Integrating stable isotope analysis and geomicrobiology studies to understand the occurrence of micrite in the Bonito region, MS, Brazil.

Under the definition of continental carbonates, the tufas correspond to porous carbonate deposits, poorly stratified and with irregular texture, mainly composed of micrite. The presence of plant remains, invertebrates, and bacteria is quite common in these deposits. The Bonito region, located in the state of Mato Grosso do Sul, Brazil, presents extensive Quaternary tufa deposits. These deposits occur in various active and inactive precipitation environments and consist of tufa dams and waterfalls, and unconsolidated micrite deposits. The high rate of deposition, fossil and active in these tufas is related to the karst system developed on Corumbá Group region, which provides the high content of dissolved calcium carbonate in the water, water low turbidity, favorable climate, and microbial influence. The main sediment in the system is micrite, which has been identified in three main forms of occurrence: micrite in micro-organisms/abiotic, micrite in plant structures, and spray micrite. The study aims to characterize and understand the origin and processes controlling the micrite formation in Bonito/MS region by means of Xray diffraction (XRD), X-ray fluorescence (XRF) and stable C&O isotopes. For the present study, 16 samples from waterfalls and springs of the Sucuri and Mimoso rivers, surrounding Bonito/MS were selected. The mineral content is mainly constituted of pure calcite (XRD-id), the chemical composition varies between 46,21% to 54,19% to CaO, 1,02% to 1,50% to MgO, 0,31% to 9,46% to SiO₂, 0,15% to 2,04% to Al₂O₃, 0,11% to 0,86% to Fe₂O₃, 0,02% to 0,06% to Na₂O and 0,01% to 0,03% to MnO. The C&O isotope composition ranges from -7,89 to -9,52 ‰VPDB to δ¹³C and -7,27 to -8,58 ‰VPDB to δ¹⁸O. The δ¹³C values indicates biogenic influence, such as the presence of C4 plants and cyanobacteria, in addition to dissolved CO₂. The δ¹⁸O values are consistent with rainwater values in the region. The next analysis will be conducted using the scanning electron microscopy (SEM-EDS and WDS), to identify structures and characterize morphologic features of the micrites. The study of these precipitation/reworking controls will provide insights about the depositional environments and climate at the time of precipitation, including variations in temperature, salinity, pH, and nutrient availability throughout of time.