REconstructing the influence of Climate change on lAterite formation – RECA

First, several weathering profiles are 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.

The first part of the project focused on the preparation of the various fieldtrips, and the implementation of analytical and methodological protocols. Three fieldtrips were carried out in 2017 and 2018 (Central Amazon, Suriname, Guyana) and about 100 samples were collected along lateritic profiles. The mineral extraction protocols and mineralogical characterization of the different generations of secondary minerals have been developed. Samples collected in 2017 and 2018 are being characterized, purified (kaolinites), or micro-sampled (iron oxides and oxyhydroxides) for dating. Results of dating kaolinites and iron oxides or oxyhydroxides from Amazonian cuirasses assimilable in the RECA project have recently been published (Allard et al., 2018). Other dating data are being acquired. In parallel, work on the retention of He in goethite was carried out and an article was submitted (Bassal et al., submitted). In addition, work on conventional and non-conventional isotopes is ongoing.

Within one year, most of the data (mineralogical, geochemical, dating) will be acquired, which will make it possible to carry out temporal weathering models, particularly on the evolution of laterite profiles. These evolution models established in the Amazonian context will provide references for the study of laterites in other environments of the intertropical belt. The processes of transformation/redistribution of Fe (and the possible role of organisms) during the formation and/or alteration of a breastplate can be described by coupling the dates (different generations of Fe oxides) and the isotopic approach (isotopic variability of Fe within the same generation as well as between different generations of minerals in the breastplate). In addition, by combining isotopic data with geochemical and mineralogical indices, we will attempt to link intra- and intergenerational isotopic variations to changes in climatic conditions during the formation of duricrust. This work also opens up other perspectives with the impact of biology on mineral crystallization, and the relationship with the evolution of biodiversity and landscapes in the Amazon.

1. Allard, T., Gautheron, C., Bressan-Riffel, S., Balan, E., Selo, M., Fernandes, B. S., Pinna-Jamme, R., Derycke, A., Morin, G., Taitson Bueno, G., and Do Nascimento, N. R., 2018, Combined dating of goethites and kaolinites from ferruginous duricrusts. Deciphering the Late Neogene erosion history of Central Amazonia: Chemical Geology, v. 479, p. 136-150.
2. Bassal, F., Roques, J., Gautheron, C., and Balout, H., 2019, Helium diffusion in pure goethite (a-FeOOH) for thermochronology applications: A theoretical multi-scale study: submitted to Computational Materials Science.

Laterites are deep weathering covers of the critical zone that occupy 80% of the total soil-mantle volume of the Earth’s landscape and significantly participate to the global geochemical budget of weathering and erosion, and greenhouse gas consumption. Despite their factual importance on Earth surface, the timing of their formation and evolution in response to climatic and geodynamic forcing are still obscure. RECA project will address both the topics of "Functioning and evolution of climate, oceans and major cycles" and "Continental Surfaces: critical zone and biosphere" from ANR Axis 1 – Challenge 1., by reconstructing the influence of climate change laterites formation. The originality of the RECA project is to combine chronometric, weathering and climatic proxies developed in the recent years in order to build a comprehensive and predictive scenario of laterite formation and evolution. We will concentrate our effort on geodynamically stable Guyana Shield and Central Amazonia regions, where laterites formed through the whole Cenozoic and can be associated with major geomorphological units. This ambitious multidisciplinary project proposes, for the first time, to perform absolute dating of lateritic duricrusts associated to five episodes of planation in the South American subcontinent. We will date mineralogically well-identified populations of iron oxides and oxyhydroxides (hematite, goethite) and clays (kaolinites) by using (U-Th)/He, (U-Th)/Ne and electron paramagnetic resonance spectroscopy, respectively. These recent methods are appropriates because they can be applied to the most common secondary minerals found in laterites and span geological time scales. The inherent complexity of weathering materials, which may contain different populations of a same secondary mineral related to distinct stages of lateritization will be taken into account. The timing of duricrust formation will then be related to paleoclimatic conditions (temperature, rainfall) derived from a combination of geochemical or mineralogical indices and proxies: (i) at global scale, through, e.g., the known continental drainage curves; (ii) at a more regional scale through the intensity of weathering, the ratio hematite/goethite or O and H isotope systems of kaolinite and iron oxides and oxyhydroxides. A second task will associate non-conventional Li, Si and Fe isotopic methods that will help to decipher the evolution of weathering processes linked to the various stages of laterite formation. Coupling weathering budget and the ages of weathering profiles will yield average weathering and erosion rates, allowing comparison with other weathering environments or paleo-environments at the Earth surface. To tackle this ambitious task, the RECA project gathers an international consortium made of skilled researchers in the identification of lateritic soils, dating methods, environmental mineralogy; "traditional" and "non-traditional" stable isotope geochemistry, and modeling approaches of the formation of weathering profiles. The synergy of the identified teams offers the highest level of guarantee to lift off the identified scientific and technical barriers, giving access to yet hidden information on soil formation as a response to climate change through geological times.