Search

In deeply weathered terrains, with high geological diversity and scarce outcrops, the lateritic regolith features support mineral research. Predictive regolith mapping uses spatial modeling and airborne gamma-ray spectrometric data to locate weathered residual products, such as the ferruginous lateritic duricrusts found in tropical regions. Although the regolith materials exhibit qualitative similarities in their gamma-ray spectrometric responses in RGB (KThU) images, the Boolean and fuzzy approaches highlighted differences that allows for mapping the regolith weathering residual products on a wide geodiversity protoliths. Two inference systems were used, one identified the ferruginous lateritic duricrusts and oxisols derived from the felsic/pelitc and mafic rocks types (TP test procedures), and the other identified solely those derived exclusively from mafic rocks (TPM test procedures). Cutoff values for the conjunction eTh/K ∩ eU/K ∩ eTh ∩ K (TP-4) using Boolean logic, provided the most accurate regolith map (accuracy = 95%) among the various tests carried out and indicate that in 28.6% of the total study area, there are lateritic duricrust and oxisols relative to rocks, saprolites, mottled horizons and other soils type. The eTh cutoff value > 4.4 ppm was used to exclude mafic rocks and associated soils, and the K value < 1.3% to exclude rocks with eTh/K and eU/K values similar to the lateritic duricrusts and oxisols. For mapping the lateritic duricrusts derived exclusively from the mafic rocks the conjunction of eTh ∩ eU/K ∩ eTh/K ∩ K ∩ eU (TPM-3) fuzzified by the large membership function was the most efficient. The eU variable with values < 1.21 ppm helps improve the effectiveness of this procedure. The generated predictive map indicated that in about ∼12% (657.9 km2) of the total study area, there are lateritic duricrusts and oxisols derived from mafic rocks. Therefore, a successful method of mapping vast areas from complicated lateritic regolith mosaic derived from a large geodiversity is to apply specific plugins for data fuzzification, supported by observations from field outcrops. • Boolean and fuzzy techniques in airborne gamma-ray spectrometric data. • Regolith with extensive lateritic residual cover. • The gamma spectrometric data reflects the chemical composition of the regolith. • Prediction of lateritic duricrusts derived from mafic rocks. • Prediction of regolith derived from metasedimentary, felsic metaplutonic and metavolcanosedimentary rocks. [ABSTRACT FROM AUTHOR]

The exceptional development of lateritic profiles make the Amazon one of the best places to conduct a refined study on the effects of long-term tropical climate on the Earth's surface rocks. Concentrations of Nd and Sr isotopes, as well as rare earth elements (REE), Sr, and other trace elements were determined in six profiles in order to shed new light into element behavior and into the processes controlling the geochemistry and the transfer of these elements among the different compartments of the critical zone. Our investigation indicates natural isotope regional pulse transformations in the Oxisol at the top of all profiles. The ƐNd(t=0) values of the parent rock, mottled horizon, and lateritic duricrust are radiogenic similar to the Amazon craton signature. The thick Oxisol has ƐNd(t=0) values which are much more radiogenic and less variable, whereas it has Zr, Th, Y, and REE in higher concentrations. These issues highlight a complex process for the Nd system, and the following possibilities are proposed to explain this behavior: formation of authigenic cerianite and/or kaolinite, penetration of low-temperature weathering solutions along zircon fractures, and some vegetation uptake. These possibilities allow keeping the more radiogenic Nd isotopes, whereas the less radiogenic Nd is released especially in the Oxisol after REE mineral(s) weathering. The Sr system contrary to that of Nd became less radiogenic along the profiles and relatively to the parent rocks content, because nearly all Sr was removed after K-Rb feldspar and mica weathering. However, a mixture return of less radiogenic Sr from plant, biogenic aerosol, and rainwater may have helped let the weathering profiles become less radiogenic. These characteristics let the Oxisol even less Sr radiogenic but still permitting to identify the general parent rocks signature. In contrast, Nd is highly fractionated in the Oxisol relative to the parent rock. Thus, the REE and Sr behavior, similar in the six profiles, does not necessarily reflect only the parent rock geochemistry, but it also depends on the multiple surficial processes typical of the critical zone: weathering, pedogenesis, plant root activity, and rainwater composition which change the inherited primary minerals isotope signatures.

Ultramafic (UM) rocks are known to be nickel (Ni) rich and to weather quickly, which makes them a good candidate to look at the Ni isotope systematics during weathering processes at the Earth’s surface. The present study aims at identifying the Ni solid speciation and discussing the weathering processes that produce Ni isotope fractionation in two deep laterite profiles under tropical conditions (Barro Alto, Goiás State, Brazil). While phyllosilicates and to a lower extent goethite are the main Ni-bearing phases in the saprolitic part of the profile, iron (Fe) oxides dominate the Ni budget in the lateritic unit. Nickel isotopic composition (δ 60 Ni values) has been measured in each unit of the regolith, i.e., rock, saprock, saprolite and laterite (n = 52). δ 60 Ni varies widely within the two laterite profiles, from −0.10 ± 0.05‰ to 1.43 ± 0.05‰, showing that significant Ni isotope fractionation occurs during the weathering of UM rocks. Overall, our results show that during weathering, the solid phase is depleted in heavy Ni isotopes due to the preferential sorption and incorporation of light Ni isotopes into Fe oxides; the same mechanisms likely apply to the incorporation of Ni into phyllosilicates (type 2:1). However, an isotopically heavy Ni pool is observed in the solid phase at the bottom of the saprolitic unit. This feature can be explained by two hypotheses that are not mutually exclusive: (i) a depletion in light Ni isotopes during the first stage of weathering due to the preferential dissolution of light Ni-containing minerals, and (ii) the sorption or incorporation of isotopically heavy Ni carried by percolating waters (groundwater samples have δ 60 Ni of 2.20 and 2.27‰), that were enriched in heavy Ni isotopes due to successive weathering processes in the overlying soil and laterite units. [ABSTRACT FROM AUTHOR]

Following an initial study of a tropical lateritic hillside system showing little iron isotope fractionation despite a strong accumulation within the soil profile, the present work investigates iron isotope signatures within the organic matter rich swamp system that represents 20% of the studied watershed surface (from Nsimi, South Cameroon). This study considers the soil–plant–water continuum in order to better understand the Fe elemental and isotopic transfer out of the ecosystem. Within the swamp system, the iron isotope compositions of gleysol samples (δ 57 Fe IRMM-14 ~+ 0.6‰) are significantly heavier than both the continental crust baseline and the reference lateritic soils from the hillslope (δ 57 Fe IRMM-14 = + 0.1‰). This enrichment towards heavy isotopes is attributed to a preferential removal of light iron isotopes during soil forming processes. Pedogenic transformations (i.e., gleyzation, organic complexation of metal and leaching) are responsible for the reductimorphic features observed in ferralitic horizons (i.e., incomplete degradation of organic matter in surface and soil whitening favored by good draining conditions). The organic carbon-rich waters of the swamp system are prone to redox processes and strong metal chelation. The dissolved iron (i.e., fraction < 0.22μm) of the Mengong stream shows positive δ 57 Fe signatures, with a downstream enrichment in heavy isotopes, from + 0.511 ± 0.266‰ to + 1.076 ± 0.240‰. The binding of iron (Fe III ) with organic matter can explain the observed enrichment in heavy isotopes in the dissolved fraction. On the contrary, plant leaves are significantly enriched in light Fe (δ 57 Fe of − 0.665 ± 0.035 and − 1.119 ± 0.080‰) relative to (i) the litter compartment (− 0.166 ± 0.078 to − 0.262 ± 0.013‰ for δ 57 Fe) and (ii) the most superficial soils. Iron isotopic compositions in plants and litter vary as a function of both plant species and season. Hence, the differences in Fe isotopic compositions between the various studied compartments suggest that Fe isotopes can be used (i) to study elemental transfers during soil pedogenesis in tropical environment and (ii) to better appraise and constrain iron biogeochemical cycle between surface horizon of soils, surface waters and the vegetation. [ABSTRACT FROM AUTHOR]