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|Title:||MUDANÇAS PALEOHIDROLÓGICAS EM LAGOS DE VÁRZEA DO RIO AMAZONAS|
|Other Titles:||Paleohydrological changes in Amazonian floodplain lakes.|
|Keywords:||Carbono; Bacia Amazônica; Várzeas; Mudanças paleoclimáticas; Carbon; Amazon Basin; Floodplain lakes; Paleoclimatic changes|
|Abstract:||Holocene paleoclimatic and paleohydrological changes have been reconstructed by semi-quantitative analyses of the clay minerals, total concentration of organic carbon (TOC) and total nitrogen (TN), carbon and nitrogen isotopic composition (δ13C and δ15N), concentration of sedimentary pigments, identification and quantification of the planktonic diatoms Aulacoseira sp and concentration of glycerol dialkyl glycerol tetraether (GDGT) membrane lipids of six cores from floodplain lake in Amazon Basin in order to understand the impact of the Amazon River hydrological changes in the floodplains lakes sedimentary process. The Comprido Lake, located near the Monte Alegre City, Pará, is characterized by an indirectly connexion with the Amazon River. This lake presented different characteristics when compared with the lakes directly connected with the river. The Comprido Lake record showed evidences of a dry climate between 10300 and 7800 cal yr BP suggested by low values of TOC (~ 0.4%) and chlorophyll derivate concentrations (~ 0.32 SPDU), with the development of a C4 grasses on unflooded mud banks. A break in sedimentation due to a complete dryness of the lake was observed between 7800 to 3000 cal years BP corresponding to the Middle Holocene dry phase. From 3000 cal years BP onwards a gradual increase of the TOC (~9,3%), chlorophyll derivate (~ 4.1 SPDU) and Aulacoseira sp suggest an increase in the productivity and in water lake level due to the high influence of the Amazon River and the catchment area as well. Santa Ninha Lake is located near Óbidos city, Pará state and is directly connected with the main channel of the River. Reduced Amazon River influence, with lower high water levels of the river, characterized the period between 5600 to 5100 cal years BP, suggested by low values of smectite (~ 26.3%) and high values of kaolinite (~ 45.6%). High values of TOC (~ 8.5%) and a C3-land plant vegetation was found in this period, also suggesting that the lake was isolated from the river. After 4000 cal years BP coarse sediments and quartz increased which suggest a higher inflow of the Amazon River and TOC showed the lowest values (~ 1.7%). The riverine inflow caused dilution of the organic material produced in the lake and consequently low rates of carbon flux in these phases were recorded. Nowadays, the Maracá Lake is directly connected to the Amazon River thought several channels. However, between 3600 and 1880 cal years BP, the low concentration of smectite (~12.4%) indicates that the lake was semi-isolated with a reduced influence of the Amazon River. During this period the d15N and d13CTOC were depleted while C/N ratios, TOC and branched GDGTs contents were high (~504 μg/gOC) suggesting that sedimentary organic matter was dominated by C3 land plants and soil organic matter directly transported from the Maracá Lake catchment area. Although the fluvial input seems to be higher after 1880 cal years BP, the abrupt sedimentation characterised by package of sediments occurred in 2700 cal years BP suggest wetter climate conditions. After 1880 cal years BP a significant shift in the mineralogical composition was observed. The increase in the smectite content (~ 39.4%) and several abrupt sedimentations suggests larger inputs of the Amazon River into the lake. The sedimentary organic matter was characterized by low values of C:N ratios (~ 9), TOC content (~ 2.4%) and branched GDGTs (~83 μg/gTOC) while d15N was enriched (~ 2.5 ), indicating that the contribution of phytoplankton-derived organic matter to sediments increased. These results show that the organic and sedimentary processes of the floodplain lakes are strongly influenced by the hydrodynamics of the Amazon River which in turn were linked to regional climatic changes.|
|Appears in Collections:||TEDE sem arquivo|
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