ONCOIDAL MICROBIALITES IN CONTINENTAL FRESHWATER CARBONATE DEPOSITS: QUATERNARY OF SERRA DA BODOQUENA FORMATION, CENTRAL-WEST BRAZIL

Depositional environments supersaturated in bicarbonate (HCO3-) and calcium (Ca2+) are highly favorable for the development of microorganisms that can extract CO2 (carbonic acid) from HCO3-. This adaption to uptake CO2 from HCO3- through the CO2-Concentrating Mechanism (CCM) favors carbonate precipitation. The excreted hydroxyl (OH-) increases pH near the cells and binds with H+ from HCO3-, causing the CO3- and Ca2+ binding, leading to newly-formed CaCO3 crystallites that can be trapped onto a microbial biofilm (extracellular polymeric substances - EPS) negatively charged. This biochemical interaction results in a carbonate construction that might be preserved in the geological record. In this context, oncoidal-coated grains exemplify a bioconstruction process as a product of microbial activity associated with bicarbonate (HCO3-) and calcium (Ca2+) in supersaturated environments. To understand these bioconstructions, mineralogical (petrographic, scanning electron microscopy, and x-ray diffraction) and geochemical (x-ray fluorescence, C&O stable isotopes) studies were carried out in oncoidal samples from Quaternary carbonate deposits (paludal and riverine tufas) of the Serra da Bodoquena Formation, Central West-Brazil, that occur overlying the Neoproterozoic metacarbonates of the Corumbá Group. The fluvial terraces consist of basal siliciclastic clay (Claystones), oncoidal allochthonous calcareous (Rudstones, Floatstones, and Grainstones), and powdery micrites layer (Mudstones). Oncoids, as the main component, show calcite micritic lamination with well-preserved EPS forming moldic porosity, as evidence of the CCM process, indicating In Vivo precipitation. We also identified smooth rhomb crystals consistent with post-mortem precipitation. Furthermore, we interpreted the bacterial shrub laminations as the remaining products of microorganism metabolism from the Rivulariaceae Family and the calcified cyanobacteria filaments as the Nostocaceae Family. Carbon and Oxygen stable isotopes in δ ‰VPDB performed in bulk oncoid samples (Rudstones and Floatstones) resulted in δ13C -5.55 to -6.13 and δ18O -7.37 to -7.51. The Grainstones showed a more negative δ13C -6.33 to -6.46, with similar δ18O -7.41 to -7.82. In this context, the isotopic data integrated with petrography and chemistry was the main tool to elucidate these research interpretations. Oncoids probably spread in shallow freshwater without macroorganisms, such as predators and higher plants. Since they require specific factors for their development, the most critical include (1) Incidence of light that allows cyanobacterial growth; (2) Water body features such as depth, temperature, energy, and low sedimentary input that enable nucleation and spread; (3) Physicochemical conditions of water and atmosphere that provide resources for cyanobacteria calcification, also predicting the oncoid framework; (4) Lack of predators and competitive biota, as a free-competition/predation environment. Our study might fill a depositional gap during the Pleistocene, with the oncoids related to deposition during a colder and drier climatic condition in Central West Brazil from (18000 yr B.P to 11700 yr B.P). They possibly can be associated with the Last Glacial Maximum (18000 yr B.P) and the Younger Dryas Event (12900 to 11700 yr B.P). Detailed studies with C14 dating and Clumped data could bring valuable information concerning the transition between the Pleistocene/Holocene in Central-West Brazil (Tropical South Hemisphere), constraining the ages and evolution stages of this Quaternary carbonate system and its climatic controls (Rodrigues et al., 2021). Rodrigues, A.C., Santos, L. da R., Cury, L.F., Rumbelsperger, A.M.B., 2021. Continental freshwater carbonate coated grains: oncoids in Quaternary deposits of the Serra da Bodoquena region, Central-West Brazil. Brazilian J. Geol. https://doi.org/10.1590/2317-4889202220210042