Quaternary Sand Deposits and Vulnerability to Agricultural Activities in Areas of Arenização in Brazilian Pampa
Dépôts éoliens quaternaires et vulnérabilité concernant les activités agricoles en zones d'arenização dans la pampa brésilienne
Feições eólicas quaternárias e vulnerabilidades agrícolas em áreas de arenização no Pampa Brasileiro
Résumé
In southern Brazil, the southwest of Rio Grande do Sul exposes sand spots embedded in Brazilian Pampa landscape, a biome that represents 63% of the state's territory. Both stable and / or unstable wind deposits represent the inheritance of a drier climate than the current one and provide important paleoclimatic evidences for understanding this landscape. In contemporary climatic conditions, the region maintains hill fields (coxilhas, in the regional denomination), covered by herbaceous-shrub vegetation and sand spots, along testimonial slopes and valleys strands, disagreeing with the current wet climate typical landscape. These sand deposits are under wind action, in dry seasons, and subject to superficial and subsurface erosion, in rainy seasons, at once. These agents remobilize the non-consolidated eolic sediments, initiating both processes of arenização (sandisation) and sand sandy stretching. The objective of this thesis is to identify, inside the study area, associations between the current geomorphological features and the quaternary surface formations, as well as the erosive / depositional dynamics acting on these features, in the context of an area historically submitted to agricultural pressure. The spatial cutting area has about 144,000 ha and it is located along the RS-377 highway, between the municipalities of Alegrete and Manoel Viana. Methodological procedures included field, cabinet and laboratory work steps. Field observations led to the initial interpretations on the morphological evolution of stable and / or unstable eolian deposits, as well as to the survey of the four points of collection of sedimentary material, for both granulometric and morphological analyzes and for absolute dating, which was established by optically stimulated luminescence method (SAR protocol). In the cabinet stage, the mappings were carried out, in order to achieve the identification and classification of the eolic deposits and their attributes, the subdivision of the relief units and the mapping of land use and land cover. The sedimentology of the environment shows variability in its characteristics. The sediments are predominantly composed by quartz sands, of Fine sand modal class, varying to lesser fractions of Medium sand and Very fine sand. Both morphological and optical characteristics of the sediments clarify the eolic competence over the particles and their mobilization, demonstrating a certain degree of selection, which is evidenced by the decrease of the grain size in SE-NO direction, due to the reworking and the evolution of Medium sand to Fine sand. Both sub-rounded and rounded grains added percentages up to 70%, which may demonstrate a shorter rework time in an aqueous medium. Associating to roundness, sphericity and surface texture characteristics, it is estimated that both Medium sand and Fine sand grains may have been transported by the winds, displaced from the sector SE-S to N-NO, having been deposited on the region, according to the direction of the paleowind that was measured in field work. However, among grains with wind characteristics there may be grains with typical water action characteristics, such as sediments from Guará Formation or from recent fluvial environments. The geomorphological features distribution associated to the relief units shows that the eolic deposits occur substantially on the tops of hills, followed by dry valleys, reflecting on erosive processes occurrence percentage, which also occur on these compartments, majority (about 37% in hill tops and 23% in dry valleys). Likewise, the sand stretch feature occurs, on a higher percentage, at the tops of hills, followed by the dry valleys, confirming the dynamics of sand stretch activation and formation, which occur in eolic deposits, usually over hills, being triggered by erosive processes, mainly due to the action of water processes (rilling and gullying), which are typical in the current humid climate. In addition, the dunefields that were studied may be associated with broader regional dynamic contexts, present in both Argentinian and Uruguayan Pampas. In relation to land uses, which was mapped between 1982 and 2016, there is a significant increase in the area devoted to crops, in detriment to the field one. Forestry is only observed in 2016, as the incentive for exotic planting began in the late 1970s. Finally, the different uses of hills, associated to both stable and / or unstable wind deposits, determine the environmental dynamics and the economy in the Brazilian Pampa. The Pampas has livestock as its primary aptitude, but the conversion of land use, especially from pasture fields to crop, intensifies the pressure on water resources, on herbaceous vegetation and on sandy soils, which are particularly sensitive to erosion.
Dans le Sud-Ouest de l’État du Rio Grande do Sul (Sud du Brésil), la Pampa brésilienne qui occupe 63% du territoire, comprend des taches de sable (areal) enclavées dans le paysage. Elles sont en lien avec des dépôts éoliens stabilisés et/ou instables, hérités d’un climat plus sec que l’actuel et constituent des indicateurs importants pour la compréhension de ce paysage. L’objectif de la thèse est d’identifier, dans l’aire étudiée, la dynamique de l’érosion à partir des traits géomorphologiques principaux du milieu (forme, matériaux superficiels). Actuellement cette dynamique restée vive, est en grande partie en lien avec la pression exercée sur le milieu par les pratiques agricoles, ce qui agit sur la mobilité des sables. Les procédures méthodologiques comprennent des travaux de terrain et en laboratoire. Les prospections de terrain ont conduit à la cartographie des formes majeures et aux premières interprétations relatives à l’évolution géomorphologiques des dépôts de sables, stabilisés et/ou instables. Elles ont permis également de déterminer des points pour le prélèvement des échantillons de sédiments en vue des analyses granulométriques et morphoscopiques et de la datation des matériaux (réalisation de datation absolue établie par la méthode de Luminescence Optiquement Stimulée - Protocole SAR). Les résultats montrent les caractéristiques très variées de l’environnement. Au plan sédimentologique, les matériaux sont composés notamment par des sables quartzeux, de classe modale Sable Fin, dont la proportion varie peu, de Sable moyen et très fin. Les caractéristiques morphologiques et optiques des grains de sable renseignent sur la compétence éolienne et sur le transport ; elles montrent un certain degré de sélection, mis en évidence par la diminution de la taille des grains dans le sens SE-NO, due au remaniement des sables et à leur évolution de Sable moyen en Sable fin. La distribution des traits géomorphologiques, associée aux unités de relief, indique que les dépôts éoliens ont lieu substantiellement sur les sommets des collines (environ 37% des cas), suivis par les vallées sèches (environ 23% des cas). Ailleurs les sables ont souvent été remaniés par le ruissellement. Ces remaniements ont par ailleurs été amplifiés par les pratiques agricoles. D’après l’occupation du sol, cartographiée entre 1982 et 2016, on note une augmentation significative de la superficie destinée aux cultures annuelles au détriment de celle destinée aux prairies. La Pampa possède comme aptitude majeure l’élevage, mais la conversion des pâturages en parcelles cultivées a intensifié la pression sur les ressources hydriques, sur la végétation herbacée et sur les sols franchement sableux qui sont particulièrement sensibles à l’érosion.
No Sul do Brasil, o sudoeste do Rio Grande do Sul expõe manchas de areia encravadas na paisagem do Pampa brasileiro, bioma que representa 63% do território do Estado. Os depósitos eólicos estáveis e/ou instáveis representam a herança de um clima mais seco do que o atual e fornecem evidências paleoclimáticas importantes para a compreensão dessa paisagem. O objetivo deste estudo é identificar, na área de estudo, associações entre as feições geomorfológicas atuais e as formações superficiais quaternárias, assim como as dinâmicas erosivas/deposicionais atuantes nessas feições, no contexto de uma área submetida historicamente à pressão agrícola. Os procedimentos metodológicos incluíram etapas de trabalho de campo, de gabinete e de laboratório. A observação de campo conduziu as primeiras interpretações sobre a evolução morfológica dos depósitos eólicos estáveis e/ou instáveis, bem como o levantamento dos quatro pontos de coletas de material sedimentar eólico, para análises granulométrica e morfoscópica e para a realização de datação absoluta, estabelecida pelo método de Luminescência Opticamente Estimulada (Protocolo SAR). A sedimentologia do ambiente estudado mostra variabilidade em suas características. Os sedimentos são compostos predominantemente por areias quartzosas, de classe modal Areia Fina, variando em menor proporção nas frações de Areia média a de Areia muito fina. As características morfológicas e ópticas dos sedimentos esclarecem quanto à competência eólica sobre as partículas e sobre seu transporte, demostrando certo grau de selecionamento, evidenciado pela diminuição do tamanho dos grãos no sentido SE-NO, devido ao retrabalhamento e à evolução de Areia média a Areia fina. A distribuição das feições geomorfológicas, associada às unidades de relevo mostra que os depósitos eólicos ocorrem substancialmente sobre os topos de colinas, seguido de vales secos, refletindo sobre o percentual de ocorrência de processos erosivos, que também ocorrem em maior número sobre esses compartimentos (cerca de 37% em topos de colina e de 23% em vales secos). Relativamente aos usos da terra, mapeados entre os anos de 1982 e 2016, há um aumento significativo da superfície destinada à Lavoura, em detrimento à de Campo. O Pampa possui, como aptidão primária, a pecuária, mas a conversão de uso, sobretudo de campo de pastoreio em lavoura, destinada a cultivos agrícolas, intensifica a pressão sobre os recursos hídricos, sobre a vegetação herbácea e sobre os solos francamente arenosos, particularmente sensíveis à erosão.
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