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Title: Distribuição mineralógica dos sedimentos superficiais da lagoa do Caçó (MA)
Keywords: Mineral;  Sedimento;  Paleoambiente;  Lagoa do Caçó (MA);  Fração Mineralógicas ( quartzo, kaolinita, sílica amorfa, goethite);  Mineralogical fraction;  Paleoenvironmental;  Lacustrine sediments;  Caçó lake (MA)
Issue Date: 1-Oct-2004
Abstract: Mineralogical fraction is important to the reconstruction of environmental and paleoenvironmental changes from lacustrine sediments. Allochtonous and Autochtonous inorganic fractions provide information about the evolution of ecosystems in the lake catchment and in the sedimentation basin and about physical and chemical water column conditions. We present results of recent mineralogical distribution from surficial sediments of Caço Lake. 59 samples were collected by diving along transverse transects and one longitudinal transect in this lake. Each transverse profile stars from a margin characterized by emergent macrophytes, crosses the central part of the lake and finishes in the other margin of the lake. All sampling points were geo-referenced and the water column depth was measured. The identification of mineralogical fraction and its quantification were done by x-ray diffractometry and by infrared spectroscopy. The distribution of the mineralogical fraction characterizes the different depth zones of the Caço lake and identify the processes that control variations of inorganic sedimentation in this lake. The most important minerals identified in this study were quartz, goethite, kaolinite and amorphous silica. Generally, along each transect quartz and amorphous silica decrease from the margin to the center of the lake. The goethite and kaolinite tend to increase in the central part of the lake. The distribution of detritic minerals appear to be mainly controlled by lake hydrodynamics, which is strongly influenced for the intensity and constancy of trade winds. For the authigenic minerals, the main factors controlling the mineral distribution must be: (1) for amorphous silica: the occupation of the littoral zone by macrophytes, which gave substrate and nutrients to the production of diatom, in larger part epiphytes; (2) for the goethite: hydrological input, by surface or sub-surface flows, which contribute with the entrance of Fe(II), which will be oxidized in the lake surface water, resulting in precipitation of goethite. Thus, the detritic minerals (quartz and kaolinite) are potentially good markers of erosion in the lake drainage basin, while authigenic minerals may give important information about the past physicochemical conditions of the environment.
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