By Saulo Rodrigues-Filho
Mineralogical-sedimentological and geochemical investigations together with pollen info of a 12.7-m-long dated sediment middle from Lake Silvana, SE Brazil, enable the reconstruction of the climatic historical past over the last 10,000 years. The lowermost part (I) displays a rough paleosol-type sediment with considerable plant particles and rootlets; pollen exhibits grassland crops. part II represents fine-grained lake sediments; pollen exhibits a savanna-like plants. part III represents a pollen-free, coarse-grained allogenic sediment deposited lower than excessive transportation strength (increased precipitation). part IV, a greenish series with expanding C content material in the direction of the head, has a pollen spectrum of present-day crops (semideciduous forest). the several sediment sections exhibit a reference to erosional stages within the catchment of the lake, in accordance with mineralogy and geochemistry, that are a reaction to weather improvement.
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Extra resources for A Holocene Sedimentary Record from Lake Silvana, Se Brazil: Evidence for Paleoclimatic Changes from Mineral, Trace-Metal, and Pollen Data (Lecture Notes in Earth Sciences)
A further indication of a major allogenic nature of this unit is 41 the contrasting behavior of Fe relative to Mn. Fe/A1 ratios present almost no variation within zone III, in contrast to Mn/A1 ratios, while in the overlying organic-rich zone IV both ratios are well correlated (Fig. 6B, C). This indicates the occurrence of authigenic Mn and prevailing detrital Fe oxyhydroxides associated with kaolinite. Microanalyses performed by SEM with EDS confirmed the occurrence of Mn-siderite. 6 (A) Pb concentrations as a function of the >20 ~m grain-size fraction; (B) relationship Fe/A1-Mn/A1 ratios in zone III; (C) relationship Fe/A1-Mn/AI ratios in zone IV; (D) sedimentation rates.
Fe % AI AI Ti AAS/ XRF .... % EG2/ColI. 2 EG2/ColI. 5 .... 9 .... 33 . . 5 .... 1 . . 52 30 EG1/ColI. 0 . 22 10 28 30 75 143 EG1/ColI. 1 . 5 32 73,5 113 EG 1/Co11. 2 . 15 8 26 14 35 75 EG1/ColI. 4 o - ° . -- not measured A light increase in Fe, Ti and Cr concentrations were also observed in the colluvial unit relative to the B horizon. , 1979). Both the weathering degree and the composition of the parent rock are controlling the metal distribution within the weathering profile EG2, with the possible exception of Hg.
Furthermore, Tardy and Roquin (1992) suggested that the iron content in kaolinite is dependent on the water and dissolved silica activity. An increase in hydrolysis and/or silica activity should induce an increase in iron relative to aluminum contents in ferruginous kaolinite. ) covers the lateritic sequence. It consists of a highly leached, yellowish sandy layer composed mainly of poorly ordered kaolinite, corroded quartz grains, gibbsite and goethite (Fig. 14). It is interesting to note the relatively high Hg concentrations and gibbsite contents associated with very low Pb and Cr concentrations.
A Holocene Sedimentary Record from Lake Silvana, Se Brazil: Evidence for Paleoclimatic Changes from Mineral, Trace-Metal, and Pollen Data (Lecture Notes in Earth Sciences) by Saulo Rodrigues-Filho