Lacustrine Geochemistry in the Pantanal Fluvial Megafans - Brazil.

Thousands of small lakes are found on the surface of alluvial deposits in the Pantanal region, a fracture basin in the interior of South America, Brazil. They have different origins and a variety of water and bottom sediments. Some lakes are saline or alkaline, but most are freshwater. The diversity of lake properties has been investigated to arrive at a genetic classification. It has a semi-humid climate with strong water deficiency, but in summer rain is abundant and runoff overflows the drainage distribution channels and extensive flooding surfaces. The diversity of lake properties must be associated with different sources of water, rain, groundwater, paleoclimate and geomorphology. Understanding the dynamics and interaction of these elements contributes to the process modeling adjustments and forecasting of continental lake deposits. Lakes in two different lobes in area were investigated for imagery and geomorphic environment, physicochemical parameters, field observations, geochemical, hydrochemical and biological aspects. The data were prepared for multivariate statistical analysis at comparing, correlating and discriminating. The conditions of mineral formation and deposits were correlated with lake history. The results show that the oldest lakes are those preserved as deflation of a Holocene paleosurface. Later phases of drought lead, by evapotranspiration, to chemical enrichment of metals, heavy isotopes and highly alkaline water. Much of the early semi-arid landscape was reworked at different levels by subsequent alluvial processes and active tectonic influence. Other younger lakes are remnants of the cutoff channels. Some older preserved lakes have active calcium and magnesium rich waters, with active formation and phase changes in the dry and hot periods with evaporation increases and wet or rainy season. At some point in the pH transition, clay gels formed from microbial action, providing carbonate and authigenic clay mineral precipitation. The importance of climate fluctuations in the formation of these lake deposits in continental river systems reveals the need to consider these changes in the simulation and modeling of these sedimentary systems, especially reservoirs.