Registro sedimentar holocênico na frente deltaica do Rio Doce (ES): uma visão sedimentológica, geoquímica e mineralógica
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Data
2022-08-22
Autores
Santos, Rayme Loureiro dos
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Universidade Federal do Espírito Santo
Resumo
The Doce River Delta is a system highlighted due its sediment load, the adjacent watershed is responsible for the largest sediment discharge on the Braziliam east coast. In the delta front, object of this study, the fluvial regime and marine dynamics exert great influence on the distribution of terrigenous materials. The main purpose of this thesis was to investigate the record of the temporal-spatial variability of Holocene sedimentary processes from the application of grain size and geochemical proxies. It was also concerned to evaluate the degree of pollution associated with sedimentary deposits formed under the influence of the release of iron ore tailings, in November 2015. The work was based on 5 cores (with depth of up to 1 m) positioned south (RPS1-13.5; R3-15), in front (R1-20) and north (RPN1- 13.5; RPN2-21) of the Doce river mouth. The grain size analysis showed, through the grain size x standard deviation method, that the core located further south presented lower standard deviation values if compared to the other cores, so RPS1-13,5 is dominated by fine population (≤ medium silt). This characteristic reflects the deposition of terrigenous mud south of the mouth. Considering this pattern as a reference, fine and coarse population increments were interpreted (combined with other parameters) as greater influence of the river system (flood events) or marine dynamics (storm waves events), respectively. The increase of coarse population in the cores situated north also indicated that this direction has been the main fate of terrigenous sands, especially when southerly winds predominate on the shelf. In the geochemical approach, the multiple factor analysis (MFA)+clustering method, involving chemical and grain size variables, showed that some chemical elements indicated the same sedimentary processes diagnosed by grain size analysis. The MFA showed that clay, silt, Al and Pb represent Dimension 1 (Dim 1), so, Al and Pb represent depositional processes as well as the muddy fractions. The clustering analysis, performed from the projection of the samples on the plane formed by the global dimensions, allowed the identification of groups which can be correlated with different sedimentary processes. The Group 1, which represents Ba concentrations and sand content, included mainly sandy samples, with higher percentages of medium, coarse and very coarse 9 sand. These same samples (situated from the southern to northern portion of the shelf) were interpreted in the grain size analysis as record of storm events induced by southerly winds. Thus, Ba is also indicative of the occurrence of these episodes. Group 2, representing Fe and Cr, is mainly composed of sandy silt samples belonging to the cores positioned in front and north of the mouth. Thus, these elements are considered to be indicators of northward sediment transport. Group 3, represented by the variables silt, clay, rare earth elements, Al and Pb, included mainly samples positioned south and in front of the mouth, again reflecting deposition processes of the terrigenous mud. The main signal associated with flood event, which was diagnosed by grain size analysis, was the existence of more positive Eu/Eu* anomaly, suggesting the occurrence of higher continental leaching. The enrichment factor (EF) index indicated higher enrichment of Fe, Cr and Cu to the north and of Mn and Zn to the south and north of the mouth. Mineralogical analysis also indicated higher occurrence of ferrous minerals in these two portions of the inner shelf. Sediment deposition in these areas is mainly influenced by the volume of material brought from the river and prevailing meteoceanographic conditions. The EF indicated moderate pollution for Cr, Fe, Cu and Mn and moderate to extreme pollution for Zn.
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Processos sedimentares , Proxies granulométricos e geoquímicos , Plataforma continental adjacente ao Rio Doce , Rejeitos de minério de ferro