Paleoincêndios como indicadores de mudanças climáticas durante o cenozoico (formação Boa Vista), Bacia do Tacutu, Roraima

Abstract

The studies involving records of paleofire from analysis of coal fragments in the Amazon region are still restricted and scarce. Fossil charcoal fragments, when well preserved, provide important information about their living environment, as well as about climate and vegetation. Based on this, this work aims to characterize the occurrence of paleo-fire events for the Cenozoic of the Tacutu Basin, associated with fragments of charcoal recovered from Boa Vista Formation deposits, and to make paleoclimatic and paleoenvironmental inferences. In order to elucidate chronostratigraphic issues and to better understand the depositional evolution of these deposits during this period, sedimentary faculties and simultaneously absolute age (LOE in quartz) were identified using the SAR protocol in fourteen sediment samples. For the paleoclimatic and paleoenvironmental interpretations, fourteen specimens of coal fragments from three locations were used within the limits of the municipality of Boa Vista, state of Roraima. The methodological steps adopted for the specimens consisted in the preparation of micrographs in three sections (transversal, radial longitudinal and tangential longitudinal) for observation and identification of the wood structures by means of Scanning Electron Microscopy (SEM), together with the 14C dating using Accelerated Mass Spectrometer (AMS), isotopic carbon analysis (δ13C) and Organic Petrography. Through the easy analysis, it was possible to establish that the deposits are constituted by five easy ones: Massive lateritic gravel (Gcm1), Ferruginous gravel (Gcm2), Massive Sandstone (Sm), Massive sand silt (Fsm) and Fossiliferous massive sandstone (Smf), with age range between 105.2 and 3.2 Ka, indicating that this unit is positioned in the Upper Pleistocene-Holocene transition. The SEM results suggest that the charcoal fragments resemble the group of angiosperms, arboreal/shrub type plants, according to isotopic data, and that these woods have undergone two fire events. One is about 3,200 years cal BP and the other at 4,820 years cal BP, with burning temperatures between 340ºC and 400ºC. The set of data presented in this research constitutes the first approximation between chronostratigraphic indicators and regional paleoclimatic proxies corresponding to the Middle / Upper Holocene for the region, and therefore are consistent with global climatic episodes over recent geological time.