Sedimentary lake records of Northern Amazon: the radiocarbon dating as a tool for paleoenvironmental assessment

The present work focused the radiocarbon technique for calculating fluxes of tracers in core sediments, which are used to point out paleoenviromental changes. This study was developed in a lake watershed located in one of the most northen portions in the Brazilian Amazon (Caracarana Lake)covering a wide boundary region, where the tropical rain forest changes suddenly to savanna. The sediment cores were taken bya avibrational-corer device placed at the top of aluminium tubes and samples in 1cm-section, considering the sedimentary facies. The samples for radiocarbon dating were analysed by radiometric technique, measuring the 14 C content in a liquid scintillation spectrometer for extended countings , or by AMs (accelerator-mass-spectrometer) measurements for poor-carbon samples. The total organic carbon (TOC) was analysed by a CHN analyser and the minerals (Kaolinite, Quartz, Calcite and Silice Amorphous) were quantified by FTIR spectrometry (Fourier Transformed Infra-Red). The Caracarana Lake core (RR94-9), considered for this study, was the most clear and continous lacustrine record amd showed a good agreement with the results obtained for the other studies in Amazonia and South America, regarding to the optimum holocenic climatic phase. We conclude, supported by the correlation between the silice amorphous and TOC and among the three detrial minerals, that the rise to Caracarana Lake as well as other ones situated in this region was hapenned in very modern times, during the mid and early Holocene, comprising the last few millennia. (author). 16 refs., 3 figs., 1 tab