Modern non-marine carbonate precipitation associated with Mg-clay mineral formation: Case study of alkaline lakes in the Nhecolândia Region, Pantanal, Central Brazil.

Modern carbonate environments can serve as possible analogs to improve our understanding of the physical-chemical processes involved in the formation of ancient microbial carbonates deposited in association with various Mg-clay minerals. Nevertheless, to define a realistic model for carbonate and clay mineral precipitation, it is difficult to study a single modern example, which fulfills all the required comparable criteria, such as geomorphological, biological, climatic, volcanic and tectonic conditions. Although we are in the process of evaluating several modern microbialite-containing locations representing vastly different conditions, e.g., in the Pantanal, central Brazil, Patagonia, Chile, and Puna, Argentina, we focus this report on the modern lacustrine system found in the Nhecolândia Region of central Brazil. Thousands of fresh-water and alkaline-saline lakes exist in the Nhecolândia region. Geographically, they are distributed closely together throughout one of the world's largest river fan systems. Despite their close proximity, the lakes display a large chemical variability, e.g., the δ¹⁸O_SMOW values of the waters vary widely from -6 to +8 ‰. Previously, the salinity of the lakes was attributed to inherited conditions, but, more recently, the chemical variability has been associated with continuous evaporative concentration of the fresh water. Also, the variable chemistry is possibly related to the precipitation of calcite or Mg-calcite and formation of Mg-silicates as the aqueous solutions become more saline. Microbial activity in many of these water bodies mediates the production of carbonates associated with authigenic clay mineral precipitation, e.g., smectite through biologically-influenced mineralization. The accompanying extracellular polymeric substances (EPS) play an important role in mineral precipitation in these alkaline lakes, serving as a labile precursor gel or binding agent to concentrate various elements subsequently incorporated into the mineral precipitates. In summary, changes in the biological, physical and chemical composition result in increasing supersaturated conditions favorable to various microorganisms promoting carbonate and silicate precipitation.