title: Organic Control of Dioctahedral and Trioctahedral Clay Formation in an Alkaline Soil System in the Pantanal Wetland of Nhecolândia, Brazil.
abstract: Recent studies have focused on the formation of authigenic clays in an alkaline soil system surrounding lakes of the Nhecolândia region, Pantanal wetland. The presence of trioctahedral Mg-smectites (stevensite and saponite types), which requires low Al and Fe contents in the soil solution for its formation, contrasts with the neoformation of dioctahedral Fe-mica (glauconite, and Fe-illite), which instead requires solutions relatively enriched in Al and Fe. This study aims to understand the conditions of co-existence of both, Mg-smectite and Fe-mica a common clay association in former or modern alkaline soil systems and sediments. The study was carried out along an alkaline soil catena representative of the region. The soil organization revealed that Mg-smectite occur in top soil close to the lake, whereas Fe-mica dominate in the clay fraction of deeper greenish horizons a few meters apart. We propose here that this spatial distribution is controlled by the lateral transfer of Fe and Al with organic ligands. Alkaline organic rich solutions (DOC up to 738 mg L-1) collected in the watertable were centrifuged and filtered through membranes of decreasing pore size (0.45 μm, 0.2 μm, 30 KDa, 10 KDa, 3 KDa) to separate colloidal and dissolved fractions. Fe, Al, Si, Mg and K were analysed for each fraction. Although the filtration had no influence on Si and K contents, almost 90% of Fe (up to 2.3 mg L-1) and Al (up to 7 mg L-1) are retained at the first cutoff threshold of 0.45μm. The treatment of the same solutions by oxygen peroxide before filtration shows that a large proportion of Fe and Al were bonded to organic colloids in alkaline soil solution at the immediate lake border, allowing Mg-smectite precipitation. The fast mineralization of the organic matter a few meters apart from the lake favors the release of Fe and Al necessary for Fe-mica neoformation. In comparison with chemical and mineralogical characteristics of alkaline environments described in the literature, the study suggests that the co-existence of trioctahedral Mg-smectite and dioctahedral Fe-mica should be regarded as a standard occurrence in alkaline soil systems with organic rich waters.
authors.affiliation: Géosciences Environnement Toulouse (GET), Observatoire de Midi-Pyrénées, Université de Toulouse, CNRS, IRD, 14 Avenue Edouard Belin, F-31400 Toulouse, France.
authors.forename: Laurent
authors.initials: L
authors.lastname: Barbiero
authors.affiliation: Institut de Recherche en Astrophysique et Planétologie (IRAP), Observatoire de Midi-Pyrénées, Université de Toulouse, CNRS, IRD, 14 Avenue Edouard Belin, F-31400 Toulouse, France.
authors.forename: Gilles
authors.initials: G
authors.lastname: Berger
authors.affiliation: FAENG-Faculdade de Engenharias, Arquitetura e Urbanismo e Geografia, Universidade Federal do Mato-Grosso do Sul, Cidade Universitária s/n, Caixa Postal 549, 79070-900 Campo Grande-MS, Brazil.
authors.forename: Ary T
authors.initials: AT
authors.lastname: Rezende Filho
authors.affiliation: Laboratoire de Chimie de Coordination (LCC), Université de Toulouse, CNRS, 205 route de Narbonne F-31077, Toulouse, France.
authors.forename: Jean-François
authors.initials: JF
authors.lastname: Meunier
authors.affiliation: Departamento de Ciências Agrárias e Ambientais, Universidade Estadual de Santa Cruz, Salobrinho 45662900, Ilhéus, Brazil.
authors.forename: Elisângela R
authors.initials: ER
authors.lastname: Martins-Silva
authors.affiliation: Departamento de Geografia (DG) Universidade de São Paulo, Avenida Pr. Lineu Prestes 338, Cidade Universitária 05508-900, São Paulo, Brazil.
authors.forename: Sonia
authors.initials: S
authors.lastname: Furian
mesh_terms: D000468:Alkalies / Q000032:analysis; D000538:Aluminum Silicates / Q000737:chemistry*; D001938:Brazil; D000077215:Clay; D019015:Geologic Sediments / Q000737:chemistry*; D008670:Metals / Q000032:analysis; D012987:Soil / Q000737:chemistry*; D053833:Wetlands
chemical_list: D000468:Alkalies; D000538:Aluminum Silicates; D008670:Metals; D012987:Soil; D000077215:Clay