Dating of weathering profiles by radioactive disequilibria : contribution of the study of authigenic mineral fraction

La datation isotopique des sols est importante pour comprendre les transferts de matière liés à l'altération. Nous présentons ici une méthode incluant l'analyse par TIMS des déséquilibres (234U/238U) de fractions inférieures à 0,2 micromètres d'une latérite du Cameroun. Les variations avec la profondeur du rapport d'activité (234U/238U) font apparaître des déséquilibres importants à la base des unités pédologiques et un retour progressif vers l'équilibre au-dessus. Ces résultats suggèrent que les fractionnements de l'uranium ont lieu à la base des unités et que les fractions analysées évoluent probablement en système clos au sein de chaque unité. L'utilisation chronométrique de ces résultats donnerait un âge global de 1,5 Ma pour le développement de cette latérite. (Résumé d'auteur

Color prediction modeling for five-channel CMYLcLm printing

In printing, halftoning algorithms are applied in order to reproduce a continuous-tone image by a binary printing system. The image is transformed into a bitmap composed of dots varying in size and/or frequency. Nevertheless, this causes that the sparse dots found in light shades of cyan (C) and magenta (M) appear undesirably noticeable against white substrate. The solution is to apply light cyan (Lc) and light magenta (Lm) inks in those regions. In order to predict the color of CMYLcLm prints, we make use of the fact that Lc and Lm have similar spectral characteristics as C and M respectively. The goal of this paper is to present a model to characterize a five-channel CMYLcLm printing system using a three-channel color prediction model, where we treat the ink combinations Lc+C and Lm+M as new compound inks. This characterization is based on our previous three-channel CMY color prediction model that is capable of predicting both colorimetric tri-stimulus values and spectral reflectance. The drawback of the proposed model in this paper is the requirement of large number of training samples. Strategies are proposed to reduce this number, which resulted in expected larger but acceptable color differences

Constraining Rates of Chemical and Physical Erosion Using U-Series Radionuclides

International audienceThis chapter relates recent developments concerning the use of several U-series nuclides, in particular 234 U-238 U and 230 Th-238 U disequilibria, for constraining physical and chemical erosion rates and sediment age. Indeed, the ability to measure these disequilibria with an extremely high precision, even in samples with low concentrations such as natural waters, has opened new avenues for investigating erosional processes. This chapter is articulated in three main parts: a brief introduction and presentation of modern technical methods is followed by a description of how 234 U-238 U and 230 Th-238 U disequilibria measured in dissolved (water) and solid (sediment) river phases can be used to provide quantitative constraints on physical and chemical erosion rates at the basin scale. In parallel, recoil effects occurring during sediment formation can now be modelled and used to estimate the residence time of a sediment within a basin. Finally, the last part of this chapter presents the latest findings concerning the study of weathering profiles and the modelling of U and Th migration within an aquifer system