A Evolução da paisagem de transição savana-floresta em Roraima durante o Holoceno tardio: base mineralógica, geoquímica e palinológica

Abstract

The Amazonia includes several vegetation physiognomies, besides the large and well-known tropical forest. The savannas, for instance, are present in several areas as discontinuous patches or covering extensive areas as it is the case of the savannas found in the north and northeast portions of the Roraima state. These savannas belong to the so-called Rio Branco-Rupununi Complex considered the largest continuous block of that vegetation type in the Brazilian Amazonian which in the state of Roraima covers about 41.000 km2 of a total of 53.000 km2 being limited to the south and west by tropical forests. In order to understand the vegetational dynamic and the evolution of the whole landscape in face of the possible climatic changes occurred along the geological time, the present study was carried out. The chosen area for this work represents a zone of transition between savannas and forests in the western portion of the savanna block, where four topossequences (FC, FH, TIA and RU) representative of the relief, vegetation and pedological cover patterns were selected for sampling. Then, samples of the regolith cover were collected starting at the base of the topossequences (veredas) until their top parts following the visible texture and color variations of the materials. Additionally, other two veredas (AM and MB) were also cored. These regoliths samples were submitted to analyses such as granulometrical by wet sieving; mineralogical by XRD and chemical (major and trace elements) by ICP-MS and XRF methods. The sediments of the veredas were still dated by radiocarbon method using the AMS (Accelerator Mass Spectrometry) technique, and also submitted to palynological analyses including the counting of charred particles. The regoliths of the studied area vary of sandy to silty sand textures being composed by quartz and kaolinite, and in minor amounts by muscovite, sillimanite, goethite, microcline and albite. The high values of SiO2 confirm the quartzose character of these regoliths, while the values of Al2O3, more expressive in the saprolite and in the sediments from veredas reflect the higher participation of the kaolinite, which is the single clay mineral identified in these samples. The mineralogical and chemical compositions of these materials indicate provenience of metamorphic rocks and laterites that faced with warm and wet climatic conditions have been suffered strong chemical weathering and leaching. The formation and wide development of vereda of Mauritia flexuosa since about 1550 years BP as showed by the pollen analyses corroborate the humidity increase in the area, which also favored the expansion of forests. In fact, trees of Virola, Alchornea, Melastomataceae and Moraceae among other were frequent suggesting that gallery forests as well as patches of secondary forests indicated by Didymopanax, Cecropia and Attalea occurred in this region in the most of the recorded period. In spite of those humid conditions, there are records of reduction of the forests about 1400-1100 years ( FC), 900-200 years (AM and FC) and between 700 and 300 years (site TIA) in favor of the expansion of the savannas. It is probable that the reduction of forests during those periods has been provoked by the increase in the intensity of fires, inferred by the highest concentration of charred particles in the sediments, prior and during the forest cover decrease. These fires are still common in the area, being more frequent in the neighborhoods of human establishments (indigenous) and cattle farms where they possibly work as a barrier to forest expansion. Although, alternate hydromorphic and hydrological stress conditions also contribute to prevent a forest expansion. The granulometrical, mineralogical and geochemical characteristics of the regolith cover combined to the sediments pollen and ages records allowed to interpret that the studied savanna-forest landscape, which is currently marked by a complex mosaic of grassy and woody savanna crossed by veredas of Mauritia flexuosa, gallery forest corridors and islands of forest covering sandy, quartzose and kaolinitic regoliths agree with the modern warm and wet climate conditions prevailing in this region since the Late Holocene. In compensation, features such as stone lines composed by fragments of quartz and crusts denote erosive events occurred probably in the Late Pleistocene and even in the Medium Holocene when dry to arid climates dominated the region. Then, the regarded landscape has been a stage of intense ecological and geomorphological transformations fomented mainly by the climatic changes imposed to the area, although the prehistoric and the modern man has also contributed, in the sense of delaying the expansion of the forests on the savannas.