Altérations dans la zone tropicale. Signification à travers les mécanismes anciens et/ou encore actuelsWeathering in tropical zone. Significance through ancient and still active mechanisms

The microscopic study of the organization of soils and alterites reveals the direction of evolution and the genetic and historical relationship between their constitutive minerals and how, subsequently, geochemical processes could be visualized. If certain soils and alterites may be considered to be the products of recent weathering, others span a much longer time interval. These different aspects of the weathering mantle coexist in the same landscape, and this situation makes them difficult to distinguish. In other words, the weathering mantle that extends over most continental areas is a real patchwork in search for an equilibrium never reached. So, in lateritic weathering mantles, most of genetic and evolutive modelling reveals the complexity of geochemical processes and time scale required to their differentiation ranges from one to several million years. The weathering rates and ages of weathering profiles are discussed as a function of processes that run at the top and at the bottom of weathering mantles. To cite this article: D. Nahon, C. R. Geoscience 335 (2003).     

Dynamics and control of towed underwater vehicle system, part II: model validation and turn maneuver optimization

This paper presents a validation of a three-dimensional dynamics model of a towed underwater vehicle system and discusses an application of the model to improve the performance of the system during a turn maneuver. The model was validated by comparing its results to experimental sea trial data, as well as to results from another independently developed simulation. The dynamics model was then imbedded in an optimization routine. This routine was used to vary turn radii in order to improve the U-turn performance. Significant improvements were obtained relative to a standard semicircular turn geometry.     

La transformation de Lyapunov de l'equation de Hill et son interpretation dynamique

Résumé La transformation de Lyapunov transforme une équation de Hill en une autre qui occupe la même place dans la classification de Yakubovich.Soit (C) une solution périodique d'un système conservatif à deux degrés de liberté. D'après le principe de moindre action de Maupertuis (C) est l'image d'une géodésique ().Nous montrons que les équations aux variations au voisinage de (C) et de () sont réductibles à deux équations de Hill qui se correspondent par une transformation de Lyapunov.

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The Lyapunov transformation of Hill's equation and his dynamic interpretation

The Lyapunov transformation carries Hill's equationÿ+F(t)y=0,F(t+T)=F(t) into another one which belongs to the same class in Yakubovich's classification.Let (C) be a closed trajectory of a Lagrangian conservative system with two degrees of freedom. By the Principle of Least action, we know that (C) is the image of a geodesic () of a certain two-dimensional surface ().We show that the two Hill equations associated with (C) and () are related by a certain Lyapunov transformation.

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Paper presented at the 1981 Oberwolfach Conference on Mathematical Methods in Celestial Mechanics.     

Imprints of wave climate and mean sea level variations in the dynamics of a coastal spit over the last 250 years: Cap Ferret,SW France

In coastal areas, sea level rise (SLR) and changing wave climates are expected to be the main oceanic drivers of shoreline adjustments. These drivers have been shown to vary on a wide spectrum of spatial and temporal scales. Nonetheless, a general rule about how this variability impacts global shorelines remains to be articulated. Here, we discuss the impacts of wave climate changes and SLR on the evolution of a barrier spit–inlet system over the last 250 years. The distal end of the Cap Ferret barrier spit, SW France, has undergone large-scale oscillations that were well correlated with variations of the decadal average of the winter North Atlantic Oscillation (NAO) index. The local wave climate hindcast supports that increased alongshore wave energy fluxes associated with the positive phase of the NAO were responsible for the updrift retreat of the spit. By opposition, the spit has elongated downdrift when waves were less energetic and more shore normal, as during the negative phase of the NAO. In addition, lower rates of SLR appeared to be necessary for the spit to develop, as higher rates of SLR very likely forced the adjacent inlet to enlarge, at the expense of the spit. These results should help to predict and detect coastal adjustments driven by climate change and by climate variability. © 2019 John Wiley & Sons, Ltd.     

Marine Sr (Ca) input in Quaternary volcanic rock weathering profiles from the Mediterranean coast of Morocco: Sr isotopic approach

Carbonate commonly accumulates in weathering profiles developed on noncalcareous parent material under arid and semiarid climatic conditions. Such weathering profile types are widespread in Morocco, making it critical to know the sources of the calcium in order to understand the genesis of carbonate-rich soils. In this study, we use ⁸⁷Sr/⁸⁶Sr as a geochemical tracer of the Ca sources in weathering profiles developed on volcanic rocks (basalt, tuff, andesite and latite) in the Mediterranean coast of Morocco. The results show that the most of the Sr, and hence Ca, is of an external origin, derived from (i) sea-water for the profiles located near to the coast or (ii) sea-water plus Liassic carbonate outcropping in the catchment for the profiles more distant from the coastline. The contribution of the host rock is small, being less than 25%. The ratio of the Sr (Ca) from the parent material to that from the external sources is controlled by the degree of porosity developed in the profile.     

The motion of a deep-sea remotely operated vehicle system: Part 1: Motion observations

Rapid and high-resolution motion and tension measurements were made of a caged deep-sea remotely operated vehicle (ROV) system. Simultaneous measurements were made of all six components of motion at the cage and ship A-frame and of the tension in the tether at the ship. Data were collected for cage depths of 0–1765 m. The most significant forcing was in the wave-frequency band (0.1–0.25 Hz) and accounted for over 90% of the variance of vertical acceleration. The vertical acceleration of the cage lagged the acceleration of the A-frame by up to 1.9 s and its variance was larger by up to a factor 2.2. For moderate displacements of the A-frame (≤2 m), the system is only weakly non-linear because the harmonics (3rd and 5th) of the vertical acceleration of the cage account for less than 2% of the total variance. The system is essentially one-dimensional because only the vertical motion of the cage and the vertical motion of the A-frame were coherent, while horizontal motions of the cage were weak and incoherent with any component of motion of the A-frame. The natural frequency of the system is 0.22 Hz at 1730 m, and we estimate that it is within the waveband for depths between 1450 m and the full operating depth of 5000 m.Large vertical excursions of the A-frame produce momentary slack in the tether near the cage. Retensioning results in snap loads with vertical accelerations of 0.5 gravity. Large rates of change of tension and vertical acceleration first occur at the cage during its downward motion and propagate to the surface with the characteristic speed (3870 m s⁻¹) of tensile waves for the tether. Six echoes are clearly detectable at both ends of the tether, and their pattern is extremely repeatable in different snap loads. Due to misalignment of the tether termination with the centres of mass and buoyancy, the cage pitches by up 14° during a snap. The resulting small radius of curvature poses the greatest stress on the tether.     

Chemical mass balance of calcrete genesis on the Toledo granite (Spain)

The chemical mass balance of calcrete genesis is studied on a typical sequence developed in granite, in the Toledo mountains, Central Spain.

Field evidence and petrographic observations indicate that the texture and the bulk volume of the parent rock are strictly preserved all along the studied calcrete profile.

Microscopic observations indicate that the calcitization process starts within the saprolite, superimposed on the usual mechanisms of granite weathering: the fresh rock is first weathered to secondary clays, mainly smectites, which are then pseudomorphically replaced by calcite. Based on this evidence, chemical mass transfers are calculated, assuming iso-volume transformation from the parent rock to the calcrete.

The mass balance results show the increasing loss of matter due to weathering of the primary phases, from the saprolite towards the calcrete layers higher in the sequence. Zr, Ti or Th, which are classically considered as immobile during weathering, are also depleted along the profile, especially in the calcrete layer. This results from the prevailing highly alkaline conditions, which could account for the simultaneous precipitation of CaCO₃ and silicate dissolution.

The calculated budget suggests that the elements exported from the weathering profile are provided dominantly by the weathering of plagioclase and biotite. We calculate that 8–42% of the original Ca remains in granitic relics, while only 15% of the authigenic Ca released by weathering is reincorporated in the calcite. This suggests that 373 kg/m² of calcium (i.e., three times the original amount) is imported into the calcrete from allochtonous sources, probably due to aeolian transport from distant limestone formations.