Gênese e geoquímica de solos em ambiente cárstico no cerrado da região de Planaltina de Goiás

• Ano de defesa: 2009
• Autor(a) principal: Lynch, Leila de Souza
• Orientador(a): Ker, João Carlos
• Banca de defesa: Moreau, Ana Maria Souza dos Santos , Abrahão, Walter Antônio Pereira
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Federal de Viçosa
• Programa de Pós-Graduação: Doutorado em Solos e Nutrição de Plantas
• Departamento: Fertilidade do solo e nutrição de plantas; Gênese, Morfologia e Classificação, Mineralogia, Química,
• País: BR
• Palavras-chave em Português: Solos; Cerrado; Calcário; Gênese; Isótopos
• Palavras-chave em Inglês: Soils; Cerrado; Limestone; Genesis; Isotopes
• Área do conhecimento CNPq: CNPQ::CIENCIAS AGRARIAS::AGRONOMIA::CIENCIA DO SOLO
• Link de acesso: http://locus.ufv.br/handle/123456789/1596

This work aimed to investigate the origin of soils in a toposequence within a karstic area in the cerrado region, with special interest on a Red Latossol that overlies the limestone, concerning the hypotheses of autocthony or allochthony of soils overlying carbonatic rocks. The geomorphological context of the area can be defined as an intraplanaltic karstic depression, developed on a pelite-carbonatic unit related to Paranoá Group. The toposequence comprises a small remnant from the Sul-Americana surface, in the form of an inlaid plateau in the depressed area. Morphological description of soils, chemical and mineralogical characterization of soils and its potential parent materials, as well as the context analysis in the field, served as a basis for this study. Chemical analyses included selective dissolution methods, as sulphuric acid attack, and extractions with citrate-dithionite and ammonium oxalate. Total analysis was also done for soil samples and potential parent materials, in which major elements and a large number of trace elements were determined. The distribution of Rare Earth Elements was also determined, as well as the Sm-Nd isotopic sistematics and the 87Sr/86Sr ratios of soils, rocks and saprolites, aiming to better discriminate the contribution from the different potential parent materials occurring in the area, to the studied soils. In order to do so, ETRs concentrations were normalized to PAAS, and the approach of binary mixture equations was applied, based on data from Sm-Nd isotopic sistematics and the 87Sr/86Sr ratios of soils, rocks and saprolites. Models based on the determined isotopic ratios (87Sr/86Sr, 143Nd/144Nd and 147Sm/144Nd) and also on the concentrations of Sm and Nd were built. Description of the profiles, along with chemical and mineralogical analyses, evince that the Paranoá pelitic material emerges on the hillslope and exerts influence upon soils in this toposequence. This pelitic material underlies the Tertiary lateritic cover and causes the in depth increase of the Ct/(Ct+Gb) ratio in soil profiles situated at high and intermediary slope positions. It also explains why most of the studied soils are kaolinitic. Mineralogical analyses also evinced the absence of carbonates in soils overlying limestone. There are chemical and morphological evidences for distinct deposition and pedogenesis cicles within the area. Such evidences indicate that most soils in the toposequence originated from the pedogenesis of pre-weathered materials eroded from higher parts, specially the soils overlying limestone. The Rare Earth Elements distribution patterns of soils show a gradation between the Tertiary lateritic cover pattern and the metapelitic saprolite pattern. The distribution patterns in limestones were distinct from all the other samples and the extremely low concentrations found for these elements could not account for the concentrations found in soils. The analyses of Rare Earth Elements distribution patterns, isotopic ratios distributions, and the application of binary mixing equations, specially the one based on Sm and Nd concentrations, allowed to constrain the following as soil parent materials: 1) the material related to the Tertiary lateritic cover, and 2) the saprolite from metapelitic rocks of the Paranoá Group. They also evinced that the limestone does not contribute to the mineral phases of soils, and only exerts influence upon its soluble phases, due to its higher dissolution rate. It was concluded that most soils in this toposequence developed from pre-weathered deposits, originated from the two parent materials mencioned above. As a consequence, the Red Latossol overlying limestone has a para-autochthonous origin, once it originated from pre-weathered materials transported in a short range. It was found that, in studies with weathering profiles, the isotopic ratios are more adequate as tracers of the more soluble mineral phases, working as a tool for estimating relative dissolution rates among minerals. On the other hand, the analysis of the distribution patterns of the Rare Earth Elements was very useful in establishing relationships between the soils and its potential parent materials, considering both the analysis of the distribution pattern of all the series, as well as through the aproach of binary mixing models based on its concentrations. A model was proposed for the evolution of the studied toposequence, which comprises four distinct phases. The inicial phase would correspond to a lacustrine environment, formed over the pelite-carbonatic unit of the Paranoá Group. Progress on the evolution of the karstic landscape would had led to the lakes regression, causing the deposition of the limestone lenses and, subsequently, of the silicatic clayey deposits over the limestone. The third phase would comprise erosion from the higher parts of this landscape, corresponding to the Tertiary lateritic cover, and deposition of coluvium over the limestone lenses and clayey deposits. The last phase would correspond to a second erosion cicle, now reaching the exposed metapelitic rocks, causing new coluvium deposition over the prior ones. According to this model, profiles P1, located at the hill top, P2, in the high slope, and P4, located in mid slope, are the exceptions, once they have not developed from such pre-weathered deposits. All the other soils show a para-autochthonous origin.