Fabrice Colin

Plant impact on the biogeochemical cycle of silicon and related weathering processes

Abstract The contribution of plants to the biogeochemical cycle of Si and related weathering processes was studied in an equatorial rainforest ecosystem (Congo) where the biologic turnover of Si is high (58 to 76 kg/ha/y). Litterfall leaves, a soil profile and groundwaters were analysed. Phytoliths and organic matter have a similar distribution with depth in the soil profile. The model of a bicompartmental distribution of organic matter is applied to phytolith distribution and shows that about 92% of the biogenic silica input is rapidly recycled while about 8% of the biogenic silica input supplies a stable pool of phytoliths, with a lower turnover. Reprecipitation of silica was observed at the base of the soil profile, indicating a local geochemical environment that is oversaturated with respect to amorphous silica. A balance in biogeochemical cycle of Si requires that the vegetation absorb dissolved silicon released from weathering of minerals, which otherwise would be available for mineral neoformation or export from the profile towards regional drainages. Plant uptake of Si increases the chemical weathering rate without increasing the denudation rate. This study shows that the uptake, storage, and release of Si by the vegetation have to be taken into account when using dissolved Si for tracing chemical weathering dynamics.

In situ produced 10Be measurements at great depths: Implications for production rates by fast muons

In situ cosmogenic 10Be values have been used to investigate a Brazilian quartz vein from the surface to a depth of 15 m. At depths greater than 1000 g/cm2, deep enough for neutron-induced reactions to be insignificant, there is only a slight decrease in 10Be concentration with increasing depth. Our results are consistent with deep production of 10Be by a mechanism, presumably induced by fast muons, with an attenuation length of 5300±950 g/cm2 and a contribution of 0.65±0.25% to the total surface production. Results are compared with values from the literature and implications of this re-evaluation are discussed.

Evidence for muon‐induced production of 10Be in near‐surface rocks from the Congo

Distributions of 10Be in vein quartz from a lateritic weathering profile from the Congo provide evidence for significant in situ production of 10Be by reactions other than neutron-induced spallation. After consideration of geomorphological and nuclear processes which might contribute to the observed depth variability, we conclude that cosmic ray muon-induced reactions are the most likely mechanism, and that such reactions lead to 1% to 3% of 10Be production at the surface. This small muon-induced component has only minor implications for the use of 10Be for quantification of exposure histories of surficial rocks. Nevertheless, the muogenic component has the potential to expand the utility of cosmogenic nuclides in examination of surficial processes, overcoming some of the limitations associated with the use of the neutron-produced component.

The development of iron crust lateritic systems in Burkina Faso, West Africa examined with in-situ-produced cosmogenic nuclides

We have investigated the development of iron crust laterites on the stable West African Craton in northern Burkina Faso using cosmogenic radionuclides produced in situ in quartz veins and pebbles. Lateritic soils develop in tectonically stable, slowly eroding, tropical environments and are a major component of the Earths surface. To examine processes affecting laterite formation, we determined 10Be and 26Al in samples of quartz from three sites representing two sequential and connected iron crust laterite systems. Results from outcropping quartz veins suggest that the mean erosion rate in this region is about 3–8 m Myr−1. In addition, data from quartz cobbles and pebbles incorporated in iron crusts demonstrate that depth-dependent distributions of these nuclides may be used to distinguish surfaces undergoing burial from those undergoing erosive loss. Results from sections of the lowland lateritic system are consistent with mean accumulation rates of a few metres per million years. Quartz cobbles, presently at depths of a few metres in a paleochannel filled with rapidly deposited fluvial-colluvial material, have10Be distributions that suggest that the lowland lateritic surface may have formed during an erosive episode, presumably associated with a wetter climate, roughly 300 kyr BP. These results illustrate the practicality and the potential of the use of in-situ-produced cosmogenic nuclides for understanding the history of formation of laterites and for differentiating between systems formed through in-situ chemical weathering and mechanical transport.

Zircon: an immobile index in soils?

Interpretation of any elements mobility in soils needs rigorous means of calculating losses or gains in relation to parent rock. The first mass-balance calculations were based on isoelement changes (major elements) during conversion of fresh rocks to weathered materials (Harrisson, 1933; Leneuf, 1959). Isovolumetric mass-balance calculations were applied for the first t ime in 1955 to determine mass transfers in saprolite (Millot and Bonifas, 1955). Since that time, numerous works have been carried out on mass-balance calculations in determining transfers during weathering (Brimhall and Dietrich, 1986 ). Recently, equations used to calculate volumetric changes and net mass gain or loss of mobile elements have been based on zirconium, mostly from the mineral zircon. The aim of the paper is to question the axiomatic immobility of zircon in soils, and then to assess the limit of mass-balance calculations in surficial weathering systems.

Rare earth element mobility during conversion of nepheline syenite into lateritic bauxite at Passa Quatro, Minais Gerais, Brazil

Abstract In a lateritic bauxite formed by weathering of nepheline syenite at Passa Quatro, Minas Gerais State, Brazil, bauxites on the hill-tops directly develop from the syenite bed-rock, while downslope, a kaolinitic layer occurs between bauxite and synetie. A petrological investigation was performed on undisturbed weathered rock samples collected from a representative upslope pit. The undisturbed weathered rocks were chemically analysed for major trace elements including REE and Zr. Mass balance calculations were applied, and the behaviour of the REE in the Passa Quatro weathering system was established compared to REE reference chondrite and to REE reference parent rock. In the lateritic bauxite, the results suggest that the first stages of weathering induce a volumetric change of 50%, i.e. collapse, with respect to the parent rock, and remove REE with a slightly larger loss of the LREE, except Ce, compared to the HREE. In the upper layers, where bauxite is more mature, a net mass gain in REE is observed relative to the underlying layers. This gain takes place during the reduction of the upper layer during the downward progression of the weathering front. Very significant REE losses occurs during the bauxitization processes throughout the upslope profile. In addition, the downslope kaolinitic system is demonstrated to be depleted in REE in the same proportions as the upslope bauxite. We proposed that the REE exported in solution from the whole weathering mantle have enriched neighbouring watershed sediments.

Quantitative approach to physical and chemical gold mobility in equatorial rainforest lateritic environment

Mobility of gold throughout the global weathering system of Dondo Mobi, South Gabon, has been quantatively addressed by calculating the gold mass transfer during lateritic weathering of auriferous lisvenite, Archean gneiss and Proterozoic schist, and by proposing a thermodynamic approach to gold solubility under equatorial rainforest acidic conditions. Both chemical and short-distance translocation processes govern the mobility of gold and gold particles during the long-term evolution of collapsed open weathering systems, which result from the combined interaction of rock, meteoric water and biological agents. Free gold particles are translocated from the lisvenite-derived soil towards formerly barren Archean gneiss and porous Proterozoic formations derived from schist, to form a 200 m wide supergene gold dispersion halo, which took shape over about 0.86-1.26 million years. Simultaneously with physical translocation of gold, dissolution of gold makes the total balance of gold transfer through the whole weathering system strongly negative, which is the converse of what is generally believed to occur in lateritic systems. Taking into account hydrolysis reactions, three main gold complexes are proposed as a function of corresponding inorganic and organic ligand concentrations. These are: Au(OH),(H,O)”, AuCIOH- and Au(OH),FA-. These gold complexes leave the Dondo Mobi weathering mantle and move towards the river system in the course of both physical and chemical processes involved in the weathering of the formation.

Formation and transformation processes of iron duricrust systems in tropical humid environment

Abstract Formation and transformation processes of iron duricrust developed under tropical humid conditions in the southeastern part of the Central African Republic have been investigated in terms of geomorphological, petrological and geochemical pathways. The study of five typical weathering profiles of the most extensive iron duricrust system now known in the world indicates that saprolitization and ferruginization are sequentially involved in such processes, as reflected by kaolinite-gibbsite and hematite-goethite replacements. Alkali earths, rare earths, metals and elements belonging to chemically inert minerals segregate during saprolite and iron duricrust formation and degradation. Mass-balance calculations performed on a typical weathering system enable us to quantify net mass element transfers deriving from lateritic weathering processes. Volumetric change, collapse or dilation, as well as Al and Ti clearly discriminate between the saprolitization and ferruginization processes. Saprolite development preserves the geochemical inheritance of parent rocks, whereas ferruginization depletes it and mostly reflects, through mineralogical pathways, pedoclimatic conditions and global climatic changes. The global iron duricrust system of the southern part of Central Africa derived from previous hematite-rich crust, and not from ancient bauxites as it is generally believed for iron crusts, formed on West African shields.

Application of in situ-produced cosmogenic 10Be and 26Al to the study of lateritic soil development in tropical forest: theory and examples from Cameroon and Gabon

Abstract Depth profiles of in situ-produced cosmogenic nuclides, including 10 Be ( T 1/2 =1.5×10 6 years) and 26 Al ( T 1/2 0.73×10 6 years), in the upper few meters of the Earths crust may be used to study surficial processes, quantifying denudation and burial rates and elucidating mechanisms involved in landform evolution and soil formations. In this paper, we discuss the fundamentals of the method and apply it to two lateritic sequences located in African tropical forests.

Equatorial rain forest lateritic mantles: A geomembrane filter

The superimposed weathering layers in equatorial rain forest lateritic mantles from Gabon, AtXca, function as interactive compartments forming a dynamic semipermeable geomembrane filter. Selectivity of the filter is controlled by a progressive downward disappearance Of Connected macropore pathways created by bioturbation and dissolution. The natural balance of root activity, translocation, dissolution, deformation, and pore evolution leads to the development of porous and permeable, mature, open geochemical weathering systems at the expense of the lithosphere. These conclusions can be useful in modeling the fate of lateritic soils, which cover one-third of the emerged area of the world and which are economically important both as metal deposits and agricultural soils.