Reconstrução paleoambiental holocênica através de registros de marcadores orgânicos na região Amazônica e Nordeste do Brasil

Abstract

This thesis is inserted in two cooperation projects between the CNPq and the IRDFrance. The project Climpast which targets to achieve more high resolution data in continental environments during the Holocene and the project Biogeoquímica dos lagos de Várzea da Bacia Amazônica . The organic geochemistry was used as the main tool in the reconstruction of the environmental and climatic lacustrine evolution in two distinct environments: Várzea do Lago Grande Curuai and Lake Boqueirão, located in the Amazon and Northeast region of Brazil, respectively. Two cores were collected, superficial sediments and samples of the vegetation around the lake in the studied areas. For the characterization of the organic matter (OM) were utilized global markers (C, N, S, Rock-Eval, palinology and δ13C e δ15N) and biomarkers as lignin, n-alkanes and pentacyclic triterpenes. The pentacyclic triterpenes and its degradation products were identified and quantified to evaluate the diagenesis processes occurred during the transportation and deposition of the organic matter. Some pentacyclic triterpenes stood out as the specific tracers: des-A-lupane as the marker of the macrophytes belt development or flooded vegetation and the PTMEs as the marker of grasslands. In the Várzea do Lago Grande Curuai the analysis of the organic matter composition indicates the presence of flooded forest vegetation 5700 to 5100 years cal BP. The following period, after 5100 years BP, was marked by the increase of the lake level, characterizing the existence of a more wetter. From 5000 years cal BP on, the floodplain starts to show progressive decrease of the lake level. The return of the current conditions, more wet, is observed beginning 700 years cal BP. In Lake Boqueirão the beginning of lacustrine sedimentation occurred 7100 years cal BP ago. Until 4400 years BP, the lake showed progressive increase of its waters level. In the following period from 4400 to 3600 years cal BP it was observed the establishment of the macrophytes suggesting a smaller level of the lake. Between 3200 and 2430 years cal BP the variability of the analysed parameters indicates the increase of the lake level in inconstant way, with alternation of wet and dry events. The period between 2340 and 1850 years cal BP was marked by the increase of des-A-lupane and PTMEs suggesting the decrease of the lake level and the development of marginal ecosystems communities as Eleocharis belt due to a drier climate. Between 1850 and 1450 years cal BP was observed the increase of the lake level. Between 1450 and 850 years BP was observed the return of the markers that indicate the development of the marginal vegetation, which happened in a progressive but extremely marked way indicating progressive increase of the dry conditions in the region. The next period, from 850 to 400 years cal BP was market by the return of wetter conditions since 650 years cal BP.