Methods

To achieve the goals of the RECA project, our multidisciplinary approach will combine several analytical methods, First, several weathering profiles have been selected in the Amazon Basin according to their presumed age and location. The secondary minerals from those profiles (kaolinites, iron oxides and oxyhydroxides) are studied from a mineralogical and crystallographic point of view from the macroscopic to the atomic scale. We will date populations of clays and iron oxides and oxyhydroxides that are well identified mineralogically. These recent dating methods are particularly suitable because they apply to the most common secondary minerals of laterites that can record geologically long times. These complementary dating methods are used to determine the major periods of crystallization of the different generations. In particular, the chronology of the formation of ferruginous duricrust will then be related to palaeoclimatic conditions (temperature, precipitation) derived from the combination of geochemical tools and mineralogical indices: (i) at the global scale, with for example known continental drainage curves, (ii) and at a more regional scale through geochemical weathering markers, analysis of goethite/hematite ratios or O and H isotope ratios on secondary minerals. In addition, « non-conventional » isotopic systems (Li, Si and Fe) will help to differentiate weathering changes related to the various stages of laterite formation and may provide indications on the redistribution processes of these elements and on the role of biological activity (vegetation, organisms) in their budget, during different stages of laterite formation. Coupling the material balance with the ages of the major weathering phases of the lateritic profiles will, in the best cases, produce weathering and erosion rates, which will be compared with other current or past weathering environments studied elsewhere on the Earth’s surface.