By Saulo Rodrigues-Filho
Mineralogical-sedimentological and geochemical investigations together with pollen info of a 12.7-m-long dated sediment center from Lake Silvana, SE Brazil, let the reconstruction of the climatic historical past over the last 10,000 years. The lowermost part (I) displays a rough paleosol-type sediment with ample 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 less than excessive transportation strength (increased precipitation). part IV, a greenish series with expanding C content material in the direction of the pinnacle, has a pollen spectrum of present-day plants (semideciduous forest). different sediment sections express a reference to erosional stages within the catchment of the lake, in line with mineralogy and geochemistry, that are a reaction to weather improvement.
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Extra info 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)
Nahon (1991 and references therein), based on studies of pedologic mantles in Senegal, have suggested that under drier climatic conditions, microaggregates of about 100 gm in diameter composed of kaolinite and iron oxyhydroxides undergo a process of segregation corresponding to a structural disorganization without major mineralogical changes. This segregation is thought to be responsible for the transition from red soils to beige soils in the center of plateaus. The formation of beige horizons at the top of weathering profiles has been recognized in several sites of the Amazonian Lowland (Irion, 1984), where beige soils are generally composed of poorly ordered kaolinite, quartz, goethite and gibbsite.
Measurements of heavy metal concentrations in sediments were performed in the <20 lam fraction. All samples were dried at 30°C until constant weight, and digested using aqua regia (HNO3/HCI, 1:3), for 3 hours at 160°C (Mailer, 1979; 29 Song and MUller, 1999). Iron, A1, Mn, Pb, Zn, Cu, Ni and Cr concentrations were determined by flame atomic absorption spectrometry - AAS (Perkin Elmer 4100), whereas Hg was measured by cold vapor AAS (Thermo Separation Products 3200). In addition, concentrations of Fe, A1, Pb, Zn, Cu, Ni and Cr were measured in selected samples using both flame AAS and X-ray fluorescence - XRF (Siemens SRS 300) for assessing the efficiency of the aqua-regia extraction.
3 Summary diagram of mineral stratigraphy and grain size distribution. Paleoenvironmental zones I to IVA as described in the text. e, ,~0"" 0 ~r~ns,z, C(org) (,/,) >20pm(%) 15 0 ° ~ - P b (pg/g) 1000 i ~ Cu (pg/g) 100 0 ~i ~ . . . - . . . C r (pglg) 100 0 Ti (%) 1200 ~ 17n ~ ~ 300 ~ = i ~ ~=- , 39o ; "== i ~ --- ~-- 420 ~' ~ ~ ~ P-"--' 480 ~. ~ ! , 550 ~" ~ ~ 1,5 ..... ~ , ~ ............ ~ ---J A oE~0 6T0 " " 700 ," J~ 740 ~' =,, m m m m i 77o; u ~800~ 830 b, 8~o ~ , i ~ i ~ ' ' m B i J 930 " 1140 i ' m m 1230 .
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