Paleoenvironmental reconstruction of the headwaters of the preto river, Minas Gerais state, Brazil, through siliceous bioindicators

Abstract

The Preto River, located in the municipality of São Gonçalo do Rio Preto (Minas Gerais, Brazil), is one of the few perennial watercourses in the semi-arid region of northeastern Minas Gerais and is essential for the socio-environmental dynamics of the region. Its headwaters are located in the Southern Espinhaço Mountain Range, which is characterized by predominantly quartzite lithologies that form a relief with dissected altimetric variations interspersed with planation surfaces, where the climate is high-altitude tropical. The soils are poorly developed and small fragments of forest, together with phytophysiognomies of the Cerrado Biome such as wet grasslands (Campo limpo úmido), colonize the peatland ecosystems. The aim of this study is to contribute to the paleoenvironmental reconstruction of the headwaters of the River Preto, using sliliceous bioindicators associated with other indicators (multi-proxy analysis). It was possible to infer that there was no change in phytophysiognomies from around 23,330 cal years BP to the present. However, there were 4 periods with oscillations between a cooler, drier environment in the Late Pleistocene, which gradually became warmer and more humid in the Late Holocene. Period 1 (Late Pleistocene) corresponds to the interval from before 23,300 to 13,500 cal years BP, in which the low tree density index (D/P) indicates vegetation similar to the current. Period 2 (end of the Late Pleistocene) corresponds to the period between 13,500 and 11,700 cal years BP. The increase in the D/P index indicates a small expansion of tree vegetation, albeit with the continued predominance of open vegetation. The appearance of Arecaceae characteristic phytoliths, associated with the drop in the climate index (Ic%), indicates an increase in temperature in relation to the previous phase. Period 3 (Pleistocene to Holocene transition) corresponds to the period from 11,700 to 10,000 cal years BP. It is a little colder and drier than the previous period, as observed from the increase in the climate index (Ic%). This period includes the Younger Dryas event, which was colder and drier than currently and had a global influence. Period 4 (Holocene) corresponds to the last 10,000 cal years BP. During this period there was no change in vegetation cover, the D/P and Pa/P indexes remain low, and there is a tendency towards a gradual increase in temperature. The use of siliceous bioindicators associated with other proxies and the regional and global events that occurred during the Quaternary in the analyzed sediments enabled the observation that, at least in the last 23,300 cal years BP, there have been no changes in the local vegetation cover. However, there were small expansion episodes or decreases in the density of tree and Arecaceae cover, as well as a tendency towards increased temperature and humidity in the Upper Holocene. The results proved to be useful for paleoenvironmental research and highlighted how peatland ecosystems are fundamental for understanding the evolution of the landscape and for reconstructing the environmental history of the Cerrado biome.