A Holocene Sedimentary Record from Lake Silvana, Se Brazil: by Saulo Rodrigues-Filho

By Saulo Rodrigues-Filho

Mineralogical-sedimentological and geochemical investigations together with pollen information of a 12.7-m-long dated sediment center from Lake Silvana, SE Brazil, enable the reconstruction of the climatic heritage during the last 10,000 years. The lowermost part (I) displays a rough paleosol-type sediment with plentiful plant particles and rootlets; pollen exhibits grassland plants. part II represents fine-grained lake sediments; pollen shows a savanna-like plants. part III represents a pollen-free, coarse-grained allogenic sediment deposited below excessive transportation power (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 plants (semideciduous forest). the various sediment sections exhibit a reference to erosional levels within the catchment of the lake, in line with mineralogy and geochemistry, that are a reaction to weather improvement.

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Additional 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)

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The geochemical and mineralogical composition, with more quartz and less Fe and Ti relative to the underlying unit, supports the assumption of a saprolite derived from the biotiterich gneiss. This interpretation is supported by another study on the behavior of Fe and Ti in saprolites relative to the parent rock. Kopp (1986) has observed that Fe and Ti concentrations in saprolites of weathering profiles in Minas Gerais show almost no variation relative to the parent rock. Trace-metal concentrations in the C1 horizon revealed the highest values for Cr and Pb (Fig.

15 Typical diffractograms of poorly (top) and well ordered kaolinite (bottom)Weathering profile EG2. 52 The B horizon consists of bleached-reddish soils composed by well-ordered kaolinite, quartz and hematite. Above the groundwater table, the decreasing water activity seems to have caused the complete crystallization of kaolinite and consequently its deferruginization. This Fe-depleted horizon also corresponds to the segment with the lowest metal concentrations, indicating their association with Fe oxyhydroxides, which were probably removed into solution as Fe 2÷.

In the soil profile, AAS/XRF ratios show more accentuated variations, probably due to a less homogeneous mineral matrix. Despite of these variations within the soil profile, the large geochemical contrasts between horizons largely compensate the low efficiency of aqua-regia extraction for certain horizons (Appendices A I A7). Mineralogical analyses of sediments and soils were performed from dry samples of the fraction <20 lam using an X-ray diffractometer - XRD (Siemens 500), and a scanning electron microscope - SEM (Leo 440) with an energy dispersive spectroscope - EDS (Oxford 7060).

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