of the tank at a slope of 5%, from a depth of 1.05 m. The slow-drift component of the measured sway motion is first compared with state-of-the-art calculations based on Newman’s approximation. At 54 cm depth a good agreement is obtained between calculations and measurements. At 21 cm depth the Newman calculations exceed the measured values. When the flat bottom setdown contribution is added up, the calculated values become 2 to 3 times larger than the measured ones. A second-order model is proposed to predict the shoaling of a bichromatic sea-state propagating in varying water-depth. This model is validated through comparisons with an extension of Schäffer’s model for a straight beach [Schäffer HA. Infragravity waves induced by short-wave groups. J Fluid Mech 1993;247:551-88] and with a fully nonlinear Boussinesq model. It appears that the long wave amplitude is much less than predicted by the flat bottom model, and that its phase difference with the short wave envelope also deviates from the flat bottom model prediction. As a result of this phase shift the actual second-order wave loads can be lower than predicted by Newman’s approximation alone. Application of the shoaling model to the barge tests yields a notably better agreement between numerical and experimental values of its slow-drift sway motion.